A Clock and Ephemeris Algorithm for Dual Frequency SBAS Juan Blanch, Todd Walter, Per Enge. Stanford University. ABSTRACT In the next years, the new GPS and Galileo signals (L1, L5) will allow civil users to remove the ionospheric delay in the pseudoranges. This will have a large impact on the Satellite Based Augmentation Systems (SBAS), as the ionospheric delay is currently the largest error. Once this source of error is removed, the Vertical Protection Levels will decrease substantially, and other error sources will dominate. The remaining terms in the error bound were much less critical than the ionospheric delay error bound, so they have received less attention. It is therefore likely that they can still be optimized. This is true in particular for the User Differential Range Error (UDRE) algorithm which computes the clock and ephemeris error bounds. In addition, new SBAS messages will be broadcast in the L5 channel, and their content is still not fixed. Therefore, it is a good opportunity to determine whether changes can be made both in the UDRE and Message Type 28 (MT28) computation and transmission to increase overall SBAS performance. In this work, we propose an algorithm to compute the error bounds on the clock and ephemeris in SBAS. As opposed to the current Wide Area Augmentation System (WAAS) UDRE algorithm, this algorithm computes the UDRE and MT28 simultaneously and takes into account receiver failures explicitly. We will evaluate the performance of the algorithm and compare it to the current UDRE and MT28 algorithm to determine whether its implementation for dual frequency SBAS would be worthwhile. INTRODUCTION L1-L5 WAAS is being developed [1] to take advantage of the second civil signal in the L5 frequency band. This second signal will allow receivers to estimate and cancel the effect of the pseudorange delay induced by the ionosphere. Since this delay is the most important source of uncertainty in single frequency SBAS [2], it is the largest contributor to the user position error bound. Once the ionospheric error bound is removed, the largest contributor will be the term bounding three different sources of error: the clock and ephemeris error, the code- carrier coherence (CCC), and the signal deformation (SQM) [1]. This term is designated in the Minimum Operational Performance Standards [3] as ı flt . In the case of WAAS, this term is the product of the UDRE and a shaping matrix contained in Message Type 28 [4]. The broadcast index UDRE is the maximum of the output of the UDRE algorithm and the floor imposed by both the CCC and SQM monitors. WAAS today provides vertical guidance in the conterminous U.S and Alaska with very high availability. However, if we want to either achieve better levels of service or be more robust against depleted constellations, it will be necessary to reduce the Protection Levels. This could be done by modifying the Vertical Protection Level [5], [6] or by reducing the term ı flt . With the development of WAAS dual frequency, there is an opportunity to upgrade the algorithms. In this paper, we outline the broad lines of a clock and ephemeris algorithm that has the potential to reduce the WAAS error bounds significantly. This algorithm uses ideas similar to the ones described in [11], but differs in some key points. In the first section we will outline the threats and the message constraints that the current WAAS clock and ephemeris algorithm accounts for, and must be accounted in a new algorithm. The second part will show how each of these constraints can be accounted for. In the third part we will summarize the algorithm. The fourth part will show the potential benefits of the new algorithm as compared to the current one. Finally, we will add a few remarks on the implementation of this algorithm. THREAT MODEL AND MESSAGE CONSTRAINTS The current clock and ephemeris algorithm has evolved to account for: - Nominal error from the network receivers - Nominal biases (antenna biases) - The use of corrections that are generated outside the safety processor - Possibly undetected errors in the network receivers (one station is assumed to return erroneous measurements)