Radar ground station characteristics
| AN/FPQ-6 | AN/FPS-16 |
|---|
| Frequency | 5.4–5.9 GHz (5.6–5.1 cm) | 5.4–5.9 GHz (5.6–5.1 cm) |
| Peak power | 2.8 MW | 1.3 MW |
| Antenna size | 29 feet (8.8 m) | 13 feet (3.9 m) |
| Antenna gain | 51 dB | 47 dB |
| Receiver noise figure | 7.5 dB | 6.5 dB |
| Angle precision (units) | 0.1 | 0.15 |
| Range precision | 9.8 feet (3 m) | 15 feet (4.5 m) |
The AN/FPQ-6 radar employed a 2.8 megawatt peak power (4.8 kilowatt average), broad band transmitter with a frequency stability of 1×108. The 29-foot diameter parabolic antenna, using a Cassegrain antenna feed, had a 0.4° beamwidth and a gain of 51 decibels. 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 optimized 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 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 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 providing 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 coupling unit located between the valve motor and pedestal drive gearing damped out undesired mechanical resonances.
The AN/FPQ-6 had a self-contained digital computer, an RCA FC-4101, whose primary purpose was to correct dynamic lag in the angular output data. As designed, both the AN/FPQ-6 and AN/TPQ-18 radars were provided with a built-in data processor referred to as the RCA 4101 Computer.
The ground floor of the two story building contained the air-conditioning, transmitter heat exchanger controls, equipment load center data input junction box and ex-Mercury atomic time standard. The first floor contained the eight equipment racks, the console, and the 3 megawatt transmitter.
Operational description
The AN/FPQ 6 radar was built by RCA and was a development of the AN-FPS 16. It was an amplitude comparison monopulse C-band radar with a 2.8 MW peak klystron transmitter tunable from 5.4 to 5.8 GHz (5.6 to 5.2 cm), and had a 30 ft (9 m) parabolic antenna. The antenna had 52 dB gain, a 0.6 degree beam width utilizing a Cassegrain feed with a five horn monopulse comparator. The system had an unambiguous maximum range of 32,768 nautical miles (37,709 mi; 60,686 km) with uncooled parametric amplifiers and a system noise temperature of 440 K (167 °C; 332 °F) giving it a noise figure of 4 dB.
A major feature of the radar was its maximum unambiguous range, despite a pulse-repetition frequency (PRF) of some hundreds of pulses per second (160, 320, 640, sometimes 80 and 1280). To combine these two features requires that the radar carry out Nth time around tracking. That is, it had to be able to track an echo resulting from a transmitted pulse other than that sent as the start of the same PRF period in which the echo was received. In order to do so, the range system employed a 2-second time base allowing the system to determine the number of PRF periods elapsing before an echo, resulting from a particular transmit pulse (Tx) received. The range system carried out a find process, then a verify process before entering the auto-tracking mode.
The FIND process is first carried out where two successive Tx trigger pulses are delayed by a time equivalent to an RF wave go-and-return distance of 16,000 yards (15,000 m). Then the range gate triggers are delayed by an equivalent time. The delayed Tx trigger pulses are counted in a zone counter, until target video pulses are detected in the delayed range gates. At this point the zone counter contains the number of PRF periods corresponding to the Nth time around.
The VERIFY process mode is where one Tx pulse is delayed for a 16,000 yards (15,000 m) equivalent distance. The range gate in the zone determined in the earlier FIND mode is also delayed to match the Tx delay. This sequence is repeated until four video returns, from eight attempts, are received. When the four returns are detected, automatic tracking is maintained. The contents of the zone counter are added to the apparent range, the range reported in the current PRF period, to determine the actual range of the target. If, during the VERIFY process four returns are not found after eight tries the FIND process is re-initiated.
As an example, if a PRF of 142 PPS is assumed, and a target at a range of 4,883,072 yards (4,465,081 m) is to be acquired, the operator initiates the FIND mode. The system carries out that process by delaying triggers, and counting zones, and finds that the zone counter has stored a count of four. The VERIFY mode then takes place, resulting in confirmation of the zone count. The target will appear, on the radar display to be at a range of 265,200 yards (242,500 m). That is the difference between four PRF period equivalents plus the additional range. The range reported will be the actual target range of 4,883,072 yards (4,465,081 m), the figure displayed on the range read-out.
As the radar was designed to track moving objects the need arose to handle targets which, in closing or opening range during Nth time around tracking, came into coincidence with the next Tx pulse. So, the radar had to be capable of tracking “through the Big Bang” (aka the Main Bang). This arises from the fact that the antenna serves both the Tx and Rx. To allow this to happen a device called the transmit-receive switch (T/R switch) is used. The antenna is connected to the receiver until the transmitter is pulsed. The T/R switch detects the Tx pulse and transfers the antenna to transmit for the duration of the pulse. At the instant the receiver is disconnected from the antenna the range system will lose track.
In order for an Nth time around tracking system to work, there has to be some arrangement to cover the loss of received signal for the transmitted pulse period. In the FPQ-6, this is achieved when the target pulse reaches an apparent range of ±16,000 yards (15,000 m) of the Tx pulse, a number of Tx pulse being the zone count, are delayed by a time equivalent to 32,000 yards (29,000 m). In the above example, when the range of the target closes to 4,633,872 yards (4,237,213 m), that is 16,000 yards (15,000 m) greater than the 4 zones, the 32,000 yards (29,000 m) delay is introduced into the Tx system. After 4 pulses the delay is transferred into the range gate generation system, and the target continues to be tracked. After the target has reached a range of 4,601,872 yards (4,207,952 m), 4 zones minus 16,000 yards (15,000 m), the delay is removed from the system. At that point the range is such that the zone counter will have been decremented by 1 and the apparent range will be 1,138,468 yards (1,041,015 m), but with the target at a real range of 3 zones plus the apparent range. Obviously, for an opening target the zone counter will be incremented and the apparent range will be slightly more than 16 yards (15 m).
