Transmitter, radiating at a right angle to the spacecraft trajectory and intersecting the surface at a shallow angle, yields accurate radar maps of lunar or planetary surfaces. Earth based station receives the signal reflected from the planetary surface. Mapping coordinates and signal strength are produced by earth based transmitter

Goldstein, R. M.

Horttor, R. L.

English

NASA Technical Reports Server

September 1969
	 Brief 69-10325 
S	 NASA TECH BRIEF 
-	 - 	
- 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151.
- 
Combination Ranging System and

Mapping Radar
72(t)
Moon 
	
cl)f)	 -
(b Mod	
Lunar Orbiter 
Spacecraft 
DSN 
Station	
\
PN PN Code
	
71)t) \	 T3lf) 
Clock	
Gererator	
Transmitter I Mapping 
	
Interference	 I Signal 
Word	 Signal 
Receiver Tape
3.3M Hz BW Recorder 
k1
IOM 
I 
Hz	
60M Hz.
Word 
	
Time	
90
71(t) Link Propagation Time 
e12)f)
Fig. 1 Combination Ranging System and Mapping Radar 
The problem: 
To develop accurate radar maps of lunar and/or 
planetary surfaces using currently available commu-
nications elements. Present earth based techniques il-
luminate a planetary surface with a time coded wave-
form, which allows resolution of constant time delay 
contours. These are circles concentric to the point 
nearest earth. Planetary rotation produces constant 
doppler contours parallel to the planet axis of rota-
tion. Thus, spatial coordinates are represented by the 
signal properties time delay and doppler frequency.
An image is formed by converting the signal proper-
ties back into spatial coordinates. Since the doppler-
time delay contours may intersect at more than one 
point, several surface features may correspond to a 
given doppler-time delay coordinate. These surface 
features can be separated only by comparing data 
from several observation periods. 
The solution: 
Use of a transmitter radiating at a right angle to 
the spacecraft trajectory and intersecting the surface 
at a shallow angle. An earth based station receives 
(continued overleaf) 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
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=Fig. 2. Combination Ranging System and Mapping Radar 
the signal reflected from the planetary surface. The 
principal features of the technique are the mapping 
coordinates and signal strength which can be pro-
duced by an earth based transmitter. The time delay 
contours are segments of concentric circular arcs, 
while the doppler contours are segments of hyper-
bolic sections. These contours intersect one point only 
and are nearly orthogonal. Thus the signal properties 
and spatial coordinates are identical, and mapping 
results may be obtained from a single observation. 
How it's done: 
Preliminary investigation has shown that the S-
band ranging transponder with a high gain antenna 
could serve as a side looking radar with the Deep 
Space Stations (DSS) a part of the system. A bistatic 
side-looking radar experiment was performed, and as 
shown in the figures, the actual radar signal is trans-
mitted from the spacecraft, reflected from the lunar 
surface, and received at the Deep Space Station (in 
this case, Goldstone, California, 210 cassegrainian an-
tenna). In order to keep time and frequency refer-
ences, the ranging modulation was actually transmit-
ted from the DSS to the spacecraft, routed through 
the transponder, and retransmitted to the lunar sur-
face on a different carrier frequency. The experi-
mental communication link contained three time-
varying propagation times: station-to-spacecraft; 
spacecraft-to-surface; and surface-to-station. Proper 
tracking of the round-trip delay and doppler is the 
crux of the data processing problem. 
The received signal is recorded in phase quadra-
ture, as shown in Figure 1, along with end of code 
word pulse, clock, and time reference. Range resolu-
tion of point scatterers on the lunar surface is 
achieved by using a pseudo noise (PN) code biphase 
modulated on the carrier. The received signal is mul-
tiplied by an identical locally generated PN sequence: 
The portion of the received signal whose modulation 
is synchronized to the local code can be separated 
from the rest by a low pass filter. This signal corre-
sponds to a narrow strip at constant range from the 
spacecraft, as shown in Figure 2. Each point within 
that strip passes through the lines of constant doppler
caused by the motion of the spacecraft. A filter which 
matches that motion-induced phase behavior can re-
solve individual point scatterers. This is the basic 
principle of the side-looking radar. 
The surface resolution is determined by the radar 
beam incidence angle, the PN code bit length, and the 
bandwidth of the spacecraft transponder, the DSN 
Receiver IF Amplifier, and the tape recorder. The bit 
rate chosen for the experiment was a one third MHz 
clock rate, allowing a 3 Asecond bit time. This is 
1.0 km in slant range, corresponding to about 1.4 km 
on the surface. 
The code length was long enough to keep the 
spacecraft's direct signal and the surface reflected 
signal unambiguous. Also, the longer the code, the 
better the suppression of the direct signal. However, 
searching for the surface reflected signal becomes 
more difficult as the code is lengthened. The code 
length chosen was 1023 bits. 
Signal strength calculations were based on resolv-
ing a one-kilometer square on the surface. Predictions 
showed a 3 db signal-to-noise ratio. Such a noisy pic-
ture should reveal a recognizable shape, such as a 
large crater. 
Notes: 
1. An analysis of the data is not yet complete. 
2. Documentation is available from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69-10325 
Patent status: 
This invention is owned by NASA, and a patent 
application has been filed. Royalty-free, non-exclusive 
licenses for its commercial' use will be granted by

NASA. Inquiries concerning license rights should be 
made to NASA, Code GP, Washington, D.C. 20546.

Source: R. L. Horttor and R. M. Goldstein of 
Jet Propulsion Laboratory
under contract to
NASA Pasadena Office
(NPO-1 1001) 
Brief 69-10325	 .	 Category 01


Combination ranging system and mapping radar

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