Effects of Multipath and Signal Blockage on GPS
Navigation in the Vicinity of the International Space
Station (ISS)
РDavid E. Gaylory, E. Glenn Lightseyy, and Kevin W. Keyz
The University of Texas at Austin, USA
Titan Corporation, Houston, TX, USA
BIOGRAPHY
David E. Gaylor is a Ph.D. candidate at the University
of Texas at Austin. His current research is focused
on GPS/INS navigation for spacecraft rendezvous applications.
Dr. E. Glenn Lightsey is an Assistant Professor in the Aerospace Engineering and Engineering Mechanics Department. He was formerly an Aerospace Engineer at NASA Goddard Space Flight Center and works on the design and testing of spaceborne GPS receivers. He received his Ph.D. in Aeronautics and Astronautics from Stanford University.
Dr. Kevin W. Key received his Ph.D. from the University of Colorado at Boulder in Aerospace Engineering Sciences. He works for Titan Corporation at the NASA Johnson Space Center where he is developing the precise relative estimator for the Mini AERCam vehicle. He is also evaluating the relative GPS navigation systems for spacecraft rendezvous applications.
ABSTRACT
A number of studies have examined GPS relative navigation
for spacecraft performing rendezvous and docking
with the International Space Station (ISS). However, these
studies have not accounted for degradation in GPS navigation
performance due to multipath signals being reflected
off of the ISS or blockage of GPS signals by the ISS. The
objective of this study is to analyze these effects on GPS
navigation in the vicinity of the ISS using simple models
for GPS signal blockage and multipath.
A simulation of a spacecraft GPS receiver operating near
the ISS has been developed. This simulation includes orbit
models for the GPS constellation, the ISS and the spacecraft.
It also includes a GPS signal blockage model and a
multipath model. These models can be used to predict the
performance obtained by combining various sensors with
GPS for navigation or attitude determination for spacecraf to perating near the ISS or other large reflecting body.
The blockage simulation shows that aiding of GPS is required
when the spacecraft approaches within 60 meters the
ISS. The multipath simulation results show the expected
trends in the range errors as a function of the GPS satellite
elevation angle, the distance from the ISS, the number of
multipath rays modelled, and the radar cross-sectional area
of the ISS. Both effects may significantly degrade GPS navigation
near the ISS.
The next logical step is to validate these models with actual flight data. If appropriate flight data cannot be found, the results of this paper may be used to justify a future flight experiment.
INTRODUCTION
A number of studies have examined GPS relative navigation
for spacecraft performing rendezvous and docking
with the International Space Station (ISS). However, these
studies have not accounted for degradation in GPS navigation
performance due to multipath signals being reflected
off of the ISS or blockage of GPS signals by the ISS as
shown in Figure 1.
The objective of this study is to analyze these effects on
GPS navigation in the vicinity of the ISS using simple models
for GPS signal blockage and multipath. In addition to
predicting GPS navigation performance in the vicinity of
the ISS, these models were developed to predict the performance
obtained by combining various sensors, such as
star trackers or inertial measurement units, with GPS for
navigation or attitude determination.
ISS SIGNAL BLOCKAGE MODEL
It has been hypothesized that the ISS will block GPS signals
needed by other spacecraft (referred to as chaser spacecraft
or chaser) to navigate during rendezvous operations.
To analyze this effect, the GPS signal blockage due to the
ISS is modelled as a sphere centered at the ISS position with diameter, d = 100 meters.
Figure 2 depicts the various vectors used in the model. The line of sight vector from the chaser spacecraft to the jth GPS satellite is found by:
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