Introduction to IFF and RADAR Beacons

IFF Introduction

The primary function of IFF equipment is to enable radar operators to recognise friendly aircraft and ships, and it was first introduced for this purpose into British aircraft in the early days of the war. Changing operational needs, and the increasing number and complexity of radar equipment, have since led to the introduction of new types of IFF, and the original version has long been obsolete. The fundamental principle underlying the method of operation, has, however, remained the same.

The IFF set, which is carried in aircraft and ships, consists essentially of a receiver-transmitter. The receiver receives and amplifies the pulse from a radar equipment and uses it to trigger the transmitter with inappreciable delay, so that if both the receiver and transmitter are tuned to the frequency of a particular radar equipment, a large echo will appear on the radar display tube. When reception and re-trans- mission occur on the same frequency the receiver-transmitter is called a responder, and when on different frequencies, a transponder. Thus, IFF sets are usually responders, but beacons are transponders.

RADAR Beacon Introduction

German Knickebein Beam Radar

In March 1940, Group Captain Blandy, Head of Y Service, the organisation that monitored Luftwaffe radio signals, had obtained a German message found in a Heinkel III bomber, that stated “Kleve Knickebein is directed at position 53 degrees 24 minutes North 1 degree West”. This meant that this cross beam was from the French border of West Germany. Later a German prisoner of War was overheard when talking to a comrade, stating that the RAF would never find the receiver.

Later a Heinkel III Bomber from a specialist unit of the Luftwaffe, entitled KGr100, had been shot down during a Scotland raid on the Firth of Forth. After a technical examination, of its radio equipment, by a Farnborough team, their attention was focused on a blind landing receiver in the aircraft. It was labelled E. Bl. I (which stood for Empfänger Blind landing receiver type 1) and was designed for the Lorenz system of blind landing, then a pre-war standard at many English aerodromes.

The Farnborough team, found that it had been modified and its sensitivity had been increased. On further examination they were able to ascertain the frequencies to which it could be tuned, and therefore on which the Knickebein beam must operate.

After further aerial photography of the proposed flight path, Dr. Robert Cockburn from TRE Worth Matravers, determined that the transmission aerial a massive 12m high x 30.5m wide dual beam German Freya aerial, mounted on a turntable, was located at Cherbourg. This transmitter provided the Approach beam that consisted of two beams, one transmitting Morse Code dots and its partner transmitted dashes these were inter dispersed, generating a continuous tone when the aircraft was positioned in the centre of both beams. In this overlapping transmission area, the pilot would hear a steady tone, known as the “eqi-signal” zone ¹.

On 14 June 1940, Air Marshall Sir Philip Joubert took command of the Air Ministry investigation on Knickebein beam. On June 16 he arranged a meeting with the Night Interception Committee and after in-depth discussions ordered that the beam, if found, should be JAMMED. In order to execute the command Dr RV Jones selected five Chain Home Radar stations, Ottercops, Staxton Wold, West Beckham, Bawdsey and Dover for the ground receiving station (Meacons) the observers were drawn from TRE Worth Matravers, near Swanage. The observers for the aircraft search came from R.A.F. ‘Y’ Service.

On 18 June, they did not detect any beams, they persevered and on the night of 21/22 the beam was discovered by R.A.F. ‘Y’ Service as it passed one mile south of Spalding, the beam width was 400-550 yds (366-503 m) ².

History of IFF

Used a system of direct interrogation. In this system the transmitter and the receiver of the IFF set both worked on the same frequency, and this frequency was varied continuously by rotating the tuning control by means of a small electric motor, so that it swept periodically through a fairly wide band. The time taken for each sweep was of the order of a few seconds. A radar equipment working at some frequency within this band received a response from the IFF responder only for a fraction of a second each time the responder tuning swept through the radar frequency. Thus, for the greater part of the time the radar display tube showed only a normal echo, but every few seconds this echo increased in amplitude for an instant. Echoes showing this periodic increase were identified as friendly. The duration of the return pulse or response from the IFF was of the order of a few microseconds. By a slight adjustment of the responder circuit, it was possible to vary this pulse width, so that the IFF set could show narrow, wide, or very wide pulses. By varying the widths of consecutive pulses, it was possible to obtain some measure of coding. In particular, a wide pulse of about 25 microseconds duration was used to denote distress. The widths of the transmitted pulses could be varied while in the air for coding purposes, by a remote-control switch available to the crew of the aircraft.

The older types of IFF used a system of direct interrogation. In this system the transmitter and the receiver of the IFF set both worked on the same frequency, and this frequency was varied continuously by rotating the tuning control by means of a small electric motor, so that it swept periodically through a fairly wide band. The time taken for each sweep was of the order of a few seconds. A radar equipment working at some frequency within this band received a response from the IFF responder only for a fraction of a second each time the responder tuning swept through the radar frequency. Thus, for the greater part of the time the radar display tube showed only a normal echo, but every few seconds this echo increased in amplitude for an instant. Echoes showing this periodic increase were identified as friendly. The duration of the return pulse or response from the IFF was of the order of a few microseconds. By a slight adjustment of the responder circuit, it was possible to vary this pulse width, so that the IFF set could show narrow, wide, or very wide pulses. By varying the widths of consecutive pulses, it was possible to obtain some measure of coding. In particular, a wide pulse of about 25 microseconds duration was used to denote distress. The widths of the transmitted pulses could be varied while in the air for coding purposes, by a remote-control switch available to the crew of the aircraft.
IFF Mk. I and II

The earliest type of IFF was the Mk. I. It was purely experimental and only about 50 models were produced; it is now obsolete.

IFF Mk. II was a production model developed after operational experience had been obtained with Mk. I. Like the Mk. I equipment responded to direct interrogation, sweeping a frequency band as described in para. 3. To be more precise, it covered three frequency bands, including the frequency ranges of CH and GL Mk. II stations and Naval type 79 equipment.

Limitations of IFF Mk II

Soon after the development of IFF Mk. II for instance, it became necessary to introduce the Mk. II G to cover the GCI band and the Mk. IIN to cover some of the higher frequencies of the newer Naval equipment.

This introduction of new types of Mk. II showed every sign of continuing indefinitely and was leading to an impossible situation, and in 1941 it became evident that a new system must eventually be introduced, using the principle of indirect interrogation.

In the indirect interrogation system, the IFF is not interrogated by the main radar equipment. Every radar equipment which needs to interrogate IFF is provided with a second subsidiary equipment.

This subsidiary equipment is itself a small radar set, and it uses a different frequency from the parent equipment. It is called an interrogator, and consists of a transmitter which radiates radar pulses in the normal way, and a receiver or responder which receives the response from the IFF.

All IFF sets work on the same frequency band, and all interrogators have a frequency somewhere in this band, so that any radar set, no matter what its frequency, can, if provided with an interrogator working in the IFF band, interrogate the IFF in any aircraft.

The interrogator is normally of very low power, so that normal echoes from aircraft are too weak to appear on the responder display tube, and it can only see the relatively strong IFF responses. It is generally locked to the main radar equipment so that its pulses are radiated at the same instant as the radar pulses, and it often has a counting-down circuit incorporated, which causes it to transmit only on every fourth, fifth, or sixth trans- mission of the main radar.

The later Marks of IFF employ the system of indirect interrogation for most purposes, although there are special cases in which direct interrogation is still used. These systems are discussed later ³.

Limitations of IFF Mk II cont.

IFF Mk. III set is a super-regenerative transponder. It has suppression circuits which stop its operation during the operation of Lucero and of other equipment which may be carried in the same aircraft. It also has an AGS circuit which automatically stabilises the gain over the whole frequency band.

The older types of IFF necessarily had horizontal aerials because the radar stations with which they operated used horizontal polarisation.

