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Assessment of Snow and Ice Reflections with In-Situ data

View from setup location, with GNSS antennas in the lower part, to the inner Kongsfjorden in north-eastern direction.
Lupe

A GNSS station with a reflectometry receiver was set up at Kongsfjorden (Spitsbergen), 78°54’14’’N, 11°52’37’’E, 512m above ellipsoid (WGS-84). This permanent station is operated by GFZ in collaboration with NPI (Norwegian Polar Institute) at the Zeppelin mountain outpost. It accumulates a data set since summer 2013 observing reflections over the fjord and the adjacent land surface. Especially the changing surface types: the fjord, adjacent glaciers and the variable snow-cover over land are of interest to investigate GNSS reflectometry applications.

The setup contains a GORS (GNSS Occultation Reflectometry Scatterometry) four-frontend receiver, which is based on commercial JAVAD hardware. The receiver is connected to an uplooking antenna and two further antennas tilted towards the horizon. The tilted antennas have right-handed and left-handed circular polarization, respectively. The antenna mount is installed approximately 476m above mean sea level with an extensive view over Kongsfjorden.

Reflections are observed at low transmitter elevation angles (between 10° and 2°) over Kongsfjorden and adjacent land areas. A topography adapted reflection model, which considers sloped surface facets, has been developed and presented [1]. It incorporates a digital elevation model, provided by NPI. The relative Doppler shift (reflected to direct signal) is computed for each facet.

Challenges in data processing persist due to drifts in GNSS satellite orbit. Reflection points at large distances are, therefore, variable even though the satellite recur after a sidereal day. The long-term objective is to map the signal-to-noise ratio (SNR) of the more than 50 recurring reflection events for a 3-year period. The influence of surface roughness and liquid water content on SNR shall be examined for soil, snow- or ice-covered surfaces. The 2014 data set of SNR over the fjord has been analysed already in a master thesis [2].

References:  

[1] Semmling, M.; Peraza, L.; Falck, C.; Gerland, S. & Wickert, J.: Doppler shift estimation for GNSS reflectometry using a land topography adapted reflection model. Geophysical Research Abstracts, EGU General Assembly, 2016, Vol. 18, EGU2016-17139  

[2] Peraza, L.: Signal Power Analysis of GNSS Reflections recorded at Kongsfjorden, Spitsbergen. Technical University Berlin, 2016 Im Original geändert:

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