Impact of Code Steps on Aviation Receivers applying Higher-Order Code Tracking Loops

Laboratory tests using a GNSS simulator and multiple GNSS receivers have demonstrated that a code step may lead to an overshoot in the output of the code tracking loop due to the employed loop filters. Although these tests have been performed using non-aviation receivers, the occurrence of overshoots in aviation receivers cannot be excluded, as current aviation standards are not imposing any restrictions on the use of higher-order code tracking loops. From an integrity perspective, an unbounded overshoot would be a concern, as a Satellite-Based Augmentation System (SBAS) or a Ground-Based Augmentation System (GBAS) has to protect the worst user being compliant with the applicable standard. The standards for aviation GNSS receivers are imposing smoothing of code measurements by using the Hatch filter. One could expect that smoothing should mitigate overshoots, at least partly. Simulations presented in this paper show that the problem of overshoot is not negligible during smoothing for particular implementations of code tracking loops. Based on the results of the simulations, new requirements have been added to the Dual-Frequency Multi-Constellation (DFMC) SBAS Minimum Operational Performance Standards (MOPS), to bound overshoots after smoothing. In order to offer flexibility to the receiver manufacturer, the use of higher-order code tracking loops is not forbidden, yet the receiver manufacturer shall demonstrate that the maximum overshoot after smoothing is limited to 5 cm.