Future satellite communication and scientific spacecraft will utilize antennas with dimensions as large as 20 meters. In order to commercially use these large, low sidelobe and multiple beam antennas, a high level of confidence must be established as to their performance in the 0-g and space environment. Furthermore, it will be desirable to demonstrate the applicability of surface compensation techniques for slowly varying surface distortions which could result from thermal effects. An overview of recent advances in performing RF measurements on large antennas is presented with emphasis given to the application of a space based far-field range utilizing the Space Shuttle and the concept of a newly developed nonuniform sampling technique

Rahmatsamii, Y.

English

NASA Technical Reports Server

N88-25745
LARGE ANTENNA EXPERIMENTS ABOARD THE SPACE SHUTTLE
-- APPLICATION OF NONUNIFORM SAMPLING TECHNIQUES
Dr. Y. RAHMAT-SAMII, Senior Research Scientist
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109
ABSTRACT
Future satellite communication and
scientific spacecraft will utilize antennas
with dimensions as large as 20 meters. In order
to commercially use these large, low sidelobe
and multiple beam antennas, a high level of
confidence must be established as to their
performance in the 0-g and space environment.
Furthermore, it will be desirable to
demonstrate the applicability of surface
compensation techniques for slowly varying
surface distortions which could result from
thermal effects. In this paper, an overview of
recent advances in performing rf measurements
on large antennas is presented with emphasis
given to the application of a space based far-
field range utilizing the Space Shuttle and the
concept of a newly developed nonuniform
sampling technique.
i. INTRODUCTION
It is very likely that antennas in the range of 20
meters or larger will be an integral part of future
satellite communication and scientific spacecraft payloads.
For example, Fig. 1 depicts the conceptual evolution of the
Land Mobile Satellite System which is anticipated to evolve
from utilizing approximately 6-9 meter reflectors to 55
meter reflectors in the era spanning the late 1980's to
early 2000. In order to commercially use these large, low
sidelobe and multiple-beam antennas, a high level of
confidence must be established as to their performance in
the 0-g and space environment. Certain ground (l-g) testing
can be performed to validate the workability of different
segments of such large structures; however, it will be a
formidable task to characterize the performance of the
entire structure on the ground. For this reason, a
conceptual study has been initiated with the intention to
describe an experiment aboard the Space Shuttle to
demonstrate the deployment reliability of the antenna
459
https://ntrs.nasa.gov/search.jsp?R=19880016361 2020-03-20T06:51:46+00:00Z
ORIGNAE PAGE IS 
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structure, to measure thermal and dynamic structural 
characteristics, and to verify performance specification 
under all expected conditions. In particular, special 
consideration is being given to the rf far-field pattern 
measurements which should provide the ultimate 
characterization for the antenna performance (Fig. 2). 
* MASURE M I - A X I S  AND o(T-AXIS BEAM PATIIRNS All-BAND 
- ' M t A S U I  UNCfR VARIOUS IHRMAL CaYDlIlONS 
* W S U R L  * F E R  MI.ORBI1 SUWACE ECOHIWRAIION 
* MMDNSTRAIf IN-SPACE R I  MEASURtMENI IECHNaOGY 
CDRRELAI~ w s u m  RI P t n i c m w t n  wirH M~ASURED SURFACE 
ANDFlEDALIGWENT 
* # R I I Y  AND UPDATE MAIHMAllCAL AND COMPUTER MODELS 01 R I  
f€RfORMAKE ANALYSIS AND PREDlCllDN 
FIG. 1. Evolution of the FIG. 2. Spaced-based rf 
proposed Land Mobile Satellite experiment objectives aboard 
System (LMSS) . the Space Shuttle. 
Several potential scenarios have been considered and 
the relative merits of each of them are shown in a 
qualitative manner in Fig. 3 .  Among all these possibilities 
the application of the scenario shown in the last row 
appeared most feasible. The rf experiment is anticipated to 
be performed on a 20-meter offset reflector at L-band using 
the Remote Mini-Flyer, a NASA-developed reusable and 
retrievable spacecraft (a modified Spartan), as the carrier 
for an rf beacon. This beacon is used to illuminate the 
antenna in a similar fashion as one does in the ground-based 
far-field ranges using transmit illuminators. An artist's 
rendition of this space-based experiment is depicted in 
Fig. 4 .  
2. RF MEASUREMENT CONSIDERATIONS 
In order to reduce the cost of the experiment, it has 
been anticipated that no gimbal mechanism will be used to 
accurately control the position of the antenna with respect 
to the illuminator. Instead, the relative motion of the 
Shuttle and free-flier in a controlled manner will be 
utilized to provide the angular range of interest. Depending 
on what the exact covered u-v space will be, several 
scenarios could be considered. The schematics of two 
possible measurement scenarios are shown in Figs. 5 and 6. 
Since without any gimbal system it will be impractical 
to measure antenna patterns in specified 4 cuts, one may 
460 
ORIGINAL PAGE 
POOR QUALITY 
have to perform the measurement in a specified u-v angular 
range in a time period in which the antenna structure is not 
changed appreciably from an rf viewpoint. Fig. 5 shows the 
possibility of measuring very dense but nonuniformly 
measured data points from which the needed 4-cuts or contour 
patterns can be constructed. If it is proven that the 
possibility of measuring a very dense set of data may not be 
realistic, an alternate scheme should be available. This 
