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 Analysis of the so called Near Field Effects of the special
tripod FG-ANA 100B
Investigations on antenna near field effects |
Reflexion and diffraction of satellite signals in the direct vinicity
of GNSS antennas leads to long periodical multipath effects. These effects do
not average out when using long observation intervals (e.g. 24h) and
consequently falsify the estimated coordinates. Tripots, adapters are prominent
examples for such reflectors. In addition, the electric properties of the GNSS
antenna may be changed due to these adations.
In order to assess the
impact of the near field effects on the phase center offset and variations,
dedicated tests were carried out at the Institut für Erdmessung
based on the absolute
antenna calibration. For this purpose two calibrations are performed: one
with antenna mounted on the model of the antenna vinicity. The results are then
compared to those obtained from an absolute calibration of the antenna only.
The tests carried out up to now underline the need of consideration of near
field effects.
Selected
references:
- Dilßner F.
(2008): Zum Einfluss des Antennenumfeldes auf die hochpräzise
GNSS-Positionsbestimmung. Dissertation. Wissenschaftliche Arbeiten des
Fachrichtung Geodäsie und Geoinformatik der Leibniz Universität
Hannover Nr.253.
- Schön, S. and F.
Dilßner (2007): Challenges for GNSS-based high precision positioning
- some geodetic aspects. In: Kaiser, T., Jobmann, K., Kyamakya,K. (Eds.)
Proceedings of the 4th Workshop on Positioning, Navigation and Communication
2007 (WPNC07). Shaker, 229 - 237, 2007.
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Analysis for precision GPS levelling
In order to
assess the near field effects on the height component determined from static
GPS field experiments, different antenna mounts and antenna were tested using
different cut-off angles (5°, 8°, 15°). Fig. (1) shows the
different mounts: (a) steal plate with Zeiss-tribranch, (b) Stativteller with
Zeiss-Dreifuß, and (c) a adapation for the direct height determination
with a levelling rod with Wild (LVG Sachsen-Anhalt).
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The impact on the coordinates are shown in fig. (2). In this study
maximum effects can reach up to 18 mm.
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Fig. 2 - These studies were financed by a donation from the
Ernst-Simons-Stiftung to F. Dilßner and carried out in close cooperation
with Geo++.
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Analysis of the GPS antennas for the GOCE satellite
Within the framework of the European GOCE mission (Gravity filed and
Steady state Ocean Circulation Explorer), a high accuracy
Satellite-to-Satellite Tracking Instrument with two hemispherical-coverage
receiving antenna will be used for real time navigation and precise orbit
determination. These GPS antennas are directly mounted on one of the metallic
solar wings of the space craft. In order to characterize the antennas with
respect to their phase center positions and carrier-to-noise decrease, several
tests were performed at IfE. The results helped to validate the simulation
studies done by ESTEC as well as the anechoic chamber measurements performed by
the antenna manufacturers.
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Analysis of the GPS Helix antenna mounted on the part of the wing of the GOCE-
Satellite at IfE.
This project was funded by ESA (ESTEC Contract No.
19069/05/NL/FF). The analysis were carried out in cooperation with Geo++,
Alenia SPAZIO, and ESA/ESTEC. |
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Selected references:
- Toso, G., M.
Manzanares, A. Torre, F. Dilßner, G. Seeber, M. Schmitz, G. Wübbena,
D. Maeusli (2006): GPS antenna phase characterisation for the ESA GOCE
mission via anechoic chamber measurements, numerical simulations and live
automated absolute calibration. - Poster presented at first European Conference
on Antennas and Propagation (EuCAP 2006), November 6-10, Nice,
France.
- Dilßner, F., G.
Seeber, M. Schmitz, G. Wübbena, G. Toso, D. Maeusli (2006):
Characterisation of GOCE SSTI Antennas. - ZfV, Zeitschrift für
Vermessungswesen, 131, Heft 2, 61-71.
- Dilßner, F., G.
Seeber, M. Schmitz, G. Wübbena, G. Toso, D. Maeusli (2005):
Characterisation of GOCE GPS Antennas. Poster presented at Geodätische
Woche 2005, October 4.-6., Düsseldorf, Germany.
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For more information, please contact:
Prof. Dr.-Ing.
Steffen Schön |
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Analysis of the so called Near Field Effects of the special tripod
FG-ANA 100B
Near field effects
may affect the precise determination of heights and height differences by GPS.
In order to avoid, or at least control such effects, a standardized tripod is
used during the measurement campaign for the update of the German national
height network (DHHN) 2006 - 2011.
Therefore a trimble antenna
(TRM33429.20+GP) combined with the standardized tripod FG-ANA 100B (Stativ
Freiberger Dreifuss SFD) and different receiver (Javad Legacy and Trimle 4700)
were used for forward analysis. This special tripod is used during the field
measurements for the DHHN campaign 2006-2011. One instrument was kindly
provided for test experiments by the LGN Hannover. The different setups for the
experiments are shown in figure 1.
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 Figure 1: Setups used during
the experiments for near field effects. In setup (1) only the pure antenna
TRM33429.00+GP was calibrated. Setups (2) to (4) show the modified tripod with
different heights to analyze the near field effects.
For strict comparisons of the phase center variations
individual offsets are transformed to a uniform phase center offset
(calibration with SFD). The resulting PCVs show a different behavior (up to 4
mm) for the signal L1 in the range of low elevations (0°-20°).
In Addition different receiver
hardware, here tested with a Javad JPSLegacy and a Trimble 4700 receiver, yield
different PCV. Here no significant differences on L1 ar visible, however on L2
differences up to 3 - 4 mm are obtained when using identical antenna setups but
different receivers. This behavior is primarily caused by individual signal
tracking algorithm of the manufacturer.
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« Figure 2: Composition of elevation depended Phase
center variations by calibration of different antennas, different setups and
different receiver hardware. Shown here are dPCV compared to calibration with
standardized tripod SFD. On Signals L1 different behavior of the tripod setups
are to notice. On the other hand Signal L2 shows a dependency on the different
manufacturer, caused by different signal tracking algorithms.
Click here to see a larger version of Figure 2. |
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Finally the impact of the near field depends on the using linear
combinations or estimating analysis strategy used. Troposphere zenith delay
will generally increase the impact.
For more information, please contact:
Dipl.- Ing.
Tobias Kersten |
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