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Investigations on antenna near field effects

Near Field Adaption FG ANA100B, special design for the German DHHN Campaigne 2006-2011 for the contact free measuring of the antenna heigth.
Fig. 1: Near Field Adaption FG ANA100B, special design for the German DHHN Campaigne 2006-2011 for the contact free measuring of the antenna heigth. The adaption was kindly supported by the LGLN.

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.

In this context some  problems were analzed in more detail:

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