FPQ-6 Tracking and Ranging

Revision as of 02:34, 7 February 2007

Noted trencher-man and transmitter technician George Allan has his on-duty meal delivered to FPQ-6. Serving him (left to right) are Frank Vinton, Colin Forbes, Nola Meiklejohn, and Geoff Goddard. Photo - George Allan

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, FPQ-6 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 FPQ-6 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 FPQ-6 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)

MSFN spacecraft were normally tracked in ‘beacon mode’ where the spacecraft transponder replied to a coded pulse group from the radar (typically two pulses 3 μsec apart) responding with one pulse generally of a different frequency to that of the interrogation. The ‘total trip time’ from the ground radar to the spacecraft and back again is used to calculate the range of the spacecraft. (read more 'Range' detail)

In ‘skin mode’ where the signal is passively reflected from the spacecraft’s metal skin, the FPQ-6 could acquire and track a 1-metre square target at a range of 500 nautical miles and hold that track out to approximately 600 nautical miles.

Angles system

The FPQ-6 horn arrangement

"The radar employs a 2.8-megawatt peak power (4.8 KW average), broad-banded (5400 to 5900 MC) transmitter with frequency stability of 1 part in 108. The 29-foot diameter antenna dish uses a Cassegrainian feed and has a 51 DB gain with a 0.4-degree beam width. Its monopulse, 5-horn feed system permits the reference and error antenna patterns to have their gains independently established as well as the slope of the error patterns optimized while supplying target return signals to the receiving system with a minimum of feed insertion loss. (read more 'Angle' detail) This design has advantages over conventional monopulse feeds and reduces the possibility of side lobe lock-on. The three-channel signal outputs of the antenna feed system [Az., El., and Ref.] are supplied directly to the receiving system without undergoing any additional, loss-inducing, signal manipulation. The three-channel monopulse receiving system has its bandwidths optimized 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 has been improved to 3.5 DB through the addition of closed-cycle, parametric RF amplifiers." [6]

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, ????
[6] TIB