Further, the sets had to work over such a wide band that it was not possible to match the aerial systems into the IFF equipment, and this naturally led to considerable inefficiency in operation. This was not very important, however, because the power of the transmitters and the sensitivity of the receivers in the interrogating stations were so high that inferior working of the IFF was unimportant.

With the new Mk. III equipment, however, which is designed to operate with interrogators of low power, it is necessary to reduce losses to a minimum and the aerial must be matched carefully into the set. The relatively small frequency band over which the Mk. III sets operate makes this possible. Vertical polarisation was chosen for the new system, principally because it enables the airborne transponder to respond to interrogation from any direction. It also gives rather better coverage.

With ground interrogators where reflection takes place from the land, the Brewster angle effect gives partial gap-filling in the polar diagram, while with interrogators where reflection takes place from the sea, although the Brewster angle is very small at the frequency of Mk. III equipment, and the effect is not marked, it gives a greater amplitude than horizontal polarisation along the surface of the sea, and is therefore useful for ship-to- ship interrogation.

The Function of Radar Beacons

Transponders of the IFF type can be used not only for identification but also as navigational aids to aircraft. When used for this purpose they are termed radar beacons. Radar beacons were first introduced into service in 1940 and since that time there have been many types produced for different purposes. They fall into two main categories; homing beacons and beam approach beacons or BABS, and the function of each of these will be described separately.

Homing beacons are transponders, working on the IFF principle, but installed at some reference point, such as an airfield or a ship. Air- craft wishing to use these beacons carry interrogators working on the beacon frequency, and can receive responses back from the beacons in the usual way. Instead of continuously sweeping a frequency band, the beacon transponders work on a fixed frequency, so that the aircraft interrogator receives a continuous response. Most beacons do not re-transmit continuously but are switched on and off for short periods so that they return “flashes to the aircraft in the same way as visual beacons, and these flashes can be coded to give mores letters. Coding can also be arranged by the transmission of wide and narrow pulses as in IFF.

Aircraft using these beacons for homing have interrogators with specially designed aerial systems. The usual scheme is to have a forward-looking aerial for transmission and two separate receiving aerials, one mounted on each wing. The starboard receiving aerial has its line-of-shoot not along the line of flight of the aircraft, but a few degrees to starboard, while the port receiving aerial has its line-of-shoot offset by the same angle to port. The horizontal polar diagrams of the two aerials overlap. If the aircraft is flying directly towards the radar beacon the signals received by the two aerials will be equal; but if its course does not coincide exactly with the direction of the beacon, one aerial will receive a signal of slightly greater amplitude than that received by the other.

These receiving aerials are switched continuously, so that they receive pulses alternately. The display is of the range-amplitude type, the time base being vertical, and the received signal from the star- board aerial appears as a deflection to the right, while the signal from the port aerial appears as a deflection to the left. By noting the relative lengths of the deflections on either side of the trace, some indication of the bearing of the beacon can be estimated, and it is possible for the crew of the aircraft to home on to the beacon. ³.

Homing Beacon History

The first beacons were used by Coastal Command. They were modified IFF sets tuned to a fixed frequency of 176 Mhz to respond to the ASV Mark II sets in the Coastal aircraft. ASV Mk. II uses horizontal polarisation, so that these beacons had to be horizontally polarised also. With the introduction of BABS beacons, to be described later, which respond on a frequency of 173.5 Mhz., it became necessary to raise the frequency of response of the ASV beacons to 177 Mhz., so that the band would not overlap with that of the BABS. Thus, the present ASV beacons receive on 176 Mhz., and re-transmit on 177 Mhz.

Fighter Command soon followed the example of Coastal Command, and introduced beacons on their aerodromes. These early Fighter beacons, like the Coastal ones, were modified IFF sets. This time, however, they used vertical polarisation, because the Marks of AI with which they were designed to work used vertical aerials. They could be interrogated by AI Mks. IV, V and VI, all of which work on a frequency of 193 Mhz. They were interrogated on this frequency, and responded on a frequency of 196.5 Mhz, so that the AI receiver had to be detuned in order to receive their signals.

Since these early days, there have been several new and improved types of Coastal and Fighter beacons. A number of these newer beacons use super-hetero-dyne receivers, although some, especially the trans- portable types, are still super-regenerative transponders. The polarisation and the frequencies remain the same, however. All Coastal beacons use horizontal polarisation, receive on a frequency of 176 Mhz., and respond on 177 Mhz. All Fighter beacons are vertically polarised, receive on 193 Mhz., and respond on 1965 Mhz.

The Naval Air Arm next developed beacons for use on ships and shore stations. These beacons were interrogated by ASV Mk. II, which many NAA aircraft carried, and, like the Coastal beacons, they were horizontally polarised. The earlier NAA beacons swept a frequency band, as did the IFF sets, but later models are tuned to fixed frequencies, and are interrogated on 176 Mhz. and respond on 177 Mhz.

Transport Command also use ASV Mk. II beacons. Bomber Command does not use homing beacons at the present time, but uses Gee for homing purposes.

With the introduction of centimetre versions of ASV and AI, the problem of providing homing facilities became acute. The earliest equipment to work on wavelengths of about 10 cm. were AI Mk. VII and AI Mk. VIII. The first of these was a pre-production model, and only appeared in relatively small numbers. The second, however, was intended for general introduction into all fighter aircraft, and some form of homing beacon was required to work with it. For this reason an AI Mk. VIII beacon was developed.

This beacon was interrogated on the original AI Mk. VIII frequency (3285 Mc/s), and responded on a frequency of 3280 Mc/s. In order to see its response on the AI display it was necessary to detune the receiver. To do this the Mk. VIII equipment was supplied with two local oscillators.

One of these local oscillators was used for normal operation, and, when its oscillations were mixed with the incoming 3285 Mhz signal, they gave an intermediate frequency signal which was fed into the IF stage in the normal way. When the operator wished to home on to a beacon he switched over to the second oscillator, which was so tuned that when its oscillations were mixed with the incoming 3280 Mhz beacon signal, the beat frequency was the same as before, and could be fed into the same IF stage.

RADAR Beacon cont.

The bearing of the beacon is indicated by a left-right display as shown in fig. 1.

Fig. 1 Beacon Display Bearing of the Beacon. Reproduced by kind permission of Philip Judkins

Coastal Command homing beacons (all horizontally polarised)

The first ASV Mk. II beacon used by Coastal Command was the TR.3111. It was a modified IFF set, and has been obsolescent for a considerable time. It was replaced by the TR.3112 which was a super- regenerative beacon. This did not come up to expectations and it is no longer used. A fighter beacon, the TR.3107, which is super-hetero- dyne, was modified for Coastal working, to replace the TR.3112. Since then other beacons have also been designed.

All the beacons mentioned below use gap coding to identify them-selves. In this form of coding, the beacon response is switched on and off repeatedly, so that instead of seeing a continuous response on the interrogator display, the operator sees a response which appears for a space of time, disappears, appears again, and so on. By varying the duration of consecutive periods of operation the beacon can be made to flash dots and dashes. It is usual to arrange for the response to give a two-letter code in this way. The two morse letters are repeated continually, and there are two such letters given to each beacon, so that the operator can distinguish between different beacons.

It is important to note the difference between this form of coding and the pulse-width coding which is used by IFF and which has already been described. In gap coding the response pulses are all of the same width, and the dots and dashes are achieved by varying the length of time that the response persists on the responder display tube. In using pulse-width coding, the transponder is switched on and off in the same way, but this time every “on” period lasts for the same length of time. The coding consists of varying the widths of the pulses. During each period of operation all the pulses radiated are of the same width, but the width in successive operations may differ. The resultant appearance on the responser display tube is a response which appears, disappears, appears again for the same length of time, and so on, but whose width can differ at each successive appearance.

