Radar System Engineering

SEC. 86] FREQUENCY CONSIDERATIONS 261
The most satisfactory presentation of beacon replies for purposes of
ordinary navigation is undoubtedly the PPI since it gives the whole
situation at a glance in a direct way. The angular widths of beacon
replies should be limited to a few degrees, if possible. For an airborne
interrogator this suggests the use of 3-cm or shorter wavelength; for
systems having ground interrogators with large antenna arrays a considerably
lower frequency can be used. Azimuth determination by lobeswitching,
involving comparison between the strengths of signals received
on two antennas pointed in different directions, is adequate for many
purposes but relatively clumsy in use. It permits the use of lower frequencies
and much broader antenna patterns, with corresponding reduction
in size of the antennas, but it results in a great increase in the difficulty
of identifying beacon replies with particular radar echoes. It is,
however, satisfactory for aircraft homing on the beacon. In the wartime
systems which provided means of precise navigation by accurate measurement
cf the ranges to two ground beacons at known positions, special
indicators were used. They displayed and measured the positions of the
steady beacon signals obtained by using omnidirectional antennas rather
than scanners.
There is an upper limit on the frequency to which it is desirable to go.
At about 15,000 to 20,000 Me/see the attenuation by water vapor in the
atmosphere of the earth begins to become of consequence. In conditions
of bad weather, when navigational beacons are most needed, the range of
beacons at such frequencies would be lowered so much that they would
be of little use.
In nearly all cases, the frequency of the beacon reply should be different
enough from that of the interrogating pulses to obviate simultaneous
recephion of beacon replies and radar echoes by one receiver. In this way
the swamping of beacon replies by strong echoes is eliminated. Even
when it is desirable that radar echoes and beacon replies be presented
simultaneously, the saturation video levels for the two kinds of signals
can be made different by using separate receivers; thus beacon replies can
be made to stand out even when superimposed on saturated ground clutter.
A very striking differentiation between radar and beacon signals from
aircraft has been achieved by putting them on separate PPI tubes that
give signals in two different colors, the signals being effectively superimposed
by optical means.
If any interrogator or radar set of a given type is to be able to locate a
beacon readily, the beacon must transmit at some known frequency and
the receiver for the signals must be readily tunable to that precise frequent
y, whether the beacon replies are rewived or not. When the lWation
of the beacon is unknown (as is usually the case) and the interrogator
is scanning, it is not tolerable to increase the general uncertainty about