User's Manual

The squitter pulses are third in the order of priority. In the absence of interrogations or identity
information, random squitter pulses are generated to maintain an average output pulse train of 800 Pulses
Pairs Per Second (PPS). The purpose of transmitting squitter pulses is to stabilize the Automatic Gain
Control (AGC) circuits in the aircraft interrogator.
The process of distance measuring originates in the airborne unit with the generation and transmission of
pulse signals called interrogations. The airborne transmitter repeatedly initiates and transmits pulse signals
consisting of pulse pairs having 12 µs spacing, a pulse width of 3.5 µs, and a gaussian or sine-squared
shape. These pulse pairs are recovered by the transponder beacon receiver, whose output triggers the
associated transmitter into transmitting reply pulse pairs. The reply pulse pairs are received by the airborne
receiver and timing circuits, which automatically measure round-trip travel time (the time interval between
interrogation and reply pulses) and convert this time into the electrical signals that operate the distance
meter.
Using the block diagram of the system in Figure 2-2, the distance measurement function can be examined
from the system stand point. The range circuits of the airborne interrogator initiate the distance measuring
process. They formulate and transmit an interrogation pulse pair, which is received at a ground station
antenna and sent to the RTC. The RTC then triggers the encoded pulse generator where the shaped pulses
are amplified and routed to the PA for modulation of the gated RF. The output RF pulses are then radiated
into space (as replies) via the antenna. The reply pulses are received by the aircraft, decoded by the
airborne receiver, and examined by the range circuits for synchronism with the airborne unit's own
randomly generated interrogation pulses.
The airborne unit measures the elapsed time between the transmission of the interrogation pulse pair and
the receipt of the reply pulse pair. It then, converts this time into a distance indication. In other words, the
distance indication is a measurement of the range time of the pulse pairs. This timing sequence is easily
seen by means of the system timing diagram of Figure 2-3. Timing starts (in the range circuits of the
airborne unit) with the first pulse of the interrogation pulse pair. After a time delay, depending upon the
distance between the aircraft and the ground station, the interrogation pulses are received at the antenna of
the ground transponder beacon. The interrogation pulses are decoded, and the reply is encoded and
transmitted after a preset time delay (the reply delay of the ground station).
This nominal reply delay duration is 50 µs, for which the airborne range circuits automatically account.
Thus, the total time lapse for any interrogation response cycle is the sum of reply pulse spacing, the
two-way transit time (range time), and the reply delay.