Meteorological Applications of GPS Signals with Ground- and Space-based Receivers
Poli, P.¹; Pailleux, J.¹; Ducrocq, V.¹; Moll, P.¹; Rabier, F.¹; Mauprivez, M.¹; Dufour, S.¹; Grondin, M.²; Issler, J.-L.²; de Latour, A.^2
1Meteo France; ²CNES

The year 2007 marks the fiftieth anniversary of the first man-made artificial satellite carrying a radio transmitter. The Sputnik satellite preceded the first constellation of Global Positioning System satellites which have sent accurate and stable radio signals since 1994. These radio signals have now become a source of information for exploratory and routine monitoring of the Earth's atmosphere.

The propagation of GPS signals within the atmosphere is subject to refraction by the three-dimensional structure of refractive index gradients. These gradients relate to electron content density in the ionosphere and to atmospheric features in the neutral atmosphere. Such weather features include fronts or rainfall events in the troposphere (the lower part of the atmosphere). Using ad hoc receivers located on the ground or in space, it has now become possible to measure the propagation delay caused by the neutral atmosphere.

For a GPS receiver on the ground, the viewing geometry imposes that the first atmospheric observable that can be collected is a measure of the zenith total delay (ZTD) at the vertical of the GPS station. That quantity conveys the superposition of two pieces of information. The first signal is the atmospheric total density above the station (i.e. the total weight of the atmosphere over a unit surface area, which relates to the surface pressure of the air at the surface). The second signal is the total amount of precipitable water above the station. The latter information is particularly valuable because the water vapour cycle is currently only partially observed and the space- and time- distributions of atmospheric water vapour are still subject to great uncertainties in weather models. Several networks of GPS stations have been deployed mostly over Northern hemisphere continental surfaces, usually for precise positioning or geodetic applications -- but rarely for meteorological purposes. The European network of meteorological services (EUMETNET) GPS water vapour programme (E-GVAP) was setup to collect and bring such measurements of GPS ZTD to operational meteorological services. Thanks to E-GVAP, more than 400 stations over Europe (some of which belong to geodetic institutes, universities, and non-meteorological entities) now send their raw data to analysis centers where ZTD retrievals are calculated and sent to meteorological services within less than 3 hours.

Using the so-called radio occultation technique, GPS receivers can also be used to probe the atmosphere from space-based platforms. The measurement process involves observing a source (one of the GPS transmitters) as it sets (or rises) on the horizon, behind the Earth's atmospheric limb. The imprint of the atmosphere on the measurement data is also a delay in the propagation, as compared to the delay that one could foresee if there was no atmosphere. These phase delays are in fact more readily apparent in the form of Doppler shifts, because the occultation links cross layers of varying densities; the propagation delays change rapidly as the linking rays between the occulted GPS transmitter and the GPS receiver scan increasingly lower (or higher) regions of the atmosphere. To date, a total of nine GPS radio occultation experiments involving fifteen receiving platforms have been conducted since 1995. Out of these, the data from eight satellites are currently collected, processed, and sent to operational meteorological services within less than 3 hours. The resulting observations extend from the near-surface up to the stratosphere, a region that was up to now routinely observed only by instruments featuring a poor vertical resolution.

We will present how the imprints of the atmosphere on the GPS measurements from both techniques are fed into numerical weather prediction systems at meteorological institutes. Doing so we will present the impact these data have on numerical forecasts at Météo-France. We will also show recent results in the field of GPS radio occultation and discuss outlooks for further improvements with the advent of GALILEO.