Projects:

Airborne
Gravimetry

GPS/INS
Coupling

ENVISAT Radar
Altimeter Calibration

Pseudolite
Research

Radar altimeter satellites allow to measure the vertical distance between the satellite and the sea level of the Earth by transmission of a radar pulse that is reflected at the sea surface. Given precise orbit information, the instantaneous sea surface height can be computed with help of the signal transmission time allowing to analyse ocean currents and tides as well as many other phenomena.

One of the most important corrections to be applied to satellite radar altimeter measurements is radar altimeter bias, an instrumental systematic error of the radar altimeter that is related to internal propagation delays of the hardware and cannot be determined a priori with help of laboratory experiments. Instead, a dedicated calibration field campaign is to be conducted during the commissioning phase in order to estimate the bias.

The method applied to derive the altimeter bias makes use of differential GPS carrier phase measurements in order to precisely determine the position of GPS-equipped buoys. The diameter of the radar altimeter footprint usually lies between 3 and 7 km depending on the geographical latitude and the roughness of the sea. Several buoys are placed on positions being covered by the EnviSat ground tracks near the island of Menorca (Balearic Islands). If the satellite orbit is precisely determined, a reference value for the altimeter measurements can also be computed with help of the buoy (by GPS) and the satellite position (by orbit determination). An advantage of this method is that a direct comparison is possible if the buoy is positioned within the altimeter footprint.

However, the Global Positioning System has become a mature technology meanwhile that is no longer solely used for positioning only. As the EnviSat satellite also carries a microwave radiometer onboard and GPS allows to determine this quantity, too, the mission goals can be extended accordingly to a radiometer check and calibration. In summary, the contributions of the Institute of Geodesy and Navigation within ESA's calibration and validation activities cover the following items:

• Derivation of precise sea surface heights at the satellite's cross-over points using GPS-equipped buoys and a nearby reference station that is to be installed at a proper location.

• Investigation of long-baseline kinematic data processing and water vapour estimation. If this method proved to be effective, the expensive installation and maintenance of the nearby reference station would become superfluous and many more locations, not necessarily very near the coast, would become candidates for further calibration missions.

• Integrated water vapour estimates by GPS using data of the nearby reference stations.

• Extraction of relevant meteorological quantities like total pressure, dry temperature, mean temperature and integrated water vapour from numerical weather fields to support the data analysis.

Calibration Site
GPS Buoys
Reference Station
Publications/Research Database
Newspaper Article in "Mallorca Zeitung"

Point of contact: Torben Schüler