FGRI.5067-TR.3107B

This is the fighter beacon TR.3107 modified for use with Coastal interrogators. It is now the standard type of fixed beacon installed in Coastal Command aerodromes.
Receiver frequency, 176 Mhz. Transmitter frequency, 177 Mhz.

FGRI.5066-TR.3213

This is a modification of the TR.3112 mentioned above. The super regenerative TR.3112 being replaced by a superheterodyne transponder.
Receiver frequency, 176 Mc/s. Transmitter frequency, 177 Mc/s

TGRI.5302C-TR.3236

This beacon is a converted 24-volt American ABX IFF set. is battery operated, with a petrol-electric charging set. It is used because Coastal Command require easily transportable beacons for use with mobile aerodromes. It will be replaced by the TR.3558 mentioned in para. 37 when this becomes available.

Because it is an IFF Mk. III responder the TR.3236 has only one tuned circuit, and it cannot transmit and receive on different frequencies. Both interrogation and response occur on a frequency of 167 Mc/s.

FGRI.5584-TR.3558 (Eureka Mk. II)

The prefix “FGRI” is a misnomer here. The equipment is easily transportable, and will replace the TGRI.5302C for use with mobile aerodromes. It is a Eureka transponder, specially modified for use by Coastal Command. It will work with a universal aerial system (aerial system, type 350). This aerial array consists of a stack of two half-wave aerials fixed one above the other, and a half-wavelength apart. The aerials can be mounted either horizontally or vertically, with or without reflectors. Aerials of slightly different lengths are supplied with the set, one being for use with Coastal interrogators (176-177 Mhz),

one for Fighter use (193-197 Mhz),

one for the Rebecca band (214- 234Mhz).

Receiver frequency, 176 Mhz. Transmitter frequency, 177 Mhz.

Eureka Beacons in Ships

It has been suggested that Eureka beacons should be installed in ships for use with Rebecca III N and IV. They offer a number of advantages, amongst which is the important fact that they will work on the Rebecca-Eureka band and will not, therefore, trigger IFF sets. If ordinary NAA beacons are used, their frequency is in the centre of the IFF band, and the aircraft Rebecca equipment must be tuned to this frequency, so that IFF sets are triggered both by the Rebecca interrogating pulses and by the beacon responses.

Eureka beacons (usually vertically polarised)

The following list includes most of the Eureka beacons used for normal Rebecca-Eureka operation. It does not contain any account of Eureka sets which have been modified for special purposes, such as those mentioned above which are used by fighters and coastal aircraft for homing.

The first type of Eureka, Eureka Mk. I, was used with Rebecca Mk. I. Both these equipment are now obsolete. Eureka Mk. II is the standard version which is used at the present time. It works on the Rebecca-Eureka band, which extends from 214 Mhz to 234 Mhz. Its receiver and its transmitter can be set independently to any two of the following frequencies:
A……214 Mhz
B……219 Mhz
C……224 Mhz
D……224 Mhz
E……234 Mhz

The Rebecca interrogator with which it operates can also be set to two of these frequencies in the same way.

Eureka Mk. III is a lightweight equipment which uses miniature components throughout its construction. It usually operates in the same way and on the same frequency as Eureka Mk. II.

Special types of Eureka Mks. II and III have been developed for special purposes, some of these versions operate outside the normal Eureka frequency band, and have special aerial systems fitted. Normally, however, Eureka sets operate on any two of the five frequencies given above, and they use vertical polarisation. They are always super- regenerative in their action. Eureka beacons are usually width-coded. The response can often be keyed.

TGRI.5666-TR.3174 (Eureka II)

This Eureka Mk. II beacon can be either mains or battery operated. It is fitted with the universal aerial system, type 350. Its transmitter and its receiver can each be set independently to one of the five frequencies of the Rebecca-Eureka band.

MGRI.5591-TR.3529 (Eureka-H)

This is a mobile ground beacon for use with Rebecca-H. It is installed in a 15 cwt. vehicle on which the aerial array is mounted. It works on the standard frequencies, as the TR.3174.

TGRI.5509-TR.3174 (Eureka Mk. II)

This beacon is similar to the TGRI.5509, but is fitted with a lighter aerial system to facilitate transportation. It is used by airborne troops, being more robust than the ultra lightweight Mk. III B which is specially designed for Paratroops, and it can be employed for purposes which may involve rough handling. It works on the standard frequencies.

TGRI.5527—TR.3514 (Eureka Mk. III A)

This is an ultra-lightweight beacon working on the standard frequencies.

TGRI.5527-TR.3563 or TR.3593 (Eureka Mk. III B)

The TR.3563 is an ultra-lightweight Eureka designed for special purposes, including the landing of airborne troops. It is supplied with a lightweight aerial, and the whole equipment, including accumulators and power supplied, packs into a small bag.
Receiver frequency, 213.5 Mhz. Transmitter frequency, 216-5 Mhz.

S-Band Beacons

S-Band beacons for use with AI Mk. VIII (vertically polarised)

The following beacons have been designed for direct interrogation by AI Mk. VIII. The way in which they operate has already been described briefly. A particular point of interest is the coding system of this type of beacon. On receiving a single interrogating pulse, the beacon can respond not once but five times.

The first response pulse occurs immediately on receipt of the interrogation; the second follows automatically after a time delay of about 21.8 microseconds, the third follows after twice this time delay, the fourth after three times the delay, and the fifth after four times the delay. 21.8 microseconds is the time required for electro-magnetic waves to travel a double journey of about 2 miles, so that the response appears on the AI display tube as a series of 5 echoes, spaced about 2 miles apart.

The position of the first of these echoes gives the range of the beacon. The first response always appears, but the other four can each be switched on or off independently, and it is possible to make the beacon transmit any combination of them. They are switched automatically, and any one of them can appear in the following way:

5 seconds on and 5 seconds off
5 seconds on and 10 seconds off
5 seconds on and 20 seconds off

Permanently on.
Permanently off.

MGRI.5518-TR.3506

This is a superheterodyne mains-operated beacon, for use with AI Mk. VIII, and is used at the present time. Like all AI Mk. VIII beacons, it uses vertically-polarised waves, its aerial systems consisting of two spun copper cones placed apex to apex, with a small gap between the two apexes in which the vertical aerial is mounted.
The system radiates equally in all azimuthal directions, and most of the radiation is concentrated at angles of elevation less than 22 deg. The whole installation is mobile.
Receiver frequency, 3285 Mhz. Transmitter frequency, 3280 Mhz.

FGRI.5600-TR.350666.

The FGRI.5600 uses the same transponder as the MGRI.5518, but it is a fixed installation.

Receiver frequency, 3285 to 3315 Mhz. Transmitter frequency, 3280 Mhz.

American S-band homing beacons (horizontally polarised)

Beacons have been developed in America for use with American S-band equipment. They are known as BGS beacons. The BGS beacons AN/CPN 3 and 8 are horizontally-polarised homing beacons designed to work with American 10-cm. equipment of the AI, ASV and H2S types. The AN/CPN3 is an early version, and the AN/CPN 8 is the main production model which embodies some small improvements.

Receiver frequency, 3270 to 3330 Mhz. Transmitter frequency, 3256 Mhz.

American X-band homing beacons

There are also American beacons, BGX beacons, working on the X-band. They are horizontally polarised, and are used with American 3-cm. equipment.
The AN/CPN-6 is a homing beacon for use with 3-cm. airborne equipment.

Receiver frequency, 9320 Mhz. Transmitter frequency, 9310 Mhz ³

BABS Mk 1 Beacons cont.

