GNSS Radio Occultations for Climate Applications: a Benchmark with Spaceborne Nadir Sounders

The FOURIER mission (Adriani et al. 2001) has been selected by the Italian Space Agency, together with other 4 projects, for a feasibility study. Within the current year one of these will be selected to fly within the next 4 years. The primary goal of FOURIER is the development of a systematic and objective observational test of the predictive capabilities of climate models, as proposed by Goody (1998). This requires long-term climate benchmark observations that differ from those presently available. High accuracy, demonstrable with respect to international standards, is the primary requirement. Other important features follow from this requirement. At least two types of space measurement satisfy these criteria: refractivity retrieved by GPS microwave occultation data; and calibrated, resolved, thermal radiances. In Fourier mission it is foreseen to put on board both a GPS-GLONASS receiver and a spectral radiometer (the Planetary Fourier Spectrometer —PFS-) to perform just such a test. In the paper will be discussed how to compare and validate data which will be obtained with both instruments for the future mission. Essentially the “main drawback” is that: while the PFS points to nadir direction, GNSS occultations are retrieved just along Earth grazing directions. So only the occultations that occur along the spacecraft velocity direction are selected and benchmarking for calibration purposes.