(7)
The length of time that the range ramp is allowed to run is proportional to the aircraft altitude. The track gate pulse is applied to the track gate coincidentally with the video return pulse. The track gate output signal consists of energy proportional to the overlapping portion of the track gate pulse and the video return pulse. When the system is properly tracking the rf echo, the track gate output will consist of pulses at a nominal pulse-repetition frequency (prf) rate of 7,000 pulses per second (pps) with individual values equal to the overlapping portion of the two pulses. The track gate output pulses are then processed so that the interval of the variable delay is retained. If the rf transmission path distance (height) distance should change, the track gate output pulse values would also change.
(8)
When the altitude of the aircraft gradually increases, the amount of overlap between the track gate pulse and the return signal decreases with each succeeding video return pulse applied to the track gate. Consequently, the positive input into the internal range voltage control is phased to cause the internal range voltage to increase, and more time will be required before the ramp voltage is equal to the internal range voltage. The result is that the track gate pulses follow the leading edge of the received signals at the track gate. When the altitude of the aircraft decreases, the area of overlap of the track gate with the video return pulse increases. The track error causes the internal range voltage to decrease. The comparison of ramp voltage equal to internal range voltage occurs earlier, causing the delay time for the track gate pulses to decrease. Therefore, with decreasing altitude, the echo signal is still tracked.
(9)
The accuracy of the altitude measurement is dependent on the slope and linearity of the ramp, and on the consistency of the rise time of the video return pulse. The gain controls keep the rise time of the video return pulse the same under all received signal strength conditions. The intermediate frequency (IF) gain controls consist of sensitivity range control (src), noise automatic gain control (nagc), track automatic gain control (tagc), and sensitivity range control assist. The IF gain controls are phased so that the gain of the IF amplifier increases with decreasing signal strength, One input is the internal range voltage which drives the sensitivity range control. Its gain schedule is a nonlinear function of altitude, decreasing the receiver sensitivity only at the very low altitudes. There are three gain controlling sources to the receiver: nagc, tagc, and src. The nagc is a wide-band control which senses all noise from the IF amplifier to maintain a usable signal-to-noise ratio. The tagc circuit is fully-keyed (gated) to allow only those IF signals which occur during the tagc pulse to control the IF amplifier gain. The tagc pulse is generated coincident with, and is wider than the track gate pulse, therefore, it overlaps more of the video return pulse so that the peak of the video remains constant under all conditions of signal strength. The src controls IF gain so that leakage pulses from antenna to antenna, and returns from aircraft appendages and rain are not received as range signals.
(10)
The system is either in the track, search, memory, or the self-test mode. In the track, search and memory modes, the system searches for an echo pulse or tracks that pulse. The time constant of the track/no-track detector is selected so that it will require at least five pulses before its output will exceed the tracking threshold. This prevents the system from being switched into the track mode by random pulses from other systems operating in the area. The momentary 0.2-second hold condition, provides for drops in signal strength. If the return pulse does not return to sufficient strength within the 0.2-second interval, the one second memory is enabled, during which time the internal range circuits search for a new signal and the last valid external range output is maintained. If a ground return pulse is not detected, the system goes into a no-track state, and remains there until a new return pulse is detected. During this no-track state, the track/search control forces the internal range voltage through its limits of zero to 25 volts at about three times a second. This causes the tagc and track gate pulses to run through their full range of delays seeking to find the video return pulse. When video return pulses of adequate amplitude return, the TAGC gate pulses will overlap them. After successfully sampling several return pulses, the track/search logic reverts to the normal track mode of operation.
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