SURVEILLANCE MONITORING OF PARALLEL PRECISION APPROACHES IN A FREE FLIGHT ENVIRONMENT. Carl Evers Dan Hicok Rannoch Corporation

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1 SURVEILLANCE MONITORING OF PARALLEL PRECISION APPROACHES IN A FREE FLIGHT ENVIRONMENT Carl Evers (cevers@rannoch.com), Dan Hicok Rannoch Corporation Gene Wong Federal Aviation Administration (FAA) ABSTRACT The FAA is conducting evaluations of Precision Runway Monitoring (PRM) surveillance techniques. This paper describes the evaluation results of the Multilateration/ADS-B system trials conducted at Atlanta-Hartsfield International Airport (ATL) in The performance of Time Difference of Arrival (TDOA) position multilateration of aircraft Mode S and Mode A/C transmissions was evaluated, in addition to the system's capability to support 1090 MHz ADS-B. The paper provides a system overview and presents preliminary coverage, accuracy and update rate results. INTRODUCTION Independent parallel operations during Instrument Meteorological Conditions are typically limited by current radar surveillance to runways separated by 4,300 feet or more. PRM provides Air Traffic Control (ATC) with a display of arrivals along the extended centerline. In the event that an aircraft blunders towards the adjacent runway s extended centerline, PRM automatically alerts ATC. The controller will respond to the blunder by issuing a breakout instruction to the threatened aircraft. By taking advantage of PRM displays and high accuracy and update rate surveillance techniques, runway separations of 3,400 feet can be safely supported for independent parallel operations, as is provided by the PRM E-scan system. The FAA has committed to buying several PRM E- scan systems and plans to install them at heavily used airports that have closely spaced parallel runways. The surveillance sensor cost constitutes a high percentage of the overall system cost. Consequently, the FAA has been evaluating alternative surveillance techniques including multilateration/ads-b that promise to be as capable, but at a lower cost. The feasibility of multilateration/ads-b was first demonstrated at ATL in 1995 by Cardion, Inc. using their Cooperative Area Precision Tracking System (CAPTS)[1]. This trial demonstrated CAPTS capability to be used for the Airport Surface Target Identification System (ATIDS) surface surveillance application. Based on the success of the multilateration trials, the system was reconfigured in February 1997 to conduct trials for the PRM application. This paper describes the results of these trials. This paper contains only the understanding and views of the authors and is not intended to represent the official position of the FAA. Success of the PRM trials at Atlanta is due to a team effort that includes the FAA PRM Product Team, FAA Technical Center, MIT Lincoln Laboratory, Cardion Inc., and the Atlanta FAA Air Traffic and Airway Facilities divisions. System Architecture and Test Setup The Multilateration/ADS-B system, illustrated in Figure 1, uses Mode S multilateration, Mode A/C multilateration, and 1090 MHz ADS-B to locate and identify aircraft. Virtually all aircraft that operate at major airports are equipped with transponders. A key element of the system is the receiver/transmitters (R/T) The R/T contains a 1090 MHz receiver, 1030 interrogator, and hardware for time stamping. The Cardion R/Ts use commercial off-the-shelf Traffic Alert and Collision Avoidance System (TCAS) hardware for the receive and transmit functions. Mode S transponders transmit identification information "short squitters" once a second. The Mode S multilateration uses R/Ts to time stamp and decode

