Estimates of the precision of GPS radio occultations from the COSMIC/FORMOSAT‐3 mission

The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC)/Formosa Satellite 3 (FORMOSAT‐3) is a six‐satellite radio occultation mission that was launched in mid‐April, 2006. The close proximity of the COSMIC satellites provides a unique opportunity to estimate the precision of the radio occultation remote sensing technique from closely collocated occultations (<10 km separation of tangent points). The RMS difference of refractivity between 10 and 20 km altitude is less than 0.2%, which is approximately twice better than previous estimates obtained from CHAMP and SAC‐C collocated occultations, apparently, due to smaller separation of the occultation pairs and due to parallel occultation planes. In the lower troposphere, the maximal RMS is ∼0.8% at 2 km altitude and decreases abruptly to ∼0.2% between 6 and 8 km altitude. The RMS difference of electron density in the ionosphere between 150 and 500 km altitude for collocated occultations is about 10 3 cm −3 .