Publications

Initial results are presented from a Lincoln Laboratory study of ACAS performance on the Global Hawk UAV. The study has been applying the process outlined in the ICAO ACAS Manual which involves developing UAV airspace encounter models and running fast-time Monte Carlo simulations of encounters. ACAS performance was examined in conventional aircraft vs. conventional aircraft, conventional aircraft vs. non-ACAS Global Hawk, and conventional aircraft vs. ACAS-equipped Global Hawk cases. The existing ICAO and ACASA encounter models were modified to reflect Global Hawk flight characteristics. ACAS performance on Global Hawk was also assessed parametrically across reaction latencies from 0 - 20 s. Global Hawk flight characteristics were shown to have a small but measurable negative impact on collision risk. Assuming no system failures or visual acquisition effects occur, performance with ACAS on Global Hawk is significantly better than without ACAS if response latencies (from the moment an RA is issued to the moment maneuvering begins) are less than 10 s. Performance drops off rapidly at latencies greater than 10 s. The needs for improved airspace models and a more in-depth study of the interaction between visual acquisition and ACAS are noted.

Advanced separation assurance concepts involving higher degrees of automation must meet the challenge of maintaining safety in the presence of inevitable subsystem faults, including the complete failure of the supporting automation infrastructure. This paper examines the types of design features and safeguards that might be used to preserve safety in a highly automated environment. The Advanced Airspace Concept (AAC) being developed by NASA is used as the basis for a fault-tree analysis. Multiple layers of protection, with carefully specified fault management strategies, appear to be important to achieving the desired level of safety.

This paper discusses inter-related studies and development activities that address the significant challenges of implementing Air Traffic Management initiatives in airspace impacted by thunderstorms. We briefly describe current thrusts that will improve the quality and precision of thunderstorm forecasts, work in progress to convert these forecasts into estimates of future airspace capacity, and an initiative to develop a robust ATM optimization model based on future capacity estimates with associated uncertainty bounds. We conclude with a discussion of the thunderstorm ATM problem in the context of future advanced airspace management concepts.

This paper is a concept study for possible future utilization of active electronically scanned radars to provide weather and aircraft surveillance functions in U.S. airspace. If critical technology costs decrease sufficiently, multi-function phased array radars might prove to be a cost effective alternative to current surveillance radars, since the number of required radars would be reduced, and maintenance and logistics infrastructure would be consolidated. A radar configuration that provides terminal-area and long-range aircraft surveillance and weather measurement capability is described and a radar network design that replicates or exceeds current airspace coverage is presented. Key technology issues are examined, including transmit-receive elements, overlapped sub-arrays, the digital beamformer, and weather and aircraft post-processing algorithms. We conclude by discussing implications relative to future national weather and non-cooperative aircraft target surveillance needs. The U.S. Government currently operates four separate ground based surveillance radar networks supporting public and aviation-specific weather warnings and advisories, and primary or "skin paint" aircraft surveillance. The separate networks are: (i) The 10-cm wavelength NEXRAD or WSR88-D (Serafin and Wilson, 2000) national-scale weather radar network. This is managed jointly by the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DoD). (ii) The 5-cm wavelength Terminal Doppler Weather Radars (TDWR) (Evans and Turnbull, 1989) deployed at large airports to detect low-altitude wind-shear phenomena. (iii) The 10-cm wavelength Airport Surveillance Radars (ASR-9 and ASR-11) (Taylor and Brunins, 1985) providing terminal area primary aircraft surveillance and vertically averaged precipitation reflectivity measurements. (iv) The 30-cm wavelength Air Route Surveillance Radars (ARSR-1, 2, 3 and 4) (Weber, 2005) that provide national-scale primary aircraft surveillance. The latter three networks are managed primarily by the FAA, although the DoD operates a limited number of ASRs and has partial responsibility for maintenance of the ARSR network. In total there are 513 of these radars in the contiguous United States (CONUS), Alaska, and Hawaii. The agencies that maintain these radars conduct various "life extension" activities that are projected to extend their operational life to approximately 2020. At this time, there are no defined programs to acquire replacement radars. The NWS and FAA have recently begun exploratory research on the capabilities and technology issues related to the use of multi-function phased array radar (MPAR) as a possible replacement approach. A key concept is that the MPAR network could provide both weather and primary aircraft surveillance, thereby reducing the total number of ground-based radars. In addition, MPAR surveillance capabilities would likely exceed those of current operational radars, for example, by providing more frequent weather volume scans and by providing vertical resolution and height estimates for primary aircraft targets. Table 1 summarizes the capabilities of current U.S. surveillance radars. These are approximations and do not fully capture variations in capability as a function, for example, of range or operating mode. A key observation is that significant variation in update rates between the aircraft and weather surveillance functions are currently achieved by using fundamentally different antenna patterns--low-gain vertical "fan beams" for aircraft surveillance that are scanned in azimuth only, versus high-gain weather radar "pencil beams" that are scanned volumetrically at much lower update rates. Note also that, if expressed in consistent units, the power-aperture products of the weather radars significantly exceed those of the ASRs and ARSRs. In the next section, we present a concept design for MPAR and demonstrate that it can simultaneously provide the measurement capabilities summarized in Table 1. In Section 3 we present an MPAR network concept that duplicates the airspace coverage provided by the current multiple radar networks. Section 4 discusses technology issues and associated cost considerations. We conclude in Section 5 by discussing implications relative to future national weather and non-cooperative aircraft target surveillance needs.

