Radar System Engineering

SEC. 4-1] RANGE, PRF, AND SPEED OF SCAN 117
After each pulse transmission enough time must be allowed for euergy
to travel to, and return from, the most distant targets, the time required
for range R being 2R/c, or nearly 11 psec for each statute mile of range.
If it is supposed that a radar set operates with a pulse-recurrence frequency
of 500 pps, during the 2000-psec interval following each pulse echoes
will be received from objects within a range of 186 miles. A target
250 miles away may, however, return a signal strong enough to be
detected. If it does, this signal will arrive 690 psec after the next transmitted
pulse, in exact imitation of an echo from a target at 64 miles
To decide how serious a complication the possibility of a “second time
around echo, ” as it is called, presents, two cases must be carefully
distinguished.
1. Targets beyond 186 miles are not of interesL The second-timearound
echo is a nuisance only. It would be well to get rid
of it if there were some way to do so; if there is not, it would be
desirable to identify it as an interloper.
2. The extension of coverage beyond 186 miles is for some reason
important. Echoes from targets beyond that range constitute
valuable information that ought to be sorted out and presented
unambiguous y.
If Case 1 applies, the difficulty can hardly be regarded as fundamental.
For one thing, there are several ways in which the echo in
question can be identified as originating from the preceding pulse. The
use of a slightly irregular pulse-recurrence rate, for instance, will prevent
overlapping of successive echoes of this type from the same target,
without affecting the superposition of echoes from a target lying within
the range limit defined above. If a more powerful remedy is needed, the
unwanted echoes can be removed altogether by some such scheme as
the following one. The frequency of the transmitter maybe changed for
each pulse, with a corresponding shift in the frequency of the local
oscillator of the receiver. For example, the transmitter frequency—
that is, the, carrier frequency—might alternate between two values, ~1
and jz, separated by a frequency interval greater than the receiver pass
band. Echoes originating from the first transmitted pulse, at frequency
j,, would not be amplified if received during the interval following the
second pulse, for during this interval the receiver would be in tune only
for echoes of frequency j,, and so on. This rather clumsy expedient,
although it is actually feasible, would scarcely be justified solely as a
means of avoiding a reduction in pulse-repetition frequency. It is mentioned
only to show that objectionable second time around, or even
“ nth time around, ” echoes could be eliminated if necessary without
reduction in PRF and without a drastic change in the radar process.