Development & Testing - Development of an Antenna and Multipath Calibration System for GPS

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Development & Testing - Development of an Antenna and Multipath Calibration System for GPS

The AMCS (Antenna and Multipath Calibration System) involves investigators from the Harvard-Smithsonian Center for Astrophysics, the MIT Haystack Observatory, and UNAVCO. We are developing the AMCS as part of a "Global Positioning System (GPS) calibration system" effort to characterize site-specific GPS phase-measurement errors, such as antenna scattering and multipath, at a level of 1 mm. This level represents an improvement in accuracy of one order of magnitude.

The AMCS system consists of a high-gain, fully-steerable, "multipath-free", paraboloid antenna, two GPS receivers, and a short-baseline to a GPS geodetic antenna to be calibrated for multipath, scattering, and phase-center variations.

Kwan-Dong Park, J.L. Davis, T.A. Herring, B.E. Corey, A.E. Niell, P. Elosegui, P.O.J. Jarlemark, J. Normandeau, C. Meertens, and V. Andreatta

Link to AMCS Homepage at Harvard CFA - AMCS Homepage at CFA

Introduction

Radio multipath effects on signals received from GPS satellites are highly dependent on the local topography and other site-specific factors. Significant improvements in the accuracy of estimates of positions, atmosphere, and satellite orbits are expected if this source of error is reduced or eliminated. We are constructing a transportable Antenna and Multipath Calibration System (AMCS) for this purpose.

Description

The AMCS consists of 2 parts:

A parabolic reflector radio antenna with high directivity coupled to a standard, geodetic GPS receiver; this component will provide the signal from the satellite free of multipath and scattering effects.
A second identical receiver to connect to the GPS antenna to be calibrated. In order to keep the cost low andto facilitate duplication, currently available commercial GPS receivers will be adapted for the AMCS. The first step in AMCS construction therefore consisted of examining different commercial GPS receivers to find suitable candidates for the AMCS.

The following memos will be posted with the most recent memo located at the top of the page

Below is a pdf version of the poster that was presented at the 2002 UNAVCO Inc. meeting in Colorado Springs, Colorado. AMCS_UNAVCO2002.pdf
Below is a memo from Kwan-Dong Park regarding the most recent AMCS testing. This memo discusses the effect of changing the baseline vector between the parabolic dish antenna and the choke ring antenna, and the effects on the phase residuals. The memo is called AMCS AZ / EL- Dependent Phase Residual test. July 20, 2001
amcs-geom.pdf
AMCS Antenna Rotation Test - Draft June 11, 2001
An antenna test was conducted at the UNAVCO facility in order to determine the effect of antenna rotation on carrier phase observations, a possible concern for the AMCS feed antenna or the parabolic antenna.
summary.pdf (PDF)
Below is a memo from Jim Davis on March 29, 2001 discussing the changes made to the Igor algorithms in order to calculate the Antenna beam pattern for the parabolic antenna used in this AMCS project.
Antenna Beam Pattern Calculation (.pdf)
Below is a memo discussing the re-analysis of the data that was taken on May 1-3, 2000, done by Kwan-Dong Park (the new post doc working on the project). Some of the graphs in the memo have prompted a discussion as to why the SNR values for the AMCS antenna were so low.
Re-analysis of May 01-02, 2000 data (.pdf)
Below is a memo discussing the geometric model of the AMCS antenna setup. This memo includes a brief description of the AMCS baseline with a Trimble choke ring antenna. This memo has a description of perfect pointing for the AMCS antenna and the deliberate mispointing of the antenna.
Memo - July 10, 2000 (.pdf file)

Photographs of Equipment

Below is a list of JPEG images associated with these tests:

	Azimuth and elevation drive mechanism for the Antenna and Multipath Calibration System parabolic reference antenna.
	The Dorne-Margolin chokering antenna to be tested is in the foreground; the Antenna and Multipath Calibration System parabolic reference antenna is in the background.
	The Trimble receivers and control computer for the Antenna and Multipath Calibration System.
	The Antenna and Multipath Calibration System reference antenna.

Screen Output

Additionally there are images of the screen output in these scenarios:

	The Front-panel display after acquiring 5 days of AMCS data with the new Andrew Phase-Stablilized RF cables (.pdf format).
	The Front-panel display after acquiring 5 days of AMCS data with the old RG214 RF cables (.pdf format).
	A generic version of the screen (.jpg format).