Radar System Engineering

SEC.58] F-M RANGE-MEASCRING SYSTE.\! 143
number of range intervals that can be distinguished is essentially the
same as the number of sidebands added. A system of this type will
be described later. If discrimination of targets is to be made by doppler
frequency, the various doppler tones can be separated with filters.
The more sidebands used, the more information is obtained per
second because of the wider band. In discriminating by frequency,
either duplicate audio systems may be used to increase the information
flow, or a sweeper of some sort may be used—with resulting loss of speed.
5.8. F-m Range-measuring System.—The next system to be
described is one that illustrates the f-m technique of range measurement.
The specific problem is that of a radio altimeter. In this case, we wish
to measure the distance to a single target, no clutter is present, and the
target radial velocity may be zero. .Mso, the system must work down
to zero range. The system described below was very successful and
Transmlthng
antenna
Recelwng
“-J-”*
FIG.5.1 1.—Simplified hiock diagram of f-m system for measuring range,
it and the proximity fuze are the two c-w systems that have been most
\riclely used.
The general method may be explained by the block diagram of
Fig. 511 and the graphs of Fig. 512. Here the transmitter emits waves
of a frequency] that varies linearly with time, oscillating above and belo~v
the mean frequency .fo, as shown in Fig. 5.12. These waves arrive at the
receiver both by a direct connection and by reflection from the target.
Since the trip to the target and return takes time, the received frequency
curve, indicated by the dotted line, is displaced along the time axis
relative to the transmitted frequency. .ilso there might be a displacement
along the frequency axis due to doppler effect. This we disregard
for the moment. The two frequencies, when combined in the mixer,
1We note that two types of “frequency analysis” areusefulin analyzingthis and
other systems. One is the Fouriermethod, in which some curve is decomposedinto
a sumof sinewaves,eachof constantamplitudeand frequencyand extendingin time
from –cato +cO. .%rcordingto the other method, frequentlyuseful in discussing
f-m systems,we say that any functionof time can be represeritedby a function of tbe
form a cos d wherea and u arefanctions of t and are so chosenas to get the best fit
at any value of t. In cases where the latter procedureis useful, a and w are slow
functionsof t.