Assessment of Alternative Positioning Solution Architectures for Dual Frequency Multi-Constellation GNSS/SBAS

G.A. McGraw, B.A. Schnaufer, P.Y. Hwang, M.J. Armatys

Abstract:	The availability of dual frequency Global Navigation Satellite System (GNSS) measurements from multiple constellations (e.g., GPS L1/L5, Galileo E1/E5a, and BeiDou B1/B2a) for civil aviation applications will provide improved capabilities, particularly with respect to operations currently supported by Space Based Augmentation Systems (SBAS). The bounding of residual ionospheric delay error in SBAS corrections is the current limiting factor in the use of GPS/SBAS for operations such as approach with vertical guidance with a decision height of 200 ft (LPV-200). The dual frequency GNSS signals enable the elimination of the vast majority of the ionospheric error. There are currently no Minimum Operational Performance Standards (MOPS) for L1/L5 operation. This paper provides preliminary assessments of alternatives in dual frequency GNSS high-integrity positioning solution computation in two high-level areas: (1) dual frequency measurement signal processing; (2) the feasibility of solution architectures that can protect against multiple satellite faults. Consideration of these algorithmic and architectural issues is needed for when standards organizations begin to develop MOPS documents. In terms of L1/L5 positioning solution algorithms, the most common approach discussed in the literature is the use of the ionosphere-free carrier smoothed pseudoranges in a weighted least squares (WLS) solution. There are a number of reasons why direct use of iono-free measurement processing may not be advantageous for high-integrity aviation applications. Compared to the current GPS/SBAS approach of conventionally smoothed L1 pseudoranges corrected with SBAS ionospheric delay estimates, iono-free pseudoranges trade elimination of the iono error with an amplification of the pseudorange noise and multipath by a factor about 2.6, assuming the same carrier smoothing filter time constant. In single frequency applications, the effects of ionospheric divergence provide a practical limit on how long the pseudoranges can be smoothed. For the iono-free case, extended smoothing time constants can be employed with no error penalty. However there are two main reasons why extended smoothing is problematic. First, extended smoothing opens up a potential risk of loss of continuity of function risk due to the time required to re-converge following signal discontinuities (such as due to maneuvers). Second, the errors induced by code-carrier divergence satellite faults are proportional to the smoothing time constant, so shorter time constants require less consideration in the design of integrity monitors for these types of faults. Finally, in the case of loss of signals on one frequency, due to interference for example, the baseline iono-free measurement processing approach basically falls back to conventional single frequency GNSS/SBAS processing which is an issue in guaranteeing continuity of LPV-200 approaches, particularly in areas with no SBAS coverage. The primary alternative that is considered in this paper to directly using iono-free measurements is the use of short time constant divergence-free smoothed L1 (or L5) pseudoranges corrected by ionospheric delay estimates derived from long time constant divergence-free smoothed L1/L5 measurements. The extended smoothing of the iono delay estimate does not present a significant continuity risk since the ionospheric delay is usually a slowly-varying error. Furthermore the effects of a code-carrier divergence fault are not as severe. This approach also provides a more graceful fallback to single frequency operations in the case of loss of a frequency to interference because the dual frequency iono estimates can be used to correct the SBAS iono estimates. Another architectural issue addressed in the paper is how multiple GNSS constellations should be handled in computation of high-integrity positioning solutions. In the absence of SBAS monitoring, current GNSS receiver autonomous integrity monitoring (RAIM) makes a single satellite fault assumption. If the same assumption can be made in the case of multiple GNSS constellations, the optimal strategy is to process all measurements in a single WLS solution. However to support LPV-200 precision approaches with Advanced RAIM (ARAIM) there may be a need to consider the possibility of multiple satellite faults, as might occur with a bad upload that affects multiple satellites. The availability of maintaining separate solutions for each constellation and cross-comparing to guarantee integrity, compared with a single all-in-view solution will be examined. The effects of receiver clock stability on the availability of LPV-200 operations is assessed for ARAIM processing. The paper also examines matters of timing calibration and prediction involved when clock stability is exploited.
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:	223 - 232
Cite this article:	McGraw, G.A., Schnaufer, B.A., Hwang, P.Y., Armatys, M.J., "Assessment of Alternative Positioning Solution Architectures for Dual Frequency Multi-Constellation GNSS/SBAS," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 223-232.
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