The Jet Propulsion Laboratory has designed and developed a mechanically steered antenna for tracking satellites in a mobile environment. This antenna was used to track an L-band beacon on the MARISAT satellite. A description of the antenna and the results of the satellite experiment are given

Bell, David J.

Berner, Jeff B.

English

NASA Technical Reports Server

THE JPL MECHANICALLY STEERED ANTENNA N88-25715
JEFF B. BERNER, Mobile Satellite Program, Jet Propulsion Laboratory,
California Institute of Technology, United States; DAVID J. BELL,
Mobile Satellite Program, Jet Propulsion Laboratory, California
Institute of Technology, United States.
CALIFORNIA INSTITUTE OF TECHNOLOGY
JET PROPULSION LABORATORY
4800 Oak Grove Drive 161-228
Pasadena, California 91109
United States
ABSTRACT
JPL has designed and developed a mechanically
steered antenna for tracking satellites in a mobile
environment. This antenna has been used to track an L-
band beacon on the MARISAT satellite. A description of
the antenna and the results of the satellite experiment
are given.
INTRODUCTION
NASA has been developing the technology for mechanically steered
vehicle antennae having I0 to 12 dBi of gain for use in L-band (1600
MHz) MSAT applications (Bell, 1986). While more expensive than omni-
directional antennae, the directional antennae offer two major
advantages. First, they save spacecraft power by a possible factor of
ten through higher gain and better multipath discrimination. But, more
importantly, due to narrower azimuthal beams, they allow two spacecraft
to provide coverage of CONUS at the same frequencies. Through proper
design of the vehicle's antenna, it is possible to keep the main beam
of the antenna pointed toward the desired spacecraft while radiating
considerably less energy toward the second spacecraft (also operating
at the same frequency).
OPERATION
Figure I is a block diagram of the mechanically steered antenna
developed at JPL. The radiating part is a linear array of four square
microstrip patches tilted with respect to the ground plane to provide
elevation coverage from 20 ° to 60 ° with a minimum of i0 dBic gain.
This configuration provides up to 6 dB more gain than a low gain
omnidirectional antenna. The rotating antenna platform is mounted on a
fixed platform that includes a stepper motor, the motor driver and the
pointing system hardware. A cylindrical radome, with a diameter of 20
inches and a height of 9 inches, covers the antenna system and produces
insignificant effects on the RF pattern characteristics.
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The initial pointing, or acquisition of the satellite, is achieved
by seeking the direction of maximumpower reception from a satellite
transmitted pilot signal. Subsequent tracking uses a pseudo-monopulse
radar technique. The i X 4 antenna array is divided into two identical
subarrays and signals from each subarray are passed through a hybrid
coupler producing sum and difference signals. The difference channel
output is modulated by a square wave and added to the sum signal via a
coupler. The composite signal is routed to a receiver which decouples
and detects the difference signal. The difference signal is then
normalized by detected sum signal to correct for signal fading up to
250 Hz. This normalized difference signal is the antenna tracking
error signal (Jamnejad, 1988).
The pointing control computer that operates the pointing control
loop uses the error signal to rotate the antenna platform azimuthally
with a stepper motor. A vehicle turn rate sensor is used to maintain
pointing toward the satellite when the received signal level fades
below a prescribed threshold. This open loop tracking modeallows the
antenna to point at the satellite even if it is blocked from the
antennaes view. The rate sensor also serves as an inertial reference
during the initial satellite acquisition (Berner, 1987).
The rate sensor, motor controller, motor, and drive assembly are
placed on the fixed base plate of the antenna system. The antenna
array, the sum/difference hybrid, the coupler, and the difference
signal modulator are placed on a rotating circular platform, which
constitutes a "ground plane" for the array and is stacked on top of the
base plate assembly. RF connection between the fixed and moving parts
is made through a single channel rotary joint which forms the common
central axis of the fixed lower platform and rotating upper platform of
