The FPQ-6 Skin-tracking radar

From Hamish Lindsay…

A close-up of the FPQ-6 dish with Don Blackman looking out of the antenna mount. The data from the FPQ6 was the first time NASA had confirmation that spacecraft, particularly the manned Apollo spacecraft, launched from Cape Canaveral had achieved its planned orbit. Photo: Hamish Lindsay.

RCA’s Missile and Surface Radar Division developed the FPQ6 skin tracking C-Band radar as a progression from their FPS-16 radar already in use at Woomera. It was capable of providing continuous spherical coordinate information at ranges of 32,000 nautical miles (59,264 kilometres) with an accuracy of plus and minus 6 feet (1.8 metres). The FPS-16 only ranged out to 500 nautical miles (926 kilometres) with an accuracy of 15 feet (4.57 metres), although it could be modified to stretch out to 5,000 nautical miles (9260 kilometres).

The Q6 radar employed a 2.8 megawatt peak power (4.8 Kilowatt average), broad banded (5400 – 5900 MHz) transmitter with a frequency stability of 1 x 10⁸.

The 8.8 metre dish antenna, using a cassegrain feed, had a 0.4° beamwidth and a gain of 51 db. Its monopulse, 5 horn feed system permitted the reference and error antenna patterns to have their gains independently established as well as the slope of the error patterns optimised while supplying target return signals to the receiving system with a minimum of insertion loss.

The three channel signal outputs of the antenna feed system were supplied directly to the receiving system without undergoing any additional loss-inducing signal manipulation with bandwidths optimised for the specified pulse widths of 0.5, 0.75, 1.0 and 2.4 microseconds and the receiver noise figure of 7.5 db was improved to 3.5 db through the addition of closed-cycle parametric RF amplifiers. This system ensured a dynamic range in excess of 120 db.

The receiving system provided simultaneous presentation of the skin and beacon returns to the console operator so that skin tracking could be used if the beacon signal was lost.

The antenna pedestal was a high precision, two axis mount, using a hydrostatic bearing in azimuth and phase roller bearings in elevation to provide mobility and support to the counterbalanced, solid surface antenna. The antenna was positioned through anti-backlash dual drive pedestal gearing via a high torque-to-inertia electro-hydraulic valve motor system. A viscous coupler located between the valve motor and pedestal drive gearing damped out undesired mechanical resonances. The Q6 had a self contained digital computer, an RCA FC-4101, whose primary purpose was to correct dynamic lag in the angular output data.

The ground floor of the two storied building contained the air-conditioning, transmitter heat exchanger controls, equipment load centre data input junction box and ex-Mercury time standard The first floor contained the 8 banks of equipment racks, the console, and the 3 megawatt transmitter.

Apart from looking after the time standard, one interesting exercise I was involved in was tracking a balloon which was rocketed to the upper atmosphere and threw out French chalk when it reached the specified height. My job was to man a 20mm Oerlikon Mark 20 gun sight modified to connect to the Q6 to slave it to follow the movements of the gun sight. I followed the rocket’s French chalk it as it drifted with the upper atmosphere currents so the Q6 could find and lock onto the signal to track the drift. I was very impressed when I pushed my shoulders into the padded shoulder brackets and watched the dish precisely follow my every movement. Gave me quite a feeling of power. I had no trouble spotting the French chalk, but had to be quick setting the sights on it as I could not see the rocket sending the balloon up until the chalk blossomed out and dissipated fairly quickly, and then we had nothing to locate the balloon.

The FPQ6 console in action. From left: Ron Burgess, diarist, George Allen and John Gerschwitz console operators, and Len Algate, Engineer-in-charge on the right. Photo: Hamish Lindsay.

The FPQ6 console in action – taken around 1969. From left: Brian Davies, Geoff Linthorne, Mick Coffey. Photo from Bob Goodman (OTC) via John Sarkissian. Thanks to Tony Green and Trevor Housley for the ID.

Len Algate, the FPQ6 supervising engineer. Photo: Hamish Lindsay.

The FPQ-6 skin tracking radar. The 8.8 metre dish antenna had a beam width of 0.4° and a gain of 51 db. Photo: Hamish Lindsay. Colour correction: Colin Mackellar.

The FPQ6 antenna. Photo: Tom Sheehan, 1971.