Projects:

Airborne
Gravimetry

GPS/INS
Coupling

ENVISAT Radar
Altimeter Calibration

Pseudolite
Research

This calibration concept adopted by the European Space Agency ESA leads to a concentration of all different calibration activities to a particular region featuring major advantages for the bias determination. The calibration region should provide ideal boundary conditions such as a dense coverage by laser tracking systems, a dense network of tide gauges, small and well-known signals due to tides and currents, a well-known response to meteorological conditions, low average wave heights, a smooth and precisely known marine geoid, a dense network of meteorological sensors (pressure, water vapour), high-resolution and high-quality atmospheric models, just to mention the most important characteristics. The European region was favoured for logistic reasons (and for matters of political correctness). In this region the possible seas are the North Sea, the Baltic, the Adriatic and the Western Mediterranean. The one which appeared to fit the requirements best, as well as being the largest, is the Western Mediterranean.

EnviSat ground tracks over the region of the Balearic Islands in the Western Mediterranean Sea. (Reference: RA-2 In-Orbit Absolute Calibration Plan, ESA, March 2000)

With the commissioning phase being as short as 9 months during which all calibration activities are to be carried out, the number of satellite cross-overs over a particular station is rather limited. For this region, only cross-over sites combining both ascending and descending EnviSat tracks are of interest in order to gain an optimal redundancy. The map above illustrates the situation over the Balearic Islands. It should be noted that the distance between a GPS reference station and the buoys is to be minimised in order to reduce atmospheric - in particular ionospheric - errors (and also, but of minor concern, GPS orbit uncertainties) and thereby to allow ambiguity fixing with a certain degree of integrity being the guarantor for a high-precision position determination. Additionally, the buoys need to be appropriately moored, i.e. the sea depth is also limited to approximately 100 meters. The cross-over point in the northern direction off the coast of Menorca is already at a region with large depth of more than 200 metres.

Map of the calibration area in the north-western part of Menorca island near the town of Ciutadella. The location of the reference station is given by the abbreviation "CIUX".

Finally, the cross-over point in the north-western part of Menorca was selected. The figure above shows that 4 buoys are arranged symmetrically around the nominal cross-over point. Ideally, the distance to the reference site "CIUX" is in the range of 8 to 9 kilometres. The sea depth, however, is between 120 and 150 metres requiring a special light-weight mooring system using stable fibre ropes.

GPS Buoys
Reference Station
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