the antenna. This rotary joint is surrounded by a set of five slip
rings, which provide low frequency and DC signals for the modulator.
This stacked configuration is easy to breadboard and simplifies the
tasks of testing and trouble shooting at this experimental stage. It
also explains the 9 inch height of the assembly (Figure 2).
DEVELOPMENTSTRATEGIES
Development of the i X 4 mechanically steered antenna was split
into the parallel developments of pointing control software, signal
processing hardware, platform structure, and RF elements. Interface
signals were clearly defined at the outset. Efficient development was
greatly facilitated by first constructing a working model of the
antenna and platform with control interface signals identical to the
final version. The RF portion of the antenna was modeled by a
photoresistor "antenna" and circuitry that mimicked the RF sum and
difference channel gain patterns. This circuitry also produced a
normalized difference/sum tracking error signal modeling the output of
the signal processing circuitry. A simple incandescent light near the
software development station becamethe "satellite" test beacon. This
model was demonstrated at the MSAT-XIndustry Briefing (Bell, 1985).
The model provided immediate feedback on software performance, speeded
software development, and increased confidence and understanding of the
entire pointing control loop. The success of this method was
demonstrated during the final integration phase when the antenna
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immediately began tracking an RF beacon in the lab after phase delay
corrections were made to the RF sum and difference channel circuitry.
The mechanically steered antenna has been the result of a low
budget, low manpower development effort. Off-the-shelf components were
employed as much as possible to speed development. JPL plans to
demonstrate a reduced 5 inch height model that will eliminate costly
prototype slip rings and rate sensor electronics. In a related effort,
Teledyne Ryan Electronics is currently under contract to JPL to develop
a flat plate mechanically steered vehicle antenna with a height of only
1.5 inches.
SATELLITE TEST
In August of 1987, the JPL MSAT Pilot Field Experiment (PiFEx)
team went to Santa Barbara, CA, to conduct satellite tracking tests on
the mechanical antenna. This test is referred to as the PiFEx
Satellite-la, or S-la, test. The antenna was mounted on the MSAT-X
Mobile Laboratory/Propagation Measurement Van (PMV). The tests used an
L-band beacon on the MARISAT satellite. After acquiring the beacon,
the tests consisted of tracking the satellite while driving through the
surrounding mountains. Over 500 MBytes of data were collected during
this two week experiment.
Test Conditions
The test conditions experienced were well below ideal for the
antenna. There are few satellites on the west coast with an L-band
beacon. In the JPL area, the highest possible elevation angle to a
satellite occurs in Santa Barbara with the MARISAT satellite and the
angle is only 13 ° . The antenna is only designed to operate in
elevation angles as low as 20 °. Also, the EIRP of the beacon is 4 dB
less than the nominal configuration proposed for MSAT applications.
The low received signal power and the additional multipath, both due to
the low elevation angle, substantially increased the noise level on the
difference channel signal, which is used as the error signal in closed
loop tracking. Two important tracking loop parameters were modified to
fit the antenna system for operation in this reduced signal level
environment. First, the tracking loop bandwidth was reduced by a
factor of 8 to combat the increased noise levels. Secondly, the closed
loop/open loop handover threshold was increased to improve the response
to quick vehicle turns that is lost when the tracking loop bandwidth is
reduced. The result was a pointing control loop that was biased to
depend more heavily on the open loop mode than is necessary for the
expected MSAT satellite signal levels.
In addition to the control loop changes, a low noise amplifier and
bandpass filter were added to the antenna just prior to the rotary
joint to increase the overall signal level and to reduce the effects of
RF losses in the rotary joint and the cabling to the receiver. Plots
of the sum and difference/sum signals for the satellite test are given
in Figures 3 and 4.
