HR: 10:35h
AN: SF52A-02    [Abstracts]
TI: Recent results of open-loop tracking of GPS occultations in the lower troposphere and implications on measuring specific humidity
AU: * Hajj, G A
EM: george.hajj@jpl.nasa.gov
AF: Jet Propulsion Laboratory, M/S 238-600 4800 Oak Grove Dr, Pasadena, CA 91109 United States
AU: Meehan, T K
EM: thomas.k.meehan@jpl.nasa.gov
AF: Jet Propulsion Laboratory, M/S 238-600 4800 Oak Grove Dr, Pasadena, CA 91109 United States
AU: Ao, C O
EM: chi.o.ao@jpl.nasa.gov
AF: Jet Propulsion Laboratory, M/S 238-600 4800 Oak Grove Dr, Pasadena, CA 91109 United States
AU: Dong, D
EM: Danan.Dong@jpl.nasa.gov
AF: Jet Propulsion Laboratory, M/S 238-600 4800 Oak Grove Dr, Pasadena, CA 91109 United States
AU: Iijima, B A
EM: Byron.Iijima@jpl.nasa.gov
AF: Jet Propulsion Laboratory, M/S 238-600 4800 Oak Grove Dr, Pasadena, CA 91109 United States
AB: GPS occultation measurements from CHAMP and SAC/C have demonstrated the ability to measure atmospheric temperature with accuracy better than 1K and a precision of 0.1K in the upper atmosphere and lower stratosphere. Most challenging has been the accurate detection of the occulted GPS signal in the lower troposphere due to atmospheric defocusing causing signal attenuation, atmospheric multipath causing strong signal dynamics, and atmospheric ducting causing temporary signal disappearance. Theoretically, accurate determination of the occulted GPS phase delay and its amplitude in the lower troposphere can lead to a determination of the atmospheric refractivity from which specific humidity can be inferred to better than 0.5 g/kg with a vertical resolution approaching 100m. Because of the signal temporary disappearance and its dynamics, traditional closed-loop or semi-closed-loop (also known as flywheeling) techniques for measuring the atmospheric refractivity in the lowest 5 km proved problematic, where systematic biases under-estimating the atmospheric refractivity and specific humidity are created. "Open-loop" tracking in this region, where the atmospheric Doppler shift is modeled and used inside the receiver, has been suggested as a means of resolving these problems. Recently, open-loop tracking has been implemented and tested on the SAC/C GPS occultation receiver. This talk will present an overview of the challenges and difficulties encountered with traditional techniques of detecting lower tropospheric water vapor from GPS occultations and evaluate improvements with the recent "open-loop" data from SAC/C.
DE: 3360 Remote sensing
DE: 1894 Instruments and techniques
DE: 0365 Troposphere--composition and chemistry
DE: 0394 Instruments and techniques
SC: Special Focus: Advances in Data Acquisition, Management, Analysis and Display [SF]
MN: 2004 AGU Fall Meeting