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4.3 Site and Set-up Effects

Site effect tests, including the effect of antenna setup heights on baseline accuracy were conducted at UNAVCO's Table Mountain facility in 1995/1996. Monuments were occupied with high (1.5 m) and low (less than 0.5 m) antenna tripod mounts. Baseline results using Trimble SSE receivers and Trimble 4000ST L1/L2 GEOD antennas with high and low antenna heights had vertical errors as large as 17 mm when tropospheric parameters were estimated. The horizontal components were not affected. The cause is L1 phase multipath error which for a low antenna is long period (more than 1 hour) and can correlate to tropospheric delay, particularly at low elevation angles (15 to 30 degrees) where multipath and tropospheric delays are strong. Based on a simple multipath model and experimental results at Table Mountain, the tests show that: (1) GPS antennas should not be placed near the ground because the scattering from the ground causes low frequency multipath that can be mismodeled as tropospheric delay resulting in vertical errors as large as several cm, and; (2) measurements using GPS antennas mounted on tripods at 1.5 m height are generally more accurate because they are subject to higher frequency multipath (Braun et al., 1995 and Johnson et al., 1995).

Multipath effects have been demonstrated to affect vertical accuracy in the high-low antenna setups. In addition, changes in multipath conditions, such as snow at the site, can affect vertical baseline solutions when tropospheric delays are estimated. Such an effect was found using two Trimble GEOD L1/L2 GP antennas on a 55 m baseline with 1.5 m antenna heights (Chris Alber, doctoral thesis, 1996). Vertical results for one day with snow cover and one without are shown in Figure 4-7. The results show a sensitivity at the 1 to 2 cm level to minimum elevation acceptance which is caused by multipath effects and is responsible for the errors and variability in vertical solutions from high-low antenna setups. Of note in this test, however, is the reversal in the sign of the effect when there is snow cover, indicating a large phase change in observed multipath. This effect is enhanced by the low height of one of the antennas. Similar effects were observed with 1-3 meter high monuments in the Swedish permanent network, at least part of which was attributed to buildup of ice and snow on one side of the radomes over the antenna (Jaldehag et al., 1995, 1996).

Figure 4-7. GIPSY-processed Vertical Solutions for a Day with and without Snow and Tropospheric Estimation. (Each point represents a 24-hour solution using data from all satellites above the minimum elevation mask. Error bars show the formal solution error.)

1996 Annual Report - 30 SEP 1997

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