Towards Airborne Multipath Models for Dual Constellation and Dual Frequency GNSS

Kurzfassung

This paper presents the initial results of an effort to develop and establish airborne multipath models for the GPS and Galileo L1/E1 and L5/E5a signals. These models will be used in dual frequency and multi constellation aeronautical navigation techniques, such as ARAIM, SBAS and GBAS. Development of the models is based on electromagnetic simulations and a large-scale data collection campaign on a variety of different airframes. In this work, results of the evaluation of the first batch of about 200 hours of flight test data with experimental equipment are shown. At a later stage, more data with avionics hardware will be collected and evaluated. This paper mainly aims at establishing the methodology and obtaining initial results as a first indication. For this purpose flight data is analyzed for periods where satellites are tracked continuously. Then, the carrier phase measurements are subtracted from the code measurements, the ionospheric divergence is removed and the bias introduced by the carrier phase ambiguities is removed. Finally, all the data is sorted, overbounded and inflated to account for the limited number of samples. The methodology also includes the removal of code errors induced by the antenna group delay, as these are suggested to be treated separately from the multipath errors. The absolute values of the bounds for GPS L1 and Galileo E1 are larger than the existing GPS L1 model. This could be expected when being conservative at each step of the evaluation process and when using experimental hardware that is not entirely compliant with current minimum performance standards for airborne equipment. Without the conservatisms applied, the observed errors are below the Airborne Accuracy Designator (AAD A) curve. The level of residual multipath on L5/E5a is significantly smaller than that on L1/E1 and the model for the ionosphere free combination is significantly larger than the bound on L1/E1. Both these properties found in our experimental results match with theoretical expectation.