Why are so many instruments needed to measure temperature and humidity?
Temperature and humidity are two important atmospheric parameters and are not easy to measure. The instruments compliment each other and will be used in synergy. Infrared Soundings, for example, will not be able to penetrate through clouds. By combining these data with those of microwave instruments, which are nearly not affected by clouds, temperature and humidity soundings can be provided also in partly cloudy and cloudy areas. One can, by using the microwave data, correct for the cloud contamination of a sounding pixel. Some parametres cannot be derived by IR instruments (for example cloud liquid water content and total liquid water content), but can be with microwave instruments. GRAS makes use of the GPS (Global Positioning System) constellation of 24 satellites distributed in six orbital planes around the globe. The GRAS instrument in near polar orbit at 824 km will observe over 500 occultations per day distributed quite uniformly over the globe. Assimilation of GRAS data into NW
