Fire Service Manual Volume 1. Fire Service Technology, Equipment and Media (1998) - part 3

 

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Fire Service Manual Volume 1. Fire Service Technology, Equipment and Media (1998) - part 3

 

 

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speed and should have enhanced the reliability of
pulling a cord. During the day most 'Sheltered'
this method, but no recent research has been pub-
alarms are monitored by the resident warden and
lished on the subject.
switched to a CAC when the warden is off duty.
Calls can also be triggered remotely by sensors in
Private circuits may, over relatively short dis-
smoke or intruder alarms, either in individual
tances, be established on a 'point to point" basis,
homes or in communal areas.
but over longer distances part of the path between
protected premises and the Central Alarm Station
When an alarm (which may relate to a range of sit-
is usually shared by many subscribers. In the latter
uations where help is required) is actuated, either
case, the many subscribers are connected via the
remotely or by a resident, an electronic signal is
alarm companies' ' Satellites' (data concentrators),
transmitted to the CAC. Detailed information
from where a large number of signals are multi-
about the caller is automatically displayed on the
plexed via private data circuits. This offers a reli-
CAC operator's computer screen.
able, fully monitored signal path at, possibly, the
highest cost.
This information could include vital data on the
caller's personal situation including, for example,
British Telecom's CARE system is quite economi-
medical details. Most of the calls received are not
cal because it uses an existing telephone line. Cost
emergencies and are usually requests for reassur-
is, therefore, independent of distance between the
ance or information. In many cases a CAC can
Central Alarm Station and the protected premises.
respond to a situation by sending out it's own
This system is now available in most areas of the
mobile warden service or alerting family, friends
country.
or neighbours. However, there will be times when
the operator needs to contact an emergency ser-
Vodafone Data Network provides periodically
vice, including the fire service.
monitored communications between a protected
premise and a Central Alarm Station, using Paknet
The policy of the national Association of Social
Radio Access. The cost is the same as a monitored
and Community Alarm Providers (ASAP) is that,
telephone line and is independent of distance
in an emergency, residents should always ring 999
between the protected premises and the Central
directly as this is the quickest way of contacting
Alarm Station. With radio coverage approaching
the Fire, Police or Ambulance Services.
95% of the UK population, Vodafone Data
Network is increasingly being adopted for alarm
There are times, however, when a call will be rout-
communications.
ed via a community alarm centre because:
6.2 Social and Community Alarms
many residents in sheltered housing do not
Centres
have a telephone and their only means of call-
ing for the emergency services is via their
Community (social) alarms are found in the homes
community alarm;
of over one million people, who are described as,
'vulnerable' in the UK. Most of those people are
some residents are able to summon help, per-
elderly and/or disabled but there are many other
haps by activating their pendant, but may not
groups including some of those discharged early
be capable of talking due to a heart attack,
from hospital. Many of the alarms are located in
stroke or fall etc. A CAC operator may,
individual dwellings and are connected to a
24
knowing the circumstances of the caller,
hour Community Alarm Centre
(CAC) via the
decide to call out an emergency service. The
PSTN telephone.
information which the centre possesses
may be vital in ensuring the most effective
A call to a CAC can be triggered by pressing the
response from the emergency services;
button on a portable pendant or on the telephone.
Similar alarms located in sheltered housing
some calls are automatically routed to the
schemes for older people are often activated by
CAC by a smoke or intruder alarm;
60
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some community alarm users are confused
In addition to the name and telephone number
and cannot easily communicate with anyone,
of the control centre and the address of the
including the BT operator. The CAC operator
incident, the CAC operator may give a refer-
is a vital intermediary; and
ence number of the housing scheme (for cross
reference to the PDA) and any access codes
some residents, perhaps because of dementia,
or instructions.
who have called the CAC do not realise that
the situation requires one of the emergency
2
Some Community Alarm Centres monitor
services.
calls over more than one Fire Service area.
These become 'out of area' calls which risk
In practice, Community Alarm Centres filter
being mis-routed. The procedure for these
out many calls which might otherwise have led
calls is that the CAC operator will dial 999,
to an unjustified 999 call.
give the name of the community alarm cen-
tre, ask for the relevant emergency service
There are approximately 350 CACs in the UK and
and instruct the BT operator to disregard the
the emergency services will regularly receive calls
CLI shown.
from them. CAC operators are in a unique position
to help emergency service control staff if a call is
The BT operator will ask for the CAC client's
routed via a CAC because they:
full telephone number which is then typed
onto the BT screen to give the connect-to
are trained and experienced in dealing with
numbers for the area from which the original
emergencies;
call was made. The community alarm opera-
tor will then talk to the EA operator and pass
are trained and used to dealing with vulnera-
details of the call.
ble people;
This process will add about six seconds to the
have detailed information about the person in
call but eliminates the risk of 'mis-routes'.
need of help;
BT operators will monitor for the duration
of the call.
will have detailed information on the address
of the emergency and on emergency access to
3
Community Alarm Centres should be given
sheltered housing schemes which can be
details of appropriate ex-directory numbers
passed to crews attending the fire;
for instant access to the relevant Emergency
Service Control Centre free of charge.
can reassure the individual and liaise with
them until the brigade arrives; and
can alert and liaise with other agencies and
carers that might need to be involved.
ASAP and CACFOA have agreed procedures
for filtering and passing emergency calls from
sheltered housing schemes to the fire service.
Should the caller require the fire service one of
three procedures will be used:
1
CAC operators will dial 999 for emergency
calls to addresses within their own area.
Communications and Mobilising
61

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Communications
and Mobilising
Chapter 7 - Automatic Vehicle Location
Systems
Automatic Vehicle Location Systems (AVLS) are
During the upgrade many of the large and expen-
not new, they have been used in the United States
sive resource display boards were replaced with
tor some time and by a number of security firms in
screen based resource displays incorporating sim-
this country. More recently Ambulance services
ple mapping or full Graphical Information
have taken advantage of the technology to enhance
Systems (GIS).
their vehicle availability and running times.
As Fire Service personnel became aware of the
Around this time many Fire Services were replacing
potential of GIS. it became obvious that questions
their mobilising and communication systems and
about AVLS, in relation to the Fire Service, would
revamping or moving into new Control Centres.
soon be asked.
Figure 7.1 An example of a screen format using an A.V.L. system. (Graphic: Fortek)
Communications and Mobilising
63
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HM Fire Service Inspectorate took the initiative,
Both systems require a data network to deliver the
and a consultancy project was set up to 'investi-
positional data to the communications centre. The
gate the applications and possible benefits of
Datatrak system utilises a national radio network
Automatic Vehicle Location (AVL) in the U.K.
set up specifically for AVL reporting. GPS based
Fire Service'.
systems require a mobile data network to be pro-
vided. This requirement can be met either by using
The consultancy contract was awarded to Fortek
a public data network or a PMR channel with a
Computers Ltd by the Fire and Emergency
data capability.
Planning Directorate as part of a Research and
Development programme managed by HM Fire
The final component of the AVL system involves
Service Inspectorate. The contract was awarded in
the processing and presentation of the AVL data to
October 1994 and ran for two years.
support the control room and management task of
a fire brigade. While the computation and delivery
The findings of this consultancy were published
of a vehicle's position can be achieved through the
in DCOL 8/1997 (in Scotland as DFM 8/1997).
use of commercially available components, the
application of this data to benefit a fire brigade
7.1 AVLS Technology
requires a degree of customisation.
An Automatic Vehicle Location System has the
capability to report vehicle positions to a central
control centre either at regular intervals or on
demand, or a combination of both. Several tech-
nologies are used world wide although in the U.K.
two systems predominate.
Systems based on GPS
(Global Positioning
System) utilise time signals received from a con-
stellation of 24 satellites moving through precisely
defined orbits to calculate the position of GPS
receiving equipment located in the vehicle. Timing
signals are transmitted by the satellites on an
almost continuous basis and hence the vehicle
position is always known providing sufficient
satellites are in view of the GPS receiver. A mini-
Figure 7.2 'Mobile Radio' data terminal. (Photo: Fortek)
mum of three satellites (ideally four to eliminate
certain minor inaccuracies) must be in view of the
receiver for it to calculate its position.
A terrestrial based system, as supplied by
Securicor Datatrak Ltd, uses a series of low fre-
quency radio base stations to distribute a matrix of
radio signals from which a Datatrak receiver can
calculate its position using a form of triangulation.
In most places both systems can determine a vehi-
cle's position to better than
100m and frequently
better than 50m. There are, however, locations (e.g.,
built up urban areas) where the accuracy of the posi-
tional calculations are compromised by physical or
geographical phenomena (e.g., high rise buildings
Figure
7.3 Securicor Datatrak Ltd data terminal.
obscuring the satellites from the GPS receiver).
6 4

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Figure 7.4 GPS Antenna
mounted in centre of
appliance roof.
{Photo: Fortek)
Figure 7.5 Data
terminal installed in cab.
Figure 7.6 Mobile Data
equipment in appliance
under rear seat.
(Photo: Fortek)
Communications and Mobilising
65

