Radar System Engineering

SEC. 15.7] AZIMUTH SCAN RATE 599
tor recovery and to allow for irregular firing of the rotary spark-gap
modulator, giving an average time interval of 2500 psec between pulses,
or a v, of 400. Thus, the value of V, was set close to the permissible
maximum for a radar set with a 200-mile range.
For some applications, presentation of greater range would have been
helpful. A pulse recurrence frequency of 350 or 300, permitting range
presentation to 250 or 300 miles, would have reduced R- very little,
but the characteristics of the modulator chosen restricted v, to 400. A
value of v, of 350 or 300 would be desirable from the standpoint of indicator
range presentation for a future radar system of this general type,
but two other factors favoring higher v, must be considered:
1. The limiting accuracy of azimuth-angle determinations is approximately
half of the angular separation between pulses. A scan
rate of 360/see with a V,of 400 gives an angular separation between
pulses of 0.09°. The contribution of this effect to azimuth error
would be about 0.045°, which is negligible for normal surveillance
and control. In operations requiring very precise control of
aircraft this error would be appreciable, and if it were doubled
by decreasing the v, to 200 the resulting error might be a handicap.
2. Cancellation of echoes from stationary objects by MT1 means is
less effectively achieved as the number of pulses per scan on the
target (N”,.) decreases (Chap. 16). If the scan rate is 360/see, with
a beamwidth at half power of 1.00 and a V, of 400, NW will have a
value of 11, which is about the lower limit, for satisfactory performance
of MT1.
16.7. Azimuth Scan Rate .—Although the relation
Smim - d azimuth scanning rate
was not yet established at the time of the development here described, it
was roughly understood; and it put a high premium on slow scanning
rates. Mechanical problems in connection with the antenna mount are
greatly simplified by a slow scan rate.
Strongly opposing these considerations is the operational need for
more complete and up-to-the-second information, in order to follow
continuously the movement of high-speed aircraft and to control their
movements intelligently. Control problems vary in difficulty from
that of a well-formed group of aircraft moving over a simple course to
that presented by the requirement for accurate following or control of
numerous aircraft moving over complex independent courses.
A plane flying 300 mph moves $ mile in 10 see, or 2* miles in 30 sec.
Data at 30-sec intervals, as given by a scan rate of 2 rpm, was considered