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The 'Lock-on' question

an Opinion by Martin Shough

How could the Venom pilot have reported 'lock on' (assuming that BOI-485's identification of the jet as a Venom is reliable) when the AI radar did not have the automatic target-following capability usually implied by this phrase? The significance of this problem has become rather over-inflated by misunderstandings both of the phrase and of the equipment. This is due to confusion between, on the one hand, the AI radar worked by the radar operator, and on the other hand the radar-controlled gunsight which is operated by the pilot.

In 1986 I had the opportunity to pose the 'lock on' question to Mike Snelling, then a Chief Test Pilot for British Aerospace (who subsumed DeHavilland) at Dunsfold, Surrey. I asked him whether such a phrase would ever have been used in connection with gunlaying radar in the Venom. His opinion was that the range 'gating' used in controlling a radar gunsight would have been described as a 'range lock'. This turned out to be the case.

At that time I believed that the Venom must have been an NF.2a from the standing Waterbeach squadron, 253 Squadron. The Mk.10 (SCR-720) radar on the NF.2a did not control the manually-ranged GGS.4 gunsight. It is now known, however, that the Venoms on QRA duty at Waterbeach at the time were NF.3s of 23 Squadron, redeployed from their home at RAF Coltishall for the purposes of Exercise Fabulous. The NF.3 did have the radar controlled GGS.5 gunsight.

Perkins originally reported the pilot as saying 'I've got my guns locked on him', and amplified this by annotating a diagram with the caption: 'point at which RAF pilot reported radar gunsight locked on UFO' [emphases added]. Nothing here specifies that the phrase is being used to describe a radar with a tracking antenna. BOI-485 states: 'Pilot advised target was on radar and he was "locking on"', but elsewhere uses the same phrase merely descriptively: ' . . . one object was observed to "lock on" to fighter scrambled by RAF and followed all maneuvers of the jet fighter aircraft', obviously not referring in this case to an electromechanical device but to a type of behaviour. BOI-485 can be said to be ambiguous here, but Perkins is precisely accurate: The Venom NF.3's AI radar did not have a tracking antenna, but its radar controlled gunsight did provide an electronic 'lock on'.

The APS-57 was designed with an altazimuth tracking function which is disconnected in the installation used by the RAF in the 2-seat Venom NF.3. With this AI Mk.21 installation the 'track antenna' and 'track lock-on' controls still appear so-labelled on the front panel, but are disabled. However although the antenna mechanism no longer tracks and locks on to the target, the range signal from the APS-57 is instead sent to a GGS.5 radar-controlled motorised gunsight, and the ranging mechanism in the GGS.5 does automatically 'lock on' to this signal. In other words, the gunsight locks on to the radar.

Appropriate excerpts of the book 'DeHavilland Venom', a detailed and authoritative history of the development and deployment of the Venom in RAF service by R. Lindsay, were kindly supplied by the RAF Museum, Hendon. Therein the AI Mk.21 radar in the NF.3 is described as follows: 'This radar . . . had a wider variety of modes, better definition [than the AI Mk.10], and a limited "lock-on" capability.' The 'limitation' of the lock-on here refers to the difference between full automatic altazimuth tracking and automatic range tracking or range lock. AP 1275E, Vol.1, Sect. 5, describes the operation of the GGS.5 gyro gunsight under control of the gated output from the AI radar.

The gunsight is mounted on top of the instrument panel in front of the pilot who sights the target through an illuminated display visible in a half-chromed glass reflector. The pilot manually centres the target in the moving graticule, and whilst at long range he has the option of manual or radar ranging by means of a selector swich on the cockpit front panel. On 'manual', the range-drive motor in the gunsight is controlled by a twist grip on the throttle control in the pilot's left hand; on 'radar' the gunsight is automatically controlled according to the range marker strobe alligned with the target on the PPI scope by the radar operator. At close range a Type R.M. Mk.3 control unit obviates the need to operate this mode selector switch by hand in order to "prevent interruption of the tracking process during the change over between manual and radar ranging". Once he is tracking the target smoothly the pilot can fully rotate the twist grip to a maximum position, and now whenever the range falls below 800 yards the radar automatically takes over control of the gunsight.

