HR: 0800h
AN: G31A-0783    [Abstracts]
TI: Power Spectral Analysis of Simultaneous VLBI and GPS Tropospheric Estimates
AU: * Ray, J
EM: jimr@ngs.noaa.gov
AF: National Geodetic Survey, NOAA 1315 East-West Hwy, Silver Spring, MD 20910 United States
AU: Boehm, J
EM: johannes.boehm@tuwien.ac.at
AF: Institute of Geodesy and Geophysics, TU Vienna Gusshausstrasse 27-29, Vienna, A-1040 Austria
AB: Observations by space geodetic techniques experience refraction and signal delay due to passage through the Earth's atmosphere. For high-accuracy positioning results, data analysts must account for these effects. Since independent path delay values of sufficient accuracy are not usually available, nuisance parameters are commonly added in the geodetic analysis. The general validity of such zenith path delay (ZPD) estimates as true atmospheric measures has been confirmed by comparison of results from independent radiometric and other techniques over many years. Biases and standard deviations in the sub-cm range are normally found, which is expected to be adequate as inputs to improve the forecast performance of numerical weather models. To better understand the noise characteristics of ZPD estimates from VLBI and GPS, we have examined the power spectra of simultaneous observations during a 15-day period in October 2002. The official combined ZPD products from the technique services have been used primarily, but series from individual analysis centers have also been included. For the seven sites studied, the power-law spectral indices over sub-daily intervals are close to -8/3, consistent with fully developed Kolmogorov turbulence, and flatten over longer periods. The VLBI series, sampled hourly, show white noise at levels of 0.7 to 1.5 mm for frequencies above 5 cycles per day. The simultaneous GPS series, sampled every 2 hours, display no indication of white noise except for one receiver with poor data analysis. The spectra of VLBI-GPS differences are generally flat but show possible signs of excess noise in some spectral bands. Based on these results, estimating VLBI ZPD values more often than every few hours should be reconsidered, especially if changes would strengthen other parameters. On the other hand, GPS-based ZPD estimates should be determined more frequently, at least hourly. Considering the greater reliability of the VLBI scale and the corresponding weakness of GPS (mostly due to uncertainties in the phase patterns of the satellite transmit and tracking station antennas), an optimal combination of ZPD results should rely on the VLBI estimates for its absolute long-term accuracy and weight GPS heavily for its high-frequency precision.
DE: 1200 GEODESY AND GRAVITY
DE: 1244 Standards and absolute measurements
DE: 1294 Instruments and techniques
DE: 0360 Transmission and scattering of radiation
DE: 0365 Troposphere--composition and chemistry
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting