Potentials for radio occultation applications during inter-satellite calibration periods

Abstract EUMETSAT has launched the first in a series of three Metop satellites in October 2006. Each satellite has a nominal 5 year life time, covering 14 years in total. Successive satellites will be launched with about 0.5 year overlap into the same sun-synchronous polar orbit, allowing inter-satellite calibration. Focusing on the GRAS RO (radio occultation) instrument, we analyze two possible applications of this inter-satellite calibration period to assess: (1) the inherent RO precision by matching observations across the two satellites; (2) possible RO contributions to calibrate Earth-viewing sounders on-board the platforms by matching occultations with these sounders. Both applications are relevant for climate monitoring. An inherent precision assessment depends on the applied collocation criteria. We investigate the impact on bending angle, refractivity, temperature standard deviations by collocating GRAS/COSMIC and COSMIC/COSMIC. Even with close collocations, bending angle standard deviations are never found below 1%; in refractivity processing they are reduced by a factor of more than two. In temperature, they are never below 1 K. A 1DVar based impact assessment of these criteria on the temperature error shows an additional error of about 0.3 K when moving from matches within 100–300 km. Using ECMWF data, the atmospheric variability over the investigated collocations is assessed. With simulations we next analyze RO matches between 2 Metops with separation times of in-between 25 min and 50 min. Fairly relaxed spatial collocation criteria are required with these separation times to find strict matches, where the same GPS satellite is observed in the same viewing geometry. Such spatial collocations are unsuited to assess application (1); separation times of For application (2), we find collocated sounder pixels for about 90% of the occultations for collocations within ⩽100 km, ⩽ 3 h, independent of whether the observations are made on the same or different platforms. Although this is reduced for close to nadir azimuth angles.