Radio occultation principle
For open air business such as agriculture, traffic, fishery or aviation accurate forecasts and reliable warnings of severe weather conditions on a daily, hourly or even shorter time base are extremely important. Today's Numerical Weather Prediction (NWP) models have reached a state where major improvements of meteorological products will depend on more fast available, precise, dense and even distributed key parameters such as temperature or humidity. A satellite sensor with all-weather capability such as the GPS occultation system fulfills these requirements. The Atmospheric and Ionospheric Profiling (AIP) technique will excellently contribute to derive atmospheric key data sets to improve the understanding and modeling of the short-term weather as well as the long-term climate and its variability. Additionally ground based and occultation GPS signals can be used to describe the vertical and horizontal state of the ionosphere. As during the CHAMP mission atmospheric profiling will be done by GFZ against which the ionospheric research will take place at DLR/Neustrelitz. Additional AIP research will be performed at JPL's GPS Environmental & Earth Science Information System (GENESIS).
The Radio Occultation Technique uses the excess doppler due to the refraction which is measured by a LEO-satellite observing a GPS satellite rising or setting through the neutral atmosphere. The duration of a typical occulation event is about 60 seconds, the observed bending angle is smaller than 1° and the cumulative excess signal delay can reach several hundred meters. Each of the 2 GRACE satellites will observe up to 200 profiles per day.
A GPS microwave signal traveling from the transmitting GPS satellite to a ground receiver (satellite-to-ground tracking mode, SGT) or to a receiver on board of a LEO (satellite-to-satellite tracking mode, SST) is influenced by the neutral atmosphere in phase and amplitude.
The primary level-1 GPS occultation product are the bending angles, which are derived from the level-0 1 Hz and 50 Hz GPS phase observables. In conjunction with the equation of state and hydrostatic equilibrium level-2 profiles of air mass, density, pressure, temperature and water vapor can be derived from the observed refractivity profiles. Additionally - depending on the tracking mode - horizontal (SST) and vertical (SGT) profiles of the total electron content (TEC) of the ionosphere can be calculated. These level-2 data can be used for higher level products such as 3D tomography of the ionosphere or updates and improvements of local and global meteorological models. The general AIP processing scheme to derive temperature and humidity profiles is shown in the following figure:
The AIP atmospheric temperature and water vapor products have to be generated within a very short time to fulfill the strong time requirements for weather forecast applications which is less than 3 hours. This can only be met on condition that
Theoretical estimations indicate that GPS radio occultation measurements are in principle capable to derive the temperature of the topside troposphere and bottomside stratosphere - depending on the above orbit accuaries - with an error of less than 1° Kelvin. There are, however, many uncertainties in present day retrieval methods, so that significantly larger errors can occur under non-optimal circumstances. Thus, the further improvement of retrieval algorithms accomplished by careful product validation is the main task to make the GRACE AIP technique successful.
As an example for the AIP capability the following figures show a comparison
of vertical temperature profiles derived from GPS/MET data analysed by
UCAR and GFZ. The independent GFZ profiles have been derived using GFZ
orbits and software. For comparison the results of an NCEP meteorological
analysis (interpolated in time and space) is added.
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