alternate scheme is depicted in Fig. 6, which assumes that 
the measured amplitude and phase are obtained at 
relatively sparse anl: nonuniformly distributed u-v points. 
The question is, then, whether or not one can construct the 
4-cuts from a set of nonuniformly distributed measured data 
points. 
TECHNIQUES CoNFlcURATlON 
M A R  FIELD TECHNIWES 
C W P A C T  R A N E S  
BEACON ON THE GROUND 
RECEIVING CITIES ON 
mE GROUND - 
lk RADIO STAR SOURCES 
GEO SAITEUITE SOURCES 
FREE-RMR AS A M C O N  
FIG. 3 .  A 
comparison of 
cosr 
@ 
qualitative 
different 
measurement techniques aboard 
the Space Shuttle. 
%+ VERY DENSE 
FIG. 5. Schematic of 
nonuniformandverydensely 
measured sampled points. 
FIG. 4 .  An artist's rendition 
of the proposed large antenna 
Shuttle experiment. 
NONUNIFORM SAMPLING ALGORITHM -
FIG. 6. Schematic of 
nonuniform and relatively 
sparsely measured sampled 
points. 
Recently, Rahmat-Samii and Cheung [l] have demonstrated 
that a two-dimensional nonuniform sampling technique which 
utilizes irregularly spaced samples (amplitude and phase) 
46 1 
can be used to generate the far-field patterns. The
mathematical developments of this two-dimensional nonuniform
sampling technique have been detailed in [i]. Additionally,
a powerful simulation algorithm has then been developed to
test the applicability of this sampling technique for a
variety of reflector measurement configurations [i]. For
example, Figs. 7 and 8 show the simulated nonuniform sample
points and the reconstructed far-field patterns in specified
4-cuts for a 20 meter offset reflector antenna with a
defocused feed operating at L-band [I]. In Fig. 8, the solid
curves are reconstructed co-polar and cross-polar patterns
using the nonuniform sampling technique. It is noted that
even though no sample points are captured in these cuts, the
reconstructed patterns agree well with the ideal patterns in
the angular range where the nonuniform sample points have
been generated, i.e., ±3.2 degrees. Many tolerance studies
have also been performed to demonstrate the required
measurement accuracies in applying the nonuniform sampling
technique [i].
FIG. 8. Reconstructed far-
field patterns from 192
sampled points using
nonuniform sampling technique.
FIG. 7. One hundred ninety
two nonuniform sampled point
distribution in (u,v)
coordinates which covers an
angular region of # = ±3.2
degrees.
To validate the accuracy of the nonuniform sampling
technique for antenna pattern construction, several
measurements have been performed as reported in [2]. In one
of the measurements JPL's 1200-ft far-field range was used,
where a 1.47-m circularly polarized Viking reflector antenna
[3] (Fig. 9) was measured at X-band (8.415 GHz) using a
corrugated horn as the illuminating antenna. The far-field
amplitude and phase were measured in the directions shown in
Fig. I0 which consisted of 585 nonuniformly distributed
sampled points in (u,v) coordinates.
462
! 
I' 
I 
i .=. 
F I G .  9 .  The 1.47-m circularly 
polarized Viking reflector 
antenna operating at X-band 
(8.415 GHz). 
a 
VI 
z 
Y) 
u = SIN0 COS@ 
F I G .  10. Five hundred 
eighty five measured data 
point distribution in 
(u,v) coordinates which 
covers an angular region 
of 0 = k16.8 degrees. 
The total system errors for amplitude measurement have 
been estimated to be less than 0.2 dB at - 4 0  dB level 
relative to the boresight power level and the phase 
measurement error has been within k 5  degrees. The pointing 
accuracy has been estimated to be better than 20.1 degrees. 
The co-polar far-field patterns for 4 = 90 degrees are 
depicted in Fig. 11. The solid curves are the standard 
azimuth cuts and the dashed curves are the reconstructed 
patterns using the nonuniform sampling technique. These 
patterns are constructed by utilizing the window concept as 
discussed in [1,2]. Note that asymmetric patterns have 
resulted even though a symmetric reflector was used. This is 
due to the feed and strut blockage effects. In the angular 
range of 216.8 degrees where the measured data are 
available, the comparison between the solid and dashed 
curves demonstrates close agreement. Note that even though 
no sampled point has been used at the boresight, the peak of 
the beam has been reconstructed very well. 
I 463 
-I0
-20
O
Z
-30
=E
I--
_ -40
-5O
4=900
I I
-- STANDARD
AZIMUTH CUT
.... RECONSTRUCTED
PATTERN
o o CAPTURED SAMPLED
POINTS
-6O
-17.00 -12.75 -8.50 -4,25 0 4.25 8.50 12.75 17.00
8 (deg)
FIG. ii. Reconstructed far-field patterns of the Viking
reflector at X-band using nonuniformly distributed measured
data.
ACKNOWLEDGEMENTS
The research described in this paper was carried out
at the Jet Propulsion Laboratory, California Institute of
Technology, under contract with the National Aeronautics and
Space Adminstration. The Author would like to thank R.
Cheung for his assistance during the development and testing
of the two-dimensional nonuniform sampling algorithm.
REFERENCES
i. Rahmat-Samii Y. and R. Cheung, "Nonuniform sampling
techniques for antenna applications," IEEE Trans. Antennas
Propagat., vol. 35, pp. 268-279, 1987.
2. Cheung R. and Y. Rahmat-Samii, "Experimental
verification of nonuniform sampling technique for antenna
far-field construction," Electromaqnetics, vol. 6, no. 4,
pp.277-300, 1986.
3. Rahmat-Samii Y. and M. Gatti, "Far-field patterns of
spaceborne antennas from plane-polar near-field
measurements," IEEE Trans. Antennas Propaqat., vol. 33, pp.
638-648, 1985.
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Large antenna experiments aboard the space shuttle: Application of nonuniform sampling techniques

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