The Effect of Antenna Covers On GPS Baseline Solutions
J. Braun, B. Stephens, O. Ruud and C. Meertens
University NAVSTAR Consortium, Boulder, CO
With the increasing number of permanent GPS monuments being
deployed, many investigators are adding covers over antennas to prevent snow
accumulation, reduce the amount of wear on antennas, and discourage animals
from disturbing the antenna. UNAVCO has tested some of these covers and has
designed a model that minimizes their effect on baseline estimation.
From the tests presented here, it is shown that antenna covers cause an
additional delay on the GPS signal that effects the height component of a GPS
baseline solution. The errors introduced can change baseline height solutions
between 2 mm and 40 mm depending on the cover type, the antenna type, and the
elevation cutoff angle used for data processing. Of the two shapes of antenna
covers tested, the ones with a conical shape have the largest effect. Covers
shaped like a hemispherical dome have a smaller effect, especially when they
are mounted so that the antennas’s mean phase center is positioned in the
center of the cover. The thickness of the cover is also important. The thinner
the cover, the smaller the effect. Finally, there does appear to be an effect
caused by the material used to make the cover, but this has the smallest effect
of all the variables described above.
The results from these tests also show that the mounting apparatus of the
cover can also have an effect on the phase center of an antenna. In particular,
mounting the cover on a metal base plate will drastically increase the near
field reflections around the antenna, [Elosegui et al., 1995] causing errors of
up to 10 mm in the baseline height solution.
While not seen in these tests, results from continuous networks in Scandinavia
show that antenna covers can cause up to a 10 mm level effect in the horizontal
baseline components (Jim Davis, personal communication, 1996).
[See attached .pdf file for more.]