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 frequency conversion ratio followed the 240:221 international IFF standard. Initially the GRARR S-band frequency conversion was downwards but later it was inverted to be compatible with the Apollo USB systems.

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 tone a finer resolution. the highest tones were modulated directly onto the carrier but those 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. An 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. Two pseudo-random codes - 255 bits and 127 bits- were combined with the result being modulated onto the 4 KHz sub-carrier at 4000 bps.
(read more 'Range' detail)

Range Rate

After range had been determined, range tones were removed from the coherent link. Range was then maintained by integrating the measured range rate – the rate of change in the phase of the return signal (Doppler effect) – with previous range measurements.

A more precise spacecraft position could be triangulated with up to three ground stations in common view; each using one of the three independent different-frequency VHF transponders carried by the spacecraft. Only a single S-band transponder was carried by the spacecraft; triangulation being of lesser value at lunar distances.
(read more 'Range Rate' detail)

Angle

Angle tracking was acheived using a monopulse technique where each antenna independantly compared four different off-axis beams simultaneously; the VHF using the four quadrants of the square plane, and the S-band with the four apertures of the receive horn.
(read more 'Angle' detail)