Radar System Engineering

SEC. 1611] COMPENSATION FOR VELOC’I TY OF SYSTEM 655
To get the greatest cancellation of an undesirable moving object, the
first blind speed, v,, should be made as large as possible. For exampie, if
U1is 30 times the radial speed of the object the response will be 10 per cent
of the maximum value, by Eq. (20). In the case of clouds moving at
30 mph, that gives us VI = 900 mph, which is a suitable value as far as
Fig. 16.24 is concerned. Since v, = Xj,/89, it can be seen that such a high
value of VIis obtainable, when a normal PRF is used, only by going to a
wavelength of the order of 100 cm. For microwave frequencies, therefore,
one can scarcely reduce storm echoes by more than about 10 db.
In the detection of ships, whose velocities are comparable with those of
clouds, it might be thought that the ships would be canceled along with
the clouds. This is not so because ships, when within the horizon, give
much larger signals than clouds, and still show up strongly after a 10-db
cancellation.
MOVING-TARGET INDICATION ON A MOVING SYSTEM
16s11. Compensation for Velocity of System.—Figure 16.25 shows
what has to be done to compensate for the velocity of the station when a
system is carried on a ship or airplane. The phase-shift unit shown in a
changes the phase of the reference
signal at the same rate as that at which
the phase of fixed echoes is being changed by the motion of the station.
The effect is to give the station a virtual velocity which cancels the actual
velocity.
Figure 16.25b is a block diagram of the phase-shift unit. An oscillator
supplies a signal at the doppler frequency which is mixed with the reference
signal. The upper sideband is then selected by a crystal filter. The
addition of the doppler frequency to the reference signal is equivalent to
shifting the phase of the coherent oscillator at the rate of je! cps. It is to
be noted that this is not the same as merely tuning the coherent oscillator
to the frequency fc + ~~, since the phase shifter is applied after the oscillator
is locked. Full details on the phase-shift unit can be found in a
report by V. A. Olson. 1
The doppler frequency depends on the radial component of velocity
and is therefore proportional to the cosine of the azimuth angle. It is
necessary to vary the frequency of the doppler oscillator automatically
as the antenna
scans.
In an airborne set, there is a further complication due to the fact that
the radial velocity depends on the depression angle, which means that
the doppler frequency of the ground clutter varies with the range. In
consequence, the phase-shift unit cannot be used for clutter at large
LV. A. Olson, “ A Moving Coho Conversion Unit, ” RL Report No. 975, Apr. 3,
1946.