The following table and fig. 2 shows how this may be done:

Ratio 1 :1

Sector Along the runway (Eqi-signal zone)

Bearing of sector relative to runway 0deg – ½ degree

Ratio 4 : 3   Sector Dots or Dashes 1

Bearing of sector relative to runway

½ degree – 2 degrees

Ratio 4 : 2  Sector Dots or Dashes 2

Bearing of sector relative to runway

2 degrees – 5 degrees

Ratio 4 : 1  Sector Dots or Dashes 3

Bearing of sector relative to runway

5 degrees – 12½ degrees

Ratio > 4 : 1  Sector Dots or Dashes 4

Bearing of sector relative to runway

12½ degrees  – 40 degrees

There are several BABS beacons of this type in use at the present time. The 1.5 metre ASV and AI sets are now becoming obsolete, so that these BABS are usually interrogated by Lucero. It is important to note, however, that fighter BABS is interrogated by fighter Lucero which is vertically polarised, so that the beacon must also use vertical polarisation, while for the same reason Coastal and Naval Air Arm BABS must be horizontally polarised. Bomber Command BABS is interrogated by a bomber version of Lucero which works on a frequency in the Rebecca band, and is used in conjunction with H2S equipment. It is vertically polarised.

Generally speaking BABS transponders receive and respond on different frequencies just as do homing beacons. Coastal, Fighter and NAA BABS usually receive on the same frequency as the homing beacons used by their respective Commands, because they are interrogated by the same airborne equipment. They re-transmit on a different frequency from the corresponding homing beacon, however, in order to prevent confusion.

The range of BABS is limited to be approximately 20 miles to avoid interference between neighbouring aerodromes, much less than the range of homing beacons. When an aircraft has approached to within a few miles of an aerodrome by using the aerodrome homing beacon, the navigator switches his interrogator over to BABS. In doing this he must detune his receiver to the BABS response frequency.

BABS Mk. II beacons

There are certain inaccuracies inherent in the design of the present BABS system. They arise primarily from faults in the aerial system, and a new type of aerial is designed to eliminate them.

One of the principal errors occurring in the BABS Mk. I system is that the side lobes of the aerial radiation patterns give rise to false equi-signal lines. It can only be effectively cured by using radiators which give polar diagrams free or almost free of side lobes, and with the Yagi and corner types of aerials used in BABS Mk. I it is difficult to accomplish this.

Another source of error in the present aerial systems is the mis- matching of arrays. If the two transmitting aerials are not equally matched into their respective transmission lines they will not radiate equal power. This will cause the beacon to squint; the locus of points of equal signal strength will no longer be the line bisecting the angle between the lines of shoot of the two aerials. Mismatches inevitably occur in any aerial system in practice, and it is almost impossible to ensure that both transmitting arrays radiate equally. Attenuation in one of the feeders would also lead to the same result.

Other inaccuracies arise owing to cross-polarisation effects. Suppose, for example, that the beacon is vertically polarised. Metal frameworks and wires in the vicinity of its transmitting aerial will inevitably give rise to horizontally-polarised radiation, and just as the two transmitting aerials have overlapping polar diagrams so far as their normal vertically-polarised radiation is concerned, each will also have associated with it a polar diagram due to horizontally-polarised waves. These horizontally-polarised radiation patterns are due to random scattering of the electromagnetic waves, and will not in general be similar for the two aerials, nor will their equi-signal direction correspond to that for the true vertically-polarised patterns. An aircraft attempting to land with the aid of the BABS will use a vertically-polarised interrogator. Whenever the pilot banks, however, the interrogator re- ceiving aerials will pick-up some of the horizontally-polarised radiation from the beacon. Metal struts and wires in the aircraft will also enable the interrogator to pick up horizontally-polarised waves, even flying on an even keel. This, of course, may lead to serious error.

In the new BABS Mk. II, the mismatch problem is overcome by using a special resonant-cavity radiator. The beacon transmitter feeds into a resonant cavity consisting of a large rectangular box. Two exactly similar half-wave slots are cut opposite to one another in the sides of the box. At the centre point of each of the slots there is a relay which, when closed, can short out the slot and prevent it from passing
radiation. If both relays were left open, both slots would radiate equal signals. In practice, however, the relays are closed alternately so that each slot transmits in turn. Behind the box is a corner reflector, which is arranged to give a suitable radiation pattern from each slot.

The advantage of this arrangement is that any mismatches or attenuation occurring in the feeder system between the transmitter and the aerial are exactly similar for both transmitting aerials. The only factor which could cause a difference between the powers radiated from the two slots would be lack of symmetry in the system. Provided that the two slots are identical, and are in identical positions relative to the corner reflector and to other conductors, the radiation pattern must be the same for each. Cross-polarisation effects are cut down to a minimum by using a cavity of such dimensions that it will not support vertical modes of vibration, by reducing the number of metal supports and cross members as far as possible, by designing the corner reflector to give as much electrical shielding as possible, and by making the whole arrangement perfectly symmetrical so that what cross polarisation there is will have the same effect for both aerials.

The same slots are used both for transmission and for reception and the system is almost entirely free from side lobes, so that there are no false equi-signal lines within 150 deg. of the correct direction. (Fig. 3). 87. In the BABS Mk. II the system of display is also changed. The slotted aerials are switched more quickly than the aerials of the old BABS, and they operate for equal intervals of time. There are ten switching cycles per second, so that each aerial transmits in turn for one-twentieth of a second. One aerial transmits narrow pulses and the other transmits wide pulses, and both pulses appear together on the interrogator display tube, see fig. 4. When the aircraft is making the correct approach, both these pulses have the same amplitude. If, how- ever, the aircraft is to one side or the other of the BABS beacon, one of the pulses appears longer than the other. It is claimed that this display makes it easier for the navigator to judge whether he is making the correct approach. The accuracy is said to be between deg. and deg. in azimuth. The same system of sectors is used as with BABS Mk. I. ³

Glide-path BABS

A new type of BABS beacon is in development; its function is to enable an aircraft to approach an aerodrome in conditions of poor visibility along the correct glide path-that is, to come down at an angle of inclination of about 24 deg. to the horizontal along a path which will enable the pilot to touch down close to the near end of the runway. This beacon receives interrogating pulses from a Lucero equipment working on a frequency of 214 Mhz, and re-transmits on a frequency of 515 Mhz. It requires a small extra receiving unit attached to the Lucero, to receive these higher frequency responses.

The glide-path beacon uses two transmitting aerial systems, situated at different heights above the ground. The transmitter is switched from one to the other, so that the aircraft responder receives pulses from each in turn, and there are 20 switching cycles per second as in the BABS Mk. II. Owing to ground reflection effects, the radiation patterns of these two transmitting aerials consist of the usual lobes and gaps, and their heights are arranged so that equal signals are received from both at an angle of elevation of about 2 deg. It is necessary, of course, to use horizontally-polarised waves for this purpose, to ensure that the ground reflection conditions are independent of the type of surface from which the reflection takes place, and that the vertical polar diagrams of the two aerials will be unaffected if the equipment is moved from one flat site to another. The radiating aerial system consists of two vertical slots in a wave guide. The transmitter output is switched continuously from one of these slots to the other.

The glide-path BABS will be used at the near end of the runway, that is the end from which the aircraft approaches, while the ordinary BABS will be at the far end. The higher and lower aerials of the glide
path equipment will transmit wide and narrow pulses respectively, so that the display will be similar to that for the new BABS. The two equipment will be used together, and their two responses shown on the same display tube. Because of the smaller range of the glide-path transponder, its response will appear on the display before the response of the ordinary BABS. The navigator can thus control both the azimuthal direction and the glide angle of approach by watching the one display and passing the necessary directions to the pilot.