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AVL fitted appliances booked in attendance would
7.2 Potential Benefits of AVLS to
also be reporting their exact position and, hence,
the Fire Service
the position of the incident. Further information,
such as the position of the appliance when it
Potentially, AVL data can be used to assist in the
booked mobile to incident could also be saved for
deployment and mobilising of brigade resources
future response time analysis.
and to improve the quality of data recorded against
each incident.
7.3 AVL System Implementation
Knowing the precise location of each brigade
resource should enable the control room to opti-
AVL systems have been implemented in many
mise resource deployment and to ensure that the
commercial organisations and other emergency
most appropriate (quickest suitable) resources are
services within and outside of the U.K. An investi-
despatched to an incident.
gation into the performance of a number of these
systems and the experience gained from the Pilot
Present mobilising policy seeks to achieve this
System installed in Avon Fire Brigade has high-
objective by using Pre-Determined Attendance's
lighted features which will compromise the effec-
(PDA's) which are compiled on the basis of appli-
tiveness of AVL in the Fire Service, the more sig-
ances being at home station, and, hence, at a
nificant of which are discussed below.
known location when they are despatched to an
incident, which in a typical brigade will be the case
Most fire brigades see the main benefit of an AVL
for approximately 80% of incidents.
system as being the ability to identify and despatch
the nearest/quickest appliances to an incident,
regardless of whether they are mobile or not.
AVL data, therefore, has the potential to assist in
While theoretically this is possible, current AVL
the mobilising of resources to some 20% of inci-
systems have not been designed to meet this
dents by providing accurate positional fixes for the
requirement and it is not easily achieved.
mobile appliances. This information can then be
used by the mobilising system to compute the
nearest/quickest appliances to the incident.
The difficulties in achieving this principal objec-
tive arise from the errors in the data that will be
Generally speaking, the resource deployment strat-
used in the calculation of the nearest appliance list.
egy used by brigades necessitates standby moves
These errors derive from:
to key stations to cover station areas when appli-
ances from that station area are unavailable.
Inaccuracies in the incident location.
Without AVL, an appliance crew report their posi-
tion as within a station ground which in many
Inaccuracies in the reported positions of each
cases is a rather imprecise location.
mobile appliance.
Through the use of AVLS, a far more precise loca-
Inaccuracies in computing, for each appli-
tion is available for each resource and, hence, the
ance, the running time from its present posi-
opportunity exists to deploy resources to more
tion to the incident location.
accurately reflect the needs of the risk areas and
their corresponding standards of fire cover.
As with the existing PDA system the acceptable
level of inaccuracy will vary according to risk and
Until recently the only parameter available for
the associated standards of fire cover. In areas
defining location in the preparation of incident sta-
where parish, or area, mobilising is used, the
tistics and analysis has been station ground. This
largest source of error will invariably come from
situation has improved slightly in those brigades
the incident location. In urban and high risk areas
where the streets and places gazetteer includes a
where street and premises locations are held in the
grid reference which can be filed with the incident
gazetteer the inaccuracies in the reported vehicle
log and used in subsequent analysis.
positions will be the more significant.
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It should, therefore, be left to the brigade to spec-
Furthermore, in busy periods it is conceivable that
ify the accuracy required from the AVL system
appliances will get drawn across the brigade area,
and for the supplier to establish whether such an
on the basis of being the nearest available appli-
accuracy can be achieved, and at what cost.
ance in a sequence of incidents, into areas with
which they are not familiar and for which they
It is difficult to set an expectation of what can be
may not carry appropriate information.
reasonably achieved. However, as a guide, it is
reasonable to expect the computed running times
With current resource deployment and mobilising
in the list of nearest appliances to be accurate to
policies AVL data will be of relevance for approx-
within 90 seconds.
imately 20% of emergency calls, i.e., when mobile
appliances are considered for mobilising.
The 'Pilot' System showed that the required func-
tionality can be met if AVL positional updates are
Through the use of AVL data, it is possible to move
transmitted with each resource status update, upon
appliances to standby points other than fire sta-
a request from the mobilising system or control
tions while still being able to identify the nearest
centre operator and, periodically, at a rate depen-
appliances to respond to new incidents. Such a pol-
dant upon the resource status.
icy, operated in a limited form in one brigade, has
already shown savings by reducing the need to
If a public data network is to be used then the inter-
turn out retained fire stations
val for periodic updates could be several minutes
without seriously compromising the system
7.5 Implementation Costs
integrity. However, if updates can be delivered at
no cost other than network loading then the inter-
The infrastructure requirements (i.e., the need to
val set should be such that it does not impact upon
provide a two-way mobile data network) of a GPS
the other data traffic.
based AVL system are such that it would be whol-
ly uneconomic to consider setting up a system
Most mobile data terminals and portable PCs can
solely for AVL. For both operational and eco-
now be fitted with a GPS transceiver and most
nomic reasons a brigade should consider the
transceivers will compute the vehicle position to
introduction of (IPS based AVL only as part of
the level of accuracy required for fire brigade appli-
a programme to introduce mobile data.
cations. Certain units will perform better than oth-
ers in difficult areas such as urban areas where
If mobile data can be justified in its own right, then
satellites may be hidden from view by high build-
the incremental cost of introducing AVL will be
ings. Therefore, the performance of the proposed
relatively small and should definitely be consid-
GPS transceiver should be checked in various
ered.
key locations throughout the brigade area.
If the cost of introducing mobile data cannot be
The same approach is recommended if a terrestrial
justified, then the benefits which could be provid-
solution is being considered.
ed by AVL at relatively minor additional cost, may
make the difference in justifying the introduction
7.4 Operational Considerations
of the mobile data network.
Present mobilising procedures and PDA's reflect
The terrestrial solution offers a different approach.
the principle that, if available, an appliance will be
Since the required infrastructure has already been
despatched to an incident in its own station
put in place by Datatrak, it is viable to introduce
ground. In the 'Pilot' site, where this policy
AVL with relatively little up-front investment.
applies, there were numerous occasions where
Such a system should provide two-way data
other appliances, sometimes at home station and
which, for mobilising purposes, is considered
sometimes mobile, were calculated to be nearer
essential.
than the appliance in whose station ground the
incident had occurred.
Communications and Mobilising
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7.6 Conclusions
An AVL system operating as part of a two-way
mobile data scheme will provide a brigade with the
opportunity to simplify, and improve, its mobilis-
ing procedures by providing information which
can be used in selecting the nearest/quickest appli-
ances to an incident.
Clearly, busier brigades with a high proportion of
wholetime crews stand to gain the greater benefits
from AVL. However, even these brigades would
need to consider changing a number of existing
operational procedures.
There would also be a need to make a significant
investment in upgrading the data, particularly the
gazetteer, used by the mobilising system and pos-
sibly to upgrade the mobilising system itself.
Without such a commitment it will not be possi-
ble to realise the benefit of improved mobilising.
AVL data will also improve the quality of opera-
tional and management information, by accurately
locating all incidents, and logging resource jour-
neys to those incidents.
The technology exists to deliver quality data to fire
fighters and control staff alike, with AVL data
being just one element. In formulating a strategy
for the introduction of new technology it is inap-
propriate to view an AVL system as an indepen-
dent item, since it will only be effective if it is
introduced as part of a broader overall scheme to
improve the quality of data brigade wide.
At the time of writing (1998), the incremental cost
of including AVL technology as part of a mobile
data scheme will be relatively low. However, the
investment required to create the environment in
which it can be exploited will be significant.
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Communications
and Mobilising
Chapter 8 - Smart and Swipe Cards
Smart Card and Swipe Card systems have been in
ferent formats such as bar codes, magnetic strip
use both in the commercial and industrial arenas
and contact or proximity smart cards, which would
for many years, but have rarely been used for fire
work in the following way:
service related purposes. Now however, several
fire brigades are using them in a number of differ-
Staff members are issued with I.D. cards that
ent applications.
can be read by remote station terminals, each
card having been programmed by the Central
These cards employ a number of tried and tested
Processor with a level of entry to selected
methods of data capture and transmission, along
sites.
with associated readers. The reading technology of
these cards may be magnetic, bar code, proximity,
At each site an external device controls
inductive or 'smart chip'. Data collected remotely
access via the entry door lock. Members of
is passed via a preferred transfer medium, PSTN.
staff are able, therefore, only to gain entry to
ISDN or radio, etc., to a central point for collation
sites that have been pre-programmed through
and analysis, and can be integrated by both man-
the Central database to allow their access.
agement information and command and control
systems. For example:
bach site also has an internal data terminal
through which all persons entering are
(a) In one brigade bar coded swipe cards are used
required to swipe their card. The data relating
for access control and resource management for
to entry and exist is transmitted (either in real
retained personnel. These integrated systems have
time or batched) to the Central Processor Unit
advantages at all management levels and enable
(CPU) for analysis. This terminal, in effect,
control staff to know at any given time, the exact
logs personnel as 'in attendance' at that sta-
number and associated skills of retained personnel
tion, where they will remain until logging
on station. Brigade managers can analyse levels of
out.
response, availability of crews and performance of
both individuals and stations.
External and internal card readers at a site are
linked and the CPU will be aware of someone
These systems can be used to replace the use of
gaining access to the site without logging in.
paper methods to record and administer personnel
movements that require payment. Swipe card data
Appliances, officers, stand-by crews and
passed to the Headquarters network makes the
ancillary staff that require access to stations
automatic payment of fees possible, which is espe-
at any time are issued with an 'all stations'
cially useful in relation to retained personnel. Drill
entry card. All attendees are required to log
and training records can be updated and stored
out thus giving accurate timing of retained
electronically.
crews for payment purposes.
Access control eliminates conventional keys and
Each terminal is fully configurable from the CPU,
increases security at remote unmanned stations.
which means that after installation and commis-
sioning only maintenance visits should be neces-
The system could be provided in a number of dif-
sary by the supplier. If a card is lost or damaged it
Communications and Mobilising
69

