Radar mapping of planets can be accomplished at lower cost and with reduced emphasis on propulsion system capability from spacecraft operating in elliptical orbit than from circular orbit

Vandenberg, Frank A.

English

NASA Technical Reports Server

August 1974	 B74-10115 
NASA TECH BRIEF 
Ames Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Variable Side-Look Angle Concept for Radar Mapping 
The radar mapping of planets can be accomplished 
at lower cost and with reduced emphasis on propul-
sion system capability from a spacecraft operating in 
an elliptical orbit (say, eccentricity of 0.5) than from 
a circular orbit. In an elliptical orbit, however, the 
change in range from the spacecraft to the planet 
surface poses problems relating to range-signal am-
biguities, radar power, and changes in zero-Doppler 
pointing direction. 
Range ambiguity occurs because the illuminated 
swath width in the range direction (perpendicular to 
Ne spacecraft orbit) increases with altitude. To cir-
umvent range ambiguity and guarantee good image 
quality, a pulse repetition frequency (PRF) for radar 
operation must be selected between upper and lower 
constraints; the upper P1QF constraint, which varies 
as a function of true anomaly or altitude, guarantees 
that the range ambiguity will be too small to degrade 
the image quality, while the lower PRF constraint 
guarantees that the Doppler ambiguity content will 
be small. Since the upper PRF constraint decreases 
rapidly with altitude, very large body-mounted an-
tennas are required; thus, large attitude control sys-
tems are required to point the antennas. Mapping to 
±300 true anomaly can be achieved by using a 
single beamwidth antenna with a constant PRF of 
4500 pps. 
The PRF-related problem can be solved by varying 
the side-look angle to obtain a better behaved upper 
PRF constraint. With a constant upper PRF con-

straint, a constant or variable PRF can be selected 
so that complete surface mapping can be achieved. 
Varying the side-look angle reduces the maximum 
range and thus reduces the power required because 
smaller antennas and also smaller attitude control 
systems can be used; for example, a power of 333
Svatts can be used for complete surface mapping
(maximum altitude of 3600 km) for an orbit with an 
eccentricity of 0.5, whereas about 600 watts are ré- 
quired when using a constant side-look angle of 300 
and mapping to ±55° latitude (maximum altitude of 
1500 km). 
If a clutterlock (automatic azimuth pointing) sys-
tem is used to point the radar antenna, it is normally 
pointed along the zero-Doppler line. At periapsis, the 
antenna is pointed essentially in the radial direction; 
as the radial velocity increases with altitude, the zero-
Doppler line deviates greatly from the radial direc-
tion. The antenna is pointed at higher squint angles 
at the higher true anomalies; when operated in the 
squint mode, it is pointed forward or aft and out to 
the side at the side-look angle. If the variable side-
look scheme is used, an electronic-offset clutterlock 
system must be used to point the antenna a pro-
grammed angle from the zero-Doppler line so that it 
is always pointed essentially in the radial direction. 
Also, the antenna must have two gimbals: The az-
imuth gimbal is controlled by the clutterlock system 
to point in the radial direction; a Doppler-frequency 
command is used to point the antenna away from the 
zero-Doppler direction. The elevation gimbal is con-
trolled to give the appropriate grazing angle. 
Alternatively, the antenna can be biased forward 
with an angle of 26.6° in the first half of the mapping 
phase and then pointed rearward when the space-
craft passes periapsis; only one gimbal is needed. A 
constant upper PRF constraint is obtained so that a 
constant PRF of 5000 pps can be used. The antenna 
is pointed radially by the antenna control system, 
and the clutterlock system points the antenna by 
using the closest ambiguity to control the aperture; 
A constant side-look angle can be built into the an-
tenna arm, so no gimbal is required about this axis. 
(continued overleaf) 
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a 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 74-10115
Patent status: 
NASA has decided not to apply for a patent. 
Source: Frank A. Vandenberg of

Martin Marietta Corporation

under contract to

Ames Research Center

(ARC-10842) 
. 
B74-10115	 Catego


Variable Side-Look Angle Concept For Radar Mapping

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