2 short squitters. R/T data is sent to the Master Work Station (MWS) for position processing. Time Difference of Arrival (TDOA) techniques are applied to determine aircraft lateral position each time aircraft SSR transmissions are received from a triad (e.g., three R/Ts). Additionally, the system multilaterates on ADS- B transmissions, thus providing ADS-B report monitoring and a back-up in the event of a GPS avionics failure. Aircraft altitude is obtained via discrete Mode S interrogations. PRM safety processing and display is provided by the Final Monitor Aid (FMA). Mode A/C multilateration also uses TDOA techniques. R/Ts interrogate Mode A/C transponders to elicit replies. "Whisper-shout" interrogations, similar to the TCAS application, are used to separate responses in time. The system may interface with either the Airport Target Identification System (ATIDS) or the Airport Movement Area Safety System (AMASS) to obtain missed approach information for aircraft near the airport surface. Where these interfaces are not available, the system will include additional R/Ts to provide this coverage. Atlanta Test Site The optimum setup for ATL would require a combination of 8 remote units, as illustrated in Figure 1, which can be receive only or R/Ts. The Government estimate is $40K for a receive only unit and $100K for an R/T unit, therefore it is important to use receive only units wherever possible. The 4 R/Ts at the airport are required for monitoring missed approaches and eliciting Mode A/C transponder emissions for approach traffic. The actual test setup (Figure 2) was constrained by the availability of only 6 R/Ts. The installation used directional cellular antennas with adjustable wings to control coverage and gain. R/T placement and antenna alignment were optimized for coverage and accuracy of west-bound approach traffic. While utilized for monitoring parallel approaches, the three R/Ts at the airport were retained to support ongoing ATIDS surface testing. transponder transmissions required for system synchronization. An airport concourse, hotel, and the FAA southern region headquarters were used for the on airport R/T installations. TEST RESULTS Mode S Multilateration Coverage Flight testing was performed to verify that Mode S multilateration would provide sufficient coverage while meeting the corresponding accuracy and update rate requirements. The FAA requires PRM coverage along the extended centerline out to 15 nmi. While the system was sited for 10 nmi. on the east, reliable coverage was provided out to 15 nmi. Reliable coverage was provided out to 10 nmi. west of the airport. Figures 3 and 4 provide examples of commercial traffic coverage plots for the off airport R/Ts. Each figure shows raw position (e.g., prior to track processing) plots for two aircraft approaches. Surveillance coverage is required along the length of the runway down to the specified threshold height level, approximately down to 50 AGL. Coverage for the outer R/Ts was recorded to be down to approximately 75 above the runways. Surveillance coverage is required from either R/Ts at the airport or ASDE- 3/AMASS. Testing was performed during the ATIDS trials that demonstrated coverage for the length of the runway[1]. Mode S Multilateration Update Rate Approach traffic was monitored to collect Mode S multilateration data to assess update rate performance. An average update interval of 0.9 seconds with a probability of update within 2 seconds of.97 was recorded for east bound approaches. An average update interval of 1.0 seconds with a probability of update within 2 seconds of.96 was recorded for west bound approaches. R/T siting flexibility was provided due to small R/T unit size and ability to displace the antenna from the R/T by distances of more than 250 feet. Two commercial towers and one FAA tower were used for the off airport R/T sites. The antennas were located between 140 and above ground level which provided sufficient height to receive the reference

3 Receiver #1 Receiver #2 ~30 Miles Dual Reference Transponder R/T #1 R/T #2 R/T #3 R/T #4 Receiver #3 ATC Tower Equipment Receiver #4 Final Monitor Aids display Redundant Master Work Stations Figure 1. Optimum PRM Multilateration/ADS-B Setup R/T #6 ~7 Miles Reference Transponder R/T #1 R/T #2 R/T #3 ~15 Miles R/T #4 R/T #5 Figure 2. Actual PRM Multilateration/ADS-B Setup for Atlanta

4 Scale is statute miles. Figure 3. Coverage Plot (West Bound Approaches) Scale is statute miles. Figure 4. Coverage Plot (East Bound Approaches)