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can be rendered void removing any risk of access
by the finder.
(b) Another brigade uses 'bar-coding' in a differ-
ent way: Each member of the Brigade, including
the Chief Fire Officer and Control Staff, are issued
with their own personal tally and bar code. At the
change of each tour of duty, the firefighters insert
their tallies onto the nominal roll board of the
appliance they are crewing. The nominal roll board
of the appliance also has a bar code detailing sta-
tion, call sign and appliance type.
In the event of a large incident, the Incident
Command Unit is mobilised and all appliance and
personnel tallies are handed in to the Unit. Each
bar code is then 'swiped' by an infra red 'pen' to
store the information on a personal computer (PC)
The bar code contains the following information:
Name, rank, station and watch.
Medical details
- blood group, allergies,
tetanus injections, etc.
Exposure to toxic substances.
Qualifications - LGV, BA wearer, etc.
Other skills which may include - foreign lan-
guages, plumber, electrician, etc.
Officer's car registration number.
At any point during the incident, the PC is able to
provide a printout of all personnel in attendance on
the Fireground. This information could be used in
the event of an evacuation. It could also be used to
determine relief crews and provide information to
personnel in attendance should there be a chemical
incident with potential long term effects.
Fire Service requirements are fairly basic at
present, but full integration with other, avail-
able computer systems could provide an effec-
tive management tool.
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Communications
and Mobilising
Chapter 9 - CCTV in the Fire Service
Most Fire Service staff will be familiar with the
can be provided in small robust units, and the
type of Closed Circuit Television (CCTV) used for
transmission and recording methods available
monitoring premises for security purposes, either
adapted to meet fire service needs.
in shops, car parks or used to survey headquarters,
remote fire stations or even Control Centres —
A number of brigades have been awarded test and
especially at night!
development licences by the Home Office to
assess the operational potential to transmit audio
Some years ago the Fire Service recognised the
and visual colour pictures within an incident
benefits of capitalising on the technical develop-
environment.
ments of video cameras, and their ability to trans-
mit images by a variety of means, having the
Microwave spectrum is used because a wide
potential to provide the Service with new tools to
bandwidth is required to transmit moving
improve the efficiency of rescue and assist with the
colour images.
command role at incidents. These objectives, as
well as the secondary, but important, benefits of
For example, one brigade concentrated on getting
improving debriefs, identifying training needs and
visual information from confined spaces, such as
informing the public accurately, led them to
collapsed buildings or sewers, and used video to
explore ways of using this equipment effectively
improve effectiveness of command at major inci-
on the fireground.
dents.
Because firefighters need to know as much as possi-
Another brigade uses two separate systems which
ble about the emergencies and the dangers they may
enhance their CCTV applications, one has a
be facing, the best substitute for seeing something
Microwave transmitter/Camera, and the other is a
directly is to have real time video of the same thing.
Cellsend System. A Bodyworn Microwave trans-
mitter/Camera System is also used.
Visual information of this kind does not add to
information overload in the way that manuals,
These systems are described below.
plans and procedural documents do. Irrespective
of whether the incident involves a collapsed build-
The Modular Remote Control Rapid
ing with casualties, a dog lost in a warren or a huge
Deployment Camera System - MAVIS, is a
fire that can only seen from one side, there are ben-
remote controlled camera sending high quality
efits in providing vision using available technolo-
vision and sound to a control unit operating either
gy which would otherwise be difficult, dangerous
in the ICLJ or, being fully portable, from a forward
or impossible to obtain. The use of video equip-
point or any remote location.
ment allows Fire Service personnel to achieve this.
The CCTV system consists of tripod mounted
Information may need to be relayed to a Strategic,
colour cameras, a maglight camera, bodyworn
Tactical or Functional Command, or the Control
camera and ISG thermal imaging camera as well
Centre. Without doubt. Officers want good quality
as a remote control decoder/microwave transmitter
information about major incidents for debrief,
and a control case. The control case is mounted
training, enquiries and public relations. Cameras
in the ICLJ video rack to receive and process the
Communications and Mobilising
71

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Figure 9.1 Detail showing camera mounting position.
Figure 9.3 One of the original tripod mounted hard
The transmitter can just be seen under the rim of the
wired cameras. These cameras are high definition colour
helmet. The camera sees approximately half the normal
but do not have a zoom facility. Both new and not so new
field of normal vision.
technology work well together.
(Photo : BedfordshireFire and Rescue Service)
(Photo:BedfordshireFireandRescurService
signals from the microwave cameras. It can oper-
ate independently of the ICU.
The bodyworn high resolution colour camera
(about
50cm long and lcm square) is mounted
under the right rim of a helmet A microwave trans-
mitter is mounted under the rear rim of the helmet.
This transmitter takes its power from a harness
mounted battery pack, the antenna is also mounted
on the rear rim protruding upwards.
The harness also has a lapel viewer in the form of
a small LCD screen which allows the wearer to
monitor the video picture being transmitted.
The camera is wired to the transmitter through a
connector to allow connection of the ISG ther-
mal/video overlay camera if necessary.
Figure 9.2 BA wearer equipped with high definition
The 'Cellsend' system uses digital technology to
helmet mounted colour camera. The combined battery
send video images from the ICU to the Control
pack and control module {mounted on the waistbelt)
Centre.
permits changeover to thermal images from the TIC. The
microwave antenna and transmitter are mounted on the
Two modems are installed, one in the ICU and the
helmet to ensure both protection and height for better
other in the Control Centre. The modem in the ICU
image transmission. (Photo . Bedfordshire fire and Rescue Service)
72

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Figure 9.4 Video rack
installed in an Incident
Command Unit. At the
top are the quad and
master monitors. Below
this is the real/lapse time
video recorder. Below
this is the Cellsend
modem that permits
transmission of pictures
to Brigade Control via
GSM telephone.
Below this is the video
processor for manipulat-
ing up to 8 inputs,
freezing images and
electronically enlarging
if required. Other facili-
ties are also available.
The unit below this con-
trols the mast mounted
camera. The bottom
(lighter) unit is the
control case for MAVIS
and the receiver for the
microwave transmissions
from both MAVIS and
the helmet camera which
normally operates within
the ICU.
(Photo : Bedfordshire Fire and
Rescue Service)
is linked to a mobile GSM phone, while the other
the images can be recorded at both ends, it is also
is linked to an extension of the Meridian telephone
possible to incorporate an audio facility. 'Cellsend'
system. The GSM link can be established from
does not require a PC to operate the system.
either end.
Results from tests in these Brigades indicated
Control Centre Operators can choose the resolution
the technology worthy of further research.
best suited to the image. In high resolution the
image is updated a few seconds behind real time, at
The general public are more aware of the Fire
the lower resolution the image is updated more
Service than they were in the past. Unusual inci-
slowly but technology will continue to improve
dents are of widespread interest, especially where
these times. This update rate is achieved by only
rescue is involved and these pictures are in heavy
updating those parts of the picture that move and
demand from news media.
Communications and Mobilising
73

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The opportunities provided by developments in
communications, computing and video technology
enable the Fire Service to provide a more effective,
efficient and safe front line service.
74

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Communications
and Mobilising
Chapter 1 0 - Radio
enable Fire Service personnel to get the best possi-
ble use from what is a highly sophisticated techni-
cal resource.
There is a continuous demand for improvements
and expansion to radio schemes; for additional
radio 'channels', which permit appliances at differ-
ent incidents to be dealt with independently; and
for new facilities of various kinds. Unfortunately,
the unlimited expansion of radio as a medium of
communication is not possible. It is a finite
resource with clear limits and, as a result, the
extent and purposes for which radio may be used
are strictly controlled.
Figure 10.1 Appliance radio in use.
The Radio Frequency and Communications
{Photo: HM Fire Service inspectorate)
Planning Unit (RFCPU) undertakes the setting of
communications standards and deals with medium
The Manual of Firemanship first contained, in
and long term planning needs to ensure that the
1954, a section dealing with Radio. At that time,
best possible communications are provided, taking
the majority of fire brigades shared the radio
into account the rapidly changing technology to
scheme (system) used by the local police force.
which radio communications, in particular, are
The situation now is completely different; for
prone. Such work includes next generation sys-
many years every fire brigade has had its own
tems for Emergency Service use, speech and data
radio scheme with a high percentage of fire appli-
security, trunking radio schemes, satellites,
ances and other vehicles, equipped with modern
replacement alerter system planning/evaluation,
'transceivers'
(radio sets capable of transmitting
mobile data, underground radio and line communi-
and receiving).
cation.
In addition to the standard radio sets fitted in vehi-
10.1 Frequency Spectrum
cles which communicate primarily with the
characteristics, selection
Brigade Control Room, there is specialised vehicle
radio equipment which can communicate with per-
and allocation
sonal radio sets. These provide on-the-spot fire-
ground communications and can have the added
10.1.1 The Frequency Spectrum
facility of being able to link personal radio sets
into the main brigade radio scheme.
Radio signals travel through space as a 'wave'
which, for the purpose of this explanation, can be
The primary objective of this book is to provide a
likened to a wave on the surface of water. Every
basic knowledge of how radio schemes work, their
such wave consists of alternative crests and
capabilities and their limitations, sufficient to
troughs to which the following terms apply:
Communications and Mobilising
75

