Globalstar Satellite Near-field Measurement Systems
Antenna Measurement Techniques Association Conference
September 30 - October 3, 1996
Greg Hindman
Nearfield Systems Incorporated
1330 E. 223rd Street #524
Carson, CA 90745 USA
(310) 525-7000
ABSTRACT
NSI recently completed installation of two large
7m x 7m horizontal planar scanners to support the Globalstar satellite
program test activity. These systems were installed at Alcatel
in France, and Alenia in Italy. These two systems are similar
to the NSI system installed at Space Systems/Loral in Palo Alto,
CA. described in previous AMTA papers. The companies are part
of the Globalstar satellite consortium, committed to launching
a constellation of satellites for mobile telephone communications.
The paper will summarize the hardware configuration and the
unique features of the two new test systems including high power
phased array testing and the interface to the Globalstar payload
for active antenna control and payload testing. In addition,
range data comparing all 3 test ranges will be shown.
1.0 INTRODUCTION
Globalstar is a low-earth-orbiting (LEO) satellite-based
digital telecommunications system that will offer wireless telephone
and other telecommunications services worldwide beginning in 1998.
Globalstar will provide low-cost, high quality telephony and other
digital telecommunications services such as data transmission,
paging, facsimile and position location to areas currently under-served
or not served by existing wireline and cellular telecommunications
systems. Globalstar will begin launching satellites in the second
half of 1997 and will commence initial commercial operations via
a 24-satellite constellation in 1998. Full 48-satellite coverage
will occur in the first half of 1999.
Each Globalstar satellite contains one S-band and
one L-band antenna each of which provide 16 fixed contoured beams
over the Earth, that collectively subtend an angle of 54 degrees
with respect to the satellite nadir. Due to the large number
of satellites, and the aggressive launch schedule, the near-field
range design included a number of challenges. Hewlett Packard
and NSI were selected to provide a test range to meet these challenges
for the Globalstar system.
2.0 TEST RANGE CONFIGURATIONS
NSI implemented the new test ranges using the planar
near-field scanning technique to provide the scan area required
for the wide beam Globalstar requirements. The system was identical
in size to a system NSI previously delivered to Space Systems/Loral
(SS/L). This choice allowed several advantages including commonality
among test ranges for the 3 partners, minimal development costs,
and a shorter delivery time for the systems. Issues involved
with design of large, precision near-field scanners are discussed
further in the book "Nearfield Antenna Measurements".(1)
The SS/L range uses separate dual-polarization probes
at the S-band and L-band frequencies, but since the 2 new ranges
would be used primarily for production testing, a new broadband
probe covering both bands was designed and implemented. A test
comparison between the ranges was performed with an antenna designated
the "Grand Tour" (GT) antenna, described later. A comparison
of the ranges is shown in the table below.
| . | SS/L
| Alcatel |
Alenia |
| Range delivery and commissioning
| March 1993
| October, 1995
| December, 1995
|
| Globalstar GT antenna testing done
| January, 1996
| March, 1996
| April, 1996
|
| AUT lift stage
| SSL 2m travel
| NSI 0.12m travel
| manual only
|
| Chamber height
| 13m |
10m | 5.8m
|
| Scaner Size
| 7m x 7m
| 7m x 7m
| 7m x 7m
|
| RF subsystem
| HP 8530A
| HP 8530A + payload I/F
| HP 8530A + payload I/F
|
| Probe used for GT tests
| L-band CP probe S-band CP probe
| L/S-band linear probe
| L/S-band linear probe
|
3.0 SCANNER AND CONTROL SOFTWARE
The 7m by 7m scanner at SS/L has been described earlier in several
referenced AMTA papers.(2)(3)(4) For the Alcatel and
Alenia implementations, a lower probe height was chosen to match
the configuration of the Globalstar satellite payload. This allowed
a simpler support structure than the SS/L configuration. Scanner
speeds up to 0.75 m/sec (30 inches per second) are possible, allowing
a complete, 7m by 7m nearfield data acquisition at the S-band
test frequency in under 30 minutes. At slower speeds, the system
can acquire up to 320 multiplexed channels on the fly (16 beams,
10 frequencies, and 2 polarizations). The Alcatel system is shown
in Figures 1 and 2.
