Abstract

Satellites in low Earth orbit with high inclination provide global coverage measurements. The Formosat-3/COSMIC satellite configuration with six single platforms, each separated in ascending node by 30° in final constellation, also allows for evenly distributed local time measurements. Especially... Show moreSatellites in low Earth orbit with high inclination provide global coverage measurements. The Formosat-3/COSMIC satellite configuration with six single platforms, each separated in ascending node by 30° in final constellation, also allows for evenly distributed local time measurements. Especially at low latitudes, where the ascending and descending branch of the orbits are separated by 12 hours, local time sampling is excellent. The inclination i and the semi-major axis a of the Formosat-3/COSMIC satellite constellation yield a secular drift in their orbit planes relative to the Sun of about 3°/d,hich corresponds to approximately 6 hours within one month. Hence, the diurnal cycle at low latitudes is sampled by Formosat-3/COSMIC within 10 days. This homogeneous local time sampling allows the reconstruction of the diurnal cycle of atmospheric parameters (e.g., temperature), and it enables the calculation of monthly climatologies with a negligible (< 0.01 K) local time component of the sampling error. At higher latitudes the ascending and descending branch of the orbit move closer together in terms of local time sampling. Thus, local time sampling at high latitudes (especially poleward of 70°) is insufficient and irregular within one month, limiting the reconstruction of the diurnal cycle of atmospheric parameters. The majority of measurements taken at high northern latitudes are temporally displaced by around 12 hours as compared to the majority of measurements taken at high southern latitudes. This leads to a small local time component of the sampling error in monthly climatologies, which are of opposite sign in the northern and the southern hemisphere. We compare local time sampling of Formosat-3/COSMIC in December 2006, July 2007, and--by means of simulation studies--in final constellation. We show preliminary results of the temperature diurnal cycle in selected months in different latitudinal domains and discuss the impact of irregular local time sampling on temperature climatologies. Show less