Multi-Frequency Kalman Filtering for Joint Ionospheric Delay and Multipath Mitigation

Abstract

Global navigation satellite systems (GNSS) are used in a wide range of applications, such as aeronautics or maritime navigation. Under nominal conditions, the GNSS based position offers a high reliability and accuracy. However, the principles used for GNSS based navigation suffer a decreased accuracy under the presence of multipath and an unknown ionospheric delay. In the case of multipath reception, the estimated delay is biased by the superposition of the line-of-sight (LOS) and the multipath signal. Conventional channel equalization techniques for code division multiple access (CDMA) based communication systems cannot easily be applied in the GNSS context. In contrast to other systems the satellite signal power is significantly less than the noise power. Additionally, the spreading code are significantly longer that other CDMA communication systems. Finally, GNSS processing suffers greatly from multipath with a delay less than the chip duration, whereas other CDMA communications systems focus largely on intersymbol interference. To mitigate the ionospheric delay, single frequency GNSS receiver generally employ a coarse model of the total electron content (TEC) [2], [3], which is transmitted in the satellite broadcast message. Dual frequency GNSS receivers, i.e. receivers which can track and process GNSS signals on two frequencies, can mitigate the ionospheric delay in post processing. They exploit the fact that the channel through the ionosphere is dispersive, i.e. the signal delay is frequency dependent [4]. Therefore, the ionospheric delay can be mitigated by appropriately combining delay measurements from two frequencies. However, this post processing ionospheric delay mitigation cannot be conducted under the presence of multipath signals. Therefore, we propose a joint ionospheric delay and multipath mitigation algorithm for multifrequency GNSS receivers. The algorithm, presented in this paper, combines the Kalman Filter based multipath mitigation method from [1] and the signal combination method for ionospheric delay mitigation [4]. By combining the measurements from two or more frequencies directly in the tracking loop, the ionospheric delay mitigation benefits from a reduced delay measurement noise even under pure LOS reception. In the case of no ionospheric delay, i.e. a no TEC in the ionosphere,the multipath mitigation algorithm benefits from using multiple frequencies.