Four-constellation Reliability in Challenging GNSS Signal Environments and the Estimation of Inter-system Time-offsets.

R. Winit, K. O’Keefe

Abstract:	The modernization of GNSS provides promising future improvements to satellite navigation users across the globe. The new GNSS constellations such as Galileo, GLONASS-K and BeiDou being planned and launched will result in increased number of available ranging sources, hence, improvement in constellation geometry, coverage and redundancy of signal observations. How to best use multiple constellations has been investigated previously for use in aviation, marine and terrestrial applications. With more satellites in orbit, improved ranging availability is expected in environments with degraded GNSS signals such as road navigation in urban-canyons. Usually, the accuracy of the navigation solution is the main focus in the development of satellite navigation systems. When the GNSS is to be used as a stand-alone navigation method in safety-critical and liability-related applications, however, system integrity becomes a major concern. In order to improve the accuracy and reliability of the position solution obtained from multi-constellation GNSS, satellite-based augmentation systems (SBAS) can be applied to one or more constellation to provide integrity messages and differential corrections which can be used by GNSS/SBAS receivers to mitigate errors in GNSS signals. When using GNSS signals, the main contribution to errors depends on the user environment. When the navigation signal is used under open sky condition, the largest source of error is due to ionospheric code delay. Also, rare but possible faults due to satellite malfunction may occur. When the GNSS is used as navigation aid in harsh signal environments such as urban canyons, however, multipath becomes the major source of error. Guaranteeing the reliability of the position solution depends on the ability to detect and exclude faulty measurements. With the increasing number of satellite constellations, a challenge of GNSS navigation is to deal with the differences among satellite systems as well as to maximize the benefit from the additional ranging sources. When using ranging signals from more than one system, additional inter-system time-offset parameters have to be estimated by the receiver at the cost of one satellite measurement from each additional system. This means that one of the satellites from additional system does not contribute to the position solution. Alternatively, depending on the satellite system, the inter-system time-offset could be broadcast by the system and this parameter can be used as additional measurement by the receiver with some assumed accuracy. Additionally, when the inter-system clock offset is unknown, a further additional satellite from the additional system must be observed in order to allow for residuals-based fault detection and exclusion. Another method to obtain the between-system time-offset parameter is to initially estimate the time-offset parameter at the receiver level during a period when the number of satellites is sufficient. Then, when the user-satellite geometry changes and the number of satellites in view is no longer sufficient, the receiver can continue using the previously estimated time-offset value until the number of satellite improves. When this method is used, however, the requirement of during which time period the navigation filter should compute the inter-system time-offset parameter and during which time period the filter should use the previously estimated value must be considered. The fundamental requirement for the receiver is to compute the between-system time-offset parameter during the time when number of satellites is sufficient. However, even when the number of satellites is sufficient, the user-satellite geometry could still be poor. This is particularly true in urban-canyon environments. Estimating the inter-system time-offset when time dilution of precision (TDOP) is high can potentially lead to an inaccurate time-offset estimation. It is thus advisable for the receiver to have the additional requirement that the navigation filter only recompute the inter-system time-offset parameter if the dilution of precision requirement is satisfied. Work to demonstrate what level of between-system time-offset accuracy can be achieved when this parameter is computed under various user-satellite geometry strengths is, however, very limited. The first part of this paper will investigate the availability and reliability performance of multi-constellation GNSS systems including GPS-GLONASS, GPS-GLONASS-Galileo and GPS-GLONASS-Galileo-BeiDou (Medium Earth Orbit (MEO)) constellations under challenging GNSS signal environments through simulation covering North America locations. As new satellite systems are well underway in the development process and started deploy into orbit, it would be of interest to see how the ranging signals from incomplete constellations start benefit GNSS users. Performance of the existing GPS-GLONASS constellation with additional signals from incomplete constellations will be investigated. Different numbers of satellites from the new constellations when using with the existing dual-constellation will be simulated and the performance will be examined. We have recently acquired and tracked real data from three simultaneously visible BeiDou satellites, and have tracked real Galileo signals, albeit without valid navigation messages. If possible, simulated results obtained above will be validated with real data from GPS, GLONASS, BeiDou and Galileo. Furthermore, the improvement in the reliability performance when applying SBAS correction to GPS measurements in the multi-constellation will be investigated. The improvement in protection level as a result of aiding existing GNSS measurements with SBAS and the improvement in protection level as a result of adding new systems in to existing constellation will be compared. Also, in order to investigate the effect of the inter-system time-offset on the system integrity and availability, the performance of multiple constellations when the inter-system time-offset parameters are known and used as input measurement and when they are estimated at the receiver level will be compared. The second part of the paper will investigate performance of multiple constellations such as GPS-GLONASS and GPS-GLONASS-BeiDou-Galileo in challenging GNSS signal environments when the between-system time-offset is initially estimated by the receiver during the time when user-satellite geometry satisfies preset requirements and uses this estimated value as time constraint during later epochs. This paper will examine the accuracy of inter-system time-offset obtained from various user-satellite geometry with the aim to determine optimum dilution of precision requirement that would provide high percentage of system availability while maintaining high estimated time-offset parameter accuracy.
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:	383 - 397
Cite this article:	Winit, R., O’Keefe, K., "Four-constellation Reliability in Challenging GNSS Signal Environments and the Estimation of Inter-system Time-offsets.," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 383-397.
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