A BMPG member, Pete Murray observed that, as the Bloodhound system was optimised against fast, high flying aircraft, firing against low flying choppers was probably not envisaged hence their were no exercise scenarios in the Launch Control Post simulator for helicopters. It would be expected that helicopter blades would create a bizarre Doppler return but did the system have any capability against low, slow helicopter targets or how a helicopter looked on the doppler trace? In answer to the first part our Historian, Richard Vernon, offered the following from his research work.
Against Rotatry Wing Targets
The LCP Mk 2 System was designed for UK Air Defence (UKAD) use so what would the chances be of engaging a Soviet helicopter? The simple answer is ZERO unless it took off from a boat in the North Sea. Even then, the Rules of Engagement for the Bloodhound force in South East England were if it was below 5000ft and below 300 knots it was friendly and was not to be engaged. The current world speed record for a chopper is 255 knots.
Trials against a Wessex helicopter were conducted with both the T86 and T87 radars at RAF West Raynham by 41 Sqn in February to May 1967 (the squadron having a deployable role in the Middle East). The Wessex was fitted with a doppler transponder so that the radar could acquire the Chopper.if the radar lost lock. The best range they got with the transponder was 20NM; the best pick up range they got without it was 9NM with the TV camera on one of the T87s.
HELICOPTER TRIAL REPORTS
20 February 1967
Following trials carried out in Australia in which Type 86 and 87 Radars were used to look onto a Hovering helicopter, No 41 Squadron was designated to take part in a similar trial in the United Kingdom. Subsequently, a Whirlwind helicopter was detached to RAF West Raynham from RAF Odiham, No 230 Squadron and underwent a series of trials at varying heights and ranges while attempts were made by the Radar Type 86 and 87 to lock on to the Doppler effect offered by the rotor blades of the helicopter. The result of the trial was undetermined and further trials will take place in the near future.
7 March 1967
The second phase of the Helicopter Trial took place this month as planned again using a Whirlwind Helicopter supplied by No 230 Squadron RAF ODIHAM. The trial was held to determine if it was possible to acquire and track a hovering helicopter using standard or non-standard operating procedures as required, with both Types 86 and 87 Radars, but with the 87 in the main. Accordingly the helicopter team were briefed to hover over two known geographical points, easily discernible from the air in turn, each point bearing respectively 130 degrees at 10 miles and 087 degrees at 7½ miles true, from RAF West Raynham. The pilot was requested to hover at 5000 feet above the selected points maintaining a ground speed of 15 knots to reduce the possibility of radar returns via aircraft skin echo. Because of the inconclusive trial held last month, when attempts were made to lock the radars on to the doppler effect offered by the helicopter rotor blades, on this occasion a Doppler Transponder was carried and Flt Lt T.J. Keeley was instructed in its use and flew the aircraft. However, the transponder was to be switched on only should the radar loses lock on the aircraft during its flight out to the hovering points. Again the trial proved inconclusive except that when the transponder was operated, the radar systems received an outstanding Doppler echo. It was decided to try again the next day forgoing doppler return produced by the blades and to concentrate on that given by the transponder. This trial proved most successful and the Doppler return was tracked out to a range of 20 miles at an altitude of 5000 feet. A most heartening side effect of the trial was that the helicopter was tracked out visually for a distance of 9 miles using the Type 87 Radar Television system. Admittedly, visibility was very good, but the clarity of the visual tracking drew remarks of admiration from both missile section crew and the Technical and Operations Officer present.
1 May 1967
The third and final phase of the Helicopter Trial took place this month as planned, using a Wessex Helicopter supplied by No 72 Squadron, RAF Odiham. The helicopter fitted with a Doppler Transponder hovered at various heights over two known geographical points to enable RAF Wattisham radar and the Type 86 and 87 Radars engaged in the "Tinsmith" trials to be accurately calibrated. The trial was successfully completed on the 3 May 1967. As it says above, there were trials by 15 JSTU at Woomera and a quick look has shown this entry from July 1964.
15 JSTU - July 1964
Single and twin engined propeller-driven aircraft and an Alouette helicopter have been tracked by both the TIR (Target Illuminating Radar) and the missile. analysis of the records for the helicopter showed, radar signals over a wide range of frequencies in the Doppler spectrum. A separate report is being prepared. Low level fast jets were top of the list of targets for Bloodhound 2 when it was designed, with high altitude targets a very close second.
COMMENT 1 - Pete Harry
During my time on 25 at Laarbruch we had a couple of helicopter incidents. One that I remember clearly was Green sections T86 going through a standard search at low level when it acquired a target at minus 500 feet! I can’t remember the speed. The cause turned out to be a helicopter in the hover at the eastern end of Laarbruch’s runway, 25’s location being at the western end of the runway. We had the time to check the aerial positioning of the T86 which was definitely below 0 degrees. The only explanation we could think of at the time was that it was reflections from the rotor blades which in turn were bouncing of the surface of the runway. The T86 treated everything below 1kHz Doppler as noise and was filtered out, if memory serves me right. Remember the light aircraft that landed un-detected in Red Square, Moscow? After that incident it was obvious that fast jets were obsolete and that to beat any air defence system you had to be very slow and low!
COMMENT 2 - Richard Vernon
Low level multipath reflections off the sea were a big problem at the Aberporth test range in very low level engagements due to the fact that the radars were higher than the target. The calmer the sea state (wave height), the worst it got. The lowest successful engagement at Woomera was 183 feet AGL (above ground level). Most of the low level failures at Woomera were either missile faults, structure failures or the target doing worst case zero doppler manoeuvers around the time of firing, Missile Lock-On or interception. As for the Helicopter at Laarbruch, that was most likely a close in noise issue due to thermal RF noise from the engines rather than doppler off the rotors. It was a known issue and had been trialled at West Raynham in 1968 against the T87 using an Army Scout helicopter and an RAF Hunter. The thermal noise for jet exhaust was something that led to the selection of targets for both the Bloodhound 1 Service Acceptance and Bloodhound 2 Missile Evaluation Trials at Woomera, along with the radar cross-section and altitude capabilities of each target. The Jindivik Viper engine was 'Quiet' as regards engine RF thermal noise. The Derwent in the Meteor was a 'medium' and the Avon in the Canberra was extremely noisy. In fact, close in site noise was a major issue with Bloodhound 2 in the early days. The operation of the original launcher HPU and ACU jammed the radar; the original fix was to line the fibreglass covers with kitchen foil! The LCP ACU operation jammed the radar. The standby generators jammed the radars both due to thermal plume out of the exhaust and RF interference. Both RAF Butterworth and RAF Seletar had issues with thermal noise from nearby towns on some search sectors.
COMMENT 3 - Pete Murray
Thanks to all. Having read the information from Richard and others, it may be possible to simulate a helicopter using the CW jamming options. I'll
give it a try sometime.