August 1965 Electronics World
Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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Side-looking airborne
radar (SLAR)
started out using a narrow beam formed by reflectors, like traditional radars, as
opposed to the synthetic aperture type most often (maybe even exclusively) used
today. Both types of side-looking radars rely primarily on the physical movement
of the airborne platform for effective azimuthal scanning rather than steering the
beam either mechanically or electronically. This 1965 Electronics World
magazine article represents early versions which used "real aperture" antennas.
Modern computer-controlled synthetic aperture radar beams can be segmented and directed
off-axis for detected areas of interest as required, but the early systems simply
gathered radar return data and presented it real-time, with some level of analog
processing, to operators.
Side-Looking Radar Imagery

Fig. 1 - Side-looking radar installation on an Army Mohawk
aircraft. Notice the long cigar-shaped radar antenna housing mounted beneath the
fuselage.
By J. L. Nelson
Military Electronics Division Motorola Inc.
Side-looking radars mounted in our military reconnaissance planes provide near-photographic
mapping coverage in clouds and darkness.
Imagery produced by electronic sensors, such as radar and infrared, is playing
an increasingly greater role in the reconnaissance efforts of the military services.
Such imagery not only supplements the aerial photograph, but in certain instances
provides information that cannot be obtained by photographic techniques. For example,
it has long been established that the ability of various types of electronic sensors
to penetrate clouds and darkness as well as to map large areas is a most desirable
characteristic.
Side-Looking Radar
An example of an electronic sensor currently used for mapping purposes is the
AN/UPD-2 Side-Looking Radar System produced by Motorola's Military Electronics Division
for the U. S. Army. The name "side-looking" stems from the fact that this form of
radar collects mapping information from the terrain to the sides of the aircraft.
In contrast, infrared and photographic systems are generally used to record terrain
beneath the aircraft. Radar as a Sensor is a ranging device; its maximum range limit
is approximately equal to the line-of-sight distance to the horizon.
For example, an aircraft flying at 3,000 feet above the terrain is capable of
mapping in excess of 50 miles to each side of the aircraft during a single run.
It is not uncommon to map areas in excess of 30,000 square miles during the course
of a single run, and this record may be contained on a strip of film less than 2
feet long. To better understand the structure of a radar image, the mechanics of
the technique must be examined.

Fig. 2 - Geometry of a side-looking radar that has been
installed on a reconnaissance aircraft.

Fig. 3 - Coverage patterns obtained with side-looking radar.
(A) Pattern as viewed from above. (B) View from in front of plane.

Fig. 4 - Side-looking radar imagery of Los Angeles area.
The black central stripe which resembles a roadway directly below the plane is the
aircraft's line of flight. Distortion Effects.

Fig. 5 - Side-look radar imagery of Phoenix, Arizona area.
Notice the outward-going shadows on both sides of the center stripe resulting from
radar beam illumination.
Fig. 1 illustrates a typical side-looking radar installation in the Army's
Mohawk aircraft. The cigar-shaped structure beneath the aircraft is the radar antenna.
Fig. 2 shows the geometry peculiar to a side-looking radar. Note that the
antenna pattern is a narrow fan-shaped beam extending from the aircraft outward
to the horizon. In general, this beam is less than 10° thick; therefore, the
radar illuminates and receives returns from only a narrow strip of terrain at anyone
time. As the aircraft moves forward, successive strips of terrain are viewed by
the radar system.
As contrasted to a camera, a radar system does not examine the entire area contained
in the strip at one time. It accomplishes its purpose by transmitting a high-energy
pulse and recording the intensity of the return echo in synchronism with the time
required for the pulse to reach a particular element of terrain and return to the
aircraft.
Thus, the basic terrain information is contained in terms of "range vs. time"
as a video signal. This signal is converted to a film record by placing the video
information on a cathode-ray tube as intensity modulation, sweeping the cathode-ray
tube in synchronism with the radar return from each element of terrain, and photographing
the resultant display. The film is caused to move at a rate proportional to the
ground speed of the aircraft, thereby building up the map in synchronism with the
illumination of successive strips of ground by the radar antenna. In general, the
ground-scanning process may be likened to the scanning spot of a television raster.
The process, as viewed from a point directly above the surveillance aircraft,
is illustrated in Fig. 3. The narrow light area perpendicular to the side of
the aircraft represents the geometry of the radar beam; hence the strip of terrain
under surveillance. The area immediately aft of this beam represents terrain already
mapped by the sensor.
Examples of Imagery
A typical example of imagery produced by a side-looking radar is shown in Fig. 4.
The horizontal dimension is related to radar slant range. The black central stripe
represents the range to the ground directly beneath the aircraft. Features such
as roads, cities, farms, and other landmarks are readily discernible. Another example
of a radar map made in the area of Phoenix, Arizona is shown in Fig. 5.
Imagery such as that of Figs. 4 and 5 does not always lend itself to direct correlation
with terrain maps or other systems of coordinates because of various types of distortion.
Two prominent forms of imagery distortion often found in radar strip maps are: drift-angle
distortion and ground-speed distortion.
Taking first the case of an aircraft encountering a side wind, it will be necessary
for the pilot to intentionally "crab" or yaw the aircraft to maintain the desired
ground track. If the antenna is rigidly affixed to the aircraft for aerodynamic
reasons, the beam will no longer be perpendicular to the ground track but will be
rotated by an angle equal to the drift angle of the aircraft. In general, side-looking
radar systems compensate for this by rotating the intensity-modulated line scan
on the cathode-ray tube a proportionate amount, thereby providing first-order correction.
To avoid ground-speed distortion, it is necessary to synchronize the motion of
the film across the image plane to that of the aircraft over the terrain. In the
event such synchronization is not achieved, the scale factor lengthwise along the
film record will not be in agreement with the scale factor laterally across the
film record. Thus, it is necessary that two vital pieces of information be provided
to render proper coordinates on the final imagery.
One device used for obtaining this basic information is a Doppler navigator.
However, present Doppler navigators provide drift-angle and ground-speed information
to accuracies on the order of 1 to 2 percent. After this information is further
processed by the radar system, this error may be increased by another 1 or 2 percent.
A number of higher order distortions are often apparent in a radar photograph.
Such second-order effects as slant-range distortion and calibration accuracy and
third-order effects such as cathode-ray tube and lens pin-cushion or barrel distortion,
film shrinkage, and other factors may be present. Although a trained interpreter
with suitable viewing equipment can often compensate for such distortions, the radar
system design should minimize them as far as practical.
For practical, day-to-day utility, a mapping radar requires other characteristics
often overlooked by systems designers. Foremost among these is stability- freedom
from drift of important parameters such as video gain, CRT intensity and focus,
etc. Since the map is recorded photographically, it is not feasible for an operator
to "tweak" controls to obtain optimum imagery.
The future of radar holds much promise in improving radar surveillance imagery.
Fig. 4 demonstrates the dramatic improvements in map realism now consistently
obtainable through better scanning geometry, higher resolution, and longer grey
scale. Further improvements in resolution could result from the use of shorter wavelengths
and/or longer antennas. The first is negated by rain and fog attenuation and the
second by aerodynamic considerations.
The effects of long antennas can be synthesized by complex signal processing.
Although such techniques hold the promise of future resolution improvements, equipment
based on these principles is not yet dependable enough for field use by military
personnel.
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