RARR Antennas and Tracking

The GRARR VHF and S-band antennas were driven by identical hydraulically-powered X-Y mounts with digital angle encoders accurate to ±0.1°.

The VHF antenna was a 8.4m (28ft) square array of cavity-backed slots operating at about 150Mc. The array was later upgraded to backward-scatter dipoles. The antenna had a beamwidth of 16° and was often used as an acquisition aid for the narrower beamwidth S-band antenna.

The S-band antenna consisted of twin 4.2m (14ft) parabolic dishes operating at around 2500Mc with a beamwidth of 2.5°. One dish transmitted and the other received; the separation of transmit and receive functions decreased noise feedback on the receive dish. Both VHF and S-band antennas had 10Kw transmitters.

Tracking was accomplished by a coherent (continuous) link from the ground station to the spacecraft and back to the ground station. The spacecraft transponder operated according to the international IFF standard – 240:221. Initially the frequency conversion was downwards but later, to be compatible with the Apollo USB transmissions, GRARR spacecraft frequency conversion was downwards.

Range

GRAAR VHF used six range tones in 5:1 frequency intervals from 20 KHz to 8 Hz. Two tones at a time were used according to spacecraft distance. The lower tone produced a coarse range reading and the higher one a finer resolution; tones below 4 KHz were modulated onto a 4 KHz sub-carrier.

In addition S-band used two higher tones - 500 KHz and 100 KHz - for even more precise range measurement. A pseudo-random Ambiguity Resolving Code (ARC) was later retrofitted to the S-band system to extend non-ambiguous ranging to almost four times the distance to the Moon. This was modulated onto the 4 KHz sub-carrier at 4000 bps.

A more precise position using VHF was usually triangulated using three independent transponders carried by the spacecraft and operating on different frequencies. Only a single S-band transponder was carried by the spacecraft, triangulation being of lesser value at lunar distance.(read more Range detail)

Range Rate

After range had been determined, range tones were removed from the coherent link and the measured range rate – the rate of change in the phase of the return signal (Doppler effect) – was integrated with range to give an updated distance value. (read more Range Rate detail)

Angle

(read more detail)