Dual Frequency SBAS Trial and Preliminary Results for East-Asia Region

T. Sakai, K. Hoshinoo, T. Walter

Abstract:	The satellite navigation is an essential technology for implementation of Performance-Based Navigation concept which is one of elements of the future air transport system. The SBAS, satellite-based augmentation system, is an international standard navigation service with continental coverage. It provides RNP (required navigation performance) integrity-assured navigation service by augmenting GPS constellation. We already have three operational SBAS; The US WAAS has been operated since 2003 and recently provided LPV approach service with vertical guidance down to 200FT above the ground; Japanese MSAS is operating for Enroute to NPA (non precision approach); and European EGNOS began its operation for safety-of-life application in March 2011. The SBAS is an international standard system defined by GNSS SARPS (standards and recommended practices). The SBAS SARPS, established by the ICAO (international civil aviation organization), prescribes RF signal characteristics, message contents, and user receiver processing procedure. The SBAS provides integrity information, differential correction information, and ranging signal by broadcast from geostationary satellite. Among them, integrity information is the most important for air navigation users. In terms of integrity, the largest threat is caused by the ionosphere distibuted at 200-400 km above the ground. The irregularities of electron density distribution cannot be corrected enoughly by SBAS ionosphere correction message given as the vertical delays at grid of 5 by 5 degrees. Under such a condition, the only way SBAS can take is giving the large uncertainty well-overbounding the possible residual error to such correction information; This degrades the performance of the SBAS. The ionosphere is the main reason of SBAS performance limitation. The essential solution for the ionosphere problem is employing dual frequency system. This means the user receiver uses L1 and L5 frequencies to make a correction against ionospheric delay instead of using SBAS message. It should be noted that the full operational capability (FOC) of GPS L5 signal is planned to be achieved in 2018. After the FOC, user receivers are possible to perform ionosphere correction by themselves with L1/L5 dual frequency signals. In any case, the current SBAS ground system using L1 and L2 signals to generate ionosphere correction information needs a transition to L5 (or L2C) due to L2 P/Y signal sunset in 2020. Recently, the US WAAS and European EGNOS are going to be dual frequency system in order to achieve CAT-I precision approach performance with enough robustness. In general, in terms of integrity, the protection levels (HPL and VPL) will be supressed largely by employing dual frequency system in comparison with single frequency system because ionosphere threat is eliminated. In order to determine operational concept and detailed specification of dual frequency system, we need to investigate the performance of dual frequency SBAS. For this purpose, the authors modified SBAS simulator into dual frequency and tested the performance. Some basic assumptions are as follows: (i) Dual frequency SBAS generates corrections based on L1/L5 dual frequency measurements; (ii) Dual frequency SBAS assumes that user receivers make ionospheric correction by themselves with L1/L5 dual frequency measurements; (iii) Dual frequency SBAS still supports single frequency user receivers on L1 signal to ensure backward compatibility; (iv) Message data rate is still limited to 250 bps on L5 (and additional 250 bps on L1). Under these assumptions, we investigate the performance we can expect and key factors which have a certain impact on operational concept such as if we can support L5 single frequency condition or not. During the investigation, the SBAS simulator developed by the authors was modified to be able to generate corrections against dual frequency measurements. The complete SBAS message stream is generated and tested with dual frequency user receiver software (DFRX). The SBAS simulator runs L1 and L2 dual frequency mode because L5 signal is not yet available. Test cases are set to cover full combination of single and dual frequency SBAS operating with single and dual frequency user receivers in order to verify compatibility of all operation modes. The performance of dual frequency system reflects the elimination of ionosphere threat in terms of integrity. Resulted position accuracy for single frequency SBAS and single frequency user receiver is comparable with the existing SBAS. The position accuracy of a combination of dual frequency SBAS and dual frequency user receiver is simular to single frequency system. It would be important that combinations of single frequency correction SBAS message and user receiver in dual frequency mode as well as dual frequency SBAS message and user receiver in single frequency mode work not well; The user position accuracy degrades to a few meters for these combinations. This result means that corrections generated for single frequency mode should not be applied to dual frequency measurements. The L5 message stream will be able to broadcast corrections for L5 single frequency users OR for L1/L5 dual frequency users in case that the current messaging capacity is still applied. In summary, the authors investigated the performance for various combinations of single and dual frequency SBAS and user receiver by using the SBAS simulator software. The performance of dual frequency system reflects the elimination of ionosphere threat in terms of integrity, and is comparable with single frequency system in terms of accuracy. The important result is the fact that the dual frequency SBAS message should not be applied to user receivers in single frequency mode. This conclusion might contribute for consideration of the operational concept of SBAS L5 users.
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:	912 - 920
Cite this article:	Sakai, T., Hoshinoo, K., Walter, T., "Dual Frequency SBAS Trial and Preliminary Results for East-Asia Region," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 912-920.
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