Experimental Validation of an Ionospheric Monitoring Scheme for Dual Frequency GBAS

Abstract

In this paper we describe a method to monitor for a difference in the ionospheric delay observed by a ground station of a Ground Based Augmentation System (GBAS) and an airborne user. In case of detection the affected satellite can be excluded or a switch to an ionospheric free processing mode can be triggered. As it is not possible to estimate the absolute ionospheric delay at the ground station from the transmitted corrections directly, we compare a pseudo-ionospheric delay estimate from the corrections with an ionospheric estimate after removing biases. We then show the performance of the proposed monitoring architecture in flight trials from our GBAS test environment for different scenarios. In order to obtain results for a full constellation we considered an L1/L2 dual frequency combination where the expected noise and multipath is larger than in the L1/L5 case which will be used in an operational GBAS. Results show that even with the larger test statistic and in the single constellation case the monitor is feasible and provides good results. Furthermore, we also show results using the available L5/E5a signals collected during flight tests. Also in this case the performance is good and the threshold was not exceeded despite the low threshold of the monitor due to the marginal geometry. In order to test the reaction of the monitor to ionospheric gradients we injected a simulated error into the airborne measurements. As the monitoring threshold depends on the satellite geometry we show some exemplary results of the impact of different gradient slopes on the test statistic of the affected satellites.

Ionospheric Gradient Threat Mitigation in Future Dual Frequency GBAS

“…The test statistic test, , as defined in (8), contains the airborne measurement noise air, from the smoothed pseudorange measurements on both frequencies , as well as the pseudorange correction noise gnd, for both frequencies. The noise in the proposed monitor can thus be described as In [20] we showed that this monitoring condition can be fulfilled rather easily. Even with the minimum number of 5 satellites and noise from our experimental system (that is expected to be larger than that of an operational GBAS), the monitoring was possible and did not cause false alarms due to very low monitoring thresholds.…”

“…The case of more than one affected satellite will probably have to be considered for monitoring as well. We therefore also considered the case of two affected satellites; that is, = 2 in (20). Figure 8 shows the minimum over all thresholds for all possible subsets of two affected satellites during the flight test in red, together with the test statistic in black.…”

“…In [20], we showed the derivation of the monitoring threshold in the case a single satellite is affected by an ionospheric gradient. This was done by starting from the requirement that the aircraft has to land within the touchdown box and the land short case (i.e., landing not too close to the runway threshold) is considered to be the driving constraint.…”

“…However, considering the cases with more affected satellites that are not excluded by other monitors becomes less likely with increasing . Equation (20) does not take into account the low probability of occurrence but this can be included in the future if necessary. Furthermore, the monitoring is conservative in the way that it does not consider the actual separation of satellites in terms of azimuth angle.…”

Airborne Ionospheric Gradient Monitoring for Dual-Frequency GBAS

“…In previous work Felux et al developed in [5] and validated in [6] a method to mitigate the threat of ionospheric gradients for singlefrequency positioning by using the second available frequency for monitoring purposes. One of the main design drivers for new GBAS service types was to maintain backwards compatibility and use the existing VHF Data Broadcast (VDB) with its limited capacity [7].…”