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GPS Radio Occultation Sounding Zhen Zeng (HAO&COSMIC)

GPS Radio Occultation Sounding Zhen Zeng (HAO&COSMIC). Occultation is from the Latin word “Occult” meaning “to hide”. Conditions: both a transmitter and a receiver off the planet of interest; Value: many “conventional” measurements (comprehensive, continuous, synoptic);

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GPS Radio Occultation Sounding Zhen Zeng (HAO&COSMIC)

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  1. GPS Radio Occultation SoundingZhen Zeng (HAO&COSMIC)

  2. Occultation is from the Latin word “Occult” meaning “to hide”. Conditions: both a transmitter and a receiver off the planet of interest; Value: many “conventional” measurements (comprehensive, continuous, synoptic); Cost: expansive or cheap? Establishment of GPS constellation

  3. Early temperature profile from GPS/MET April, 1995 GPS/MET

  4. CHAMP Sunsat IOX SAC-C GRACE Ørsted T. P. Yunck

  5. Principles of occultation technique Basic measurements: precise measurement of phase delay and amplitude of GPS signals.

  6. Tangent point The LEO tracks the GPS phase while the signal is occulted to determine the Doppler vGPS LEO vleo Principles of occultation technique The velocity of GPS relative to LEO must be estimated to ~0.2 mm/sec (velocity of GPS is ~3 km/sec and velocity of LEO is ~7 km/sec) to determine precise temperature profiles

  7. Atmospheric refractive index whereis the light velocity in a vacuum and is the light velocity in the atmosphere Refractivity (1) (2) (3) • Hydrostatic dry (1) and wet (2) terms dominate below 70 km • Wet term (2) becomes important in the troposphere and can • constitute up to 30% of refractivity at the surface in the tropics • In the presence of water vapor, external information information is needed to obtain temperature and water vapor • Liquid water and aerosols are generally ignored • Ionospheric term (3) dominates above 70 km

  8. COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) • 6 Satellites was launched: • 01:40 UTC 15 April 2006 • Three instruments: • GPS receiver, TIP, Tri-band beacon • Weather + Space Weather data • Global observations of: • Pressure, Temperature, Humidity • Refractivity • Ionospheric Electron Density • Ionospheric Scintillation • Demonstrate quasi-operational GPS limb sounding with global • coverage in near-real time • Climate Monitoring

  9. Launch on April 14, 2006 Vandenberg AFB, CA • All six satellites stacked and launched on a Minotaur rocket • Initial orbit altitude ~500 km; inclination ~72° • Will be maneuvered into six different orbital planes for optimal global coverage (at ~800 km altitude) • All satellites are in good health and providing initial data COSMIC launch picture provided by Orbital Sciences Corporation Chris Rocken

  10. COSMIC Ionospheric Data GPS Receiver From presentation by Stig Syndergaard, UCAR/COSMIC TEC Profiles Scintillation TBB TIP UV Radiation TEC Scintillation

  11. Absolute TEC Processing • Pseudorange local multipath • Phase cycle-slip & outliers (Blewitt, 1990) • Phase-to-pseudorange leveling • Differential code bias (DCB) calibration From presentation by Stig Syndergaard, UCAR/COSMIC

  12. Pseudorange Local Multipath Calibration From presentation by Stig Syndergaard, UCAR/COSMIC

  13. Pseudorange Local Multipath Calibration From presentation by Stig Syndergaard, UCAR/COSMIC

  14. LEO Differential Code Bias Estimation From presentation by Stig Syndergaard, UCAR/COSMIC

  15. LEO Differential Code Bias Estimation From presentation by Stig Syndergaard, UCAR/COSMIC

  16. Electron Density Profile Processing • Subtracting positive elevation angle data from negative elevation angle (Schreiner et al., 1999) • Model independent estimate of upper boundary electron density (Syndergaard et al., 2006) • Profile retrieval based on straight-line and spherical symmetry assumptions