Lucero interrogators

Lucero interrogators are used with most British S-band and X-band radar equipment. There have been three marks of Lucero, and there is in addition a new miniature type which is under develop- The situation is complicated, however, by the fact that each radar device requires a Lucero of its own which differs from that of any other radar device, so that there are a number of models of each mark. The reasons for this are:

1. Different radar equipment require Lucero equipment of different frequencies. Thus Lucero used with ASV must interrogate on 176 Mhz and receive on 177 Mhz. Lucero used with AI must transmit on 193 Mc/s, and receive on 196.5 Mhz, and so on.

2. Lucero uses the IF stages of the various equipment with which it is used, and different S- and X-band radar equipment have different IF frequencies.

Thus the IF frequency of H2S Mk. II is 13.5 Mhz.

Thus the IF frequency of H2S Mk. III is 45 Mhz.

Thus the IF frequency of ASV Mk. III is 13.5 Mhz

Thus the IF frequency of later marks of ASV is 45 Mhz

Thus the IF frequency of AI Mk. VIII is 30 Mhz

Thus the IF frequency of AI Mk. IX is 45 Mhz

The Lucero Mk. I now obsolete and the equipment at present in service are all variations of Lucero Mk. II. Lucero Mk. III is now in service with the NAA. The various models of Lucero Mk. II are included in the following list. Generally speaking, except in the case of Bomber Command aircraft which do not use homing beacons, these Lucero sets are capable of interrogating either homing or BABS beacons.

The Lucero Mk. I now obsolete and the equipment at present in service are all variations of Lucero Mk. II. Lucero Mk. III is now in service with the NAA. The various models of Lucero Mk. II are included in the following list. Generally speaking, except in the case of Bomber Command aircraft which do not use homing beacons, these Lucero sets are capable of interrogating either homing or BABS beacons.

A miniature Lucero is being developed. It will have its own power pack, IF stages, and display, and so will effectively similar to a Rebecca equipment. It is, in fact, called Rebecca Mk. V. ³.

Rebecca interrogators cont.

The question of polarisation is important with this equipment, because it will interrogate both vertically and horizontally polarised transponders. It may be necessary to carry both horizontal and vertical aerials, although this will add slightly to the weight. The following facts must be considered in deciding what type of aerials the set must use :-

(1) It interrogates shipborne horizontally polarised homing beacons on a frequency of about 176 Mhz.

(2) It interrogates vertically polarised IFF sets, probably on the same frequency, 176 Mhz.

(3) It interrogates vertically polarised Eureka beacons, on a frequency of 214 to 234 Mhz.

(4) It may interrogate horizontally polarised BABS beacons on a frequency of about 176 Mhz.

(5) It will probably be used as a low power ASV, in which case it will be better to use horizontal polarisation, as this gives smaller sea returns.

If both horizontal and vertical aerials are fitted, the set will use the one or the other according to the transponder with which it is working, viz:

Horizontal aerials for interrogating shipborne homing beacons such as types 251 M and 251 P, BABS beacons, and for normal ASV use.

Vertical aerials for interrogating Eureka and IFF.

The horizontal aerials will be tuned to the ASV Lucero band (172- 182 Mhz) and the vertical aerials to the Eureka band (214-234 Mhz). The mismatch when the vertical aerials are used to interrogate IFF will have to be tolerated; the only alternative is to use horizontal aerials which will be tuned to the correct frequency and to tolerate the consequent reduction in range due to cross-polarisation.

It may be possible to fit horizontal aerials only, and to use cross polarisation for both IFF and Eureka working. One difficulty of this scheme is that it may be difficult to design an aerial system with a sufficiently wide frequency band to work efficiently on both the ASV Lucero frequency and on the Eureka frequencies.

Rebecca Mk. V

It has now been decided to dispense with this equipment, and to use Rebecca Mk. IV for the purposes for which it was intended.

Rebecca Mk. VI

This equipment is an independent interrogator comprising a Lucero II plus an IF amplifier and an improved indicator, so that it is independent of any other radar.

American interrogators equivalent to Rebecca

The following American interrogators are used by the RAF and the NAA to fulfil the functions of British Rebecca and Lucero equipment.

SCR-729 A (Horizontally polarised)

SCR-729 A is an airborne interrogator which can be used either in conjunction with a centimetre radar equipment or independently. The equipment with which it is used are the ASD, ASG, AN/APS-3, AN/APS-4, and the AN/APS-15. Its transmitter frequency is preset to 176 Mc/s, and it can work on either one of two receiving frequencies, 177 Mhz or 173 Mhz. These receiving frequencies are selected by a switch; one is for use with homing beacons and the other with BABS. The equipment uses vertically-polarised directional aerials with which it can home on to beacons.
With NAA and Coastal beacons this involves working with cross- polarisation.

SCR-729 F

This is a modification of the SCR-729 A, which has facilities for interrogating fighter beacons as well as IFF. It is used with AI Mk. X (SCR-720). It can transmit and receive on 183 Mhz, or alternatively it can transmit on 193 Mhz and receive on either 190.5 or 196 Mhz. The aerial system is vertically polarised and is similar to that of the SCR-729 A.

AN/APX-2

The AN/APX-2 is an equipment which combines the functions of an interrogator and an IFF transponder. It is considered somewhat more fully later, and its IFF operation is described. The interrogator of AN/APX-2 transmits and receives on a frequency band of 160 to 184 Mhz. Its transmitter and its receiver can be tuned independently to any two frequencies of this band. The set is intended primarily to interrogate IFF Mk. III, and it is supplied with a single vertical aerial for this purpose. The aerial system is not directional, and it cannot be used for homing on to beacons. The equipment can operate independently with its own display tube, but it is usually used with a centimetre radar. It will be used primarily as an IFF interrogator with aircraft carrying American radar equipment.

AN/APX-8

The AN/APX-8 is an AN/APX-2 transponder fitted with vertical Yagi aerials to enable it to home on to beacons. It also includes, in addition to the AN/APX-2 set, a separate AN/APA-1 radar repeater. indicator and an antenna switch unit to switch the two aerials. Like SCR-729 it must use cross-polarisation when interrogating beacons. The AN/APX-8 interrogator works on the same frequency band as the AN/APX-2 ³.

The IFF Mk III System cont

The RAF usual ground interrogator is the T.3117 transmitter used with the R.3118 receiver. The frequency can be varied over the whole A-band and with different radars it uses different spot frequencies. The type 242 is used with one ground equipment and various airborne radar sets also have their own irregularities.

RAF Equipment Ground. Reproduced by kind permission of Philip Judkins

IFF Mk. IIIG and Mk. III G(R)

For certain operations aircraft require other identification facilities in addition to those provided by IFF Mk. III. This is particularly true in the case of fighters operating with GCI stations. IFF Mk. III responds to the GCI interrogator, but this is not sufficient since the response appears only on the IFF display and not with the normal echoes on the PPI tube. It is necessary for the controller to be able to identify echoes appearing on the PPI tube quickly and conclusively, and he must therefore be able to see IFF responses in this tube. The old Mk. II G fulfilled this requirement because it was directly interro- gated by the GCI, and if the Mk. III equipment is to work satisfactorily with GCI stations; it also must have similar facilities for direct interrogation, in addition to its normal A-band working. For this reason the Mk. III G was developed. This transponder is capable either of sweeping the A-band in the normal way or of responding directly on the frequency of the main GCI equipment.

The frequency band allotted to GCI stations is called the G- band, and it extends from 200 to 210 Mc/s. At the present time all GCI stations use a spot frequency of 209 Mc/s although future equipments may use other frequencies within the band. The Mk. III G transponder will respond either to normal interrogators or directly to a GCI station working on any frequency in the G-band. It normally sweeps the A-band in the usual way, but the pilot can, when requested by the GCI Controller, press a button which will temporarily put the set into a state known as G working. The set remains in this state for about twenty seconds, during which time it gives direct responses on the GCI frequency and after which it automatically reverts to the normal A-band sweep.

