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receiver gain control also lessens beam-width distortion.
7.7.2. Pulse-Length Error. Pulse-length error is caused by the fact that the radar transmission is not
instantaneous but lasts for a brief period of time. There is a distortion in the range depiction on the far
side of the reflector and this pulse-length error is equal to the range equivalent of one-half of the pulse
time. Since pulse length error occurs on the far side of the return, it may be nullified by reading the
range to, and plotting from, the near side of a reflecting target when taking radar ranges.
7.7.3. Spot-Size Error. Spot-size error is caused by the fact that the electron beam which displays the
returns on the scope has a definite physical diameter. No return which appears on the scope can be
smaller than the diameter of the beam. Furthermore, a part of the glow produced when the electron beam
strikes the phosphorescent coating of the CRT radiates laterally across the scope. As a result of these
two factors, all returns displayed on the scope will appear to be slightly larger in size than they actually
are. Spot-size distortion may be reduced by using the lowest practicable receiver gain, video gain, and
bias settings and by keeping the operating range at a minimum so that the area represented by each spot
is kept at a minimum. Further, the operator should check the focus control for optimum setting.
7.7.4. Total Distortion. For navigational purposes, these errors are often negligible. However, the radar
navigator should realize that they do exist and that optimum radar accuracy demands that they be taken
into account. They are usually most significant when the target is a thin, no-show (river), when it is very
reflective but small, or when it is in close proximity to another show target. Thin no-shows are erased
except for their wider points. With tiny but very reflective targets, the cross-section of the return would
normally be negligible on the PPI. Their extremely strong reflectance, coupled with the inherent errors,
causes them to appear larger and of seemingly more significance on the indicator. When show targets
are close to each other, these errors will cause them to blend together, thus, diminishing the scope
resolution. Generally, the combined effects of the inherent errors cause reflecting targets to appear larger
and nonreflecting targets to dwindle (Figure 7.9).
AFPAM11-216 1 MARCH 2001 183
Figure 7.9. Combined Effects of Inherent Errors.
Section 7D— Radar Enhancements
7.8. Basics. The airborne radar sets used throughout the Air Force vary slightly in the navigational
refinements offered. The following is a description of enhancements designed to overcome some of the
common problems encountered in radar navigation.
7.9. Variable Range Marker and Crosshairs. Most radar sets provide a range marker that may be
moved within certain limits by the radar operator. This variable range marker permits more accurate
measurement of range because the marker can be positioned more accurately on the scope. Furthermore,
visual interpolation of range is simplified when using the variable range marker. On many radar sets, an
electronic azimuth marker has been added to the variable range marker to facilitate fixing. The
intersection of the azimuth marker and the variable range marker is defined as radar crosshairs.
7.10. Altitude Delay. It is obvious that the ground directly beneath the aircraft is the closest reflecting
object. Therefore, the first return that can appear on the scope will be from this ground point. Since it
takes some finite period of time for the radar pulses to travel to the ground and back, it follows that the
sweep must travel some finite distance radially from the center of the scope before it displays the first
return. Consequently, a hole will appear in the center of the scope within which no ground returns can
appear. Since the size of this hole is proportional to altitude, its radius can be used to estimate altitude. If
the radius of the altitude hole is 12,000 feet, the absolute altitude of the aircraft is about 12,000 feet.
7.10.1. Although the altitude hole may be used to estimate altitude, it occupies a large portion of the
scope face, especially when the aircraft is flying at a high altitude and using a short range. This may be
seen in Figure 7.10. In this particular case, the range selector switch is set for a 50/10-mile range
presentation. Without altitude delay, the return shown on the inside part of the scope consists of the
altitude hole and the return shown on the remaining part is a badly distorted presentation of all of the
terrain below the aircraft.
7.10.2. Many radar sets incorporate an altitude delay circuit which permits the removal of the altitude
hole. This is accomplished by delaying the start of the sweep until the radar pulse has had time to travel
　
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