1-6
The frequency of the rf energy in the pulse radiated by a radar is referred to as the CARRIER
FREQUENCY of the radar system. The carrier frequency is often a limiting factor in the maximum range
capability of a radar system because radio frequency energy above 3,000 megahertz is rapidly attenuated
by the atmosphere. This decreases the usable range of radio-frequency energy. Therefore, as the carrier
frequency is increased, the transmitted power must also be increased to cover the same range. Long-range
coverage is more easily achieved at lower frequencies because atmospheric conditions have less effect on
low-frequency energy.
Radar systems radiate each pulse at the carrier frequency during transmit time, wait for returning
echoes during listening or rest time, and then radiate a second pulse, as shown in figure 1-3. The number
of pulses radiated in one second is called the pulse-repetition frequency (prf), or the pulse-repetition rate
(prr). The time between the beginning of one pulse and the start of the next pulse is called PULSE-
REPETITION TIME (prt) and is equal to the reciprocal of prf as follows:
Figure 1-3.—Radar pulse relationships.
AMBIGUOUS RETURNS.—The radar timing system must be reset to zero each time a pulse is
radiated. This is to ensure that the range detected is measured from time zero each time. The prt of the
radar becomes important in maximum range determination because target return times that exceed the prt
of the radar system appear at incorrect locations (ranges) on the radar screen. Returns that appear at these
incorrect ranges are referred to as AMBIGUOUS RETURNS or SECOND-SWEEP ECHOES.
Figure 1-4 illustrates a radar system with a 1 millisecond prt. The pulses are shown at the top, and
examples of two transmitted pulses hitting targets and returning are shown at the bottom. In the case of
target A, the pulse travels round trip in 0.5 millisecond, which equates to a target range of 82,000 yards.
Since 0.5 millisecond is less than 1 millisecond, displaying a correct range is no problem. However, target
B is 196,800 yards distant from the radar system. In this case, total pulse travel time is 1.2 milliseconds
and exceeds the prt limitation of 1 millisecond for this radar. While the first transmitted pulse is traveling
to and returning from target B, a second pulse is transmitted and the radar system is reset to 0 again. The
first pulse from target B continues its journey back to the radar system, but arrives during the timing
period for the second pulse. This results in an inaccurate reading. In this case, the first return pulse from
target B arrives 0.2 millisecond into the second timing period. This results in a range of 32,800 yards
instead of the actual 196,800 yards. You should see from this example that pulse returns in excess of the
prt of the radar system result in ambiguous ranges while pulse returns within the prt limits result in