While in the condition of G-working, the IFF does not entirely abandon its A-band operation. It continues to sweep the A-band in the usual way, but gives chopped A and G responses, replying alternately on the A-band and on the fixed frequency, usually 209 Mc/s, in the G-band, in such a way that the A-band operation continues for one- tenth of a second and is followed by G-band operation for one-twenty- fifth of a second. Thus, for a period of twenty seconds after the pilot has depressed the G button, the set gives a rapid succession of short responses on the GCI frequency, which appear on the PPI tube of the GCI, and meanwhile it responds in the normal way to A-band interrogation.

The G facility requires the inclusion of a second tuned circuit in the transponder. This circuit, the G circuit, is permanently tuned to the frequency of the GCI station with which it is to operate and it will give responses only to interrogation on this preset frequency. During G operation the tuning of the A-band circuit continues its normal frequency sweep and the chopped responses are obtained by switching from the G to the A circuit.

Mk. III Transponder cont.

(2) G working when it gives a chopped response, replying alternately for 1/10 second on the A-band and for one twenty-fifth second on a preset frequency on the G-band. It can be switched on to this state either by depressing the G button, in which case it automatically returns to normal A-band working after 20 seconds, or by depressing the R switch, when it remains in this state indefinitely.
(3) R working, when it responds continuously to a preset frequency on the R-band. It can be switched on to this condition by depressing the R switch and will continue to work on the R-band until switched back to the A-band condition. The responses can be keyed.
The equipment can be set up to work in conditions (1) and (2) or in conditions (1) and (3).

IFF Mk. III G can work in conditions (1) and (2) only. IFF
Mk. III can work in condition (1) only.

In the past it has been customary for certain aircraft of Coastal Command and of the Naval Air Arm to carry IFF Mk. II N for use as Rooster. The continuous frequency sweep was stopped and the equip- ment was set up on a fixed frequency so that it operated as a Rooster beacon only. Certain Mk. III sets have now been modified for this purpose and converted into Mk. III R equipments. They will replace the old Mk. II N and will operate either as ordinary Mk. III IFF or as Rooster beacons. The Mk. III transponder has, of course, only the normal A-band tuned circuit and is not provided with a circuit for G working and in order to change from the A-band to the R-band state it is necessary to stop the sweeping of the A-band at fixed Rooster frequency. The usual Rooster frequency is 176 Mc/s. This is in the A-band so that it is not difficult to make the necessary modification on switching from the A to the R condition, the variable tuning mechanism is brought to rest by a stop whose position can be adjusted to give fixed frequency working on any required spot frequency in the band.

Certain Rebecca interrogators used in H.M. ships work on a frequency of 214 Mhz and require Roosters to respond on this frequency. This has led to the installation of two modified Mk. II N sets in some aircraft; one to respond on 176 Mhz and the other on 214 Mhz. A British Mk. III G(R) equipment is modified to perform both these functions. The normal A-band sweep can be stopped as before at 176 Mhz, while the G circuit is tuned to respond on 214 Mhz. The set modified in this way therefore becomes a Mk. III (R) with double R facilities.

The two types of specially modified sets mentioned are, of course, very specialised, and rather different from the principal types of Mk. III set which are being produced. They are mentioned because they apply particularly to the Naval Air Arm and because they illustrate the way in which modifications, which have been demanded from time to time by special requirements of the different services, have complicated the whole history of the IFF and beacon situation, and have led to so many different types of equipment.

Because GCI stations and ASV Mk. II and certain Lucero equipment use horizontal polarisation, horizontal aerials were designed for Mk. III for use when working on the G- or the R-band. These have been abandoned, however, and the equipment now works with cross polarisation. This does not decrease its efficiency very materially; and in any case the extra power of GCI and ASV Mk. II over that of ordinary interrogators more than compensates for the loss entailed.

The general details of IFF Mk. III G(R), are summarised in the following list. Marks III and III G only perform part of these functions.

Normal Mark III Operation

Normal Mk III Operation. Reproduced by kind permission of Philip Judkins

British and American version of IFF Mks. III, III G and III G(R) cont.

All types of Mk. III G(R) IFF equipment comprise the following units:
(1) Main transponder unit: transmitter-receiver complete with power supplies.
(2) Control unit assembly, type 1, comprising two small control units’ side by side: –
Control unit, type 89-with six-way selector switch for selecting any one of the six possible codes.
Control unit, type 90-with on/off switch, and distress switch which gives normal distress signal.
(3) G button.
(4) R switch and Morse key for keying Rooster reply.
(5) Plugs, sockets, connectors and switch units for detonating an explosive charge to destroy set in emergency.

In the Mk. III G installation there is no Morse key, and in the Mk. III installation there is no G button, R switch, or Morse key. The original intention was to make British and American installations as nearly as possible identical, so that in any aircraft fitted with any type of IFF Mk. III, III G or III G(R), if the main transponder unit were removed and any other type of transponder substituted in its place, the new set would work satisfactorily in the old installation, using the old original switches, control units and connections. For this reason, the various types of transponders and their associated units were made as nearly as possible identical in size and it was arranged that all use the same plugs, sockets and connectors and all have the same control knobs and switches. The following paragraphs deal with the interchangeability of transponders, and show to what extent this object has been achieved in practice.

Interchangeability of airborne IFF sets

Although the various types of British and American IFF trans- ponder are to some extent interchangeable there are several factors which cause difficulty when a transponder is fitted into an installation other than its own. For example, if any type of Mk. III G(R) set is fitted into a Mk. III installation it will not operate as a Mk. III G(R), because the necessary controls for switching on to G and R working will not be present. Conversely, if a Mk. III transponder is fitted into a Mk. III G(R) installation it can operate only as a Mk. III set, as it has no G circuit, and pressing the G switch or the R button will have no effect on its mode of operation.

These limitations would be expected, but there are others, which, while not immediately obvious, are equally important. Difficulties arise where a British Mk. III G or Mk. III G(R) transponder is fitted into an American AN/APX-1 installation.

This is due to the difference between the control circuits. The American transponders use miniature valves, and can therefore employ much more elaborate circuits than the British sets which use standard components throughout. Thus, whereas the British Mk. III G(R) transponder has only 14 valves, the AN/APX-1 has 28.

When the G button is depressed in the AN/APX-1 equipment, the set is automatically brought back to A-band working after 20 seconds by a fed-back time-constant valve circuit. In the British equipment space limitations prohibit the use of an extra valve for this purpose, and the automatic time delay is supplied by a thermal delay switch. This difference between the two sets leads to complications when one is used in the other’s installation. When the AN/APX-1 is used in the British Mk. III G(R) installation it works satisfactorily. When the British Mk. III G(R) set is used in the AN/APX-1 installation, however, when once switched to G or R working it continues indefinitely in that state and will not return to A-band working until the LT has been switched off ³.

Interchangeability of Airborne IFF sets Cont.

When certain American equipment are used with Lucero they refuse to accept the two suppression pulses. The AN/APX-1, for example, differentiates the suppression pulses, and whatever width the pulse may have, suppression cannot occur for more than about 70 micro- seconds. This will be satisfactory with a 20 microsecond prepulse, but the longer noise suppression pulse will not pass, and noise suppression will be incomplete. Furthermore, with AI Mk. VIII which does not use a prepulse, there is a second difficulty. The Lucero used with this AI must manufacture its own prepulse, and it uses a special phantastron circuit for this purpose. The pulse which it produces is of about 300 microseconds duration, and when this is passed to the AN/APX-1 it is quite inadequate, and gives no form of suppression whatever. The AN/APX-1 equipment is being modified for use with Lucero and it is hoped that the defect will be cured.

