Meteorological Applications of GNSS from Space and on the Ground
Authors from France’s national meteorological agency and the French space agency describe the techniques and applications that use the behavior of GNSS signal propagation through the Earth’s atmosphere to predict the weather and monitor climate change.
The year 2007 marked the 50th anniversary of the first man-made artificial satellite carrying a radio transmitter. Launched by the former Soviet Union in 1957, the Sputnik satellite preceded the first constellation of GPS satellites by more than three decades.
Beginning with launch of its first satellite in 1978 through declaration of full operational capability in 1995, the U.S. Global Positioning System has broadcast accurate and stable radio signals. These GPS signals have now become a source of information for exploratory and routine monitoring of the Earth’s atmosphere, using data collected by GPS receivers located on the ground or in space.
Experiments exploiting these ground- or space-based GPS techniques were first reported as far back as 1992 and 1995, respectively. (For further discussion of these pioneering efforts, see the articles by M. Bevis et alia  and E. R. Kursinki et alia  in the Additional Resources section at the end of this article.) Similar measurements are now routinely available to national meteorological services worldwide.
The measurements collected by these two techniques can be useful to meteorologists provided that ancillary (external) information is available with which to retrieve such data as integrated water vapor (for ground-based GPS measurements) or profiles of temperature and humidity (for space-borne GPS measurements).
However, the addition of such information convolves the error patterns in the meteorological retrievals, making them more difficult to use in an error-reduction scheme such as those commonly employed in numerical weather prediction (NWP) and assimilation systems. For that reason, NWP assimilation makes direct use of data closer to the raw measurements, for which the error patterns are better characterized.
This article presents the methodology for assimilating the GPS-based measurements into the NWP system of the French national weather service (Météo-France). We describe the error reductions obtained by this method in estimating atmospheric conditions and making subsequent weather forecasts.
The article further considers the application of these observations for climate monitoring. Finally, we discuss the perspectives on these questions that will be allowed by the future Galileo system (as, of course, the perspectives allowed by Compass, GLONASS, IRNSS, etc., could have been similarly discussed).
Ground-Based GPS Measurements
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Spaceborne GPS Measurements: Radio Occultation
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NWP Applications and Methodology
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Ground-Based GPS. The observation operator for simulating ground-based GPS ZTD observations involves the integration of equations (1) and (2) using the model temperature, humidity, and pressure above the vertical of each GPS station location.
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Space-Based GPS: Radio Occultation. The observation operator for simulating GPS radio occultation bending angles derives from work conducted at the European Centre for Medium-range Weather Forecasts. (Additional Resources: article by S. B. Healy and J. N. Thépaut). It involves the integration of equations (3) and (2) using the model fields, without applying a bias correction.
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Effect of GPS-Derived Data on NWPs
Meanwhile, data collected by the GPS radio occultation technique have been available in near real-time to national meteorological services since February 2007. Several impact studies of these data have been conducted at Météo-France.
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Applications for Climate
With GPS-based measurements, the raw observable is a number integer (and fraction) of the GPS wavelength, whose stability is guaranteed by the atomic clocks onboard the GPS satellites, which are themselves calibrated daily with atomic clocks on the ground. Hence, GPS-based measurements of atmospheric delay are calibrated indirectly with atomic clocks.
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New Receivers and Signals
The receiver is also compatible with ground-based C/A-code pseudolites having an RNSS uplink frequency (Lp signal = 1340 MHz).
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Prospects for Galileo
First, for the second frequency required for ionospheric correction Galileo will present a higher signal-to-noise ratio than the current GPS L2. This is not so critical for the ground-based GPS measurements (except for a real-time meteorological application) as it is for the GPS radio occultation.
Indeed, that latter technique currently relies on ad hoc receivers (semi-codeless) or complex antenna schemes in order to be able to collect L2 measurements, while extrapolation is needed for tracking in the lower atmosphere. With Galileo the measurement process should be simplified in that respect.
Second, the ability for a future dual-constellation receiver to track both GPS and Galileo satellites will mean that the number of links (raw measurements) is multiplied by about two, thus yielding potentially twice as many independent observations.
Third, the future climate records from ground-based GPS or GPS radio occultation measurements on the GPS system itself would not rely on a single GNSS system. Galileo will make it possible to compare the potential drifts observed in measurements collected from either system.
Fourth, the surge in commercial applications from Galileo may contribute to making GPS and Galileo receivers’ basic components more affordable and supporting improved baseline designs for scientific receivers.
Fifth, the generalized deployment of several ground (or spaceborne) networks (or constellations) of GPS and Galileo receivers could enable meteorologists to collect a wealth of measurements of opportunity. This would be possible if GPS receivers were able to conduct their primary functions of positioning (orbit determination) and timing while at the same time collecting scientific measurements with a common baseline receiver design.
The amount of data collected (especially for spaceborne receivers) could be tremendous and achieved at a very little additional cost.
The more promising frequencies to be used in the future for multiple constellation radio occultation receivers will be the ones commonly used by several systems. For instance, the case of two worldwide GNSS constellations with two confirmed common central frequencies is today unique, with GPS and Galileo and E1/L1 + E5a/L5 frequencies.
Of course, for high-end, heavy radio occultation receivers, “all frequencies in view” or “all constellations in view” (GPS/Galileo/Compass/GLONASS/IRNSS/QZSS, for instance) are better from a radio science perspective.
In particular, simultaneous use of the four Galileo frequency bands — E5a, E5b, E6, and E1 — offers the possibility to directly solve the high-order terms of the ionospheric delays, to improve carrier phase ambiguity resolution and related robustness, and to obtain more accurate measurements.
The use of Compass and Galileo E5b signals already being transmitted on a common frequency also looks promising, like the use of common E6 central frequency by QZSS and Galileo and probably at least another GNSS system.
Conclusions and Perspectives
Measurements of atmospheric refraction can also be collected via the GPS radio occultation technique, which involves GPS receivers in space. Recent examples include the six-satellite constellation FORMOSAT-3/COSMIC and the GRAS (GNSS Receiver for Atmospheric Sounding) instrument on-board the MetOp satellite.
Since September 2006 and 2007, the national weather service of France, Météo-France, has been using data sets from European ground-based GPS networks and from satellites equipped with GPS radio occultation receivers for updating its operational weather prediction analyses.
This decision was made after it was shown that these respective observations helped improve quantitative precipitation forecasts, and predictions of temperature and wind. More progress is expected as the number of such observations may increase; the ground network of GPS receivers is due to expand worldwide, and more satellites may carry GPS receivers of radio occultation grade.
Overall, the use of the GPS signals for meteorological observations is a good example of a measurement of opportunity in which large number of transmitters and all-weather availability have enabled meteorologists to gather ever more measurements of our atmosphere in near real-time.
The potential of these observations for climate studies is significant, but further detailed sensitivity studies are required in order to evaluate the effects of possible sources of interruptions in measurements as well as trends in the stability of time-series measurements being collected.
The prospects offered by the Galileo system to meteorologists using GPS-based measurement may include improving ionospheric characterization (and correction), increasing the number of collected measurements, avoiding the reliance of such measurements on one system only (GPS), and (perhaps most importantly) collecting a large number of measurements of opportunity at very little cost.
For the complete story, including figures, graphs, and images, please download the PDF of the article, above.
ManufacturersThe dual-frequency GPS receiver to be used on the PROBA-2 satellite is a modified version of the TOPSTAR 3000 from Thales Alenia Space, France, with involvement of Sideral, Switzerland, and Nordspace, Norway.
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