Coverage Improvement for Dual Frequency SBAS

In the next few years GPS will start broadcasting civil signals suitable for aviation use on both the L1 and L5 frequencies. In addition, Galileo and other constellations will offer an even greater number of dual frequency ranging measurements. The Wide Area Augmentation System (WAAS) and the other Satellite Based Augmentation Systems (SBASs) can also be updated to exploit these new signals. Such updates offer a variety of improvements over existing single frequency systems. These dual frequency systems will be fully robust against ionospheric gradients that currently limit vertical guidance during severe ionospheric disturbances. Further, they offer improved resistance against interference as operations can proceed when aircraft lose access to one frequency or the other. However, the largest benefit to a user taking advantage of both frequencies is that their availability can extend much farther away from the reference station network. The uncertainty in the ionospheric behavior at the user is essentially eliminated, allowing this increase in coverage. Importantly, this availability can be extended into equatorial areas where the current single-frequency, twodimensional grid can be a very poor fit to actual behavior. Thus, availability to these regions can be reliably provided for the first time. This paper examines the coverage that will be offered by SBAS systems when they upgrade to dual-frequency operation. It will examine the coverage offered to the user through the combined coverage of existing and planned systems. Further, we will examine how, with small extensions in their reference station networks, nearly global coverage of LPV-200 service may be achieved. Finally, we will present the improvement to coverage that can be provided by also integrating additional constellations into the SBAS coverage. Additional ranging signals dramatically improve the users geometry, even further extending coverage from reference station networks. INTRODUCTION The Wide Area Augmentation System (WAAS) monitors the Global Positioning System (GPS) and provides both differential corrections to improve the accuracy and associated confidence bounds to assure the integrity. It was the first of the Satellite Based Augmentation Systems (SBASs) and was commissioned for service in 2003 [1]. The Japanese system MTSAT-based Satellite Augmentation System (MSAS) followed next and was commissioned in 2007 [2]. The European system, European Geostationary Navigation Overlay Service (EGNOS) [3] was declared operational in 2009, but has not yet been commissioned for safety-of-life service. That commissioning is expected to occur in mid-2010. Two other SBASs are in the developmental stage. The Indian system, GPS Aided Geo Augmented Navigation (GAGAN) [4], and the Russian SBAS, System for Differential Corrections and Monitoring (SDCM) [5], have fielded equipment and are planning to become operational in the next few years. These SBASs are also planning improvements to expand their coverage areas and strengthen their performance. These include near-term improvements such as additional monitoring stations and algorithmic enhancements. There will also be longer-term improvements such as the incorporation of a second civil signal in a protected aeronautical band and the addition of new GNSS constellations. An SBAS utilizes a network of precisely surveyed reference receivers, located throughout its coverage region. The information gathered from these reference stations monitors the GNSS satellites and their propagation environment in real-time [6]. Availability of SBAS service is a function of two quantities: the arrangement of the pseudorange measurements used to determine the user’s position, referred to as geometry; and the quality of each individual measurement, referred to as the confidence bound. Although very small confidence bounds can make up for poor geometries, and strong geometries can overcome large confidence bounds, both values are generally required to be good to obtain high availability. Geometry is determined purely by the locations of the ranging satellites relative to the user. Currently the basic geometry is provided by the GPS constellation. Historically it has exceeded commitments and there are currently 29 healthy satellites in orbit when only 21 are nominally guaranteed [7]. However, as satellites are taken off-line in critical orbital slots, the quality of the geometry can degrade significantly. There could be short duration losses of service daily at some locations. Since the goal is to provide service more than 99.9% of the time, these outages can have a dramatic impact. WAAS currently mitigates this concern by adding geostationary satellites with a ranging function virtually identical to the GPS satellites. These satellites are always in view and improve the overall geometry, although they do not eliminate the problem completely. The confidence bounds relate to the expected error sources on the range measurements. Currently three error sources are corrected via broadcast to the user: satellite clock error, satellite ephemeris error, and delay error due to propagation through the ionosphere. These error sources are described by two confidence bound terms: the User Differential Range Error (UDRE) for the satellite errors, and the Grid Ionospheric Vertical Error (GIVE) for the ionospheric errors. For single frequency SBAS, this last error source is the most significant. Users may sample the ionosphere anywhere in the service volume, but the SBAS only has measurements from its reference station locations. Thus, there is always the possibility of undetected ionospheric disturbances [8]. This leads to larger confidence bounding terms and lower availability. The combination of geometry and confidence bounds yields the Protection Levels (PL). Protection Levels are the real-time confidence bound on the user’s position error. To match aviation requirements these are broken into a Vertical Protection Level (VPL) and a Horizontal Protection Level (HPL). Each SBAS guarantees that the user’s actual position error will be smaller than these values 99.99999% of the time. The PLs are calculated in real-time using stored and broadcast information. They must be compared to the maximum allowed value for a desired operation. The upper bounds are called Alert Limits (AL) and they are fixed numbers whose values depend on the operation. In this paper we are interested in the LPV-200 approach with a VAL of 35 m and HAL of 40 m [9] [10]. Because GPS and SBAS generally perform better at horizontal positioning than vertical, the requirement that the VPL be below the VAL is nearly always the limiting constraint for these operations. This paper will present the current performance of WAAS, EGNOS, and MSAS. Then we will study expected performance for the future. Specifically we will look first at the set of network improvements that could expand LPV-200 performance around Europe and Japan. Next, we will look at the benefit of GPS L5 and how it will improve SBAS performance. Then we will add in the GAGAN and SDCM systems to evaluate their impact on global coverage and also examine southward expansions for the original three SBASs. Finally, we will examine the impact of a second constellation of navigation satellites and evaluate the performance for a user taking advantage of two core constellations.