Validation of GPS Radio Occultation Measurements and Retrieval Methods

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Validation of GPS Radio Occultation Measurements
and Retrieval Methods

• Grace S Peng, John E. Wessel, Robert W. Farley
• The Aerospace Corporation

This work was supported under The Aerospace
Corporation's Independent Research and
Development Program This work was supported under
The Aerospace Corporation's Mission Oriented
Investigation and Experimentation program, funded
by the U.S. Air Force Space and Missile Systems
Center under Contract No. FA8802-04-C-0001.
Space Science Applications Laboratory Electronics
Photonics Laboratory
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Aerospace Transportable Lidar System (ATLS)
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ATLS

• 355 nm laser, frequency tripled to UV
• Rayleigh scattering off O2 24-75 km above surface
• density as a function of distance
• Raman scattering from surface upwards
• Ratio of N2 to H2O
• lt 1K accuracy from 24-45 km
• Scattering from atmospheric aerosols near surface
  prevent accurate T meas.
• Works only in from dusk to dawn, no thick clouds
• Currently sitting on the beach at Barking Sands,
  Kauai (the wet, windy side)
• http//www.aero.org/publications/crosslink/summer2
  000/02.html

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Dry Retrieval for Temperature

• Both techniques measure density
• Density converted to temperature using
  hydrostatic integral
• Lidar measures a cone about 30 meters in diameter
  at 30 km above instrument
• Time delay translated into 37.5 meter thick
  slices of cone
• GPS/RO measures tube 1 km in diameter and 300 km
  long
• Lidars optical wavelength affected by scattering
  off aerosols
• Clouds and volcanic debris can cause cold bias
• GPS/RO not affected by aerosols

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GOES 10 IRJan 17, 2004 00UT
Champ 418 UT (22.3,-156.6) Lidar 540
UT (22.05,-159.8) 428 km Angle 7.79
Imagery courtesy of NOAA
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NCAR GFZ -- JPL Lidar Raob Hilo Lihue
Lihue and Hilo raobs courtesy of NOAA
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GOES 10 IR Jan 18, 2004 00UT
Champ 455 UT (24.5,-158.7) Lidar 600
UT (22.05,-159.8) 290 km Angle 29.9
Imagery courtesy of NOAA
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NCAR GFZ Lidar Raob Hilo Lihue
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GOES 10 IR Jan 19, 2004 00UT
Champ 359 UT (22.2,-155.4) Lidar 600
UT (22.05,-159.8) 448 km Angle 2.7
Imagery courtesy of NOAA
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NCAR Lidar Raob Hilo Lihue
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GOES 10 IR Jan 20, 2004 00UT
Champ 436 UT (23.4,-158.5) Lidar 600
UT (22.05,-159.8) 202 km Angle 24.05
Imagery courtesy of NOAA
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NCAR GFZ -- JPL Lidar Raob Hilo Lihue
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Tsuda et al JGR 2000
Raob-GPS 20-30 km x Lidar-GPS 24-34 km
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Water Vapor Retrieval

• Above 220 K altitude, use water vapor from
  background
• Below 220 K, assume BG temperature, calculate
  water vapor from refractivity
• Can cause discontinuity in water vapor
• N 77.6P/T 3.730e5Pw/T2

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NCAR -- JPL Lidar Raob 1DVAR ECMWF
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NCAR Lidar Raob 1DVAR ECMWF
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NCAR Lidar Raob
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NCAR -- JPL Lidar Raob 1DVAR ECMWF
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Concluding Remarks

• GPSRO, lidar and radiosonde temperatures in
  agreement below 35 km
• Gravity waves in GPS/RO profiles validated
• GPSRO cold bias above 35 km
• Lower signal to noise of GPS signal
• Smoothing of bending angle using climatology
• Influence from hydrostatic seed temperature
• Test retrieval algorithm with lidar temperature

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Concluding Remarks 2

• Water vapor generally in agreement
• Differences arise mainly from representativeness
  of measurement type and offset in time and space
• classic retrieval can sometimes lead to
  unphysical solutions (negative water content)