GNSS Reflectometry

GNSS (GPS, Galileo, GLONASS, BeiDou) reflectometry belongs to the class of bistatic radar systems which can be used as a tool for Earth remote sensing. In a bistatic configuration transmitter and receiver reside at different locations. The receiver passively uses emitted signals of sources of opportunity. Beside the direct line-of-sight signals a GPS reflectometry receiver can potentially use all GPS signals that are reflected from the Earth's surface. As the GPS signals are transmitted continuously, the receiver can continuously receive the signal, allowing for continuous measurements of the reflecting surface.

The transmitted GPS signal interacts with the reflecting surface and some part of the signal is reflected and can be registered at the receiver. In comparison to the directly received signal the reflected signal arrives at the receiver with a certain delay. From the associated path length difference and the knowledge about transmitter and receiver position the altimetric height of the specular reflection point can be calculated from geometric relations. Observations of this kind belong to the field of GNSS altimetry.

As the surface roughness of a reflecting sea surface scatters the energy pattern of the reflected signal, significant wave height and wind speed can be derived from the temporal development (the waveform) of the received reflected signal. Observations of this kind belong to the field of GNSS scatterometry.

One of the relevant new aspects introduced by the bistatic character of GNSS reflectometry is the dependence of its capabilities on the measurement geometry. Different measurement geometries occur due to different positions of the transmitting GNSS satellites at the same time. Thus, the receiver can acquire reflector height and surface roughness information, e.g., sea state, from different reflection point locations simultaneously.

GeoHALO - Airborne GNSS Remote Sensing over the Mediterranean
GeoHALO denotes a corporate campaign of different partners in Geoscience to use the German research aircraft (HALO, High Altitude and Long Range Aircraft). The flight campaign was successfully completed in June 2012 over the Mediterranean. The flight covered large areas of the Adriatic, Tyrrhenian and Ionian Sea. Collected GNSS-R data offers the opportunity to advance altimetric methods for airborne ocean remote sensing.

GEROS-ISS - GNSS Remote Sensing aboard the ISS
GEROS-ISS is an experiment, foreseen for realization aboard the International Space Station ISS. GEROS-ISS was proposed by a top-class international science team. The work of the team is coordinated by GFZ scientist Dr. Jens Wickert. Involved are scientist and engineers from Germany, Sweden, Spain, U.S., Denmark, and Switzerland.

GORS Receiver
Within the GITEWS project, the development of GORS (GNSS Okkultation Reflektometry Scatterometry) receiver was started. Based on commercial hardware of a JAVAD GNSS two-frequency receiver firmware is adapted to receive reflected signals or those in occultation next to the direct signal. This receiver is used for ground-based or airborne setups. Spaceborne setups are planned.

GPS-SIDS (Sea Ice Dry Snow) Ground-based Sea Ice Campaign in Disko Bay
Disko Bay is situated west of the Greenland coast. Near by the port of Godhavn (69.2717° N; 53.5435° W) a GPS Reflectometry setup was installed. On a cliff about 670 m above sea level, observations over the bay were recorded for several months (in winter season 2008/09). Regular changes of the ocean surface due to ocean tides and sea-ice coverage could be examined with reflection data. This study has been initiated by ESA connected to further measurements of dry snow properties at Dome-C, East Antarctica.

Feasibility study for spaceborn tsunami detection
Within GITEWS a feasability study was conducted for satellite-based tsunami detection using GNSS-R. Lastest results show that a constellation of multiple low earth orbit satellites would be suitable to detect tsunami waves within 15-25 minutes on a global scale. This estimation implies that the altimetric signal is strong enough.

OpenGPS Prototype
Based on OpenSource components an independent single-frequency GPS-R receiver is developed at GFZ.

Spectral Retrieval of reflected GNSS signals for Ocean Altimetry
The L-band carrier of GNSS signals is subjected to permanent frequency shifts due to permanently changing geometry of transmitter and receiver (Doppler effect). This shift affects direct and reflected signals as well. In principal spectral retrievals are not suitable to examine such signals as precise frequencies cannot be derived due to permanent changes.

Nowadays excellent models for GNSS signal propagation are available and allow to derive residuals of the observed signal where only a small frequency shift is left. For reflected signals such residuals are dependent on the height of the reflecting surface (e.g. sea level). Spectral retrieval of residuals allows accumulation of observations. Disturbing roughness effects of the ocean surface can thus be mitigated and ocean altimetry with L-band carrier signals is even possible for moderate sea state. The applicability of reflected GNSS signals for ocean altimetry is thus improved.

Small satellite studies
Several small satellite studies (MicroGEM, NanoGEM, NanoX) have been conducted since 2010. The main goal is the realization of a prototype satellite for GNSS reflectometry and radio occultation. Significant benefit is expected due to a future multiple satellite constellation with a high coverage for GNSS Earth Monitoring, that is needed e.g. for tsunami detection.

Walchensee Experiment
Fahrenberg is a mountain in the Bavarian Alps (47.61° N; 11.32° E). Its summit rises 1625 m above mean sea level. At the foot of the moutain lake Walchensee spreads on an area of about 16 km² with a lake level of about 800 m above mean sea level. The vicinity of lake and mountain offers an ideal location for Reflectometry experiments with the GORS receiver.

WISDOM - Water level monitoring of the Mekong Delta in Vietnam
The Water-related Information System for the Sustainable Development Of the Mekong Delta (WISDOM) project is a German Vietnamese initiative. Its objective is to build an information system for the Mekong Delta that supports and assists planners and authorities for an optimized and integrated resource management. The system will comprise hydrologic, hydraulic, ecologic and sociologic data. One of WISDOM’s tasks is to test the usability of GNSS-R for water level monitoring of the Mekong Delta. In February 2012, a first measurement campaign in Can Tho City (9.99°N, 105.74°E) was successfully conducted.

ZOIS - Zeppelin-based GNSS Remote Sensing over Lake Constance
In collaboration with DLR and Zeppelin Luftschifftechnik (ZLT) experiments are conducted for Zeppelin-based Occultation, Interferometry and Scatterometry (ZOIS) measurements. Flight campaigns were successfully completed over Lake Constance in October 2010 and September 2012 starting from Friedrichshafen close-by. The Zeppelin airship has an excellent manoeuvrability even at small flight speeds (<100km/h). Due to the manoeuvrability the airship can easily pilot height anomalies on the rather small lake surface. These experiments allow important altimetric studies for following flight experiments over the ocean.