GBAS Enhancement by Using the Ionospheric Gradient Correction

D. Weng, W. Chen, S. Ji, Y. Xu

Abstract:	In GBAS, the range correction is generated for the corresponding location of reference station at a specific epoch. The validity of the correction decreases as the latency time increases and as the separation distance between the reference station and the user increases. They are called temporally and spatially decorrelated errors of GBAS. To compensate the temporally decorrelated errors, the range rate correction is also generated and sent to the user for the solution. The term improves the DGPS performance significantly in case of Selective Availability (SA). Since the Selective Availability (SA) was switched off, the effects of the temporal decorrelation has been reduced, and the remaining temporal errors become negligible when using the rate of change of corrections. The spatial decorrelation is caused by the ionospheric error, the tropospheric error, the ephemeris and clock error. Among them, the spatial variability of the ionosphere causes a significant performance degradation of the local area GBAS. In previous studies (Datta-Barua 2002; Skone and Coster 2009), cases of the large spatial variability have been reported, and their effects have also been evaluated, in both mid-latitude and equatorial regions, under both the storm and the storm-free conditions. The largest gradient reported in terms of the ionospheric delay at L1 reaches hundreds of mm/km, which happened in the mid-latitude areas during the geomagnetic storms. But the ionospheric gradient in the mid-latitude areas is very small during the quiet ionosphere conditions. Over equatorial latitudes, the spatial variability is extremely large under both the quiet and the severe conditions. It not only degrades the GBAS accuracy, but also affects the integrity which is critical for safety critical applications such as the aircraft approaching and landing. The purpose of this paper is to compensate the difference of the ionospheric delay between the user and the reference station by using an ionosphere delay gradient model. The model is actually a vector that describes the spatial change rates of the ionospheric delay along the east and the north directions for each satellite in GBAS. In the proposed GBAS, these two more parameters for each satellite are transmitted to the user together with the range correction. By applying the gradient correction, the user receiver is able to estimate and correct the ionospheric delay difference between the reference station and the user, and the improvement of the GBAS position performance can be achieved. To construct the model for each satellite, the between-station ionospheric delay differences should be calculated accurately to guarantee the performance of the model. The noise level of the code measurements is much higher that of the carrier phase measurements. However, the carrier phase measurement introduces the unknown ambiguities. In this study, only the carrier phase measurements are involved in the model construction. To estimate the single difference ambiguities, a new two-step algorithm for estimating the between-receiver difference of the ionospheric delay is developed in this paper. The proposed algorithm include two steps, one is to estimate ionosphere spatial difference during the lowest spatial variation of the ionosphere, and the other is to extend the results to other epochs for the day by combining Single Differencing (SD) and Double Differencing (DD) techniques. The study places emphasis on the characteristics of the spatial ionosphere variability in low-latitude areas and the validity of the ionospheric gradient model. The assessment of the ionospheric gradient model for mitigating the uncorrelated error caused by the spatial ionosphere variability is performed by processing GPS data collected in low-latitude areas. The assessment of the proposed model includes three baselines, and two of them are used for constructing the gradient model, and the other is a simulated DGPS baseline. The quiet and the severe ionosphere activities are also considered in this assessment. The estimated gradients show that the ionosphere in low latitude is characterized by the large variability along the S-N direction. The largest gradient along the S-N direction reaches 50 mm/km during the quiet ionospheric conditions, and introduces 0.5 m range error into the satellite measurement assuming that the separation distance is 10 km. The gradient like this degrades the GBAS performance, and limits its use in the applications that requires better than 1 m position accuracy. The gradient obtained is then used for estimating the ionospheric delay difference of the simulated baseline, and the position results show that the proposed model can reduce the position error significantly. Furthermore, some changes to the original DGPS are required by the proposed DGPS. The additional facility of the proposed DGPS is the data link used for the carrier phase measurements sharing among the reference stations in the local area. And the data link is not a challenging issue for the communication technique nowadays. The other change is that two more parameters should also be included by the GBAS message from the reference station to the user. Despite these changes, the proposed GBAS shows a great improvement, and can be applied in the real-time application.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1417 - 1427
Cite this article:	Weng, D., Chen, W., Ji, S., Xu, Y., "GBAS Enhancement by Using the Ionospheric Gradient Correction," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1417-1427.
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