Analysis of 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.
- Fig. 1: Trimble Antenne TRM33429.20 with Groundplane.
- Fig. 2: Standard Tripod FG ANA100B and head of Leica GST120.
- Fig. 3: High adaption with Near Field Enviornment.
- Fig. 4: Additional high adaption with additional metal plate.
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.
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.
- Fig. 5: 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.
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