Application of ray-traced tropospheric slant delays to geodetic VLBI analysis

Abstract

The correction of tropospheric influences via socalled path delays is critical for the analysis of observations from space geodetic techniques like the very long baseline interferometry (VLBI). In standard VLBI analysis, the a priori slant path delays are determined using the concept of zenith delays, mapping functions and gradients. The a priori use of ray-traced delays, i.e., tropospheric slant path delays determined with the technique of ray-tracing through the meteorological data of numerical weather models (NWM), serves as an alternative way of correcting the influences of the troposphere on the VLBI observations within the analysis. In the presented research, the application of ray-traced delays to the VLBI analysis of sessions in a time span of 16.5 years is investigated. Ray-traced delays have been determined with program RADIATE (see Hofmeister in Ph.D. thesis, Department of Geodesy and Geophysics, Faculty of Mathematics and Geoinformation, Technische Universität Wien. http:// resolver.obvsg.at/urn:nbn:at:at-ubtuw:1-3444, 2016) utilizing meteorological data provided by NWM of the European Centre for Medium-Range Weather Forecasts (ECMWF). In comparison with a standard VLBI analysis, which includes the tropospheric gradient estimation, the application of the ray-traced delays to an analysis, which uses the same parameterization except for the a priori slant path delay handling and the used wet mapping factors for the zenith wet delay (ZWD) estimation, improves the baseline length repeatability (BLR) at 55.9% of the baselines at sub-mm level. If no tropospheric gradients are estimated within the compared analyses, 90.6% of all baselines benefit from the application of the ray-traced delays, which leads to an average improvement of the BLR of 1 mm. The effects of the ray-traced delays on the terrestrial reference frame are also investigated. A separate assessment of the RADIATE ray-traced delays is carried out by comparison to the ray-traced delays from the National Aeronautics and Space Administration Goddard Space Flight Center (NASA GSFC) (Eriksson and MacMillan in http:// lacerta.gsfc.nasa.gov/tropodelays, 2016) with respect to the analysis performances in terms of BLR results. If tropospheric gradient estimation is included in the analysis, 51.3% of the baselines benefit from the RADIATE ray-traced delays at sub-mm difference level. If no tropospheric gradients are estimated within the analysis, the RADIATE ray-traced delays deliver a better BLR at 63% of the baselines compared to the NASA GSFC ray-traced delays.

Refined discrete and empirical horizontal gradients in VLBI analysis

“…On the basis of ray-traced delays through numerical weather models (NWMs) using the highly sophisticated VieVS ray-tracer (Hofmeister and Böhm 2017 ), we developed new discrete horizontal gradients for a priori use in VLBI analysis referred to as GRAD, as well as a new empirical gradient model GPT3 in the two grid sizes and . All of these models are capable of outperforming existing models in our comparisons; this is shown through baseline length repeatabilities (BLRs) from VLBI analyses as well as theoretical delays.…”

“…As the NWMs are available globally, ray-traced delays can be produced for any point on Earth. The ray-tracing software developed by Hofmeister and Böhm ( 2017 ) as part of the Vienna VLBI and Satellite software (VieVS) (Böhm et al. 2017 ) can not only be used for the derivation of highly accurate mapping functions [see Landskron and Böhm ( 2017 )], but provides the basis for the determination of horizontal troposphere gradients through 2D ray-tracing at several azimuth angles, too.…”

VMF3/GPT3: refined discrete and empirical troposphere mapping functions

“…For this reason, ray-tracing is, unlike surface measurement methods, able to consider the effect of the whole atmosphere. Current ray-tracing software such as RADIATE (Hofmeister and Böhm 2017) manages to compute ray-traced delays for a more or less limited num-ber of observations such as those from VLBI (∼10 million since advent in 1979); however, it is evidently not possible in terms of computational effort to do this for every single GNSS observation. The concept of mapping functions provides remedy as the information of the variability of delays over the whole elevation range is condensed in them, more precisely in three mapping function coefficients a, b and c. The first mapping function to adopt information from ray-tracing through NWMs was the Isobaric Mapping Functions (IMF) by Niell (2000), which induced a major leap in accuracy at that time.…”

Evaluation of the ZWD/ZTD Values Derived from MERRA-2 Global Reanalysis Products Using GNSS Observations and Radiosonde Data

“…Tropospheric delay is a key factor affecting high-precision spatial positioning, such as the Global Navigation Satellite System (GNSS), very long baseline interferometry (VLBI) and satellite laser ranging (SLR), which are also the basic data in atmospheric scientific exploration [ 1 , 2 , 3 , 4 ]. In GNSS data processing, the empirical tropospheric delay model is generally used to calculate zenith hydrostatic delay (ZHD) or zenith tropospheric delay (ZTD) information, which is taken as a prior value, and the zenith wet delay (ZWD) or ZTD residual error and position parameters are taken as unknown parameters to be solved [ 5 ].…”