SCR-695 also has the same fault, but a British modification has already cured this. The modified SCR-695 is called R.3598.

The difficulty of suppression to Lucero of the AN/APX-1 and SCR-695 also occurs when other interrogators similar to Lucero are used. It happens in particular with the American SCR-729 interrogator. The following tables summarise the facts stated above, and show the limitations imposed on the operation of various types of IFF when used in different installations.

The following table shows the degree of interchangeability in installations designed for R.3067 or R.3090.

Interchangeability designed for R.3067 or R.3090. Reproduced by kind permission of Philip Judkins

Naval Mk. III transponders

Special models of Naval IFF Mk. III have been produced, primarily for installation in H.M. ships, although they are similar to the airborne types and can be used in aircraft. Generally speaking these Naval transponders are primarily used as beacons, but they have identification as a second function. Some of these sets have already been mentioned in the part of this report which deals with Beacons.

Limitations of IFF Mk. III and Proposed Future
Systems

IFF Mks. III, III G and III G(R) suffer from serious opera- tional limitations. The failings of the Mk. III system as it exists at the present time, and a number of suggested improvements and alternative systems are described in the following paragraphs.

Clutter

Many aircraft and ships now carry some form of IFF Mk. III that the traffic-handling capabilities of the system are no longer adequate, and ground and ship-borne interrogators, during periods of great activity, receive so many responses that the trace of the IFF display tube shows one solid mass of echoes through which it is impossible to recognize any one individually. This appearance of crowded responses on the IFF display is called clutter, and it arises from four main causes.

Over-interrogation

The chief factor in producing clutter is simple over-interrogation. If a large number of aircraft are operating in one area, and all their trans- ponders are switched on, clutter is the inevitable result; it occurs also on the main radar display tube when the concentration of aircraft in any one area is too high. In certain cases, in fact, IFF clutter is not an im- portant operational limitation. If the normal radar echoes cannot be identified individually, there is little point in worrying about clutter on the IFF tube; a single hostile aircraft which happened to be present among the others could not be seen in any case. This is true when CH and CHL stations are plotting bomber raids leaving and returning to the coast.

The IFF clutter can still be troublesome, however, when the concentration of aircraft is not sufficiently great to cause clutter on the main radar. This suggests that there are also other factors which add to the trouble, and although no controlled experiments have been per- formed to prove this conclusively, it is almost certain that the following factors are contributory.

Mutual triggering

It is possible for the response from one IFF set to trigger a second set in its neighbourhood. Because IFF transponders have a bandwidth of some 6 Mhz this can happen if the two sets are tuned to slightly different frequencies, and it can be shown that if a large number of IFF Mk. III sets are working in one neighbourhood complex mutual triggering effects may occur and a single interrogator can cause multiple response on all frequencies in the A-band.

Triggering by engine noise

A transponder can be triggered at random by the engine of the aircraft.

Triggering by Chain Home Low (CHL)

There has been evidence recently that IFF sets are being triggered by CHL stations working on a 193 Mhz. Although this is 6 Mhz outside the A-band it appears that the bandwidths of the CHL transmitter and the IFF transponder are sufficient to permit overlap.

All these factors are probably instrumental in producing clutter. The Operational Research Section of Fighter Command are investigating the whole question, and are hoping to obtain more conclusive experimental evidence. Whatever the causes, however, the problem is so serious that it often renders IFF Mk. III virtually useless, so that the number of aircraft and ships allowed to show IFF had to be seriously reduced.
Methods of overcoming clutter

There have been many suggestions of possible methods of over- coming clutter. The following appear to be the most promising: –

Switching of interrogators

Interrogators should be switched on only when required, and all interrogators should be fitted with spring-loaded switches which automatically return to the OFF position when released. In this way, each interrogator will work only during occasional periods of a few seconds, and the interrogation will be materially reduced.

Limitations of IFF Mk. III and Proposed Future
Systems cont.

Beam Width of an Interrogator Aerial System. Reproduced by kind permission of Philip Judkins

It would be possible to work with narrower beams if the interrogator aerials were not continuously rotated, but were turned by the operator when he wished to look at a particular echo whose identity was required. The beam could then be held on this echo indefinitely. Many interrogators now in use, however, have their systems mounted on the same framework as those of the main radar, so that stopping the rotation of the interrogator aerial means also stopping that of the main equipment.

8-bay and 12-bay aerial systems have been used with RAF interrogators, and the beaming has probably been carried to its practical limit. CHL and GCI equipment fitted with these highly beamed aerials usually carry them above the normal radar aerial, and have to stop their continuous rotation during interrogation. Army equipment usually have a much wider interrogator beam, whose width is of the order of 50 deg. This is not a great disadvantage, however, because the interrogators have low power output (about 30 watts) and being used with precision radars such as GL, the timebase is necessarily fast and gives good range resolution. The problem is most acute in the case of the LW set which had a slow timebase, and this is now being fitted with a more highly-directional interrogator aerial.

Naval interrogators have simple interrogator aerials which give wide beams. The problem of clutter has not been serious with the concentrations of aircraft so far used in Naval operations, but it is most necessary to bear it in mind if greater densities are expected in the future. With the advent of greater concentrations of aircraft, it will certainly be necessary to use beamed arrays.

Sippi

A technique known as Sippi has been developed for artificially narrowing the beam-width of the responsor. This does not reduce the total amount of clutter, because the interrogator can still transmit over a wide azimuthal range, but it does reduce the amount of clutter scan by the interrogator. It is very effective, but again necessitates stopping to “look” at the target which is to be identified.

Double interrogation

 It is possible to introduce a system of double interrogation which would completely cure clutter arising from mutual triggering and from triggering by engine noise and by CHL stations. There are two alternative methods of accomplishing this. The first of these methods involves the use of a double pulse. The interrogator must be modified to transmit two pulses spaced a few microseconds apart. The transponder must be so designed that it will not respond unless interrogated by two such pulses.

The second method uses the ordinary single-pulsed interrogation system, but arranges that the transponder will not reply to the interrogation pulse unless it is receiving simultaneously a priming signal on a second chosen frequency. The priming signal may be radiated continuously from the interrogating station, or may consist of a second series of pulses radiated on the priming frequency. This latter alternative is operationally better and more convenient, as it is possible to use the pulses from the parent radar for priming purposes; the interrogator is usually locked to the parent radar in any case, so that the pulses are always coincident. This system of using a transmission on a second frequency to put the transponder into an operational state is called the system of prime-and-poop.

Both these systems are discussed in paras. 194 to 197, dealing with security problems. Either one could eliminate mutual triggering and triggering by engine noise and by other radar equipment.

²¹

Split interrogation

It has been suggested that a new system might be developed whereby the interrogator transmits pairs of pulses, one of each pair being radiated from one of the two halves of a split array. The lines-of- shoot of the two halves of this array would be inclined at a small angle, and the transponder would be so designed that it would respond only if the amplitude of the two pulses which it received were almost equal. The effective beamwidth could then be made as small as required. To put this method into operation would require an entirely new IFF system.

Raster presentation

In the case of interrogating equipment where it is possible to display IFF echoes as brightness modulation on an afterglow tube, it may be helpful to give the timebase of the tube a slow vertical sweep, synchronised as nearly as possible with the time of frequency sweep of the IFF transponder. The echoes will then appear on a raster and signals from aircraft at the same range will not usually be superimposed. Integration

It is possible to obtain better range resolution with equipment where the timebase of the display tube is slow, by introducing a range strobe with an interrogator. A portion of the timebase can then be strobed and magnified.

Security aspects

There were two serious problems of security that arose in the operational use of IFF during the last war.

(1) The enemy could interrogate IFF Mk. III, and use the responses to detect and identify allied aircraft.

