Development of an antenna and multipath calibration system for Global Positioning System sites
K.-D. Park(1,2), P. Elo´segui(1,3), J. L. Davis(1), P. O. J. Jarlemark(1,4), B. E. Corey(5),
A. E. Niell(5), J. E. Normandeau(1,6), C. E. Meertens(7), and V. A. Andreatta(7)
Received 3 November 2003; revised 21 May 2004; accepted 17 June 2004; published 29 September 2004.
Abstract
Site-dependent errors such as antenna phase-center variations, multipath, and
scattering can have a significant effect on high-precision applications of the Global
Positioning System (GPS). Determination of these errors has proven to be elusive since no
method has been developed to measure these effects accurately in situ. We have
designed and constructed a prototype Antenna and Multipath Calibration System (AMCS)
to obtain such in situ corrections. The primary components of the AMCS are a steerable
parabolic antenna, two GPS receivers, and a computer for control and data-logging
functions. We obtain phase corrections for site-dependent errors by forming the difference
between the carrier-beat phases from the GPS antenna to be calibrated and from the
AMCS antenna, which is relatively free of such errors. Preliminary ‘‘sky maps’’ of the
antenna phase and multipath contributions show root-mean-square (RMS) phase
variations that are a factor of 10 or more greater than the AMCS system noise, which is
0.5 mm. To explore the source of this ‘‘noise,’’ we acquired observations over small (few
degrees) patches of the sky. From the analysis of these experiments we concluded that the
source of the phase variations was antenna and multipath errors that vary by 5 mm
amplitude over small changes in satellite direction. Thus, for example, differences of 1 in
elevation angle can result in several millimeter variations in phase. Similarly, small
variations in azimuth angle can also result in significant phase variations. We have also
observed day-to-day millimeter-level changes in the calibration. We hypothesize that these
phase variations are due to changes in multipath caused by changes in the local
electromagnetic environment associated with, e.g., weather.
Citation: Park, K.-D., P. Elo´segui, J. L. Davis, P. O. J. Jarlemark, B. E. Corey, A. E. Niell, J. E. Normandeau, C. E. Meertens,
and V. A. Andreatta (2004), Development of an antenna and multipath calibration system for Global Positioning System sites, Radio Sci., 39, RS5002, doi:10.1029/2003RS002999.
[See attached .pdf file for more.]
|