Sensitivity of zenith total delay accuracy to GPS orbit errors and implications for near‐real‐time GPS meteorology

Abstract

[1] Global Positioning System (GPS) measurements have been demonstrated to provide precipitable water vapor (PWV) estimates with a level of accuracy that is comparable to that of radiosondes and microwave radiometers. GPS measurements therefore have the potential to become a significant source of data for operational weather forecasting, provided that PWV (or the intermediate zenith total delay (ZTD)) can be made available in near real-time with a minimum accuracy degradation. Despite the recent decrease in the latency and increase in accuracy provided by the International GPS Service (IGS) ultrarapid predicted GPS orbit products, we show that the accuracy of these orbits continues to be a limiting factor for the accuracy of near real-time GPS-derived atmospheric estimates. In this work, a coefficient matrix is derived from the normal equations of the least squares adjustment model for the GPS observables that maps the orbital parameter errors into ZTD errors. This is used to analyze the sensitivity of GPS derived tropospheric errors to an extensive set of parameters, including their time dependence, in a computationally efficient manner. We show that ZTD errors are dominated by biases in the orbital semimajor axis, with minor contributions from the inclination and argument of perigee, and that this error increases significantly after the fourth to fifth hour of the prediction window. We implemented a GPS data processing strategy based on an iterative estimation of the three most critical orbital parameters (semimajor axis, inclination and argument of perigee) together with the ZTD parameters. We tested this strategy in a 3500 Â 3500 km network of 15 GPS sites in western Europe providing hourly data files. We show that the standard deviation improvement compared to a strategy based only on the orbit quality index provided with the predicted orbit products is on the order of 20%. The analysis of one month of data in near-real-time shows a bias lower than 1 mm ZTD and a standard deviation lower than 6 mm ZTD compared to using the most precise IGS final orbits. We also show that this strategy is robust and capable of dealing with very large orbit errors appropriately. We demonstrate that the same quality is achievable with a 1500 Â 1500 km network which has positive implications for decentralized processing strategies. The near real-time processing methodology described here meets the current timeliness requirements of operational meteorology (30 mn to 2 hours, depending on the application), while ensuring a level of accuracy similar to that provided in postprocessed mode with precise final IGS orbits (1 mm ZTD bias, 6 mm ZTD RMS). The method we propose can also be considered as an ''on-the-fly'' orbit quality control for near real-time GPS applications.

West African Monsoon observed with ground‐based GPS receivers during African Monsoon Multidisciplinary Analysis (AMMA)

“…On average, the two precise solutions show a difference of 2.6–4.1 mm ZTD (0.4–0.6 kg m −2 PWV) and a standard deviation of 4.2 mm ZTD (0.6 kg m −2 PWV) mainly due to changes in the tropospheric models. The rapid and NRT solutions show a bias of ∼5 and ∼7 mm ZTD (0.8–1.1 kg m −2 PWV), respectively, and a standard deviation of ∼7 and ∼15 mm, respectively, in ZTD (1.1–2.3 kg m −2 PWV) compared to “precise/repro.” These results are consistent with, for example, those of Ge et al [2002] and Pacione and Vespe [2003]. Figure 5 shows the differences between processings at Niamey for the month of August 2006.…”

Observations of precipitable water vapour over complex topography of Ethiopia from ground-based GPS, FTIR, radiosonde and ERA-Interim reanalysis

“…The estimation error is obtained from the residual of constrained least squares solution as described by Ge et al (2002) and Jin and Park (2005) and references therein. Figure 4 shows PWV estimation error for all sites.…”

Efficient LEO Dynamic Orbit Determination with Triple Differenced GPS Carrier Phases

“…The variable order and step size numerical integrator is used which provides the output at 30-second intervals and corresponds to the time interval of the RSO solution. The total tropospheric delay for the IGS ground stations was corrected using the modified Hopfield model (Goad and Goodman, 1974). The NRLMSISE-00 model is used for the atmospheric drag force modelling, with the area-to-mass ratio of the CHAMP satellite given as 0 .…”