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2.8.4 Measurement Reference

The final measurement required in altimetry is the surface height. The altimeter, however, measures the range, which is the distance between the altimeter, mounted on the satellite, and the surface the satellite is flying over. In order to derive the surface height, the satellite altitude with respect to the ellipsoid is required. By subtracting the range measured by the altimeter from the satellite altitude we obtain the surface height with respect to the ellipsoid (see Figure 1.8 in Principles Of Measurement 1.1.3. ).

Satellite Altitude defined for the ENVISAT orbits is referenced to the satellite Centre of Mass (CoM). Whereas the range measurement of the RA2 is referenced to the phase centre of the antenna. To obtain a precise surface height we must know the vertical offset of the phase centre from the CoM.

The RA2 instrument measurement is actually made at some point (the FEE circulator) inside the instrument electronics. However, by applying on-ground characterisation measurements and the in-flight calibration corrections in the level 1b processing, the measurement in the data products is referenced to the antenna phase centre.

Figure 2.6 Reference point for Altimeter range measurement

The satellite orbit is calculated using DORIS data and information from ground laser stations which track the on-board Laser Retro Reflector (LRR). To relate the orbit to the CoM we require precise knowledge of the DORIS antenna position and the LRR position.

Figure 2.7 Positions of the RA2, LRR and DORIS instrument on EnviSat

The positions of all these elements, in the satellite reference frame, were determined in tests performed at ESTEC and Kourou. For RA2, LRR and DORIS the accuracy of the measurement was better than ±1 mm in each of the x, y, z, axies. These positions are taken as fixed for the mission duration.

The CoM position at the start of mission was determined to an accuracy of 3.65mm rms. The position of the CoM changes with time due to consumption of on-board fuel and a new offset is calculated after manoeuvres.

Figure 2.8 Showing the range offset between the CoM and the RA2 antenna

The relative offset of the antenna phase centre from the CoM is stored in the auxiliary file RA2_PLA_AUX which is updated whenever the CoM changes due to manoeuvers.

This correction is included in the range measurements of the Level 2 data products.

Full details of the Measurement reference Points are documented in an ESA Tec-note PO-TN-ESA-RA-1319.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry