Carphone 1976 - System 3

SYSTEM 3 - The last GPO operator controlled Radio Phone system

SYSTEM 3 - 9 Channel Control unit

System 3 first incarnation

System 3 started with mobiles on which the user must manually select 9 out of 36 speech channels

The mobile only let him select an idle channel, an idle channel being one with WITHOUT carrier.

SYSTEM 3 - 55 channel control Unit

Storno System 3 55 Channel control unit

Later , but still called System 3, idle channels were identified automatically by mobiles as those WITH carrier and WITH 2 audo tones (eg 2.2 Khz and 2 Khz for London area), these mobiles know as 55 channel could tune to all the 55 channels used allocated to the GPO system

This picture and others of a 55 channel brief case unit with kind permission of

before system 4 was launched in London in July 1981, some of the radiophone 3 channels were taken out of service for system 4, this had to be done care fully as many of the 9 channel mobiles were still in use then.

GPO System 3 Radio Telephone 55 channel Handset

System 3 Time line

Launched in London in 1971(or was it 72?) with 37 25 Khz system channels - each radiophone could tune to 10 channels - estimated capacity 700 mobiles

In 1973 system 3 was launched in south lancashire( replacing the first ever GPO radiophone system)

In mid 1973 there were around 1000 users in the london area (system 3 wasnt available anywhere else)

18 more channels were added for the system to use in 1973 to make upto 55 channels. This allowed the expected london capacity to increase from 1750 to 3000

In 1974 the service was launched in the midlands

In 1974 a traffic study was done that concluded if all mobiles could tune to all 55 channels (instead of 9), the number of mobiles per channel would be increased by 30%

In 1975 the service was launched in "east penines"

In 1976 there were about 2500 users nationwide

In mid 1976 a 55 channel prototype was developed and the type approval was completed.

In 1976 the service was launched in "central scotland", "severnside and south wales","tyne-wear-tees"

In Dec 1978 there were around 6052 users country wide, 3264 in London

In April 1979 an article was published in the POEEJ on 55 channel mobiles ( 3 years after type approval)

Sometime before July 1981 some of the 55 channels were handed over for use on System 4.

In Jan 1983 dealers were given 3 years notice of switch off, customers were told a year later

System 3 and 4 ran in parallel

System 3 was switched off in England and Wales on 31 Jan 1986 ( A year after cellular launched)

System 3 was switched off in Scotland on 30 Dec 1986

Carphones cover new ground

The POTJ article below recaps on the UK's radiophone development so far. It mentions the "55 channel" radiophone that is documented more fully in a POEEJ article in April 1979.

It tells us that System 3 was launched in south Wales and severnside in April 1976, and that "central Scotland is to go live later in 1976

J Valentine Summer 1976

Radiophone coverage  map 1976

DURING the time it takes a busy executive to drive, say, 50 miles from one appointment to another it may be vital for him to contact his office, a client in another part of the country or even associates on the Continent. At one time it would probably have meant a delaying diversion in his journey to find a telephone. Today, when on the move in and around major business centres he can simply pull into the roadside and make the call on a telephone at his fingertips. Such fast and convenient communications for motorists have been made possible by a series of major extensions and technical improvements to the Post Office’s car radiophone service since its introduction in South Lancashire in 1959, The service now covers all England’s major industrial areas. With the eventual intention of providing national coverage, another extension is scheduled to become operational later this year which will cover a central belt of Scotland, including Edinburgh and Glasgow.Basically, the service enables a driver to make and receive calls in his vehicle over the public telephone network while in any of the areas covered by the service’s strategically located base radio stations. The driver can be linked with any of the 21 million telephones in the United Kingdom and provided the operator has direct International Subscriber Dialling(ISD) access to most countries in Western Europe, Canada, the United States of America, Australia, New Zealand, South Africa, Hong Kong, Israel, Singapore, Cyprus and Finland as well as main towns in the Irish Republic. From its introduction in South Lancashire, the Post Office radiophone service has been expanded to cover areas in London, the Midlands and, last year, the industrial area of the East Pennines.

The latest extensions, completed in April this year, take in 3,000 square miles of Severnside and South East Wales and an area of 2,000 square miles in the North East. Although all Britain’s major industrial areas will soon be covered by the radiophone service there is still much work to be done in extending the capacity of the system even further and in examining the marketing aspects. The Post Office hopes to organise meetings with approved equipment manufacturers with a view to staging and taking part in exhibitions to promote the service, and there might also be selective advertising and postal sales promotion to prospective customers.

