GRAS – Metop’s GPS-Based Atmospheric Sounder

Introduction Occultation methods based on effects connected with the refraction of electromagnetic waves in both the optical and radiofrequency domains have been used by astronomers for numerous investigations of planetary atmospheres over the last decades. The first experiment started with the Mars flyby by Mariner-IV in 1964. Since this first attempt, radio-occultation experiments have been carried by the majority of planetary missions. Measurement principle The GPS constellation consists of 24 satellites distributed in six orbital planes around the globe. Each satellite orbit is circular with an inclination of 55 deg, a period of 12 h and an altitude of 20 200 km. A schematic of atmospheric profiling by GPS radio occultation is shown in Figure 1. For the receiver, an occultation occurs whenever a GPS satellite rises or sets and the ray path from its transmitter traverses the Earth’s atmospheric limb. With 24 GPS satellites, a single GPS receiver in a near polar orbit at 824 km will observe over 500 occultations per day, distributed fairly uniformly about the globe (Fig. 2).