The integration of Unmanned Aerial Vehicles (UAVs) into civil airspace requires new methods of ensuring collision avoidance. Concerns over command and control latency, vehicle performance, reliability of autonomous functions, and interoperability of sense-and-avoid systems with the Traffic Alert and Collision Avoidance System (TCAS) and Air Traffic Control must be resolved. This paper describes the safety evaluation process that the international community has deemed necessary to certify such systems. The process focuses on a statistically-valid estimate of collision avoidance performance developed through a combination of airspace encounter modeling, fast-time simulation of the collision avoidance system across millions of encounter scenarios, and system failure and event sensitivity analysis. Example simulation results are provided for an implementation of the analysis process currently being used to evaluate TCAS on the Global Hawk UAV.

Aviation planners have called for increasing the capacity of the air transportation system by factors of two or three over the next 20 years. The inherent spatial capacity of en route airspace appears able to accommodate such traffic densities. But controller workload presents a formidable obstacle to achieving such goals. New approaches to providing separation assurance are being investigated to overcome workload limitations and allow airspace capacity to be fully utilized. One approach is to employ computer automation as the basis for separation-assurance task. This would permit traffic densities that exceed the level at which human cognition and decision-making can assure separation. One of the challenges that must be faced involves the ability of such highly automated systems to maintain safety in the presence of inevitable subsystem faults, including the complete failure of the supporting computer system. Traffic density and flow complexity will make it impossible for human service providers to safely reinitiate manual control in the event of computer failure, so the automated system must have inherent fail-soft features. This paper presents a preliminary analysis of the ability of a highly automated separation assurance system to tolerate general types of faults such as nonconformance and computer outages. Safety-related design features are defined using the Advanced Airspace Concept (AAC) as the base architecture. Special attention is given to the impact of a severe failure in which all computer support is terminated within a defined region. The growth and decay of risk during an outage is evaluated using fault tree methods that integrate risk over time. It is shown that when a conflict free plan covers the region of the outage, this plan can be used to safely transition aircraft to regions where service can still be provided.

The integration of Remotely Piloted Vehicles (RF'Vs) into civil airspace will require new methods of ensuring aircraft separation. This paper discusses issues affecting requirements for RPV traffic avoidance systems and for performing the safety evaluations that will be necessary to certify such systems. The paper outlines current ways in which traffic avoidance is assured depending on the type of airspace and type of traffic that is encountered. Alternative methods for RPVs to perform traffic avoidance are discussed, including the potential use of new see-and-avoid sensors or the Traffic Alert and Collision Avoidance System (TCAS). Finally, the paper outlines an established safety evaluation process that can be adapted to assure regulatory authorities that RPVs meet level of safety requirements.

The integration of Remotely Piloted Vehicles (RPVs) into civil airspace will require new methods of ensuring traffic avoidance. This paper discusses issues affecting requirements for RPV traffic avoidance systems and describes the safety evaluation process that the international community has deemed necessary to certify such systems. Alternative methods for RPVs to perform traffic avoidance are discussed, including the potential use of new see-and- avoid sensors or the Traffic Alert and Collision Avoidance System (TCAS). Concerns that must be addressed to allow the use of TCAS on RPVs are presented. The paper then details the safety evaluation process that is being implemented to evaluate the safety of TCAS on Global Hawk. The same evaluation process can be extended to other RPVs and traffic avoidance systems for which thorough safety analyses will also be required.

Estimating the relative safety of a Remotely Piloted Vehicle (RPV) equipped with ACAS will require several extensions to the methods developed in previous ACAS studies. This paper outlines several of these redesign issues. First, it may be necessary to compute the probability that an RPV will experience a critical encounter relative to that for a conventional aircraft. Performing a safety study on only the incremental impact of equipping an RPV with ACAS would circumvent this need. Additionally, methods are proposed to adapt existing encounter models to better represent the likely characteristics of encounters with RPVs. Finally, modifications to the level of detail included in dynamic simulations and fault trees are discussed. It is proposed to shift all dynamic elements out of the fault tree and into a new more complex Monte Carlo simulation.

This paper presents an analysis of the Doppler processing technology currently in use in the nation's terminal airport surveillance radars, and examines possibilities for performance improvement, particularly in the presence of moving clutter. The research focuses on five- and eight-pulse waveform methodologies and their respective detection capabilities given clearly defined rain clutter scenarios. Performance with fixed coefficient filters similar to those used in the existing radars is calculated, followed by performance using an adaptive Doppler filtering technique. Performance is quantified in terms of signal-to-interference ratio at the output of the Doppler filters and resultant probability of detection given a specified probability of false alarm. The results will show that a substantial improvement in detection in the vicinity of rain clutter is realized for both the five- and eight-pulse waveforms when using the adaptive coefficient Doppler filters as compared to the performance observed with the fixed coefficient filters. For constant filter weights, the eight-pulse Doppler filters give significantly better performance in most diverse rain clutter than the five-pulse Doppler filters.