GPS Reviews & Technology

COSMIC – Using GPS for Weather Forecasting

In many scientific endeavors the full range of beneficial outcomes are not, and often cannot, be foreseen. It is an element to consider in the current debate over the value of moon bases or manned missions to Mars. It was one such unexpected beneficial outcome which allowed the
COSMIC Satellite Constellation
August 2006 formation of Hurricane Ernesto in the eastern Caribbean to be predicted two days in advance when contemporary weather models failed to predict its formation at all.

When the United States launched the Global Positioning System (GPS) satellites it believed it was launching infrastructure for accurate terrestrial position determination. It is now apparent that it was also launching the part of the infrastructure for a weather observation system capable of vastly improving the accuracy of weather prediction models.

Using the Radio Occultation Method to Determine Atmospheric Conditions
GPS receivers use measurement of the travel time of radio signals from a number of GPS satellites to determine distance from their known positions and triangulate their position in latitude, longitude and elevation. A curse on this position determination method is the disturbance of GPS satellite radio signals as they pass through the atmosphere making corrections for atmospheric disturbances necessary for accurate GPS location fixes. However, In an immensely beneficial twist, measurement of annoying atmospheric disturbance of GPS satellite radio signals by specialized GPS receivers in orbital satellites can be utilized by scientists to determine the underlying atmospheric conditions causing the disturbances.

Since the early 1960’s, when Mariners 3 and 4 passed behind Mars, as seen from Earth, the radio occultation (limb sounding) technique has been used to discover the atmospheric properties of numerous planets and moons in the solar system. For such a technique to be used on the Earth’s atmosphere, two satellites, sending and receiving, are required.

Building on earlier satellite experiments, space based infrastructure, consisting of a set of six micro-satellites (70kg each), was launched in April 2006 for applying the occultation method to collect accurate weather observation data. The micro-satellite constellation is called the Constellation of Observing System for Meteorology, Ionosphere and Climate (COSMIC).

As the GPS satellites rise or set on the Earth’s horizon in relation to the COSMIC micro-satellites, their specialized GPS receivers measure with great precision the bending and slowing effects (occultations) which have occurred as GPS satellite signals have traveled through the atmosphere. These very accurate measurements allow atmospheric conditions such as air density, winds, temperature, moisture, refractivity, atmospheric pressure and electron density to be computationally inferred.

More Weather Data and Wider Coverage
The accuracy of any weather model is dependent on the accuracy and size of the weather data set used for prediction and COSMIC should provide improvements to both. COSMIC produces data sets for over 1,000 points around the globe each day and in 2007 will produce some 2,500 vertical profile data sets. In comparison, currently weather data is currently sourced from satellite based radiometers and instrument equipped weather balloons launched in some 900 locations around the Earth twice daily. Being ground launched, few balloon launches are over oceans or from hard-to-reach locations such as Antarctica and the remote Pacific. Improved data from COSMIC should improve forecasts in such areas and others by providing a more complete model data set.

Improved Weather Predictions
In addition to the accurate prediction of Hurricane Ernesto using a main US forecast model, COSMIC data had demonstrated significantly improved stratospheric temperature forecasts over the Northern Hemisphere for the European Center for Medium-Range Weather Forecasts (ECMWF) and improved tropical storm tracking predictions in Taiwan.
It has also been suggested that COSMIC data may sufficiently accurate, precise, and stable to be the best data source for measuring global and regional climate change.

Future Possibilities
While the improvement of weather model predictions from radio occultation data cannot be quantified at this stage, it is apparent that weather agencies are keen to apply what data is available at this point. It is expected that from January 2007, Cosmic data will be used by the U.S. National Oceanic and Atmospheric Administration in operational forecasts. Canada is also expected to follow. Grants for the advancement of GPS remote sensing science through Cosmic data have been announced by NASA.

JPL, who developed the COSMIC satellite reveivers, is working on a third-generation GPS satellite receiver which will take advantage of the greater capability next generation of GPS satellites and the greater number of them with the European Galileo system coming on line and rejuvenation of the Russian GLONASS GPS satellite constellation. Naturally the more observational data taken, the more accurate and wide ranging weather prediction models can be.

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