Radar System Engineering

SEC, 5.11] PULSE-MODULATED DOPPLER SYSTE.\f 155
estimated as 100 db above the noise in a 103-cycle band, or 130 db above
that in a l-cycle band.
A third estimate can be made from measurements on the completed
system at this same location, which showed a clutter 100 db above the
noise in a l-cycle band. Part of the discrepancy between this and the
above 130-db estimate is no doubt due to crudities in the estimation,
the rest to the fact that the clutter in a 500-~sec pulse will certainly be less
than 500 times that in a l-~sec pulse because the clutter is not spread
uniformly in range. In any case, whatever the reason, the clutter is
experimentally found to be about 70 db above the noise in the 1000-
cycle audio band, or about 80 db above the noise in the effective band of
100 cycles.
The system, as actually made and tested at this same location, had
92-db attenuation at the “infinite attenuation” points of the filters and
experience showed that this was just comfortably adequate.
From the above results we conclude that the filters must have about
90 db of rejection for satisfactory operation at this particular site.
Measurements at other sites would be valuable in answering the question
as to how representative this site is. Lacking such data, we can only
venture the opinion, based on personal observation, that the location
did not appear unusual in any way-in fact it appears likely that mountain
areas with even higher ground returns may be common.
The width of the rejection bands depends on the variability of the
ground returns. Until now, the ground returns have been assumed
constant, so that the voltage at Point 1 in Fig. 5.15 is actually periodic
and could therefore be removed by filters with infinitesimal bandwidth.
Actually the ground returns vary, with the result that the filter rejection
bands must have a finite width. Using some data of H. Goldstein, 1 a
width of 4 cycles 12 db up from the 92-db bottom of the’ curve was
chosen. Even if the ground returns had been constant, much the same
bandwidth would have been needed because of modulation due to
scanning. The rest of the curve was then made as narrow as possible.
This turned out to be about 200 cycles at the 3-db point.
Additional filters for eliminating “window” signals or rain clutter
were also provided and could be switched in when desired. These
filters had much less attenuation than the clutter filters, but the attenuation
extended over a wider frequency range, designed to exclude
doppler frequencies due to motion of “window.”
Although the audio amplifier began to cut off rapidly above 1000 cps,
and the strength of the harmonics of j, decreased fairly rapidly, it was
found necessary to have infinite attenuation filters at 2000 and 3000 cps,
1See Sec. 6.20, Vol. 13 of this series.