Real-Time Data Link Implementation Aspects of a Measurement-Based ADS-B System for Conflict Detection and Resolution

P. Duan, M. Uijt de Haag, J.L. Farrell

Abstract:	Future Aircraft Surveillance Applications (ASA) systems for situational awareness applications, airborne spacing applications, airborne separation applications, and self-separation applications will benefit from the introduction of Automatic Dependent Surveillance-Broadcast (ADS-B). Certain future surveillance applications, however, such as aircraft self-separation demand higher performance requirements and are currently seen as too immature to be specified in a Minimum Operational Performance Standard (MOPS). An alternative measurement-based ADS-B system has been proposed previously to improve the surveillance performance and act as an enabler for self-separation applications such as Conflict Detection and Resolution (CDR). In measurement-based ADS-B measurements from the Global Navigation Satellite System (GNSS) are transmitted rather than processed aircraft state vectors and performance parameters in the current implementation. This paper will discuss specific aspects that must be addressed for a real-time implementation of measurement-based ADS-B for CDR. The discussion will include a data link bandwidth utilization analysis, interpolation and extrapolation methods in the presence of dissimilar GNSS receivers onboard ownship and traffic, and estimation methods for the conflict prediction and detection tasks both spatially and temporally. As a core technology of the Next Generation Air Transportation System (NextGen) program, ADS-B is designed to improve surveillance services, and, hence, safety and capacity of the National Airspace System (NAS). With its unprecedented air-to-air surveillance capability, ADS-B provides a datalink that aircraft can use to share their real-time information. The transmitted information includes the aircraft state vectors (i.e., position and velocity), and a set of performance-related parameters such as the navigation accuracy category for position, NACp, the navigation accuracy category for velocity, NACv, the navigation integrity category, NIC, and the surveillance integrity level, SIL. The values of these performance parameters are completely determined by the navigation sensor that is used as an input to the ADS-B receiver such as GNSS Space-Based Augmentation System (SBAS) receivers and Ground-Based Augmentation System (GBAS) receivers. Future ASA systems will use ADS-B as the primary information source to perform situational awareness applications (e.g., conflict detection and resolution, final approach and runway occupancy awareness, and airport surface situational awareness methods), airborne spacing applications, airborne separation applications, and self-separation applications. As a result, the quality of the ADS-B data is an important factor in determining the performance of ASA systems. The values of NACp and NACv in the current ADS-B implementation will be used by separation methods to specify the uncertainties for traffic position and velocity, but they may prove to be conservative in nature. By determining ownship and traffic uncertainties separately, it is assumed that these uncertainties are uncorrelated. Since ownship and intruder are in close proximity to each other in a conflict scenario, their position and velocity errors are in fact highly spatially and temporally correlated if they use the same navaid. With the proposed measurement-based ADS-B system, participants would transmit GNSS measurements (pseudorange and carrier phase) rather than processed aircraft coordinates and performance parameters. Given the measurements from both traffic and ownship, range single differences, double differences, and triple differences can be formed and used to accurately derive the separation vector or relative position vector between ownship and traffic as well as the change in separation vector. The latter quantity is directly related to the average velocity over a time epoch. This approach essentially calculates the separation vectors and separation vector changes between ownship and traffic in the measurement domain. Since, in a conflict scenario, most errors in the measurement domain are highly correlated spatially and temporally, they will cancel out resulting in a far superior estimate of the relative position and velocity. Previous simulation and flight test results have shown that the measurement-based ADS-B system provides meter-level relative position accuracy and millimeter-per-second-level relative velocity accuracy. Particularly, the relative velocity accuracy is orders of magnitude better than the one provided by the current ADS-B implementation, and will significantly reduce the uncertainty growth over time and increase predictability of target future position and potential conflict. Previous flight test has also demonstrated the feasibility of the measurement-based ADS-B system in real-time. A radio data link was selected to mimic ADS-B for measurement sharing between ownship and traffic. However, the bandwidth limitations of the actually ADS-B data link were not taken into consideration. This paper will present a realistic architecture for a real-time implementation of the measurement-based ADS-B system. An optimal utilization of the actually ADS-B data link (both Mode-S and UAT) bandwidth for the measurement-based ADS-B system will be explored. On the transmitter side differential carrier phase measurements will be transmitted instead of absolute carrier phase measurements to save bandwidth. Additionally, the satellite identification numbers (PRN codes) will be omitted from the message set. On the receiver side, the ownship measurements must be associated with the corresponding traffic measurements in the absence of identification data using test similar to an integrity test. Furthermore, to compensate for possible temporal misalignment between the ownship and traffic measurement, an interpolation/extrapolation step is performed to align the measurements. For the relative position and velocity estimation, a Kalman Filter has been implemented to not only address some of the alignment issues during dynamic maneuvers, but also to perform the conflict prediction and detection functions. Performance from different dynamic models (e.g., constant velocity and constant acceleration) in the context of CDR will be examined and a trade-off study between update rate and number of incorporated measurements will be performed.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	282 - 291
Cite this article:	Duan, P., de Haag, M. Uijt, Farrell, J.L., "Real-Time Data Link Implementation Aspects of a Measurement-Based ADS-B System for Conflict Detection and Resolution," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 282-291.
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