The mobile equipment consists of a handset and control unit which may be fitted under the vehicle dashboard, a special aerial and a transmitter and receiver unit which is usually housed in the car boot. Recently a self-contained, portable set has become available which can be removed from the vehicle and carried over the shoulder.

Most present mobile units operate on any of 10 radio channels which have 25 kHz channel separation. One channel is for control and is used by the operator at the radiophone exchange to signal to mobiles, while the remaining nine channels are available for transmitting calls.

The nine traffic channels are allocated to different sectors into which a service area is divided. A channel selector switch enables the user to select a suitable free channel, and he then presses a call button to contact the operator. Ringing tone indicates that the call has reached the radiophone switchboard. Incoming calls to a mobile unit are indicated by a call lamp and a short buzzing tone. The user then contacts the operator using the procedure as for making a call.

A positive step to help cater for increasing demand, particularly in the London area, is the introduction of new 55-channel mobile equipment. This will gradually replace the existing 10-channel equipment, giving customers a much greater choice of channels and, in turn, affording an increase in user capacity of several hundred sets. Further increases would depend on achieving improvements in operating procedures to avoid any degradation in service.

A current problem is that the radiophone frequency spectrum allocated to the Post Office is being used to capacity and the Home Office may not be able to release further frequencies before the 1979 World Administrative Radio Conference. It is essential, therefore, to make more efficient use of the bandwidth currently available.

One way of doing this would be to switch from 25 kHz channel spacing to 12.5 kHz. Tests are being carried out to see whether this could be done without disrupting or modifying the existing system. If modification is necessary the high cost would need to be balanced against the fact that the capacity of the system would be doubled. And if this step was combined with a change to an automatic system, currently all calls are connected by operators, less channel time would be used setting up each call. As a result, capacity could be further increased. Even so, maximum use would still not be made of each channel. Pauses would remain while calls were set up and during breaks in conversation, but to utilise this time would require sophisticated electronic monitoring and switching equipment at all points in the system. At present this would be neither economic nor practical, but as both the size and cost of large-scale integrated circuitry reduces it does become a more realistic proposition.


Technical developments to date have already provided considerable advances in user capacity and improved service. The first system in South Lancashire had a maximum capacity of only 300 users, and while the second, opened in London six years ago, had a similar capacity it was based on an improved service, known as System 1. This provided the major advantage of selective calling, by which individual customers could be called exclusively. Previously a customer had to listen in to all calls, only “switching off” when he knew the calls were not for him.

The first two systems quickly became used to capacity and in London a waiting list built up. To allow for early expansion in London and other conurbations a new system was designed around standard, commercially available radio equipment which required minimal modification. Known as System 3, it was introduced in London in 1972 and today is the basis of all services now in operation. Although the new system was similar to its forerunner in some respects, there were some major changes. The use of 25 kHz spaced FM channels instead of 50 kHz allowed a more efficient use of the frequency spectrum, contributing greatly to increased capacity. Its initial capacity of some 700 customers was, in fact, more than twice that of the old system and was later developed to serve up to 3,000. Perhaps the most important advance in System 3 however, was the introduction of very much faster selective calling, call-connection times between the radiophone switchboards and customers being reduced from up to 30 seconds to less than two seconds. Another significant innovation was that customers could use their radiophones in any of the available service areas, regardless of their “home” area.

Before System 3, operators had no indication that a call had been received by a mobile unit until the customer lifted his handset and announced his identity. It could be many minutes before a driver found it possible to stop and answer, so operators often had to dial the same number several times. Under the new system the car set automatically acknowledges a call without the customer having to lift his handset, and operators are given a lamp signal to confirm that the call has been picked up by the vehicle. Lamps also provide positive clear down signals to indicate that a call has been completed.

Currently there are about 2,500 Post office radiophone customers throughout the country and there are clear indications that this number will continue to increase. In London alone applications are still being received at the rate of 50 a month, and demand for this service by 1980 has been estimated at 6,000. The Post Office is anxious to meet the challenge, and the next long-term major step could well be a switch to an automatic system.

Mr J. Valentine is product manager of new services in the Sales and Installation Division of new services in the sales and installation Division of Telecommunications Marketing Department

PO Telecommunications Journal, Summer 1976