Airborne GNSS reflectometry using crossover reference points for carrier phase altimetry

GNSS reflectometry (GNSS-R) measurements were conducted in Sep 2012 over Lake Constance between Austria, Germany and Switzerland. A Zeppelin NT (New Technology) type airship equipped with a GORS (GNSS Occultation Reflectometry Scatterometry) receiver setup conducted flight transects (up to 64 km long) at about 500 m altitude above the lake surface. The setup uses two downwards tilted antennas with right-handed and left-handed circular polarisation (RHCP and LHCP) to acquire the reflected signal. An up-looking RHCP antenna acquires the signal on the direct link. The GORS receiver uses a Master-Slave sampling to process direct and reflected signals. An additional geodetic GNSS receiver provides direct link observations to calculate an airship trajectory with centimeter precision. A phase altimetric method is applied to the GORS samples in postprocessing. The height retrieval is initialised with an apriori height of the lake surface. Ancillary water gauge data provides reference at crossover points to adapt the apriori height. This height referencing allows to mitigate the phase-ambiguity induced bias. In this demonstration study interpolated water gauge data serve as reference. Height retrievals of an example event agree with lake surface undulations predicted by the GCG-05 (German Combined QuasiGeoid 2005) model. For RHCP (LHCP) observations a mean difference of 7 cm (5 cm) and a corresponding standard deviation of 3 cm (4 cm) is reported that confirm the phase altimetric performance.