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Acquisition Tests
Acquisition testing consisted of manually moving the antenna
platform off-point and issuing the start acquisition command. These
tests were performed both while the PMV was stationary and mobile.
Figure 5 provides a histogram of the Time to Acquire. During the
mobile tests, it was possible that the satellite was blocked. If the
signal was not located, the antenna platform was scanned again, until
the satellite was found. The mean time to acquire was 11.97 seconds,
below the requirement of 15 seconds.
Closed Loo R Tracking
As was mentioned earlier, the closed loop tracking capability was
limited due to the adverse test conditions. For the closed loop to be
used, the road that the PMV was on had to have a clear line-of-sight to
the satellite and had to have turn rates less than i0 degrees/second.
Figure 6 provides a scatter plot, generated from over one hour of data,
of the PMV Turn Rate versus the Degrees Off Point when tracking in
closed loop. As can be seen, the pointing error remains between ±2 °
for turn rates in the range of ±8 Degrees/sec. For turn rates greater
than this, the antenna tracked in open loop mode. The specification
that the antenna was designed to was a closed loop pointing error of
±2 °
O__p_enLoo_Tracking
The purpose of the open loop tracking is to keep the antenna
platform pointed at the satellite while the signal is blocked. This
tracking method uses an angular turn rate sensor to provide the
tracking information. The problem with using a rate sensor is that the
sensor has a drift that varies with temperature and time. For the S-la
experiment, the rate sensor was temperature controlled by an oven,
leaving only the drift with time. Because of the drift, the antenna
pointing system was designed to operate in open loop for time periods
of less than I0 seconds. The goal was to keep the pointing error
between ±2 ° in the i0 second period. Figure 7 provides a scatter plot
of the Time Spent in Open Loop versus Degrees Off Point which shows
that the open loop tracking keep the error between ±1.2 ° Figure 8
shows a scatter plot of the open loop error versus the turn rate. Both
figures were generated from I0 minutes of data. As can be seen, over a
range of ±8 degrees/sec, the antenna tracked within ±i ° .
Closed Loop/OpenLooD Handover
When the signal fades the tracking mode switches to open loop.
When the fade ends, closed loop tracking takes over again. If the
transfer to open loop is too slow, closed loop tracking will continue
with a difference/sum signal that is noise dominated, causing the
antenna pointing error to grow. If the transfer occurs too soon, the
antenna will be rapidly switching back and forth between open and
closed loop. Due to the poor signal conditions, the transfer had to be
quicker than it would be for the MSAT system. Figure 9 shows a one
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minute history of the pointing error as the antenna switches between
closed and open loop. The closed-to-open loop transfers occur at 35
seconds, 45 seconds, and 52 seconds. As can be seen, the transfer,
which occurs 70 msec after the fade occurs, should have occurred
sooner.
CONCLUSIONS
The JPL mechanically steered antenna has demonstrated that
mechanically steered antennae can track a satellite. This low cost
__ __ _l.^J _-
........ =u L,_ satelliLe over a variety of
environmental conditions and in all cases the antenna tracked the
satellite within ±2 °
ACKNOWLEDGMENTS
The authors would like to thank the people involved in the
experiment and the data analysis that followed: Richard Emerson,
Loretta Ho, Walt Mushagian, Jim Parkyn, and William Rafferty.
REFERENCES
Bell, D., and Naderi, F.M. 1986. "Mobile Satellite Communications
Vehicle Antenna Technology Update," AIAA Space System Technology
Conference, June 9-12, 1986.
Bell, D. 1985. "Vehicle Antenna Pointing Techniques for Mobile
Satellite Communications," Mobile Satellite Industry Briefing,
November 13-14, 1985.
Berner, J. B., and Winkelstein, R. 1988. "Antenna Pointing System,"
MSAT-X Quarterly, Number 13, January 1988.
Jamnejad, V. 1988. "A Mechanically Steered Monopulse Tracking
Antenna for PiFEx: Overview," M SAT-X 0uarterly,. Number 13, January
1988.
Figure i: Antenna System Block Diagram
245
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The JPL mechanically steered antenna

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