Park, K., J.L. Davis, P.O.J. Jarlemark, P. Elosegui, J.E. Normandeau, B.E. Corey, A.E. Niell, C.M. Meertens, V. Andreatta, Multipath Characteristics of GPS signals as determined from the Antenna and Multipath Calibration System (AMCS), ION GPS Meeting, September 24-27, 2002.
Multipath Characteristics of GPS signals as determined from the Antenna and Multipath Calibration System
Kwan-Dong Park(1,4), James L. Davis(1), Per O. J. Jarlemark(1,5), Pedro Elosegui(1), James E. Normandeau(1), Brian E. Corey(2),
Arthur E. Niell(2), Charles E. Meertens(3), and Victoria A. Andreatta(3)
(1)Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts
(2)MIT Haystack Observatory, Westford, Massachusetts
(3)UCAR/UNAVCO, Boulder, Colorado
(4)Now at Korea Astronomy Observatory, South Korea
(5)Now at Swedish National Testing and Research Institute, Sweden
ABSTRACT
Geophysical applications of the Global Positioning
System (GPS) for studies such as global sea level change
and glacial isostatic adjustment require very high
accuracy (1 mm yr-1) determinations of site velocity,
especially of its vertical component. Despite the many
efforts devoted by investigators to the calibration of sitespecific
errors, signal scattering and multipath remain an
unsolved problem. We have developed an Antenna and
Multipath Calibration System (AMCS) for characterizing
site-specific GPS phase measurement errors. The system
consists of a high-gain, multipath-free, 3-m diameter
parabolic antenna, a test antenna, and two Trimble GPS
receivers. There are two modes of operating the AMCS:
Zero-baseline (ZBL) and AMCS modes. In ZBL-mode,
the two receivers simultaneously record the signal from
the test GPS antenna. In this operating mode, one can
determine the receiver clock synchronization error and the
phase biases for each satellite. Typical RMS accuracies of
ZBL-mode phase residuals are sub-millimeter level,
ranging from 0.4 to 0.7 mm. In the AMCS-mode, one
GPS receiver records the signal received at the test
antenna, and the other records the signal from the
parabola. Thus, one can compare the phases from the two
receivers, and determine the antenna and multipath
calibration errors of the test antenna. In our test cases with
the test antenna located in a multipath-rich environment,
the phase residuals obtained by tracking the same satellite
over several days show large amplitude variations over
small elevation angle ranges with highly repeatable
patterns. The amplitude is 4-6 mm for low elevation
angles and 1-2 mm for high elevation angles. Modeling
and subtracting the repeating patterns from the phase
residuals results in RMS of about 1 mm. We have
recently installed a second GPS antenna at a nearby
location where the multipath effects are presumably less
significant than at the location of the first GPS antenna.
To further reduce multipath effects, all-weather
microwave absorbers surrounded the second antenna. The
amplitude of the phase residuals obtained for the second
antenna location is significantly smaller than for the first
antenna, implying that the second antenna is less affected by multipath. These independent results also served to
confirm that the origin of the phase patterns measured is
multipath.
[See attached .pdf file for more.]
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