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CYCLE - the portion of the wave between succes-
The highest frequency currently in use for radio
sive crests or troughs, which is repeated over and
communication within the fire service is about
over again to form the continuous wave.
2,300,000,000 Hertz, corresponding to a wave-
length of 0.13 metre
(13 cms). The Police use
WAVELENGTH - the distance between succes-
higher frequency bands up to 50 GHz for very
sive crests, or successive troughs.
short (5 kms) links.
FREQUENCY - the number of cycles of wave-
1000 Hertz (Hz) is called 1 kiloHertz (kHz)
lengths, which appear to pass a given point in a
10,000 Hz
=
10 kHz
specified time, usually one second. Wavelength,
1000 kHz
=
1 MegaHertz (MHz)
frequency and velocity are related in a very simple
1000 MHz =
1 GigaHertz (GHz)
way:
Thus
2,000,000,000 Hertz is more compactly
Velocity = Frequency x Wavelength.
called either 2000 MHz or 2 GHz.
However, this formula does not show the relation-
The following are two worked examples using the
ship very clearly. Normally the velocity is a con-
above formulae:
stant for a particular type of wave in given condi-
tions so more specifically:
If a transmission has a wavelength of 4 metres, what is
the frequency?
Velocity (constant)
Frequency =
Wavelength
Frequency =Velocity (constant)
Hertz
Wavelength
or,
where
Wavelength =Velocity (constant)
Frequency
Velocity
300,000,000 metres per second
Wavelength
4 metres
Radio waves are just one form of what is known as
'electromagnetic radiation', other forms being
Thus
'micro-waves', infra-red (heat), visible light, ultra-
300,000,000
violet and X-rays.
Frequency =
= 75,000,000 Hz or 75 MHz
4
These all have one very important common charac-
teristic which is that they all travel through space
with the same very high velocity. This is 300 mil-
If a transmission has a frequency of 450 MHz, which
lion metres, or
186,000 miles per second. For all
is a similar frequency to the Fireground channels, what
earthly distances this is virtually instantaneously.
is the wavelength?
The only difference between the various forms of
Wavelength =Velocity (constant)
electromagnetic radiation is that they each occupy
Frequency
different ranges of frequency and, hence, different
where
ranges of wavelengths. Radio waves occupy the
lowest range of frequencies
(and, hence, the
Velocity
300,000,000 metres per second
longest range of wavelengths) followed by infra-
Frequency
450 MHz or 450,000,000 Hz
red, visible light, ultra-violet, and X-rays. Even
though they occupy the lowest part of the spec-
Thus
trum, the frequencies of radio waves are quite high
in numerical terms. The lowest usable frequency
300,000,000
for radio communication is about
10,000 Hertz,
Wavelength =
metres = 0.88 metres
450,000,000
corresponding to a wavelength of 30,000 metres.
76

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Figure 10.2 The electro-
magnetic spectrum
Type of
Radio Waves
Infra
Ultra
X
Gamma
Radiation
Micro
Violet Rays
Rays
Red
Waves
Microwave
Ovens
Frequencies
30 300 3000 KHz
3 30 300 3000 MHz
Frequencies too high to express in
3 30 300 3000 GHz
"RADIO" units
The result is that the higher the frequency the
The result is that, whereas the various colour com-
shorter the wavelength. For radio waves, 'wave-
ponents of white light normally all behave in the
length' is measured in 'metres' and 'frequency' is
same way, the lowest range of radio frequencies,
measured in 'Cycles per second' for which a spe-
e.g., below 100 kHz, will behave quite differently
cial name 'Hertz' is used.
from the highest range, e.g., above
1 GHz. This
leads to the 'radio frequency spectrum' being
Figure 10.2 shows how the various forms of elec-
divided into relatively small frequency bands,
tro-magnetic wave occupy different parts of the
within each of which all frequencies behave in
range of frequencies which are known as the 'elec-
much the same way and are therefore, suited to a
tro-magnetic spectrum'. With the exception of vis-
particular purpose. Since every frequency has a
ible light the boundaries of the various forms are
unique corresponding wavelength the different fre-
not sharp and there is considerable overlap.
quency bands correspond to different 'wave-
bands'.
Our interest is with the radio wave portion, extend-
ing slightly into the micro-wave portion, and that
10.1.2
Characteristics of the different
is expanded in Figure 10.3 with the corresponding
Frequency Bands
wavelengths added.
Figure 10.3 shows, in very broad terms, how the
Radio waves occupy a wide range of frequencies
different frequency bands (wavebands) differ with
with the maximum being several million times larg-
particular reference to the way they travel through
er than the minimum. This contrasts with the very
space - their 'propagation characteristics', the size
narrow range occupied by visible light in which the
of the aerials and the power required.
maximum is only about twice the minimum.
Frequencies
KHz
30
100
300
1000
3000
KHz
GHz
1
3
10
30
MHz
1
3
10
30
100
300
1000
3000
Low
Medium
High
Very High
Ultra High
Super High
BandNames
Frequencies
Frequencies
Frequencies
Frequencies
Frequencies
Frequencies
Long Waves
Medium
Short Waves
Short Waves
(UHF)
(SHF)
(LW)
Waves (MW)
(SW)
(VSW)
1000
3000
1000
300
100
30
10
3
1
0.03
0.1
0.03
0.01
Wavelength
(metres)
CMS 100
30
10
3
1cm
Figure 10.3 Divisions of the radio spectrum
77

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Consider the size of the aerial. It is common
Low Frequencies (LF) or Long Waves (LW)
knowledge that, for receiving, the size of the aeri-
30 - 300 kHz
10,000 - 1000 metres
al is not very important, indeed the vast majority of
Follow earth's curvature. Not screened by moun-
transistor radio receivers operate very well with no
tains etc. Consistent long range both by day and
visible aerial at all. The aerial is a coil wound
by night. Requires very high transmitter powers
round a magnetic rod (ferrite aerial). However, for
and very big aerials. The top end of the band is
transmitters the position is totally different; for
widely used for broadcasting.
effective transmission an external aerial is essen-
tial and its length must be carefully matched to the
Medium Frequencies (MF) or Medium Waves (MW)
wavelength being transmitted. For the type of aer-
300 - 3000 kHz
1000 - 100 metres
ial fitted on vehicles the correct length is almost
Longer ranges by night than by day. Rapidly
precisely one-quarter of the wavelength; e.g., at 30
varying effects at sunrise and sunset. Requires
MHz, wavelength 10 metres, this would be 2.5
high transmitter powers and big aerials. Widely
metres. For higher frequencies it is shorter, but for
used for broadcasting, Ship-shore radio, marine
lower frequencies it is longer.
navigational aids, etc.
High Frequencies (HF) or Short Waves (SW)
From that, and Figure 10.3, it can be seen that, cur-
3-30 MHz
100-10 metres
rently, only two parts of the radio frequency spec-
trum are suitable for land-based, mobile and per-
Short range over ground, but reflection from
sonal radio schemes: the VHF and UHF parts.
upper atmosphere gives very long range both by
Unfortunately these parts are also eminently suit-
day and by night with very little power.
able for many other uses, notably the entertain-
Vulnerable to atmospheric disturbances, sunspots,
ment side, i.e., broadcast radio and television.
etc. Frequency changes needed every few hours
to maintain continuous communication. Widely
used for long range communication.
There are also allocations to marine, aeronautical,
armed services, public utilities, and other commer-
Very High Frequencies (VHF) or Very Short Waves
cial user requirements. There is, therefore, only a
30 - 300 MHz
10-1 metre
limited allocation available to the emergency ser-
vices, of which the Fire Service is only one.
Screening and reflection by hills, large buildings,
etc., becomes noticeable, gradually approaching
visible light characteristics giving significance to
10.1.3 Frequency Selection and Allocation
line-of-sight. Generally short range over ground,
20 miles (30 km) average, almost wholly depen-
From the spectrum characteristics in Figure 10.4 it
dent on upon line-of-sight, i.e., height of aerial.
is clear that the allocation of radio frequencies is
Fairly constant results both by day and by night
not a matter which can be handled in isolation by
but vulnerable to long-range interference during
any one service, by any one government depart-
abnormal weather conditions. Ideal for two-way
ment, or even by any one country. Agreement has
land mobile schemes due to relatively short aeri-
to be reached on an international basis as to how
als and moderate power requirements.
the different parts of the spectrum are to be shared
between the different types of service for which
Ultra High Frequencies (UHF) or Ultra Short Waves
they are best suited. For broadcasting, civil avia-
300 - 3000 MHz
1 - 0.1 metre
tion and the mercantile marine, operation in the
Broadly similar to VHF but closer still to visible
same bands of frequencies may be either by
light characteristics. Screening and reflection
regional cover or world-wide.
more noticeable, but less long-range interference.
Shorter range over ground, but line-of-sight even
Block allocations of frequencies, by function, are
more significant. Lower part of the band is ideal
agreed from time to time at conferences of the
for two-way, hand-held personal radio schemes
International Telecommunications Union, of which
due to very short, but efficient aerials and low
practically all countries are members. These block
power requirements.
allocations by broad function are then divided nation-
ally among the various users of each type of service.
Figure 10.4 Characteristics of different frequency bands
78

