B. RADAR HORIZON.
C. FADE CHARTS.
D. USING THE FADE CHART.
E. CALIBRATION FLIGHTS.
F. ADVANTAGES OF FADE CHART.
G. DISADVANTAGES OF FADE CHART.
H. SM, SP RADARS.
I. ADVANTAGES OF THE SM AND SP.
J. DISADVANTAGES OF THE SM AND SP.
K. THE SX RADAR.
Successful aircraft control is dependent on accurate information in a three-dimensional plane. This is particularly true in aircraft interceptions where the controller officer must have continuous accurate information on the relative position of his own planes with respect to their target. Bearing and range of air contacts are read directly from the radar, but positive determination of altitude is a more difficult problem. If the SM/SP model radar is available, altitude may be read directly within the known limits of variation and accuracy for the specific set. If no such altitude-determining radar is at hand the fade chart and radar horizon methods will serve to provide a reasonably approximate altitude solution.
The fact remains that accurate height reading on target aircraft is one of the hardest problems which a controller faces. Despite improvements in equipment, such as SM/SP and SX sets, no one solution will suffice. Other sources of altitude information, such as fire control and zenith search radars, intelligence information and knowledge of combat tactics, observation post reports, other aircraft, and optical height finders must be utilized to the fullest extent possible.
B. RADAR HORIZON
Radar transmissions are of extremely high frequency and so fall into the category known as line-of-sight propagation. This means that, except under special conditions of anomalous propagation, targets below the visual horizon cannot be detected by radar.
An aircraft approaching a radar from below the optical horizon will pass through a critical point at which it enters the field of coverage of the radar. Due to peculiar characteristics of high-frequency transmissions this point generally is at a slightly greater distance than the visual horizon. Because of this an artificial horizon known as the "Radar Horizon" is used to determine the maximum range of detection of an aircraft.
If the controller or CIC officer is familiar with the radar horizon of his sets, as he should be, he can obtain an initial estimate of the
Figure 45.--The radar horizon.
altitude of an air target from the range at which it is first detected. When the characteristics of the radar being used are not known, the altitude of a plane may be judged to be not less than 0.86 (E)2 feet, where R is the maximum range of detection in miles.
In using this method of altitude determination it must be remembered that if the plane changes altitude after appearing on the screen this change will not be evident unless the plane drops below the radar horizon. Furthermore it must be assumed that the radar is sufficiently powerful to detect any plane in its field of coverage.
C. FADE CHARTS
Figure 46.--In phase, strong echo.
Within the field of coverage of a radar there exist areas where the echo strength from a target will be at a maximum or a minimum. These areas are caused by the energy echo from a target returning along two paths, the direct path and the reflected path. When the waves along both paths arrive at the antenna in phase, they reinforce each other and create a maximum echo. When the waves arrive 180° out of phase they tend to cancel each other and produce a minimum echo. It has been found that these points of maximum return occur in lobes, and the minimum returns are in nulls between the lobes as shown below.
Figure 47--if it were visible, energy from the antenna would look like this.
A vertical cross section would look like this :
Figure 44.--Vertical cross section of lobe pattern.
A "fade chart" can be drawn that will locate these areas for any radar of given frequency and antenna height.
D. USING THE FADE CHART
Best results will be obtained by stationing a trained person thoroughly familiar with the chart and the radar at the "A" scope where he can observe the echoes. The echo amplitude should be observed as it varies in range, to determine at what ranges the echo height is at a minimum (or in a "fade") and at what ranges the echo height is at a maximum (disregarding rapid flutter of the echo and considering only average echo height). The height should be
Figure 49. Sample fade chart.
determined by noting on the "fade" chart the ranges of successive minima and maxima which coincide with the observed successive minimum and maximum echo heights. The "pick up" range will yield two or three possible heights. The first minima or maxima will most likely pin the aircraft height to a specific height. The second minima or maxima will establish the aircraft height. At least two ranges (either a successive maximum and minimum or two successive minima or maxima) are needed for an altitude determination, but all further minima (or "fades") and maxima help corroborate the result. Best results are obtained on aircraft in level-flight. This system requires time in determining heights and does not quickly detect changes in aircraft altitude, but it is of considerable value in the absence of height finding radar and can always be used to advantage in supplementing results of those radars (SM, SP, SX). The height data determined by fades should be correlated with cloud conditions and known enemy tactics in arriving at a decision as to the altitude of the aircraft. Unusual atmospheric conditions (weather fronts, different humidity at varying altitudes) can change the position of the radar horizon but will not materially affect the positions of the nulls and lobes. The information within 20 miles or less of the ship is of little if any value since the lobes and nulls are so close together. At extremely high altitudes (20,000 feet or higher), the echo from an aircraft or group of aircraft may be visible only a small portion of the time at short ranges (less than 30 miles). This fact makes it difficult to get a track under such
conditions. It also is possible for aircraft to glide down a fade and get in close to the radar set before it is detected. Among the ships of a task force or task group fade areas will vary for the different types of radar and for different antenna heights so that fairly adequate over-all coverage is insured. However, small groups of planes or single planes will often close a task group undetected at high altitude due to the inherent weakness of present air search radars at altitudes above 25,000 feet. Fade charts must be utilized at all times even though altitude determining radars (SP, SM, SX) are available and in use.
