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.
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 is 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 appears 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.
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. [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.
Many radar sets incorporate an altitude delay circuit that 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 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 that preserves the actual relationships between the various ground objects.
Sweep delay is a feature that 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 is enlarged. [Figure 7-11] The more this range is reduced, the greater 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.
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 display capabilities to display these dynamic 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.
The iso-echo control compensates for this deficiency by presenting a void area on the display 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 display, 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.
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, the iso-echo is not selective in the targets it blocks. If ground returns are received by the radar and a portion of their intensity falls into the range selected to be blocked, they too are blocked from the scope.
Radar beacons have been used for many years in aviation. In the past, airfields had beacons visible on radar much like a nondirectional beacon (NDB), but most are now decommissioned.
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 display. The time needed for generation of the return pulse causes a range error amounting to one-half mile, generally.
Beacons are sometimes coded with a mixture of aircraft identification and flight parameters for air route traffic control centers (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 appear on the interrogators display. The first pulse is 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.
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 display. In such cases, excessive gain causes returns to be picked up by side lobes of the antenna. Figure 7-13 is an example of a beacon return on the scope.
Sensitivity Time Constant (STC)
Most radar sets produce a hot spot in the center of the radarscope, because the high-gain setting required to amplify the weak echoes of distant targets overamplifies the strong echoes of nearby targets. If the receiver gain setting is reduced sufficiently to eliminate the hot spot, distant returns are weakened or eliminated entirely. The difficulty is most pronounced when radar is used during low-level navigation; to make best use of the radar, the navigator is forced to adjust the receiver gain setting constantly. Sensitivity time constant (STC) solves the problem by increasing the gain as the electron beam is deflected from the center to the edge of the radarscope, automatically providing an optimum gain setting for each range displayed. In this manner, the hot spot is removed while distant targets are amplified sufficiently. STC controls vary from one model radar set to another. Refer to the appropriate technical order for operating instructions.