UNIVERSITY OF ANBAR ADVANCED COMMUNICATIONS SYSTEMS FOR 4th CLASS STUDENTS
COLLEGE OF ENGINEERING by: Dr. Naser Al-Falahy ELECTRICAL ENGINEERING
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RANGE AMBIGUITY
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 below. The number of pulses radiated in one second is called the pulse-repetition
frequency (prf) , pulse repetition time .
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.
The maximum unambiguous range Runamb is:
Example:
A radar system with a 1 millisecond prt, scanning two targets A & B.
For target A, the pulse travels round trip in 0.5 millisecond, which equals to a target range of
75,000 meter.
Since 0.5 millisecond is less than 1 millisecond, displaying a correct range is no problem.
However, target B is 180,000 meter 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 30,000 meter instead
of the actual 180,000 meter.
WEEK 9
