Spatial Gradient Monitor for GBAS Using Multiple Baseline Antennas
Jaymin Patel, Samer Khanafseh, and Boris Pervan, Illinois Institute of Technology
In this paper, we develop a ground monitor capable of detecting anomalous signal-in-space (SIS) spatial gradients for rising, newly acquired, and re-acquired satellites in the Ground Based Augmentation System (GBAS). These gradients can be caused by satellite orbit ephemeris faults and ionospheric fronts. The monitor utilizes differential code and carrier phase measurements across multiple reference receiver antennas as the basis for detection. We show that the new monitor significantly improves the detection performance over existing fault detection algorithms and that it is capable of meeting Category III precision approach and landing requirements. The error models for measurements used in the monitors are experimentally validated using data collected from GBAS ground installations.
GBAS is a safety-critical navigation system intended to support all phases of approach, landing, departure, and surface operations at an airport. It will include multiple spatially separated Global Positioning System (GPS) antennas and receivers at each Ground Facility (GF). The primary reasons for the use of this redundant hardware are to provide a means for detection and isolation of a failed receiver and also to allow for a net reduction in ranging error by averaging measurements for a given satellite. However, another benefit of antenna separation is that differential carrier phase measurements across the antenna baselines can be used to detect and isolate certain SIS anomalies that are hazardous to GBAS.
Large ionospheric gradients caused by ionospheric storms are one type of anomaly that has been studied extensively in GBAS [1-3]. Gradients larger than 400 mm/km have been observed in CONUS, which can cause up to 20 m vertical positioning errors . Orbit ephemeris failures are another type of fault that can be characterized as a spatial gradient . There are three types of orbit ephemeris failures : Type-B failures occur when the broadcast ephemeris data is incorrect but no satellite maneuver is involved; Type-A1 failures occur when the broadcast ephemeris data has been updated incorrectly following a satellite maneuver, and a Type-A2 failure exists if the broadcast ephemeris remains unchanged after the maneuver.
There has been prior work utilizing multi-antenna code and carrier phase measurements to detect gradient faults [2, 3, 5, 7, 8]. Most recently, a ground monitor with runway-parallel antenna baselines was developed to satisfy Category III integrity requirements for all types of orbit ephemeris faults. . However, this monitor, while sensitive to spatial gradients, was also overly sensitive to other nonthreatening measurement errors. Specifically, short duration localized neutral atmospheric disturbances (sometimes called tropospheric turbulence) can also appear as gradients . This type of tropospheric activity typically exists in the lower atmosphere, and the associated structures have small physical scales relative to ionospheric fronts. They produce much smaller differential measurement errors than ionospheric fronts and are not hazardous to GBAS users. But a major problem remains: they can significantly elevate probability of monitor false alarms. One easy solution would be to loosen the monitor’s detection thresholds; we show, unfortunately, that that this approach degrades the monitor’s detection performance to the point that it would no longer be effective for the Category III application . Another alternative is to attempt to somehow distinguish hazardous ionospheric faults from benign tropospheric turbulence. This was done to some extent in , but the solution worked only for certain speed and tropospheric error parameters.
In this paper, we propose two new monitors, respectively using single- and dual-frequency measurements, to detect hazardous gradients and ensure Category III integrity for all runway directions at an airport. The single-frequency monitor is relevant to today’s GBAS. Its basic structural element is a co-linear pair of baselines with two different lengths, created using three antennas. Two or more similar baseline pairs, rotated relative to one another, and either translated or interlocking (i.e., sharing antennas) make up the monitor’s composite structure. Using this small network of baselines, we can isolate tropospheric turbulence because not all antenna pairs will be affected by it. The L1 cycle ambiguities along each baseline are initialized using a code-plus-carrier combination , which will be free of ionospheric divergence errors, even in the presence of a moving ionospheric front. This is necessary so that the monitor can detect fronts present immediately at the time of satellite acquisition. Otherwise the effect of the gradient would be largely absorbed in the estimated cycle ambiguity, and therefore mostly invisible to the monitor itself. We show that about 5 minutes of filtering of the code-plus-carrier observable will be sufficient to resolve the ambiguities well enough to meet Category III false alarm (continuity) and missed detection (integrity) probability requirements. The ground station will only be allowed to broadcast GBAS corrections for the newly acquired satellites after this initialization is complete.
When civil dual frequency L1/L5 measurements become ubiquitous, we show that the GBAS monitor architecture can be simplified considerably. Widelane ambiguities can be resolved quickly using geometry-free and ionospheric-free combinations of code and carrier measurements; these, in turn, can be used to resolve the L1 ambiguities. We prove that a single pair of baselines – again of different lengths – would be sufficient to meet the CAT-III GAST-D false alarm and missed detection requirements within 150 seconds for any given runway direction and that two such pairs could cover all possible runway directions.
The error models for the code and carrier phase measurements used to quantity performance of the monitors are validated using experimental data from GBAS installations at Newark Liberty International and Houston Intercontinental Airports.
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