E. CALIBRATION FLIGHTS
Calibration flights furnish the best means by which to check the calculated fade chart and to gain accuracy in its use. A single plane, a small group, and a large group should be used at different times to observe the echoes of targets of different size. Depending on the type radar, its range and characteristics, a schedule of altitudes and distances should be made up. Usually flights at 1,000, 5,000, 10,000, 15,000, 20,000, 25,000, and 30,000 feet out to distances slightly beyond the maximum range of detection are desirable. The runs should be made on different relative bearings so as to detect any effects of the ship's structure on the radiation pattern. For best results the planes should fly straight out and straight back toward the radar. This will facilitate tracking them through null areas, and will enable radar operators to anticipate where they will reappear. Remember that IFF will often furnish good ranges when the blip is not visible in a fade.
A good system for recording the results of a flight is to make up a list of ranges with a blank space alongside for noting blip sizes. This should be measured as a ratio of the blip height to the height of the grass, and recorded as E-1, E-2, etc.
Additional sources of information on fades are: (1) Tracking own searches and patrols flying at known altitudes, and (2) noting fades on other planes that pass near enough to own CAP to check altitude. Both of these checks, carefully applied, will increase the accuracy of the chart and afford valuable practice in estimating sizes of targets.
F. ADVANTAGES OF FADE CHART
(1) It is a fairly accurate system for use with each air search radar.
(2) It provides some degree of height-finding in the absence of more precise height-finding radars.
(3) It is of value as a check on the more precise radars.
G. DISADVANTAGES OF FADE CHART
(1) It requires excessive time before a definite estimate of height can be made on planes in level flight; i.e., ambiguities exist on the initial range of detection.
(2) It doesn't detect changes in altitude quickly. It is only good for aircraft in level flight.
(3) Weather affects the lower edge of the lowest lobe, whereby a possible error may be introduced. Weather conditions, however, do not affect the fades.
H. SP RADARS
This type of radar differs from others in that it has a tiltable antenna and a very narrow beam similar to that of a searchlight. When the antenna is trained on a plane, the altitude can be read directly from a height meter on the set. However, it is possible to supplement this source of information with an elevation angle altitude chart. When on target, the angle at which the antenna is tilted can be read from the radar, as well as the range. This angle which is measured from a horizontal plane is the elevation or position angle.
Knowing it and the range, altitude can be computed from the formula:
Altitude=sin Elevation Angle × range
This formula, however, is accurate only for ranges of a few miles as it does not take into account the earth's curvature.
In the illustration it is clear that the difference between the plane's true altitude and the apparent altitude derived from the above formula will increase as the range increases so this error
must be corrected for. The chart to be used in conjunction with the SP radar will give true altitude quickly without lengthy calculations, thereby supplementing meter readings.
I. ADVANTAGES OF THE SP
(1) They provide reasonably quick- and accurate altitude determination.
(2) They can detect changes in altitude.
J. DISADVANTAGES OF THE SP
(1) The equipment is heavy and complicated.
(2) The number of aircraft that can be followed simultaneously is small as the antenna must be stopped on each to obtain the altitude.
(3) The accuracy of results is largely dependent upon the skill of the radar operator.
(4) Below an elevation angle of 2° the results are less accurate due to reflections from the sea.
K. THE SX RADAR
The SX radar, the latest type of altitude determining radar, employs really two radars with their antennas mounted on one antenna pedestal. The search system is similar to other air-search systems except for details as to power, frequency, beam width and pulse repetition. The altitude determining radar is an entirely separate radar, except that its antenna is permanently fixed ninety degrees behind the antenna of the search system. By a mechanical drive, the altitude beam is "rocked" vertically, thus giving a vertical scan which can be presented on an RHI scope, enabling us to read elevation in feet directly.
Figure 51.--Apparent vs. true altitude.
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