4.0 RF SUBSYSTEM FOR ANTENNA AND PAYLOAD TESTING
The RF subsystem was based on Hewlett Packard's HP 85301 antenna
measurement system. The heart of the system is the HP 8530A receiver,
along with the dual HP 8360 sources. The custom content of the
RF subsystem includes a computer controlled Transmit/Receive reversal
unit, attenuation and power level control, 16 port high speed
pin switch assemblies for S-band and L-band, and a high speed
frequency converter designed to allow testing the antennas and
their associated transponders in the payload at the appropriate
transponder frequencies. The HP RF switching network, source
frequency control, and receiver triggering, are controlled by
NSI's DSP-based multiplexing software. This software allows real-time
position correction for all beams with the probe scanning at maximum
speed without suffering from position errors normally encountered
in bi-directional scanning. Figure 2 shows the location of the
HP equipment racks and figure 3 shows the RF equipment block diagram
and interface to the NSI control electronics and computer
The probes were designed specifically to support the Globalstar
frequency bands at 1.6 and 2.5 GHz. A dual-polarization design
was required to minimize test time, and a single probe covering
the full frequency band was desired to eliminate the need to change
probes when changing between L-band and S-band antenna test modes.
SS/L had (much earlier) chosen to optimize the probes at the individual
bands, since their priority was less on high throughput production
and more on design verification. The probes were calibrated at
3 frequencies per band, for gain and patterns.
5.0 SYSTEM VALIDATION
NSI performed a several week validation of each of the new ranges
using the accepted NIST 18 term model(5). A combination
of analyses and "self-comparison" tests were performed
to assess the system accuracy for testing the Globalstar antennas
and payload. An engineering model of the Globalstar antenna was
used for the evaluations, and 2 of the 16 beams were selected
for analysis - beam 1 (on-axis) and beam 8 (beam tilted 44 off
axis). The results of the 18 term budget analysis is shown in
figure 4 for the beam 1 principal polarization. Included in the
table are the gain and EIRP measurement uncertainty, and the pattern
level uncertainty at various pattern levels. Further dscription
of the EIRP measurement process is in a companion paper(6).
During the evaluation period there were also a number of trades
performed, to determine a reasonable choice for certain of the
test parameters. For instance, the AUT-to-probe spacing needed
to be increased to minimize the AUT-to-probe mutual coupling,
but kept within acceptable limits based on the effects of truncation
of the off-axis beam. The use of the dual-Z scanning technique
described in a prior NSI paper(7), helped control the
mutual coupling errors. The results achieved were consistent
with the goal of making the range suitable for high volume production
testing on the Globalstar antennas and payload, and maintaining
acceptable measurement accuracy.
6.0 RANGE COMPARISON TESTING
A confidence test was performed to insure the three Globalstar
near-field test ranges gave comparable performance. An S-band
planar array antenna, nicknamed the "Grand Tour" antenna,
was used for the comparison. This same antenna was used for much
of the range evaluation activity. The antenna was first tested
at SS/L, then at Alcatel, and finally at Alenia. The testing
was performed over a several month period in early 1996.
The results of the comparisons are shown for the beam 8 principal
and cross polarization patterns, in figures 5 and 6. The largest
discrepancy was noticed in the beam 8 cross polarization, under
the area where the main beam energy is concentrated. Part of
the discrepancy could be from some part of the main beam "leaking"
into the cross polarization due to the error involved in reconstructing
circular polarization from the 2 linear polarizations acquired
with the Alcatel and Alenia probes, whereas the SS/L probes were
circularly polarized (CP).
Other possible error sources include chamber reflections, changes
in the antenna during shipment, alignment of the antenna to insure
the same cut through the antenna is compared. Additional confidence
in the results could be gained by re-testing the "Grand
Tour" antenna back at the first test facility to insure its
stability at some time in the future.
7.0 CONCLUSION
This paper has described the general characteristics of three
test ranges used for design and production testing for the Globalstar
satellite program. The Alcatel and Alenia ranges were based on
the earlier SS/L range implemented by NSI and HP, and then further
optimized for specific use on the Globalstar program with a custom
RF subsystem, and RF probe design.
Tests between a "Grand Tour" antenna at the three test
ranges show quite good agreement in the performance of the ranges,
especially considering the compromises inherent in implementing
state-of-the-art satellite antenna measurement facilities.
REFERENCES
- Near-field Antenna Measurements, by Dan Slater, pp 253-256, Artech House, Norwood, Ma, 1991.
- Error Analysis Techniques for Planar Near-field Measurements, by Allen C. Newell, IEEE Transactions on Antennas and Propagation, Vol 36, No.6, June 1988.
