Questions and Answers on COSMIC and GPS/MET

1. Where can I obtain detailed background of the radio occultation technique and its application to meteorology, climate and ionospheric research and operations?
Melbourne, W. G. E.S Davis, C.B. Duncan, G.A. Hajj, K. R. Hardy, E.R. Kursinski, T.K. Meehan, L.E. Young, T.P. Yunck, 1994: The application of spaceborne GPS to atmospheric limb sounding and global change monitoring. JPL Publ. 94-18, 147 pp.

2. Have the results from the GPS/MET experiment been evaluated and peer reviewed?

The GPS/MET proof-of-concept instrument was launched 1 April 1995. Since then over 70,000 occultations have been processed and there are an increasing number of peer-reviewed articles appearing in the scientific literature. Appendix A lists ten peer-reviewed publications and includes their abstracts.

3. The refractivity derived from GPS/MET data in the troposphere depends on both water vapor and temperature. How is this ambiguity resolved to make the data useful in weather prediction or meteorology and climate research?

Given accurate measurements of refractivity, water vapor pressure may be calculated using an independent estimate of temperature. As shown by several studies, this calculation is relatively insensitive to small uncertainties in the temperature. For example, in the lower troposphere, water vapor pressure may be estimated to within 0.5 mb if the temperature is known within 2 K.

Alternatively, refractivity or bending angles may be assimilated directly into numerical models in which case the temperature and water vapor adjust through the model's dynamic and thermodynamic processes in a mutually consistent way toward atmospheric values.

4. Many of the GPS/MET soundings fail to penetrate into the lower troposphere, which limits their use. What is the cause of this and how will the problem be addressed in future experiments such as COSMIC?

The problem has been identified as being related to the tracking firmware in the GPS receiver and inadequate antenna gain in the proof-of-concept experiment. In one of the prime-time periods of the GPS/MET experiment, changes in the tracking firmware allowed 45% of the occultations to reach to within 1 km of the surface (with the original firmware, only about 10% reached the surface). With a new GPS receiver, a higher-gain antenna, and improved firmware, we expect 90% of the soundings in the COSMIC experiment to reach to within 1 km of the surface.

5. Many of the GPS/MET soundings appear to have a negative bias in the lower part of the sounding. What is the cause of this and how will the problem be addressed in future experiments such as COSMIC?

This problem is closely related to the depth of penetration problem discussed above, and the changes proposed for COSMIC are expected to eliminate this problem.

6. Radio occultation data do not give any information on the winds field, which is important for initializing numerical models. How will COSMIC data improve the wind field in models?

As shown by data assimilation and observing system simulation experiments, assimilation of either bending angle or refractivity will cause temperature, water vapor, pressure and winds to adjust toward atmospheric values in a mutually consistent way.

7. What data will COSMIC produce?

COSMIC will produce about 4,000 soundings of bending angle and refractivity globally in all weather each day for at least two years after launch in 2001. From these soundings, estimates of electron density in the ionosphere and temperature, water vapor and pressure in the stratosphere and troposphere will be derived. Desirable characteristics of this data include:

• High accuracy

• High vertical resolution

• All weather (clouds and aerosols do not affect measurements)

• No calibration of instrument required

• No instrument drift

• Require no first guess

• Modest cost

8. What will be the value of COSMIC data? How will the data be used?

A. For operational numerical weather prediction

The most likely use of COSMIC data will be in the assimilation of bending angles or refractivity. Early studies have shown that assimilation of accurate bending angle or refractivity data will cause all of the model's variables--temperature, water vapor, winds and pressure--to adjust toward the true atmospheric state.

B. For climate monitoring and research

The characteristics listed above under Question 7 make radio occultation an ideal tool for monitoring global climate variability and change.

C. For ionospheric research and "space weather"

The COSMIC suite of instruments (radio receiver, Tiny Ionosphere Photometer, and Beacon) will permit three-dimensional tomography of the ionosphere with unprecedented resolution and accuracy. Assimilation of these data will be useful in numerical prediction of "space weather."