The GGS.5 takes ranging information from the radar and inertial information from the gyro. It senses the rate-of-turn, the direction-of-turn, predicts the deflection due to these accelerations and the changing range, then compensates to allow for the ballistic fall due to gravity, for the weight of the ammunition being used, and even for the varying atmospheric pressure due to change in altitude. It computes the firing trajectory and continually adjusts the motorised sight accordingly, so that as long as the target is kept centred in the floating graticule the guns (or rockets) will be accurately layed. If sudden manoeuvres disrupt the tracking the pilot can then rotate the twist grip back to regain manual control of the gunsight until the proceedure can be repeated and the radar takes over again. Appendix 1 to AP 1275E (GGS.5 Control Unit, Type R.M., Mk.3, AL 53, Aug.1954) describes this automatic ranging control function as the radar equipment 'locking on'.

In operation, at night, as long as the pilot has no visual target then he obviously can't achieve accurate tracking with the gunsight. But if he knows that something is there, because ground radar and his onboard radar operator are telling him so, then his natural response will be to end-stop the throttle twist grip so that the radar will automatically begin to range his gunsight as soon as the range falls below 800 yards.

The pilot sees the illuminated pattern of the sighting graticule reflected in the half-chromed glass of the sight, and whilst the range is still (say) 1000 yards he rotates the twist grip. In this position the manual ranging is disconnected and the circuit 'waits' for a voltage from the radar unit. The diameter of the aiming graticule is frozen at a minimum position whilst the nav/rad now directs him towards a radar target on the AI Mk.21. When the range closes to 800 yards a voltage derived from the radar's PPI range strobe causes a motor in the gunsight housing to rotate the range graticule and thereby visibly enlarge the circle of illuminated diamonds displayed in the glass of the sighting-head in front of the pilot, thus telling him that the gunsight is now slaved to the radar. As the range changes the drive motor now keeps the sight automatically adjusted. When using radar ranging the illuminated display is usually pre-callibrated before take-off so that it will indicate to the pilot when he is approaching his minimum reliable gunnery range of 200 yards, below which the radar range lock does not operate.

This sort of fleeting range lock without visual cues would be well described by what Perkins remembers hearing on the radio as the incident ended: ". . . [pilot] #1 also made a remark at this time to #2 that he had his radar locked on whatever it was for just a few seconds so there was something there solid." BOI-485 confirms that the phrase 'lock on' was not simply interpolated in Perkins' later memory but was reported by the RATCC personnel to SAC intelligence officers within a couple of days. This does not prove accuracy, but it should be borne in mind that written logs were available, made up on the night in question. Moreover, according to Perkins tape-recordings of all radio transmissions and controller 'chit chat' were made throughout the incident, although it is not known how much, if any, of this missing material was consulted by Capt. Stimson.

In summary, it is unreasonable to argue that the phrase 'lock on' could not have been used as described. The official technical manual for the equipment, dating from two years prior to the incident, uses the phrase 'lock on' to describe the Venom's radar-controlled gun ranging, as do other informed sources. The phrase was certainly not unfamiliar to Venom aircrews since it appeared stencilled on the front panel of every AI Mk21 radar set, and a pilot familar with the operation of the GGS.5 radar control unit might very well have used it to correctly describe the electronic lock of its range-control circuit.

Evidence that the phrase was in fact not used becomes a separate issue, and would depend on the testimony of the pilot in question. Since there is evidence that the pilot in question was probably not one of those who have been interviewed, this is a moot point. (NB. It is interesting to note that the standard GGS.5 radar gunsight installation provides an output to a recording device which would enable the response of the gunsight to be reconstructed in later analysis.)

M.L.Shough 2003

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