5 Mode S Multilateration Accuracy Position accuracy, Table 1, was measured for east bound approaches. The achieved accuracies are substantially better than the PRM requirements. Accuracy performance is affected by Dilution of Precision (DOP) with highest accuracy inside the R/T triads. A DOP model, Figure 5, was developed for the ATL installation. The airport "AP" is in the center of the plot. Accuracy contours units are stated in feet (95%). R/Ts are located at each "". The model is a conservative representation of the measured performance. Overall accuracy is improved by relocating the receive units, as illustrated in Figure 6. Modeling has shown that there is a significant amount of flexibility in placement of R/Ts to achieve the accuracy required to support closely spaced parallel approaches. Mode A/C Multilateration Results Preliminary Mode A/C flight testing was performed with Rannoch s Piper Turbo Arrow IV. Mode A/C flight testing was performed during the last week of the trials, therefore there was no opportunity to resolve problems identified during flight testing. Figure 7 provides a plot of an approach to 27L. The system was setup to perform two whisper-shout interrogation sequences per second. Update rate was measured with an average update interval of 2.6 seconds and a probability of update within 2 seconds of.58. Update rate performance was hampered by a faulty transmitter. Based on the reply efficiency of the R/T located at the FAA region headquarters site, it has been estimated that a single R/T would support an update interval of.7 seconds and a probability of update within 2 seconds of.99. Further system optimization to improve update rate performance can be achieved by refining the whisper-shout parameters (i.e., signal levels). The accuracy results of Mode A/C, Table 1, were similar to the Mode S results. False Targets A problem with airport primary radar and tracking systems is the generation of false targets due to reflections from airport structures. Where it is possible to track false targets using secondary surveillance techniques, during this testing for PRM, and surface testing [1], no false targets were tracked by the system. Application PRM E-scan Requirement Mode S Multilateration Approach Mode A/C Multilateration Approach Mode S Multilateration Surface[1] ADS-B Operation Accuracy +/-.06 degrees (RMS) or cross track +/- 60 ft. (RMS) at 10 nmi Cross track +/-17 ft. (RMS) Along track +/- 25 ft. (RMS) Cross track +/-29 ft. (RMS) Along track +/- 19 ft. (RMS) 27 ft. (Mean) 51 ft. (95%) Table 1. System Accuracy Testing was performed during surface trials[1] to demonstrate system operation in a mixed equipage environment (e.g., users equipped with either 1090 MHz ADS-B or transponders). The ability of the system to determine ADS-B target position by multilateration was demonstrated. Independent position determination provides a means to enhance ADS-B surveillance continuity, and is essentially a back-up in the event of a loss of differential corrections or failure of DGPS avionics. Independent position determination also provides a means to automatically identify erroneous ADS-B position reports. This verification ensures integrity at a level of accuracy currently not achievable via other ground based systems. Testing indicated that two or more 1090 ADS-B receivers may be required to perform PRM surveillance in an environment where all aircraft are 1090 ADS-B equipped and multilateration is not implemented. ADS-B reception performance drops to approximately percent[1] at each airport based R/T when the aircraft are on the surface. This low reception performance was determined to be the result of multipath interference from reflections off airport buildings (i.e., hangars, concourses). Diversity with multiple receive sites is required to ensure no coverage gaps. Multipath was not a major factor for

6 monitoring arrivals down to threshold. Airport based R/T reception performance for Mode S short squitters was 93% during periods of peak traffic. Mode S short squitter performance can be used as a reliable indicator of 1090 ADS-B performance. While the 1090 ADS-B long squitters are 56 bits longer then the short squitters, our testing has shown that there is no measurable difference in reception performance between short squitters and long squitters. Additional work should be performed to determine how many receivers are required to support 1090 long squitter ADS-B. CONCLUSIONS The multilateration/ads-b system has demonstrated Mode S multilateration accuracy, coverage and update rate that exceed the current FAA PRM requirements. Mode A/C multilateration exceeds accuracy and coverage PRM performance, but has not demonstrated sufficient update rate performance. Based on analysis of individual R/T reply efficiency performance, there is high confidence that update rate requirements can be readily demonstrated. The system has shown the capability to perform 1090 MHz ADS-B surveillance. However, additional work is required to demonstrate parallel runway operations with ADS-B equipped aircraft. The system supports both multilateration and ADS-B in an environment where some aircraft are ADS-B equipped and other aircraft are not ADS-B equipped traffic (e.g., transponder only). There is significant flexibility for siting and installing the system. The R/T hardware requires a small enclosure of about 1 meter cubed and can be placed anywhere including building roof tops. Additional siting flexibility is gained through the capability to offset the antenna from the R/T by distances of 250 feet or more. Accuracy model results and test results with a less than ideal R/T placement configuration have shown that there is significant flexibility in locating R/Ts. REFERENCES 1. Federal Aviation Administration, Positive Identification of Aircraft on Surface Movement Areas Results of FAA Trials, SPIE AeroSense 96 Symposium, 1996.

7 20 Theoretical Accuracy Best Triads AP Y (miles) (miles) Figure 5. Accuracy Contours for the ATL Test Setup Theoretical Accuracy Best Triads Y (miles) AP (miles) Figure 6. Accuracy Contours for an Operational Setup

8 Scale is statute miles. Figure 7. Mode A/C Coverage