9. How will radio occultation data, such as provided by COSMIC and other future systems, be used with other satellite systems?

COSMIC data will be highly complementary to other satellite sounding systems, including radiometric sounders on the Polar-orbiting Operational Environmental (POES) and Geostationary Operational Environmental Satellites. The independence and the high-vertical resolution of the radio occultation soundings complement the high horizontal resolution of the radiometric soundings and together the two systems can likely be combined to yield composite soundings of temperature and water vapor with unprecedented accuracy, horizontal and vertical resolution, and global coverage.

10. What is the policy on COSMIC data?

COSMIC data will be made available free and openly, or at the marginal cost of distribution, to the national and international science and operational community.



Appendix A

GPS/MET Publications

[1] Eyre, J.R., 1994: Assimilation of radio occultation measurements into a numerical weather prediction system. ECMWF Tech. Memo. No. 199, 34 pp.

[2] Gorbunov, M.E. and A.S. Gurvich, 1998: Microlab-1 experiment: multipath effects in the lower troposphere. J. Geophys. Res. , 103, (in press).

[3] Kuo, Y.-H., X. Zou, and W. Huang, 1997: The impact of GPS data on the prediction of an extratropical cyclone: An observing system simulation experiment. J. Dyn. Atmos. Ocean, 27, p. 439-470.

[4] Kuo, Y.-H., S.-J. Chen, Y.-R. Guo, W. Huang, X. Zou, R. Anthes, D. Hunt, M. Exner and R. Reed, 1998a: A GPS/MET sounding through an intense upper-level front. Bull. Amer. Met. Soc., 78 (in press)

[5] Kursinski, E.R., G.A. Hajj, W.I. Bertiger, S.S. Leroy, T.K. Meehan, L.J. Romans, J.T. Schofield, D.J. McCleese, W.G. Melbourne, C.L. Thornton, Y. P. Yunck, J. R. Eyre and R. N. Nagatani, 1996: Initial Results of Radio Occultation Observations of Earth's Atmosphere Using the Global Positioning System. Science, 271, pp. 1107-1110.

[6] Kursinski, E.R., G.A. Hajj, J.T. Schofield, R.P. Linfield and K.R. Hardy, 1997: Observing Earth's atmosphere with radio occultation measurements using the Global Positioning System. J. Geophys. Res. , 102, No. D19, p. 23,429-23,465, Oct. 20, 1997.

[7] Leroy, S.S., 1997: The Measurement of Geopotential Heights by GPS Radio Occultation. J. Geophys. Res. , 102, No. D6, 6971-6986, March 27, 1997.

[8] Rocken, C., R. Anthes, M. Exner, D. Hunt, S. Sokolovskiy, R. Ware, M. Gorbunov, W. Schreiner, D. Feng, B. Herman, Y.-H. Kuo, X. Zou, 1998: Analysis and validation of GPS/MET data in the neutral atmosphere. J. Geophys. Res.-Atmospheres, 102, No. D25, 29,849-29,866, Dec. 27, 1997.

[9] Ware, R., M. Exner, D. Feng, M. Gorbunov, K. Hardy, B. Herman, Y. Kuo, T. Meehan, W. Melbourne, C. Rocken, W. Schreiner, S. Sokolovskiy, F. Solheim, X. Zou, R. Anthes, S. Businger and K. Trenberth, 1996: GPS Sounding of the Atmosphere from Low Earth Orbit: Preliminary Results. Bull. Amer. Meteor. Soc., 77, 19-40.

[10] Zou, X., Y.-H. Kuo and Y.-R. Guo, 1995. Assimilation of Atmospheric Radio Refractivity Using a Nonhydrostatic Adjoint Model. Mon. Wea. Rev., 123, 2229-2249.

last updated July, 1999