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| Titel |
Airborne Interferometry using GNSS Reflections for Surface Level Estimation |
| VerfasserIn |
Maximilian Semmling, Georg Beyerle, Steffen Schön, Ralf Stosius, Thomas Gerber, Jamila Beckheinrich, Markus Markgraf, Maorong Ge, Jens Wickert |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250074115
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| Zusammenfassung |
| The interferometric use of GNSS reflections for ocean altimetry can fill the gap in coverage
of ocean observations. Today radar altimeters are used for large scale ocean observations to
monitor e.g. global sea level change or circulation processes like El Niño. Spacial and
temporal resolution of a single radar altimeter, however, is insufficient to observe mesoscale
ocean phenomena like large oceanic eddies that are important indicators of climate change.
The high coverage expected for a spaceborne altimeter based on GNSS reflections
stimulated investigations on according interferometric methods. Several airborne
experiments have been conducted using code observations. Carrier observations have a
better precision but are severely affected by noise and have mostly been used in
ground-based experiments. A new interferometric approach is presented using carrier
observations for airborne application. Implementing a spectral retrieval noise reduction is
achieved.
A flight experiment was conducted with a Zeppelin airship on 2010/10/12 over Lake
Constance at the border between Austria, Germany and Switzerland. The lake surface with an
area of 536km2 is suitable for altimetric study as its decimeter range Geoid undulations are
well-known. Three GNSS receiver were installed on the airship. A Javad Delta receiver
recording direct signals for navigation. The DLR G-REX receiver recording reflected signals
for scatterometry and the GORS (GNSS Occultation Reflectometry Scatterometry) receiver
recording direct and reflected signals for interferometry. The airship’s trajectory is
determined from navigation data with a precision better than 10cm using regional
augmentation.
This presentation focuses on the interferometric analysis of GORS observations. Ray tracing
calculations are used to model the difference of direct and reflected signals’ path. Spectral
retrieval is applied to determine Doppler residuals of modelled path difference and
interferometric observations. Lake level is estimated inversely, based on the correlation of
Doppler residuals and trial heights, with decimeter precision. Estimates are validated with
tide gauge reference data.
Wave-induced surface roughness disturbs interferometric observations introducing noise on
short time scales < 1s. This effect is mitigated by the spectral retrieval. Geoid undulations
affect the observations on longer time scales > 100s and pose a challenge to spectral
retrieval. To address this challenge phase data is considered to overcome the poor
resolution of spectral retrievals. The phase data shows residual Doppler shifts. The
Doppler induced by Geoid undulation (< 25cm) is small (typically about 1mHz).
Other effects, typical for GNSS observations, are important to reach this precision.
The Doppler related to unmodelled variations of water vapour and the airship’s
trajectory is in the same range and masks the altimetric effect of Geoid undulation. |
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