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.
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.