  17. First Collocated Ionospheric Profiles From presentation by Stig Syndergaard, UCAR/COSMIC

  18. COSMIC Occultation Over South Atlantic: 1815 UTC Black: Cosmic Blue: IRI Red: IDA3D Horizontal Lines Representation Error From presentation by Gary S. Bust, Atmospheric & Space Technology Research Associates

  19. COSMIC NmF2 - 1 week From presentation by Bill Kuo, UCAR/COSMIC

  20. COSMIC #2 GAIM COSMIC #2 GAIM Comparisons During Quiet and Disturbed Conditions Quiet Storm From presentation by Ludger Scherliess, Utah State University

  21. Comparison of NmF2 and HmF2 between COSMIC and GAIM during Apr. 21-28, 2006 Good agreement of NmF2 between COSMIC and GAIM; Higher peak heights from GAIM than those from COSMIC

  22. Comparison of Ne(h) between COSMIC (red), Ionosondes (green)and TIEGCM (black) on Aug. 17 - 21nd COSMIC agree well with ionosonde obs, especially the HmF2; Vertical structures from COSMIC coincide well with TIEGCM in the mid-lat, but not in the tropics. TIEGCM shows a bit higher HmF2 compared with obs.

  23. Maps of NmF2 for: COSMIC (dots), Ionosondes (stars), TIEGCM (contour) COSMIC agree well with ionosonde observations; Global map of NmF2 revealed from COSMIC is well represented by TIEGCM model, though TIEGCM shows higher peak density in the low latitude.

  24. Formosat-3/COSMIC Observations of Scintillations RED = COSMIC sat BLUE = GPS sat From presentation by Chin S. Lin, AFRL

  25. TIP 135.6-nm passes 14 Sep 2006 FM1 FM3 FM6 0-24 UT (2100 LT) From presentation by Clayton Coker, NRL

  26. COSMIC Soundings in 1 Day COSMIC Radiosondes About 90,000 soundings / month Or ~10 soundings / 2.5 x 2.5 pixel / month All local times sampled every day! Chris Rocken

  27. COSMIC Education Module • A joint effort by COMET and COSMIC. • It covers: • Basics of GPS radio occultation science • Applications to weather, climate, and ionosphere • COSMIC Mission description http://www.meted.ucar.edu/COSMIC/

  28. COSMIC Data Access • http://www.cosmic.ucar.edu • * Select the 'Sign Up ' link under • COSMIC • Accept data use agreement • * Enter information: • Name, Address, email, user_id, • Password, planned use of data • An email will be sent within 2-3 • business days to indicate • access has been granted. More than 335 users have already registered

  29. Applications • Atmospheric science and weather forecasting • Global change and Climate • Ionospheric monitoring and space weather

  30. Using GPS temperatures to study the extratropical tropopause 1) Observational characteristics of double tropopauses Occurs frequently, related to interaction between tropics and extratropics acd.ucar.edu/~randel (see also Schmidt et al., GRL, 2006) 2) The tropopause inversion layer W. Randel

  31. Equatorial QBO -- T. Schmidt, Atmos. Chem. Phys., 2005 Diurnal tides

  32. Some promising forecast scores in the stratosphere. EG, verification against radiosonde at 100 hPa. Red = COSMIC experiment. SH NH The results in the NH are new. We had not seen improvements in NH with CHAMP. Note that the improvements are generally statistically significant at the 5% level. TR Sean Healy

  33. Validation of Microwave Lower Stratosphere Temperatue using CHAMP RO data Shu-peng Ho

  34. Study ionospheric Irregularities : Amplitude scintillations averaged in 40-80 km interval Pavelyev, GPS solution, 2005

  35. Questions for the future • Retrieval Problem • -- Ionosphere residual • -- Spherical symmetry assumption • -- Orbits • -- Multipath • -- Cycle slips, qualtiy control … • What is optimal assimilation variables? • What is optimal processing strategy for climate records?

  36. THANKS!

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