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In the United Kingdom, control of the frequency
which can be made available within a given fre-
spectrum is vested in an inter-departmental com-
quency bandwidth. A 100 kHz allocation will take
mittee comprising representatives of all
two 50 kHz channels or eight 12.5 kHz channels.
Government departments with responsibility for
This is shown in Figure 10.5.
frequency-using services. These include the
Radiocommunications Agency of the Department
Nevertheless, there are still not nearly enough
of Trade and Industry, the Home Office and the
radio channels available to meet the growing
Ministry of Defence.
(See the section on
demands from would-be mobile and personal radio
Regulatory Issues.)
users. Further reductions in channel widths and
channel spacing will inevitably be sought as tech-
10.1.4 Channel Spacing
nology continues to improve. The alternative,
using digital technology, is to place multiple
It is not possible to convey information by using
speech channels onto one radio carrier by giving
just a single frequency. A narrow band of frequen-
each one a time slot. The TETRA system which
cies is required which is known as a 'Channel'.
is proposed for the Public Safety Radio
Different channel widths are required for different
Communications Project (PSRCP) has four speech
services: for example, a television video channel
channels in a 25 kHz bandwidth channel, whereas
must be many times wider than a speech channel.
GSM, which is the system used for digital cellular
Channels are normally known by their centre fre-
radio, currently has
8 speech channels in a 200
quencies and the centre frequencies of adjacent
kHz bandwidth channel. Advances in technology
channels must be separated by at least the required
will soon increase this to 16 speech channels in a
channel width in order that there shall be no over-
200 kHz bandwidth channel.
lap which would result in unacceptable interfer-
ence.
10.2 Radio Scheme Engineering
In fact the centre frequency spacing of adjacent
10.2.1 Modulation methods
channels are slightly greater than the 'bandwidths'
occupied. Several technical factors, including the
The technique of super-imposing a speech signal
design, build standard and achievable frequency
on a radio wave is called 'MODULATION'. The
stability all determine allowable channel spacing.
radio wave then becomes the 'carrier' for the
Technical advances have made it practicable to
speech and it is often referred to as the 'carrier
reduce channel spacing progressively from 50 kHz
wave', or simply, the 'carrier'. Basically the radio
to 25 kHz and, currently, to 12.5 kHz. Reduction
wave is a single frequency of constant 'amplitude'
of the channel spacing specification to which all
which means that all the peaks in the wave have
users and, hence, all manufacturers must comply
the same height and all the troughs have the same
has the effect of increasing the number of channels
depth.
100 kHz Channel
2 x 50 kHz Channels
4 x 25 kHz Channels
8 x 12.5 kHz Channels
Figure 10.5 Channel spacing.
79

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Modulation can be superimposed by varying either
not possible to receive, so the person receiving
the frequency or the amplitude of the basic radio
cannot interrupt. Any attempt to do so means nei-
carrier. Thus, there are two techniques currently in
ther person hears anything. A vital part of 'sim-
use in the fire service:
plex' operating procedure is the use of the word
'over'. The speaker must say the word before
(1) Frequency Modulation (FM)
switching from 'transmit' to 'receive', and the lis-
tener must hear the word before switching from
(2) Amplitude Modulation (AM)
'receive' to 'transmit'.
Whichever method of modulation is used, the
All equipment normally rests in the 'receive'
result is to produce 'side frequencies' just below
mode, and operation of a 'press-to-speak' key,
and above the carrier frequency. It is the presence
sometimes known as a 'pressel switch', switches
of these side frequencies which causes the radio
the equipment from 'receive' to 'send'. The key or
signal to require a small band of frequencies, and
switch must be released before transmissions from
they determine the 'bandwidth' of the signal.
other stations can be received.
Within the narrow channels used for mobile radio
'Simplex' working makes it impossible to speak
(12.5 kHz) there is little difference between AM
and listen simultaneously, but it has the advantages
and FM in terms of 'user-noticeable' performance.
of encouraging a concise and efficient operating
In schemes originally provided by the Home
procedure and an economy in the use of words,
Office prior to
1989, the main and mobile trans-
and of discouraging lengthy conversations.
mitters use amplitude modulation (AM). Whereas
Further, the equipment required is simpler than
in schemes provided and maintained by commer-
that needed for 'duplex' working.
cial suppliers they may use both AM and FM
depending upon a brigade's stated need to commu-
Single-frequency
nicate with any adjacent AM brigades.
Single frequency radio equipment is designed to
Simplex and Duplex
transmit and receive on the same frequency.
Clearly such equipment can only operate in the
The two terms can be taken as a pair. Within the
'simplex' mode and, in such equipment, the
context of emergency services' radio, 'simplex'
receiving portion is always effectively switched
working is that, while transmitting (sending), it is
off when the transmitter is activated.
Figure 10.6 Principles of single
frequency simplex working.
Aerial
Aerial
Transmitter
Transmitter
relay
relay
Receiver
Receiver
MAIN CONTROL
OUT STATIONS
80

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Single frequency working is not used in main VHF
sage. It also permits the engineering of multi-sta-
radio schemes between brigade control rooms and
tion, wide area coverage schemes.
mobiles, but single frequency personal radio
equipment is commonly used by fire brigades for
10.2.2 Talk-through
direct person-to-person working over short dis-
tances both with VHF and UHF. (Figure 10.6)
An important difference between 'single frequen-
cy' working and 'two-frequency' working is that
Home Office supplied VHF vehicle-fitted radios
'single-frequency' provides an 'all-hear-all' sys-
are capable of operating on the two VHF channels
tem, whereas 'two-frequency' does not. In 'two-
allocated to manpack working.
frequency' working, all the mobiles can hear con-
trol, and control can hear all the mobiles, but the
Double-frequency or two-frequency
mobiles cannot normally hear each other.
Double or two-frequency equipment is radio
A pip-tone 'busy' signal (short 'beeps' about one
equipment which is designed to receive on one
second apart) is, therefore, transmitted by control
frequency and transmit on another and all fire
whenever it is receiving from a mobile. It is an
brigade main radio schemes operate on this prin-
important aspect of radio scheme discipline that no
ciple (Figure 10.7). The need to occupy two chan-
mobile transmits when the 'pips' are on except in
nels of the limited available spectrum is a disad-
urgent, high priority circumstances.
vantage but that is outweighed by the advantages
it affords.
Although the mobiles in a 'two-frequency' system
cannot normally hear each other, there are occa-
'Two-frequency' working permits 'duplex' opera-
sions when it is more convenient for them to com-
tion but, in practice, all fire brigade mobiles are
municate directly rather than requiring the control
'two-frequency simplex', mainly because of the
operator to relay a message. To make this possible,
advantages of 'simplex' already given.
'two-frequency' systems are provided with a facil-
ity known as talk-through. 'Talk-through' is
The advantages of 'two-frequency' working are
selected by the control operator and, when it is
that it permits the control station to operate in the
selected, the incoming speech from any mobile is
'duplex' mode, which in turn allows a mobile to
'turned round' and re-transmitted. It is, therefore,
'break-in' to a control station transmission when
received by all other mobiles in exactly the same
urgent attention is required due to a priority mes-
way as speech from the control operator.
Figure
10.7 Simplex working - out
stations only
(double frequency).
Communications and Mobilising
81

