Calibration of the CONSERT/ROSETTA radar

The ROSETTA/ESA probe will probably rendezvous Comet 67P/Churyumov-Gerasimenko in 2014 and launch a Lander at the nucleus surface. The CONSERT instrument will perform the sounding of the comet nucleus by measuring a 90 MHz electromagnetic wave propagation from the Lander to the orbiter throughout the nucleus. The goal of this sounding is to determine the internal structures of the comet nucleus at different scales and to deduce information on its composition (density, type and abundance of the refractor). CONSERT is an original concept of spaceborne transmission radar based on the propagation throughout the nucleus while the classical radars are based on the reflection. This instrument fulfills three functions: a chronometer to measure the propagation delay, an imager based on the multipath propagation and a radiometer to estimate the wave attenuation The calibration of the instrument should characterize these three functions but due to the novelty of the instrument concept we have no classical set of parameters to quantify the instrument performances from a data-inversion point of view. The instrument design and the absence of calibration channel due to mass and power constraints require specific signal processing and calibration protocol. In this paper, we review the instrument, its objectives and its time transponder structure. Then we present the philosophy of the calibration. In particular we present the results of the instrument ground calibration and the planed in-fly calibration.