Metop (Meteorological Operational Satellite Program of Europe) is a series of three polar orbiting meteorological satellites which form the space segment component of the overall EUMETSAT Polar System (EPS). EUMETSAT is an intergovernmental organisation founded in 1986 with the purpose of supplying weather and climate-related satellite data, images and products – 24 hours a day, 365 days a year – to the National Meteorological Services of our Member and Cooperating States in Europe, and other users worldwide. The EPS programme consists of a series of three polar orbiting Metop satellites, that have been flown successively for more than 14 years, from 2006, together with the relevant ground facilities.
Metop-A (launched on 19 October 2006), Metop-B (launched on 17 September 2012) and Metop-C (launched on 7 November 2018) are in lower polar orbits, at altitudes of about 820 kilometres, to provide detailed observations of the global atmosphere, oceans and continents. The the satellites are operating in parallel for as long as their consumables allow since their combined capabilities bring benefits to users.
Metop carries a set of "heritage" instruments provided by the United States and a new generation of European instruments that offer improved remote sensing capabilities to both meteorologists and climatologists. The data generated by the instruments carried by Metop can be assimilated directly into Numerical Weather Prediction (NWP) models to compute forecasts ranging from a few hours up to 10 days ahead.
Among these instruments, the GRAS is a receiver for the reception of signals from the GPS (Global Positioning System) which is part of the Global Navigation Satellite Systems (GNSS). Unlike all other instruments on Metop, GRAS is not viewing the Earth by vertical scanning but instead looks tangentially through the Earths atmosphere. The instrument measures the time delay of the refracted GNSS radio signals as the ray (signal path) skirts the Earths atmosphere on its way from the transmitting GNSS satellite to Metop.
By precisely computing position and velocity of Metop and the GNSS satellite the measured time delay is converted to the bending angle of the ray path, which again is converted to values of temperature, pressure and water vapour content in the atmosphere.
The IEEC, though the Institute for Space Sciences (ICE, CSIC), is part of the EUMETSAT Radio Occultation Meteorology Satellite Application Facility (ROM-SAF). The ROM-SAF is a decentralized facility under EUMETSAT, that runs an operational radio occultation system responsible for delivering bending angle, refractivity, temperature, pressure, and humidity profiles in near-real time and offline for NWP and climate users.