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The control operator can, of course, still hear all the
deliberately off-set the frequencies of the main
mobile transmissions, and retains full control of the
transmitters within the allocated channel width.
scheme. When 'talk-through' is selected, 'pip-tone'
The 25kHz channel spacings used at the time per-
is automatically inhibited, either completely or
mitted at lest three slightly off-set main transmitter
whenever speech is received from a mobile, and
frequencies, which a mobile would receive as a
there may be an increase in the level of background
single integrated signal without any noticeable
noise and some degradation of speech quality,
interaction.
which may be noticed because of the link-up
between the incoming and outgoing channels.
However, with the compulsion to reduce channel
spacing to 12.5 kHz, the spaced carrier system has
'Two-frequency' working provides a measure of
had to be abandoned because the narrower channel
security because unauthorised listeners can only
width channel does not permit sufficiently large
hear one way, normally the 'outgoing' transmis-
offsets to prevent noticeable interaction.
sions from control to mobiles. However, in fire
brigade communications security is a lower priori-
10.2.5 The 'Quasi-Synchronous' or
ty than speed and it is usually more important for
'Common Frequency' System
two mobiles to talk to one another. 'Talk-through'
provides that speed, and some fire brigades choose
Fortunately, technical advances have improved the
to operate their schemes permanently on 'talk-
stability of the frequency generators used for
through'.
scheme main transmitters to the stage where they
can maintain an almost constant frequency over
10.2.3 Wide Area Coverage
long periods of time in spite of changing tempera-
tures, etc. The main transmitters at different sites
This implies that radio communication is required
are not exactly synchronous, but they are almost or
over an area greater than that which can be served
'Quasi' synchronous.
by a single base station, no matter how favourable
its location may be. All county fire brigade radio
For all practical purposes they all have the same,
schemes fall into this category with the result that
common frequency.
at least two, and in some cases more, base stations
are required. Hence they are known as 'multi-sta-
As an example, fire brigade scheme main trans-
tion schemes'.
mitters currently operate at about 70 Mhz. The sta-
bility of the 'quasi-synchronous' frequency gener-
One approach would be for the individual main
ators is such that the individual main transmitters
stations
(hill-top sites) to operate on different
in a scheme keep to within less than 0.5 cycle per
channels, in other words a number of single station
second of each other.
systems, and not an integrated scheme. However,
mobiles receiving from one main station would not
In conjunction with the method of modulation
benefit from the 'fill-in' effects of other main sta-
used, a scheme can be described as
'quasi-syn-
tions as they moved into difficult areas.
chronous amplitude modulation' or 'quasi-syn-
chronous frequency modulation', with the alterna-
The system adopted must appear to be a single
tive term 'common frequency' in place of 'quasi-
station system even though two or more stations
synchronous'.
are involved. It might be thought easy to set all the
main transmitters at all the hill-top sites on exact-
Doppler Effect
ly the same frequency, so that their signals merge
into one in the mobile receivers but it is, in prac-
An effect which may be apparent in 'quasi-syn-
tice, virtually impossible.
chronous'
(common frequency) systems, which
was never apparent in spaced carrier systems is the
10.2.4 The Spaced Carrier System
'Doppler' effect. This is the effect where there is
an apparent change of frequency whenever there is
The original, classic solution to the problem was to
relative motion between a transmitter and receiver.
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If the vehicle is moving towards a fixed transmit-
(1) Outgoing - main station transmit and mobile
ter the frequency appears to increase slightly, but if
receive.
the vehicle is moving away the frequency appears
to decrease slightly.
(2) Incoming - mobile transmit and main station
receive. These main frequencies use aerials
'Doppler' effect is of no consequence in a single
at, or near, the tops of the masts, and the aeri-
station scheme because the change is so small
als are almost always omni-directional to
compared with the channel width. Likewise, it was
cover the largest possible area.
unnoticeable in spaced carrier systems because the
changes are so small compared with the deliberate
10.2.7 Links
off-sets. However, Doppler effect may well be
noticeable in quasi-synchronous
(common fre-
The links between the control station and the main
quency) systems when a mobile is in an area where
radio stations can be by land line but the majority
it receives more or less equal signal strengths from
use radio links (see Figure
10.8). Each radio link
two hill-top sites and is travelling towards one but
has its own dedicated pair of frequencies so that
away from the other. One frequency appears to
there is no mutual interference. Directional aerials
increase while the other appears to decrease with
are used - commonly known as 'yagis' - similar,
the result that the difference super-imposes a 'war-
apart from size, to TV or FM sound broadcast aeri-
ble' or flutter on the received speech which varies
als. These aerials 'look at each other' from oppo-
with vehicle speed.
site ends of the link to 'beam' the signals and pro-
vide the 'point-to-point' mode.
Every effort is made to engineer schemes, by loca-
tion of hill-top sites, by adjustment of transmitter
Although, in general, each link has its own trans-
power, by use of directional transmitter aerials,
mitter and receiver and its own pair of dedicated
etc., so as to minimise the effects, but because its
frequencies, the outgoing links from control are
cause is a natural phenomenon it can never be
identical. Channels can be saved and made other-
completely avoided.
wise available if a single outgoing link transmitter
is used and its output is split between two or more
When a mobile is stationary in a position where it
aerials, each pointing at a main station. There is
receives more or less equal signals from two sta-
then only one outgoing link frequency but, there
tions, the small difference between the two fre-
must always be independent incoming link fre-
quencies may be noticeable as a slow 'whoosh-
quencies and link equipment.
ing'. Normally it is only noticeable when the trans-
mitter is on without any speech and it does not
At each main station, the link transmitters and
impair speech intelligibility. If it is intrusive, a
receivers are interfaced with the main transmitters
small change of position, to take advantage of
so that, for example, a signal from control is:
local screening from one station, can be advanta-
geous.
(1) Transmitted by the link transmitter at the con-
trol station.
10.2.6 Scheme Engineering
(2) Received by the link receivers at all the main
stations.
A number of carefully sited main stations (hill-top
(3) Re-transmitted by all the main transmitters.
sites) are required to give brigade-wide radio com-
(4) Received by the mobile receivers.
munication coverage. Figure
10.8 illustrates the
way in which such schemes are engineered. There
A similar sequence, in the opposite direction,
are a number of variations, particularly as far as
occurs when a mobile transmits to control.
the linking arrangements are concerned.
In general the above descriptions refer to the use of
Under 'two-frequency working' there are two
VHF High-Band linking. Some brigades also
main frequencies:
deploy onward linking at UHF. However, in recent
years there has been a move towards vacating both
Communications and Mobilising
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VHF and UHF linking in favour of microwave, or
10.2.8 Frequencies
land line links. RFCPU have issued policy state-
ments addressing this subject
(see section on
It is of interest to add up the number of frequencies
Microwave).
which are permanently needed in multi-station.
Mobile station
Main station "B"
Main station
Brigade H.Q.
(main control)
Mobile station
Hand-Portable
Station
Fixed
Mobile
station
Key
Main station "C"
Main outgoing frequency
Main incoming frequency
Radio link frequency
Figure I0.8 Multi-station double frequency area coverage scheme.
84

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wide-area coverage radio schemes. In the example
Remote control facilities are also provided to
shown (Figure 10.8), a permanent assignment of
enable control room staff to switch main station
eight frequencies is required with independent out-
equipment from 'Main' to 'Stand-by', such
going links to all stations, and this can only be
equipment can be changed either individually,
reduced to six if they all share a common outgoing
i.e., just one faulty piece of equipment, or collec-
link, that is, two main frequencies and either six or
tively, i.e., all equipment. In addition to the dupli-
four link frequencies to provide just one opera-
cation of equipment, a further safeguard is nor-
tional radio channel.
mally provided against complete link failure, per-
haps due to aerial damage. The equipment at all
10.2.9 Equipment
main stations is arranged so that in the event of a
link failure the station changes to 'automatic
It is normal practice in Fire Service radio schemes
talk-through'.
to provide two sets of equipment at every station,
known as the 'main' and the 'stand-by' equipment.
This means that any signals received from mobiles
Basically, only one is operational at any one time
by the main receiver are automatically re-transmit-
and their purpose is to ensure continuity of service
ted to the mobiles by the main transmitter instead
in the event of failure. Change-over from 'Main' to
of, or in addition to, being transmitted via the link
'Stand-by' equipment and vice-versa is normally
transmitter to control. This system provides some
under the control of the control station operator or
measure of service, which is better than none, until
supervisor. In addition, every station will have
the faulty link can be repaired.
several items of ancillary equipment.
Under the automatic talk-through system the
At the main control station of a radio scheme,
mobiles can at least talk to each other but the con-
facilities are provided to enable control room staff
trol is isolated if the link is completely severed.
to isolate the main stations individually when for
Even if the link still works one way the control
one reason or another they are troublesome (e.g.,
will either hear what is going on without being
when a temporary, very high noise level is caused
able to participate or will be able to speak out with-
by the effects of static electricity during a severe
out knowing whether anyone is receiving.
storm in the vicinity of a site).
Figure 10.9 Voice
infrastructure.
Duplicated
Appls.
Base
Fixed
Stations
mobiles at
Secondary
WCars
location
To hilltop sites (6)
Inter-Bde
Main ch's +
&
ch&ch 21/22 &
ch 21/22
Local Control
ICCS
OPERATORS
Communications and Mobilising
85

