AN/FPQ-6

The AN/FPQ-6 Missile Range Instrumentation Set is a fixed station long-range precision tracking set to be used for tracking intercontinental ballistic missiles for range safety and range user's trajectory measurement data. It will also be used for tracking during staging and parking orbit (trilateration) of a synchronous satellite, The operational objective is to provide a fixed station radar capable of skin tracking a square metre target to ranges in excess of 300 nmi (560 km).

The specifications for the FPQ-6 radar installation at Patrick Air Force Base, Florida, are shown in the following table.

Apollo Land-Based C-Band Radar Characteristics

Station    Station        Radar     Antenna      Peak    Receiver   Receiver    Unambiguous
           Location       Type     Diam.  Gain   Power    Noise    Bandwidth      Range
                                   (ft)   (dB)  (MW)  Figure     (MHz)      capability
                                                          (dB)                  (nmi)
-------------------------------------------------------------------------------------------
PAFB    Patrick Air       FPQ-6     29     51      3        4          1.6       32,000
        Force Base, FL

NOTE:
The nominal frequency range is 5.4 to 5.9 GHz.
Receiver bandwidth is for 1 microsecond pulsewidth.
FPS-6 radars have circular polarization capability.

At least two of the AN/FPQ-6 systems were given the "CSP" (Coherent Signal Processor) modification. This added a synthesized transmit signal frequency and Local Oscillator frequency exciter, a TWT and Crossed Field Amplifier transmitter, an I&Q Pulse Doppler receiver with very narrow IF crystal filters, a Doppler Velocity tracking servo loop that digitally tuned the LO synthesizer, a real-time digital computer program to resolve pulsed-doppler frequency/velocity ambiguity (compared to noisy Range-Rate data), a hardwired logic Doppler acquisition processor that generated time compressed ambiguous velocity and acceleration profiles for correlation against stored, time compressed, replayed I&Q received samples to detect the best match to initiate ambiguous track (until ambiguity resolution corrected the track).

The AN-FPQ 6 radar was built by RCA and was, effectively, a development of the AN-FPS 16. The Q6, as it was known by those who worked on 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, which had a 9-meter parabolic antenna, having 52 dB gain, a 0.6 degree beam width, utilizing a Cassegrainian feed with a five horn monopulse comparator.

This radar had an unambiguous maximum range of 2¹⁵ or 32,768 nautical miles (60,686 km), and employed uncooled parametric amplifiers with a system noise temperature of 440 K, [a noise figure of 4 dB].

A major features of the radar was its maximum unambiguous range of 32,768 nautical miles (60,686 km) despite a pulse repetition frequency [PRF]of some hundreds of pulses per second. (160, 320, 640, sometimes 80 & 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 which allowed the system to determine the number of PRF periods elapsing before an echo, resulting from a particular transmit (Tx) pulse was received. The range system carried out a find process, then a verify process before entering the auto-tracking mode.

[Note: All the following discussion uses ranges in yards—the radar was designed to work in those units and converting them to metric units would not add any clarity to this description.]

The FIND process is first carried out. In this process 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 an auxiliary counter, the 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 is then entered. In this mode one Tx pulse is delayed for a 16,000 yards (15,000 m) equivalent distance. The range gate in the zone determined in the 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, that is 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.

Take this example, in which a PRF of 142 PPS is assumed: A target at a range of 4,883,072 yards (4,465,081 m) is to be acquired. At the radar console the operator initiates the FIND mode, and the system carries out that process by delaying triggers, and counting zones, as described above, 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, in this case, 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, in the jargon, 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 Rx until the Tx is pulsed. The T-R Switch detects the Tx pulse and transfers the antenna to the Tx for the duration of the pulse, say, 1 microsecond. [That period is that in which a radio wave would travel 164 yards (150 m). The period is radar range, the round trip distance to and from the target.]

At the instant the Rx 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 Rx signal for the Tx pulse period. In the radar under discussion this is achieved as follows. When the target pulse reaches an apparent range of ±16,000 yards (15,000 m) of the Tx pulse a number of Tx pulse, the number being the zone count, are delayed by a time equivalent to 32,000 yards (29,000 m) Take our example above. When the range of the target reduces 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).

• AN/FPQ-6 Missile Range Instrumentation Set, USAF MIL-HDBK-162A, Volume 1, 15 December 1965. Radio Corporation of America, Moorestown, New Jersey.

• AN/FPS-16