Using LEO GNSS Data for Precise Calibration of Space HW Biases

J. David Calle Calle, Irma Rodriguez Perez, Marta Cueto Santamaria, Esther Sardon Perez, F. Amarillo

Abstract:	A new algorithm for the calibration of the hardware delays that affect GNSS data is being investigated by GMV under the ESA’s General Studies Program. This new algorithm relies on the promising collaboration between two completely independent space systems: GNSS systems, based on MEO (Medium Earth Orbit) satellites, and systems based on LEO (Low Earth Orbit) satellites. Precise calibration of hardware delays is key to improving ionosphere monitoring performances and also to ensure that GNSS-based positioning can benefit in the future from a combination of more than two signals at different frequencies. The idea of receiver and satellite hardware bias calibration/estimation is by no means new. Indeed there is a large body of literature on the estimation of inter-frequency hardware biases, as they are the main source of error when estimating ionospheric delays from GNSS data. Both software and hardware calibration techniques have been developed and the standard approach is to estimate the hardware biases using the ionosphere combination, typically with static receiver on ground. Due to the high correlation of the ionosphere and the hardware biases estimation in these processes, the accuracy of the estimates is limited by the accuracy that can be got when modeling the ionosphere effects. It is in fact the ionosphere dependency on the geometry and its relationship with other measurements what permits to separate hardware biases and ionosphere estimation in the process. The algorithm herein presented makes use of observations from GNSS receivers that are on-board LEO satellites orbiting above the main ionospheric region. The main advantage of this approach is that it represents an alternative option to minimize the ionospheric delay in the ionosphere combination, so that the hardware biases can be estimated more easily and with higher precision. Signals broadcast by GNSS satellites and received by LEO satellites firstly cross the plasmasphere and secondly the ionosphere. Depending on the relative location of both satellites, the signal will only cross the plasmasphere and the topside region of the ionosphere above the LEO height, or will go down to lower ionospheric regions (as in the case of radio occultation´s geometries). The presence of both the ionosphere and the plasmasphere affects GNSS signals broadcast by delaying the propagation of the code carried by the signal and advancing the phase. Besides, it is well-known that the ionosphere presents high spatial (latitudinal, longitudinal and altitudinal) and temporal (solar cycle, seasonal and local time) variability. As far as the plasmasphere is concerned, the plasmaspheric electron content does also vary with location, time and geomagnetic conditions, although the spatial and temporal dependency of the plasmasphere is not correlated with the ionosphere behavior. All these considerations have been deeply analyzed and taken into account to define the subset of GNSS observations from LEO satellites that are to be used by the hardware biases calibration algorithm. The algorithm filters input observations depending on the geomagnetic location of the GNSS and the LEO satellites, season and the local time associated to position of the LEO satellite. An experimentation campaign with GPS data is being carried out with the purpose of evaluating the performance of the new algorithm. Not only the accuracy in the estimation of the hardware biases together with the prediction performance over a 24-hour arc will be tested but also, and above all more remarkable, the improvement in the estimation of the slant Total Electron Content (sTEC) with a local fitting over Europe and making use of precisely calibrated biases will be assessed. Moreover, the prediction of the vertical Total Electron Content (vTEC) will be better thanks to the enhancement in the estimation of the sTEC and thus both single-frequency GNSS positioning and ionosphere applications will benefit from the availability of precisely calibrated hardware biases. Additionally, it will be also evaluated the impact of the hardware biases calibration accuracy on the Single Frequency UERE (User Equivalent Range Error) and on the future Multiple Frequency UERE. In order to define the experimentation scenarios, several flying LEO satellites, with dual-frequency GPS receiver on-board and whose GNSS data are freely accessible, have been identified as suitable for the calibration of GNSS satellites’ hardware biases. The selection of the final candidate LEO satellites have been based on several criteria. First of all, GNSS data must be freely accessible. For example, CHAMP products and Jason-1 and Jason-2 data can be freely downloaded. Secondly, satellites with higher orbits are in principle more appropriate as the impact of the ionosphere and plasmasphere on GNSS measurements decreases with altitude. Finally, the geometry of the orbit and the measurement sampling rate are also important as they determine the number of GNSS measurements that will be available and the observability of the ionosphere and plasmasphere. The intention of this paper is to present the new approach for calibrating hardware biases together with the architecture and detailed design of the new algorithm.
Published in:	Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012) September 17 - 21, 2012 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2249 - 2258
Cite this article:	Calle, J. David Calle, Perez, Irma Rodriguez, Santamaria, Marta Cueto, Perez, Esther Sardon, Amarillo, F., "Using LEO GNSS Data for Precise Calibration of Space HW Biases," Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012), Nashville, TN, September 2012, pp. 2249-2258.
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