Different geoscientific instrumentation was set up for the GEOHALO mission aboard the German research aircraft HALO (High Altitude and Long Range Aircraft, see Fig. 1). Four mission flights in June 2012 covered major parts of Italy and the adjacent Mediterranean Sea. In addition to GNSS measurements for reflectometry (GNSS-R) and positioning, measurements for laser altimetry, gravimetry and magnetometry were accomplished during these flights. The recorded reflectometry data is currently under analysis to finally retrieve the topography of the sea surface.
The GEOHALO experiment allows for advancing ground-based altimetric GNSS-R methods for flight platforms. The main focus of the GFZ activities here lies on carrier phase observations. The carrier phase precision is significantly higher than for common code observations. A retrieval of carrier phase information, however, requires specularly reflected signals.
Compared to previous coast-based measurements a significantly reduced coherence of the reflected signals is detected, related to the altitude of the aircraft (about 3500m above mean sea level), the roughness of the open sea surface and the dynamics of the flight trajectory.
The results of the GEOHALO experiment still show that the GNSS-R carrier phase can be observed from flight platforms. Based on these observations the sea surface height can be estimated along the reflection track. Aim of the study is to resolve sea surface height anomalies (dynamic sea surface topography) with a precision of a few centimeters. Fig. 2 shows the mean dynamic topography (MDT) according to a global model in the destination area of the GEOHALO mission. The values refer to the geoid.
The altimetric analysis with phase precision provides topography estimates along multiple events.For dedicated geometries an agreement with residuals of less than 10 cm standard deviation reported. However, unfavourable geometries and atmospheric uncertainties affect the method's accuracy [Semmling et al., 2014].
Semmling, A. M.; Beckheinrich, J.; Wickert, J.; Beyerle, G.; Schön, S.; Fabra, F.; Pflug, H.; He, K.; Schwabe, J. & Scheinert, M. Sea surface topography retrieved from GNSS reflectometry phase data of the GEOHALO flight mission Geophysical Research Letters, 2014, 41, 954-960