Laser telemetry

Laser telemetry has become the only space geodesy technique being at the same time, precise and absolute (exact distance measurement), continuously controllable on ground at the metrology level. This is due to its maintenance availability and its worldwide development. See the campaigns using ultra-mobile laser station (in french) (FTLRS).

• Jason-1, the first satellite from the Jason oceanography family developed by Cnes and Nasa, is the outcome of a sea state survey concept initiated several years ago with Topex/Poseidon (T/P). On a geodetic point of view, the expected variations of the mean sea level are about a millimeter per year; so long uninterrupted time series of oceanographic data are essential. In the same way, we expect, thanks to laser telemetry as well as other space techniques, a better knowledge of the orbit of altimetric satellites, and the determination of the Earth's crust displacements, including vertical motion, with the same precision level.
• The calibration of the on board altimeter radar is a key point for the validation of an altimetry mission [Ménard & al., 1994]. The time drift of an unexpected bias, due to electronic aging, is even more worrying for the analysis of secular mean sea level variations. The T/P mission experience has shown that is absolutely necessary to control this calibration at mm/year level over several years [Bonnefond et al., 2003a].
• Another major limitation of the altimetric system is related to the orbit determination quality, which is required to achieve the proper accuracy of the sea level determination in a geocentric reference frame. This because altimetry measurement is performed from a spacecraft.

This implies the use of global satellite tracking systems such Doris and / or GPS and outstanding numerical orbit processing methods in order to reach an orbit determination accuracy compatible with the centimeter level precision of data [Nouel & al., 1994]. Laser telemetry is the only absolute technique (the Doppler technique provides information on time varying distances) providing the essential scale factor with a constant quality even on intercontinental distances [Bonnefond & al., 1999]. The troposphere effects (moisture effects) are also much smaller than with radio-electric techniques.

In the frame of space oceanography like for space geodesy, it is a prime necessity to develop laser instruments with much more stable biases typically less than 5 mm over several years. Grasse is one of the 3 sites in Europe, reaching a sufficient accuracy for such measurements. It's a very interesting site that covers a large geographic area in the western Mediterranean Sea.

The economic interest to survey this area is evident, from a scientific point of view. The western Mediterranean Sea has become a unique calibration and experimentation zone in the world of the 90's beginning, due to its low oceanic variability as a quasi-closed Sea and its economical abilities. That's why we proposed to develop a double calibration site, in Grasse (Calern Plateau) and in Corsica (Aspretto naval base and Cap de Senetosa in the south).