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Voice Infrastructure
tion' transmit frequency. Transmissions from such
a unit are received by control in exactly the same
Figure 10.9 shows a typical voice communication
way as transmissions from true mobiles and, apart
system infrastructure which comprises a number
from an identifying call-sign, are totally indistin-
of Control Officer operating positions, each of
guishable from them. Within the context of a
which is provided with a headset and microphone,
mobile radio scheme the expression 'fixed mobile'
a loudspeaker and means of making a radio chan-
is, therefore, quite logical and understandable.
nel selection. The operating positions provide the
Control Operator interfaces to the Integrated
If a 'fixed mobile' is installed in, or can be operat-
Communication Control System (ICCS) which in
ed from, the control room then, in the event of total
turn provides access to individual radio channels
link failure, a control room operator will be able to
and also to telephone circuits.
fully participate in whatever remains of the radio
scheme through the 'auto-talk-through' facility. In
The Main Fire Service voice channel is broadcast
this context, the 'fixed mobile' may alternatively
on low band VHF from a number of hill top radio
be described as 'reverse frequency' equipment
sites, at each of which is located either a single or
because its transmit and receive frequencies are
duplicated Base Station. Each Base Station is con-
the reverse of those for the normal outgoing and
nected to the control site by means of either a
incoming control channels. It can also be used as a
microwave link network or by a private wire cir-
realistic way of checking hill-top site performance
cuit. At the control site the receive and transmit
within radio range of control.
signals are brought together in a Voting unit. The
main purpose of the voter is to accept all of the
Although individual fire brigade systems are total-
receive signals from the hill top sites and select
ly independent, it is very useful, for the control
(vote) the best signal to pass to the ICCS.
rooms at least, to have access to the schemes of
neighbouring brigades. This facility is useful when
A local base station at headquarters is connected to
incidents occur over brigade boundaries or when
the ICCS to provide for fallback control of the
assistance is sought at large incidents. Such access
main VHF channel when it is operating in talk-
is also provided by a 'fixed mobile', each control
through mode, by operating on the mobile fre-
room having a radio which operates on the mobile
quencies. This base station may also provide com-
transmit and receive frequencies of the neighbour-
munication on fireground channels 21 and 22 in
ing brigade(s).
the locality of Fire HQ. Further levels of fallback
protection are also provided by means of a desk
The fixed mobile originally supplied by the Home
mounted mobile radio located at Fire HQ or at an
Office can also be programmed with the two-man-
alternative location.
pack frequencies (Channels 21 and 22) allowing,
when radio range permits, direct radio communi-
A local base station at headquarters is also con-
cations in an emergency when main scheme fail-
nected to the ICCS. This provides for inter-
ures occur, between control and vehicles.
Brigade communications with adjacent county
Fire Services.
10.2.11 Main Control
10.2.10 Fixed Mobiles
A radio scheme with a considerable number of
users all operating on the same channel is almost
A unit can either be fixed or mobile but it cannot
unworkable unless one station is made responsible
be both. However, in the radio sense, a 'fixed
for its overall control. That station is known as the
mobile' is a radio transmitter/receiver which has
'Main Control', or simply 'Control'. The 'two-fre-
all the attributes of a mobile radio (it might even be
quency' system automatically gives the control
physically identical) except that it is installed in a
station the ability to 'dominate' all other radio
fixed location, within a building, instead of in a
scheme users. Normally they can only hear control
vehicle. Such a unit transmits on the 'mobile'
and not each other unless talk through is a perma-
transmit frequency and receives on the 'main sta-
nent arrangement.
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In the Fire Service the main radio control is invari-
cle battery, 12 volts, DC, with a 'mains power unit'
ably in the centralised mobilising control room for
which converts 240 volts, AC, direct to 12 volts,
the brigade, whilst the radio equipment is located
DC, without the need for a battery.
in an adjacent room or building known as the 'link
room'. Outside is a tower or mast on which the
10.2.14 Microwave
directional link aerials, each pointing at a distant
hill-top site, are fitted, along with simpler aerials
In the section on 'scheme engineering' it was
for any 'fixed mobile' equipment in the control
explained how as many as eight separate frequen-
complex.
cy channels are required in a three-station scheme
to support just one operational channel for a
The control equipment will be duplicated at two or
brigade. Additional stations require at least one
more operating positions, the number of such posi-
additional frequency (possibly two) whilst an addi-
tions depending upon the size of the brigade and
tional operational channel will require a complete
the number of separate radio channels it uses.
additional set of frequencies. Only two of the fre-
quency channels supporting each operational
10.2.12 Transportable Equipment
channel are used to actually communicate with the
mobiles - the main outgoing and incoming fre-
'Mobile' equipment in 'hand-portable' form can
quencies shown as 'f.a' and 'f.b' in Figure
10.8.
either be in a briefcase, haversack or 'backpack'. It
The remainder are 'link' frequencies, shown as L1
operates on the mobile transmit and receive fre-
to L6 in Figure 10.8 and they serve to carry speech,
quencies and contains its own (usually) recharge-
on a point-to-point basis, between the control sta-
able batteries. It permits direct contact with control
tion and the hill-top sites.
whilst away from a parent vehicle and is an alter-
native to the personal radios. Because of the lower
Until very recently the frequencies used for link
transmitter power, imposed by the limited weight
channels have always been in either the VHF band
for the batteries, and the less effective aerial, this
or the lower part of the UHF 2 band. An obvious
type of equipment is not as good as a vehicle radio
disadvantage is that frequency channels which are
particularly for transmitting back to control.
ideally suited for mobile communication on a
broadcast basis are being used for point-to-point
10.2.13 Power Supply Arrangements
links and, with the intense competition and
demand for additional mobile channels, that
The control and main scheme radio equipment at the
'waste' of mobile channels can no longer be toler-
control station, and all the radio equipment at hill-top
ated.
sites, are operated from the normal domestic electric-
ity supply of 230 volts, 50 Hz, AC. Fire Service per-
To release currently used VHF and UHF 'link'
sonnel should be aware of the potential danger aris-
channels for 'mobile' use, regulations now require
ing from the presence of such voltages and should
that all new point-to-point links shall immediately
never attempt to go inside any equipment.
operate in the microwave part of the frequency
spectrum and that all currently used VHF and UHF
At all key stations there will usually be a stand-by
'link' channels shall be moved to microwave as
power supply in the form of a diesel driven gener-
part of a 'rolling plan'.
ator with automatic start-up and change-over to
ensure the scheme is never put out of action by a
Definition
mains supply failure.
The term 'microwave' is one which has no precise
'Fixed mobile' equipment may be designed to
and universally accepted definition which fits in
operate direct from the AC mains, i.e., genuinely
with the generally accepted frequency and wave
fixed equipment made to operate on 'mobile' fre-
bands given in Figure 10.3. However, for our pur-
quencies, or it may be a 'mobile radio' made to
poses, 'microwaves' means frequencies above 1000
operate from a vehicle battery, 12 volts, DC, with
Mhz (1 Ghz), that is wavelengths shorter than 0.3
a 'mains power unit' made to operate from a vehi-
metre (30 cms). 'Microwave' ovens operate at 2.45
Communications and Mobilising
87

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Ghz so the frequencies which will be used for emer-
gency services links - in the range 1.8 to 2.3 Ghz -
can be legitimately described as 'microwaves'
although Figure
10.3 clearly shows such frequen-
cies to be in the upper part of the UHF band.
As far as the operational user of a radio scheme is
concerned, its linking arrangements - its 'scheme
engineering' - should be completely transparent,
in other words operationally 'invisible'. The user
may work on the assumption that radio signals
pass directly between the vehicle aerial and the
mast at brigade HQ, although it is better to appre-
ciate the limitations of radio communication over
the ground, and the need for linked multi-station
schemes.
Limitations
Figure 10.10 Micro-wave link in 'daisy-chain'
At microwave frequencies, an unobstructed line-
configuration.
of-sight path between aerials at opposite ends of a
link, is essential. This is in contrast to VHF links
for which a degree of obstruction from hills, trees,
10.2.15 Multiplexing
or buildings, was acceptable. As a result it is not
always possible to replace a VHF link path with an
'Multiplexing' on a wide band microwave link
identical microwave link path. Some reconfigura-
does not save on frequency spectrum occupied
tion, either re-routing between existing stations or
because the total width of a number of multiplexed
additional stations, may be necessary. In VHF
channels in a single wide band channel is greater
linked schemes every effort was made to link
than the sum of the widths of the same number of
every main station (hill-top site) direct from con-
channels in individual narrow bands. The big sav-
trol in a radial 'cartwheel spoke' configuration.
ing is on equipment - one anologue microwave
link can, for example, carry up to
36 separate
That was not always possible and in some cases a
speech channels — and on the number of aerials
very remote main station is linked to control
required on the masts.
through another main station which is then known
as a 'master' station or 'repeater' station. With
The multi-channel capability results in another
increasing congestion of the VHF band, the use of
potential change in linking philosophy. Whereas in
'master' or 'repeater' stations was becoming pro-
the past all the individual emergency services have
gressively unworkable, but the move to
had independent radio systems with perhaps their
microwave has removed that particular problem,
equipment in different rooms or even different
and microwave links are equally likely to be
buildings at hill-top sites, microwave linked sys-
arranged in a 'daisy-chain' configuration.
tems planned on a 'combined user service area'
basis to meet all the operational requirements of all
Microwave links are wide-band in contrast to VHF
sharers. Each system will be designed individually
links which were narrow-band. In this context nar-
taking into consideration:
row-band is
12.5 kHz, just one channel width,
whilst wide-band is several hundred kilohertz
Topography of coverage area, e.g. a county or
which is many channel widths. This means that it
counties.
is possible for a single microwave link to carry
many separate speech channels using a technique
Disposition of existing, and possibly future,
known as 'Multiplexing'.
hill-top sites.
88

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Disposition of operational controls, e.g., Fire
and Police Headquarters etc.
The number of channels and the routing
required by each user. Figure
10.10 illustrates a
possible linking arrangement with Fire and Police
HQs in different parts of the county.
10.3 Mobile, Transportable and
Personal Radio Equipment
10.3.1 Conventions
Although the actual use of individual channels is
not subject to regulations there are obvious dan-
gers if there is not disciplined use. DCOL 4/88 (in
Figure 10.11 A typical vehicle radio control unit.
Scotland DFM 5/1988) recommends that use of
{Photo: Simoco)
channels should be identified.
DCOL 6/1992 Item 12 Appendix 1 (in Scotland
The standard 'mobile radio' consists of two main
DFM 4/1992) recommends the primary and sec-
parts:
ondary use for each channel.
(1) the transmitter/receiver unit; and
Since 1 January 1993, 6 UHF 'at incident' chan-
nels and 2 VHF channels have been available, in
(2) the control unit.
addition an inter-agency channel is provided.
Other users, such as airport fire brigades and
The transmitter/receiver unit is the larger of the
works fire brigades may be permitted to use one
two and is placed within the vehicle. The smaller,
channel if the local authority Chief Fire
control unit is mounted in a convenient position
Officer/Fire Master considers this could improve
for operation by the driver and/or the front seat
operational efficiency and subject to the approval
passenger. A multi-core control cable with multi-
of RFCPU.
way plugs or sockets connects the two units
together. Connected to the control unit are the
This, for instance, enables an airport fire officer
handset and the loudspeaker. Provision is made for
instant radio contact with responding local author-
two loudspeakers so that one can be fitted in the
ity appliances equipped with UHF facilities.
cab and one at the rear of an appliance if required.
Connected to the main unit are the aerial and the
10.3.2 Mobile Equipment
battery.
The World Administrative Radio Conference in
Installation
1979 directed that all emergency services in the
UK still operating in the
88-108 Mhz VHF
The installation of radio equipment in all vehicles
Broadcast Band must move to alternative bands by
(motor cars in particular), is controlled by the
the end of 1989. Conversion of old equipment to
requirements of Health & Safety and compatibili-
operate in the new bands was not considered
ty (non-interference) with other sophisticated vehi-
worthwhile and the opportunity was taken to re-
cle electronic systems which are now fitted as
equip and standardise.
standard equipment.
Current mobile equipment for the Fire Service nor-
'Standard fits', in which the precise location of
mally receives in the 70-72 Mhz band and nor-
every part of the radio equipment is defined,
mally transmits in the 80-82 Mhz band.
should be agreed by the vehicle manufacturers, the
Communications and Mobilising
89

