Radar System Engineering

SEC.5.8] F-M RANGE-MEASURING SYSTEM 145
(7)
‘I’his is of the nature of a maximum error and can be much reduced
by averaging over a number of modulation cycles.
Design Procedure. —Design of such an altimeter is begun by using
the above equation, together with an allowable altitude error, to pick a
suitable Af. In the present case Aj was chosen as 40 Xfc/see, which
corresponds to ah = 6 ft, and so, because of averaging, to a rather smaller
operational error.
The mean transmitter frequency is now chosen. This must be
fairly high in order to keep. the frequency variation from being an impossibly
large fraction of the mean trequency. On the other hand there is no
gain in very high frequencies and there may be some loss in intensity
over such terrain as forests. These considerations, plus a consideration
of the tubes available, led to the choice of a mean frequency of 440
Me/see, which can be obtained from acorn tubes; the frequency modulation
is accomplished mechanically.
The modulation frequency is next chosen to give a convenient range
of beat frequenciesf~, subject to the restriction that the time of a modulating
cycle shall be long compared to the maximum signal transit time.
A value of 120 cps was picked. This gives ~a = 8000 cps at an altitude of
400 ft. A second range of O to 4000 ft is obtained by reducing Aj to
4 Me/see.
This completes the major specifications, except for the power. Practically
speaking, power must be decided on the basis of experience,
calculations of available power and thermal noise power being quite
useless since, in practice, the limitation is not thermal noise but microphonics,
etc. Thus if we consider the earth as a diffuse reflector }\-e
easily find the received power is (1/2m) (A/r2) times the earth’s reflection
coefficient and times the transmitted power. Even for very small
transmitter powers and reflection coefficients, this power is large compared
to thermal noise. On the basis of experience, then, it was decided that
the 0.3 watt available from an acorn tube would be sufficient for altitudes
up to 5000 ft, provided certain points of apparatus detail were correct,
as explained below.
Apparatus Considerations. —The leading parameters having been
specified, one can proceed with the detailed design. This is straightforward,
except for certain steps necessary to reduce various spurious
signals in the receiver which might limit the range.
These signals, for the most part, are due to amplitude modulation in
the transmitter. This amplitude modulation comes from microphonics
and fwm slight variation of transmitter amplitude with fTE!qU(311Cy.