FPQ-6 Tracking and Ranging

Tracking techniques

All tracking systems go through four phases of activity for each track - pre-pass checks, acquisition of spacecraft (or satellite), tracking data output, and post-pass checks – except that for a precision radar, such as the FPQ-6, there are additional refinements. [1]

Pre-pass checks

In addition to the usual slew tests and collimation tower tests of reflector and transponder tests, Q6 recorded the local humidity and pressure entered into the RCA computer to compensate for the refraction coefficient which depends on the atmospheric factors in a complex manner and affects the calculation of range. [2]

Acquisition

Usually, the Q6 antenna was controlled by the data processor from pre-programmed orbital parameters which provided antenna pointing angles and a ranging distance. If no clear radar signal was received, the console technician would modify the pre-programmed tracking path by choosing an antenna angle scan (usually an 8 mil diameter circle) and a range search (usually pm 10,000 yards either side of the predicted range) or through the station ‘acquisition bus’ by slaving to a station antenna which had already locked onto the spacecraft.

Part of the skill of acquisition required the patience of the technician to allow the spacecraft to move ‘through’ the antenna side-lobe into the main signal lobe before locking onto the spacecraft.

Tracking data output

Once the spacecraft had been acquired, the RCA computer proceeded to update its orbital parameters to provide more accurate orbital data to down range radars and to the data processor should a premature loss of signal (LOS) occur and re-acquisition became necessary. Meanwhile high speed tracking data (10 sets/sec) was recorded on magnetic data and low speed tracking data (1 set/sec) was dispatched by teletype to the tracking network.

Post-pass checks

After LOS a final collimation tower check was made and the local atmospheric data was again recorded to enable possible further adjustments to the tracking data recorded and to the orbital parameters.

Following the Apollo-8 mission, NASA queried a range tracking difference of 60 metres between the Carnarvon FPQ-6 and the Unified S-Band ranging systems at an elevation of about 13 degrees. [2] Local static tower tests and dynamic testing with the STADAN simulation aircraft revealed no significant local differences. [3] Finally Carnarvon engineers suggested that NASA had applied the Q6 refraction coefficient in the US in addition to its automatic inclusion at Carnarvon. The station heard no more about the ‘error’.

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Ranging

The FPQ-6 Digital Ranging Machine (‘machine’ - a heritage of the days of mechanical control rather than digital electronics) used nth-time-around techniques to achieve a maximum unambiguous range of 32,768 nautical miles (60,686 Km) with a precision (accuracy) of ±2 yards (1.8m). The FPS-16 standard radar had only a maximum range of 500 nautical miles with a precision (accuracy) of ±5 yards although later it was modified to 5000 nautical miles as could be the FPQ-6 radar to 256,000 nautical miles. [4]

The concept of precision/accuracy needs clarification. Lindsay Sage, an early Chief Engineer of the station, is remembered as saying, “You can be very precise and be precisely wrong”. Precision is the closeness of the individual readings to each other and is nothing to do with how accurately – how close - they are to the true value. Accuracy depends on other factors apart from the structural integrity of the antenna. [5] (read more 'Accuracy' detail)

Beacon mode

Skin mode

(read more 'Range' detail)

Angles

(read more 'Angle' detail)

Side-lobe detection

Computer assistance

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References:

[1]
[2] Heald, B. personal communication, 15 October 2005
[3] NAA: PP583/1 C358A, Contractor Performance Report, June 1969
[4] Anderson, K. and Hocking, R. personal communications, 2005
[5] Housely, T. email, ????