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Fire Service and the service provider. The staff
limited number are used. Channels
1 to 20 are
who actually install the equipment have no author-
'brigade allocated' and each brigade has made its
ity to deviate from the 'standard fits' because, in
own selection. The brigade's own channel or chan-
attempting to meet the wishes of vehicle owners, a
nels will normally be on channels 1, 2, 3, etc., as
physically or electrically dangerous situation may
required, followed by the channels of neighbour-
result.
ing brigades by mutual agreement. Channels are
selected using a numerical key pad and the illumi-
Further guidance on installation of mobile
nated display will show the channel number
radio equipment in fire appliances is available
entered.
from RFCPU.
Channel numbers
21 and
22 are allocated for
10.3.3 The Aerial
working both with man-pack VHF radio equip-
ment and directly between vehicles fitted with
It is a truism that any mobile radio is only as good
suitable radio equipment.
as its aerial, hence the design of the aerial, and its
location on the vehicle, largely determine the over-
10.3.5 Squelch
all performance obtained. The type of VHF aerial
currently fitted on fire mobiles is known as a
All mobile radios operating at VHF (and UHF)
'quarter-wave rod', its length being almost exactly
have an automatic'squelch' or 'mute' which com-
one quarter of the transmitted wavelength.
pletely switches off the receiver output to the loud-
speaker and earpiece when no transmission is
For a transmitter frequency of
80-82 Mhz, the
being received. The 'squelch' is necessary to sup-
wavelength is about 3.66 metres, so a quarter wave-
press the noise which would otherwise be heard in
length is just under one metre.
the absence of a signal. The receiver automatically
'opens up' when a signal of sufficient strength to
Ideally the aerial should be mounted in the centre
over-ride the noise and give an intelligible output
of a flat electrically conducting (i.e., metal) sur-
is received. An incorrect setting of squelch levels
face, such as the roof of a car or van.
aimed at reducing unwanted noise could mean that
very weak operational signals will not open up the
Many modern fire appliances have fibre-glass bod-
receiver.
ies and it is customary for an area of metal foil or
mesh to be moulded into the roof of the cab during
10.3.6 Transmission Timer
manufacture to which the aerial must be fitted. If
other roof-mounted equipment, such as ladders,
To avoid the risk of the transmitter being perma-
are fitted first, care is needed to avoid encroaching
nently locked on transmit due to a faulty handset
on the critical 'aerial space'. Metal close to the aer-
pressel switch or the handset falling into a position
ial will absorb the radio energy resulting in inferi-
where the switch is jammed on, the transmitter is
or performance.
fitted with an automatic transmission timer. A
jammed-on transmitter would block the complete
The aerial is connected to the transmitter/receiver
radio scheme, and, being 'simplex', the receiver
unit by a coaxial cable, similar to that used to con-
would be inactive so that no signals could be
nect a TV aerial to a TV set. The performance of
received. The transmission timer automatically
such cable is impaired if it is sharply bent or
switches off the transmitter after about 30 seconds
squashed even though there may be no visible sign
continuous operation. Normal transmissions rarely
of damage.
last that long but, if necessary, the pressel switch is
simply released and pressed again to continue
10.3.4 Channel Selection
transmitting. The 30 seconds time-out applies to
Home Office supplied mobile radio equipment.
The mobile radio (as originally supplied by the
However, similar principles, but possibly with
Home Office) has the capability of accessing up to
actual different time-outs, will normally apply to
255 channels but, at the time of writing, only a
all transceiver equipment.
90

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10.3.7 Power Supplies
230-volt domestic AC mains supply via a power
unit whose output is 12 volts DC.
The standard mobile radio is designed to operate
from the standard
12-volt DC vehicle battery
The radio can be controlled either 'locally' at the
source. Connection is made between the transmit-
rack or cabinet, or 'remotely' by a control unit
ter/receiver unit and the vehicle battery via a heavy
designed to fit in a console or be free-standing on
duty two-wire cable with a suitable fuse in the
a desktop. Emergency power can be made avail-
'non-chassis' (usually positive) wire, the fusehold-
able either via the building backup generator, from
er being as close to the battery terminal as possi-
the uninterrupted power supply, or a direct 12v DC
ble. The use of two wires, avoiding 'earth return'
switchcable or plugged battery supply.
through the vehicle chassis, helps with equipment
'compatibility' by reducing the risk of mutual
10.3.9 Special Features
interference with vehicle electronic systems.
(1) Single frequency working
No problem arises in standard 12-volt vehicles but
many larger vehicles and fire appliances have 24-
The standard mobile radio normally works in the
volt electrical systems. Radios can be built to work
"two-frequency simplex' mode communicating
off 24 volts, but it is not economic to have two
with 'Control' over the main VHF radio scheme
standards so 24-volt vehicles are fitted with
12-
via the hill-top sites and the linking system. The
volt radios.
outstanding feature of 'two-frequency' working is
that the mobiles can only hear 'Control'; they can-
There are two ways in which this can be done:
not hear each other unless the control operator has
engaged 'talk-through'. Direct 'mobile-to-mobile'
(1) By battery tapping. The 24 volts is normal-
communication is possible with 'talk-through'
ly provided by two
12-volt batteries 'in
engaged, but that ties up the whole of the 'main-
series' so the radio can be connected across
scheme'. The control operator may wish to moni-
the 'lower' one (the one with one terminal to
tor the messages, but all other mobiles are unnec-
chassis). This works reasonably well,
essarily involved.
although the battery supplying the radio will
be discharged more than the other one and
To provide greater flexibility the 'standard' mobile
this can cause battery maintenance problems.
radio will have one or two 'single frequency' chan-
The main disadvantage of this method is the
nels (usually channels 21/22) and any two or more
risk of a vehicle mechanic, unfamiliar with
mobiles switched to one of those channels will be
the unusual arrangement, re-connecting the
able to communicate directly and totally inde-
radio across the full 24 volts when replacing
pendently of the main scheme, within a very lim-
the batteries. This method is, of course, not
ited geographical area. The size of the area will be
possible if the 24-volt battery is a single unit
almost entirely determined by the intervening ter-
with no access to the intermediate
12-volt
rain and is likely to be severely restricted in heav-
point.
ily built-up areas.
(2) By using 24-volt to 12-volt converters. These
It is of course necessary to pre-arrange the switch to
units, which are readily available, are easily
the single frequency channel. It is even more impor-
fitted to 24-volt vehicles and are far more sat-
tant to switch back to the normal two-frequency
isfactory. However, there are cost, installation
channel because there is no way in which 'Control'
and maintenance overheads to consider.
can contact a mobile switched to the single frequen-
cy channel. Under normal circumstances permis-
10.3.8 Fixed Mobile Version
sion will be requested from 'Control' before a
mobile switches to Channels 21 and 22. The fixed
A fixed mobile version of the standard mobile
mobile can also operate on Channels 21 and 22 for
radio is available, designed to be fitted in a rack or
direct emergency communications with mobiles
cabinet in a building and powered from the normal
when no other normal channel is available.
91

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(2) CTCSS
10.3.10 Transportable Equipment
CTCSS stands for 'continuous tone controlled sig-
'Transportable' in this context, as distinct from
nalling system'. It is an optional feature, already
'mobile' or 'personal', means equipment which is
fitted in a small number of brigade hilltop
completely self-contained with its own batteries
receivers, mobile radios may be similarly fitted.
and aerial, which can thus be transported from
place to place and used anywhere, but which is
In the normal way the squelch or mute of a receiv-
usually set down, rather than operated whilst being
er is opened by the reception of a 'carrier' signal of
carried, as is the 'norm' for personal equipment.
adequate strength. The audio output of the receiv-
The distinction is, however, somewhat vague and
er is then fed to the loudspeaker or earpiece to
some transportable equipment is certainly capable
reproduce any speech modulation superimposed
of being used 'on the move' as is illustrated by the
on the carrier.
'hand-portable' (see Figure 10.2).
This normal system works reasonably well but it
The standard equipment has a 99 channel capabil-
has two disadvantages:
ity and the first twenty channels are 'brigade allo-
cated' in the same way as a standard mobile. It also
(1) There is no way in which individual mobiles,
has the same 'single frequency' capability using
or groups of mobiles, can be called indepen-
Channel 21 or 22. It has all the features and facili-
dently so that only those for whom a particu-
ties of a standard mobile except 'public address'.
lar message is intended will hear it; and
The biggest demand on the battery is during 'trans-
(2) There are circumstances in which a radio
mit' and a compromise must be made between the
receiver can be 'fooled' by natural or man-
transmitter power and the acceptable size and
made radio noise' so that its squelch opens
weight of the battery. Of necessity, the transmitter
when no real signal is present resulting in a
power is about half that of a standard mobile but in
'noise' output. This particularly affects main
all other respects the performance is identical.
VHF scheme hilltop receivers which control
rooms need to maintain a constant listening
watch.
CTCSS overcomes those disadvantages by super-
imposing a continuous low-pitched tone upon the
radio 'carrier' at the transmitter, in addition to the
speech. The continuous tone is used to open the
squelch at the receiver, after which it is 'filtered
out' so that it is not heard at the audio output. In the
outgoing direction
(hilltop-to-mobile) different
tones can be selected by the control operator and
different mobiles, or groups of mobiles, will
respond to different tones. This provides what is
known as 'selective calling' in which only selected
mobiles will receive the transmission.
When CTCSS is fitted to inhibit hilltop radio
receivers, remote technical arrangements must be
fitted to allow Control to switch off the brigade's
CTCSS. This arrangement is necessary to allow
non-CTCSS fitted mobiles to access the brigade's
radio scheme upon such a request from the non-
Figure 10.12 Hand-held radio in use.
CTCSS fitted brigade Control.
(Photo: Hertfordshire Fire & Rescue Service)
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