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to the ground point directly below the aircraft and back. Hence, the name altitude delay circuit. The
altitude delay circuit also minimizes distortion and makes it possible for the radarscope to present a
ground picture which preserves the actual relationships between the various ground objects.
184 AFPAM11-216 1 MARCH 2001
Figure 7.10. Altitude Delay Eliminates the Hole.
7.11. Sweep Delay. Sweep delay is a feature which delays the start of the sweep until after the radar
pulse has had time to travel some distance into space. In this respect, it is very similar to altitude delay.
The use of sweep delay enables the radar operator to obtain an enlarged view of areas at extended
ranges. For example, two targets that are 75 miles from the aircraft can only be displayed on the scope if
a range scale greater than 75 miles is being used. On the 100-mile range scale, the two targets might
appear very small and close together. By introducing 50 miles of sweep delay, the display of the two
targets will be enlarged (Figure 7.11). The more this range is reduced, the greater will be the enlarging
effect. On some sets, the range displayed during sweep delay operation is fixed by the design of the set
and cannot be adjusted by the operator.
Figure 7.11. Sweep Delay Provides Telescopic View.
7.12. Iso-Echo. Detecting hazardous weather is not difficult in the normal mapping mode with most
radar units. The weather mode offers increased sensitivity to weather phenomenon. But to discriminate
between areas of varying hazards presents a dilemma. Reflected energy from weather is dependent on
the density of the rain and hail it contains. The limitations of PPI capabilities to display these dynamic
AFPAM11-216 1 MARCH 2001 185
characteristics make detection of the more intense areas difficult. Also, computer circuitry is more
effective at judging slight variations in shading than the human eye.
7.12.1. The iso-echo control compensates for this deficiency by presenting a void area on the PPI
corresponding to a hazardous area in the weather environment. This void area, the black hole, is
dependent on a control that the operator sets to define the intensity of the area that is to be avoided. For
instance, say only the largest cells of weather are desired to be displayed. The operator would set the
appropriate control and, on the PPI, the weather depiction would be present. The areas within the
weather where the most hazardous cells were located would be no-show areas or black holes.
7.12.2. The iso-echo circuits are capable of sensing the variation in the received signals and act like a
radio squelch control to block presentation of selected intensities (Figure 7.12). A word of caution! Isoecho
is not selective in the targets it will block. If ground returns are received by the radar and a portion
of their intensity falls into the range selected to be blocked, they too will be blocked from the scope.
Figure 7.12. Iso-Echo.
7.13. Radar Beacon. Radar beacons have been used for many years in air-to-ground operations. In the
past, airfields had beacons visible on radar much like a nondirectional beacon (NDB) but most are now
decommissioned. Aircraft IFF-SIF transponders are the outgrowth of this earlier equipment. Radar
beacons are still used in air-to-air operations by Air Mobility and Air Combat Commands for types of
rendezvous.
7.13.1. Radar beacons consist of interrogator and responder units operating from different locations. The
interrogator transmits a pulse that causes the responder to transmit a corresponding pulse. The
interrogator receives the coded return and uses time lapse and azimuth or sweep relationships to display
the returns on the PPI. The time needed for generation of the return pulse causes a range error
amounting to one half mile, generally.
7.13.2. Beacons are sometimes coded with a mixture of aircraft identification and flight parameters for
ARTCC. Aircraft equipped with beacons like the APN-69 can interrogate and respond to like-equipped
aircraft. Beacons like the APN-69 use a pulsed code of up to six pulses. The pulse codes are set by the
responder aircraft and will appear on the interrogators PPI. The first pulse will be in the relative position
of the responder with successive pulses trailing. The range between aircraft is equal to the range of the
first pulse (minus one half NM) and the azimuth is measured through the middle of the pulse length.
186 AFPAM11-216 1 MARCH 2001
7.13.3. Two blocking circuits are included in the units to prevent interference from radar on other
frequencies or a return of the interrogating pulse. This sometimes prevents a ring around where false
azimuth inputs are presented on the PPI. In such cases, excessive gain causes returns to be picked up by
　
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