(2) Many of our IFF equipment inevitably fell into enemy hands, and he was able to fit them into his own aircraft and ships to confuse our identification system. With sufficient time and labour, he even manufactured copies of our sets himself and used them for this purpose.

The second of these problems was the more important, and it might be expected that the enemy would attempt to compromise our system by carrying British and American IFF sets. IFF Mk. III as it exists at the present time can be both easily interrogated by enemy equipment and if captured, used by him to cause confusion. It is, therefore, essential either to develop some new system or to increase the security of the present IFF.

Any of the following schemes could be adopted to alleviate the problem of security.
Double-pulse interrogation

This method has already been mentioned in dealing with clutter. Its advantage, from a security point of view, is that the enemy would have some difficulty in interrogating our transponders. He would first have to discover that we were using this method of interrogation and then he would have to build suitable double-pulsed interrogators. The system offers no cure for the second security problem.

Exponents of this method say that the necessary facilities could be provided by the addition of a small box to the airborne transponder. It would also require modification to all existing interrogators including Lucero, and it would be impossible to interrogate IFF with ASV Mk. II. Another difficulty would also arise from the fact that the interrogator response would be delayed, and to obtain correlation between the radar responses and those from the IFF it would be necessary to modify the existing forms of display.

The U.S. Army and Navy have considered the possibility of adopting the system and have modified the ABK and the SCR-695 sets to take double pulses. They are also contemplating modifications to the AN/APX-1 and AN/APX-2 for the same purpose ³.

The Oslo Report: A Remarkable Tale of Espionage and Scientific Insight

The annals of military intelligence are replete with tales of espionage and intrigue, but few can match the audacity and impact of the Oslo Report. This clandestine document, written by the German mathematician and physicist Hans Ferdinand Mayer during a business trip to Oslo, Norway in November 1939, stands as one of the most spectacular leaks in the history of military intelligence.

Mayer, motivated by a desire to see the Nazi regime defeated after Hitler’s invasion of Poland, embarked on a mission to divulge as many military secrets as possible to the British. He arrived in Oslo on October 30, 1939, and checked into the Hotel Bristol. In a daring move, he borrowed a typewriter from the hotel and, over the course of two days, meticulously typed the seven-page Oslo Report in the form of two letters. In an act of covert communication, he mailed the first letter on November 1, 1939, challenging the British military attaché to alter the introduction to the BBC World Service’s German-language program as a signal for receiving the report.

The Oslo Report contained a treasure trove of information about German weaponry, both in service and under development. It covered diverse topics, from remote-controlled gliders to radar technology, aircraft rangefinders, torpedoes, and electric fuses for bombs and shells. The report was received by the British Embassy in Oslo and eventually reached MI6 in London, providing a wealth of critical information that would aid the British in developing counter-measures.

The report covered various aspects of German military developments, shedding light on different weapons and technologies.

Ju 88 Programme
Mayer’s report mentioned the production levels of Junkers 88 medium bombers, asserting that around 5,000 were produced monthly, with a predicted total of 25,000–30,000 by April 1940. However, it is noted that this turned out to be an exaggeration.

The Franken
The report revealed the existence of the German navy’s first aircraft carrier, referred to as Franken, located in Kiel and expected to be completed by April 1940. There is a discussion about potential misidentification, as some suggest Mayer may have confused it with the Graf Zeppelin, but the details suggest otherwise.

Remote-Controlled Gliders
The report described remote-controlled gliders with a 3m wingspan and 3m length, equipped with an explosive charge. These gliders were intended for use above water, powered by a rocket engine. This may have connections to specific German designs like the Blohm & Voss BV 143 and the Henschel Hs 293.

Autopilot

Mayer briefly mentioned another remote-controlled system, this time for aircraft instead of rockets, referring to an autopilot system.

Remote-Controlled Projectiles
However, not all was smooth sailing for Mayer. He navigated a web of indirect communication paths, using Danish colleague Niels Holmblad and his British friend Henry Cobden Turner to relay information, while Germany and Britain were at war and Denmark remained neutral.

Mayer’s commitment to the cause was unwavering, even in the face of personal danger.

The impact of the Oslo Report was profound, particularly in the hands of Dr. R. V. Jones, a young Ph.D physicist who led the new field of “Scientific Intelligence.” While the report was initially met with indifference or scepticism by British Intelligence, Jones recognized its value. He argued that the technical details within the report were accurate, and that the source was reliable.

Simplified RV Jones Report: Aims of Scientific Intelegence

Information leaks out in five ways:
(1) Accidental indiscretions (including deciphered messages) of which there are always a large number and if these are pieced together a valuable impression may be gained.

(2) Indiscretions encouraged by alcohol and/or mistresses. The results obtained by these methods are all that can be expected.

(3) Information that cannot be kept secret and yet can give useful information to an enemy. R.D.F. transmissions are one example and, in time of war, loss of apparatus by contact with the enemy is another.

(4) Direct acquisition of information by placing agents in Military Research Departments. Such a method is difficult and hazardous, and comparatively little is obtained; its value is large.

(5) Information from disaffected nationals. Frequently this is unreliable and must always be checked.

A Scientific Intelligence Service starting at the present time would have to concentrate on (1) and (3), but the other ways should be exploited to best advantage.

After surveying what he had done to encourage the collection of more scientific and technical information he drew attention to an important weakness in our Intelligence cover in that there was no organization to listen to German transmissions that might be connected with radar or radio navigation:

Enquiries have shown that there has been no systematic observation of evidence indicating German R.D.F. transmissions….

He outlined an organization in which there would be a central section with the following terms of reference:

(1) To ascertain the development of new weapons and improvement of existing ones
by other countries.

(2) To mislead potential or actual enemies about our own weapons.

(3) To mislead the enemy about the success of his own weapons.

(4) To assist technically in espionage and its counter (including codes and ciphers, where technicians are becoming important at the expense of classical scholars).

(5) To coordinate Scientific and Technical Intelligence between the Services.
In addition to the Central Section there should be Scientific Intelligence branches attached to the Director of Naval Intelligence, the Director of Military Intelligence, and the Director of Air Intelligence. As regards the size of the staffs that would be needed I wrote that they should be kept as numerically small as possible, and that quality was much the most important factor.

When the report was completed it was circulated to the Directors of Intelligence and the Directors of Scientific Research in the Service Ministries and to Sir Henry Tizard, who promptly wrote congratulating me on the report and hoping that it would be accepted.

The Director of Scientific Research at the Air Ministry, D. R. Pye, who was still nominally my Chief, wrote in the same vein.

All three Directors of Intelligence agreed

Two of the Directors of Scientific Research. D.S.R. Admiralty, advised by his Deputy Director J. Buckingham, disagreed

This one disagreement caused the whole scheme to founder.

Buckingham’s argument appeared to have something to it. He stated:
“He would like to see the collection of scientific information about the enemy encouraged, but should be assessed and interpreted not by an Intelligence organization but by scientific experts in the Scientific Directorates”.

He argued that these were the men best qualified because they were working on our own weapons, and that they would be in a better position to assess the incoming information if they had it undigested by someone who was less expert.

References

1. Brettingham L. 1997 Royal Air Force Beam Benders: 80 (Signals) 1940-1945. Leicester Midland Publishing Ltd  
2. Jones R.V. 1998 Most Secret War: British Scientific Intelligence 1940-1945 Wordsworth Editions Ltd
3. Air Ministry 1946 (Declassified 1977) Introductory Survey of Radar Part II Air Publication 1093D Vol. 1 Chap 6
4. Goebel G. 2004 Oboe Navigation: Radio Navigation Systems: Operational History [Online] Available from: https://military-history.fandom.com/wiki/Oboe_(navigation)#Technical_details