- Implementation of a 22' x 22' Planar Near-Field System for Satellite Antenna Measurements by G. Hindman and G.F. Masters, 1993 AMTA Symposium
- An Automated Test Sequencer for High Volume Near-Field Measurements by G. Hindman and D. Slater, 1993 AMTA Symposium
- Position Correction on Large Near-Field Scanners Using an Optical Tracking System by G. Hindman, 1994 AMTA Symposium
- Automated EIRP Measurements on a Near-Field Range by G.F. Masters, 1996 AMTA Symposium
- Error Suppression Techniques for Near-field Antenna Measurements, by G. Hindman and D. Slater, 1989 AMTA Symposium
Figure 1 - Alcatel NF Range Side View
Figure 2 - Alcatel Nearfield Range Showing HP Equipment and Control Room Location
Figure 3 - RF Block Diagram for Antenna Test Mode
| . | . | . | .
| Beam-1 Co-pol
| . |
| Term
| Item |
Gain | EIRP
| -10 | -20
| -30 | -40
| Source |
| 1
| Probe relative pattern
| 0.00 |
0.00 | 0.09
| 0.09 |
0.34 | 0.34
| . |
| 2
| Probe polarization ratio
| 0.00 |
0.00 | 0.01
| 0.04 |
0.14 | 0.40
| . |
| 3
| Probe gain
| 0.09 |
0.09 | 0.00
| 0.00 |
0.00 | 0.00
| Does not affect relative measurements
|
| 4
| Probe alignment
| 0.05 |
0.05 | 0.02
| 0.08 |
0.08 | 0.11
| . |
| 5
| Normalization constant
| 0.05 |
0.10 | 0.00
| 0.00 |
0.00 | 0.00
| Repeatability of connectors and accuracy of attenuators
|
| 6
| Impedance mismatch
| 0.05 |
0.06 | 0.00
| 0.00 |
0.00 | 0.00
| Assumes a -30 dB match at all locations
|
| 7
| AUT alignment
| 0.00 |
0.00 | 0.00
| 0.00 |
0.00 | 0.00
| Does not affect relative measurements
|
| 8
| Data point spacing
| 0.03 |
0.03 | 0.09
| 0.28 |
0.88 | 2.80
| Compares Alenia file Gtour18, includes interpolation error
|
| 9
| Measurement truncation
| 0.01 |
0.01 | 0.04
| 0.12 |
0.39 | 1.12
| . |
| 10
| XY errors
| 0.00 |
0.00 | 0.00
| 0.01 |
0.04 | 0.11
| Based on a measured X-Y RMS error of 0.13mm
|
| 11
| Z errors
| 0.00 |
0.00 | 0.00
| 0.01 |
0.02 | 0.06
| Based on a measured Z RMS error of 0.07mm
|
| 12
| Probe-AUT reflections
| 0.06 |
0.06 | 0.09
| 0.27 |
0.86 | 2.70
| Compares Alenia file Gtour63
|
| 13
| Receiver linearity
| 0.02 |
0.02 | 0.06
| 0.19 |
0.60 | 1.90
| Simulation with HP8530 linearity specs.
|
| 14
| Systematic phase
| 0.00 |
0.00 | 0.01
| 0.02 |
0.05 | 0.20
| Based on cable phase tests. Files:Cable_x1.dat, Cable_y2.dat
|
| 15
| Dynamic range
| 0.00 |
0.00 | 0.01
| 0.02 |
0.05 | 0.20
| Measured 70 dB dynamic range
|
| 16
| Room scattering
| 0.04 |
0.04 | 0.11
| 0.34 |
1.10 | 3.40
| Based processing Gstar008 and comparing Z1Z3 vs. Z1aZ3a
|
| 17
| Leakage and crosstalk
| 0.01 |
0.01 | 0.04
| 0.11 |
0.34 | 1.10
| . |
| 18
| Random errors
| 0.00 |
0.00 | 0.00
| 0.00 |
0.00 | 0.00
| This term is already included in item 16
|
| . | RSS total
| 0.15 |
0.18 | 0.21
| 0.59 |
1.87 | 5.76
| . |
| . | Specification
| 0.25 |
0.25 | 0.25
| 0.50 |
1.00 | 2.00
| . |
Figure 4 - 18 Term Error Budget - Beam 1 Principal-Pol
Figure 5 - "Grand Tour" Beam 8 Principal-Pol
Figure 6 - "Grand Tour" Beam 8 Cross-Pol
© Copyright 1996, Nearfield Systems Inc., All Rights
Reserved