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The Effect of Antenna Covers On GPS Baseline Solutions

Introduction

As the UNAVCO facility became increasingly involved in the deployment of permanent GPS stations, the facility soon realized that many permanent GPS monuments require some type of cover over the antenna to protect it from weather, animals, and general wear and tear. Some manufactures, including Ashtech and Trimble, now sell antenna covers for this purpose. Also, principal investigators have been designing and deploying covers of their own designs. With the results of the Ashtech dome tests reported in the ARI report, the facility decided that further tests needed to be conducted to determine what effect these covers have on baseline solutions.

The facility tested two general shapes of covers. One of the shapes was conical while the other was spherical. Ashtech and Trimble both offer conical covers, but they are made of different materials and do not mount over the antenna in the same way. UNAVCO has made a spherical cover using two different thicknesses of acrylic (one-eighth inch and one-quarter inch). We tested all these designs, and report their effects in this report.

The domes were tested by measuring very short baselines (<10 meters) with and without a cover on one of the antennas. The data were then analyzed with different estimation parameters and elevation cutoff angles. The solutions were then compared to a "ground truth" measurement to determine the effect the cover has on the baseline solution.

Results of these tests show that when an antenna cover is used at one end of the baseline, the height component of the solution is different from ground truth. These problems are amplified by estimation of tropospheric parameters, which produce errors in tropospheric delay as well as station height. The shape and design of the covers appears to vary the magnitude of the effect. The conical covers produce vertical errors of up to 44 mm, while the thin spherical domes appear to minimize the problem and produce an error of a few millimeters or less. The solutions also have a strong dependence on variations in the elevation cutoff angle.

These tests also point out the fact that it is important to mount the cover on or around the antenna in such a way as to minimize multipath. For example, a metal plate like the one provided with the Trimble conical cover has up to a 20 mm effect in the height component, whereas mounting the cover using some sort of plastic or acrylic plate produces an effect of less than a few millimeters.

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Last modified Thursday, 17-Nov-2005 03:58:00 UTC

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