With the emergence of air vehicles like Northrop Grumman's Global Hawk, General Atomics' Predator and the NASA/AeroVironment Pathfinder, the reality of unmanned aircraft is clear. Today, unmanned air vehicles (UAVs) or, to include their ground control components, unmanned aircraft systems (UAS), are proliferating worldwide. A September 2006 study by the Teal Group predicts that the world UAV market will exceed $54 billion over the next 10 years. The U.S. Army alone expects to have 10,000 UAVs by 2011, compared with around 1,200 today. While most of the anticipated expenditure will be directed toward military applications, wide civil use is expected eventually. FAA recently hired Lockheed Martin to "refine" its UAS budgetary requirements and operating plans by developing a five-year roadmap of government and industry UAS initiatives.
But with that approaching surge in activity comes the vital necessity of assuring UAV safety in avoiding collisions with other unmanned and manned aircraft, particularly commercial airliners. Video images taken in 2004 from a military UAV in Afghanistan show it flying head-on toward an airline Airbus A300 on its approach to the Kabul airport, carrying 100 passengers. Only a last-second avoidance maneuver by the Airbus pilot prevented a mid-air collision.
Consequently, the task of detecting, seeing and avoiding (DSA) airborne objects, ranging from balloons, ultralights and other UAVs up to airline jets, is the next challenge for the unmanned aircraft community. The challenge is multifaceted, and also very demanding. In congressional testimony in July 2006, Nick Sabatini, FAA associate administrator for aviation safety, stated: "Currently, there is no recognized technology solution that could make these [UAV] aircraft capable of meeting regulatory requirements for see and avoid, and command and control."
The requirements Sabatini referred to cover operations in the U.S. National Airspace System (NAS), the safety of which is his specific responsibility. Within the NAS, air traffic controllers continuously monitor separation between aircraft, while most larger aircraft supplement this by carrying traffic alert and collision avoidance systems (TCAS), which perform cooperative DSA functions with smaller aircraft equipped only with conventional transponders.
TCAS, however, is not commonly employed in military aircraft, other than transports and certain other types that use civil airspace, and in any event, the system's cost, size and power drain is impractical for any but the very largest UAVs. Most UAVs, in fact, cannot even accommodate the general aviation transponders which TCAS requires on smaller aircraft in order to detect them. And TCAS cannot "see" any aircraft which are neither TCAS- nor transponder-equipped, making it unlikely that UAVs without transponders would be allowed to fly in civil airspace. There is intense pressure within the military and civil UAV community to develop innovative non-TCAS DSA solutions, which could be applied after completing the hierarchy of avoidance processes, starting with air traffic control (ATC) procedural separations.
The Air Force Research Laboratory's (AFRL's) Air Vehicles and Sensors directorates, at Wright-Patterson AFB, Ohio, have been working to develop systems which will provide UAVs with the essential equivalent level of safety (ELOS) to manned aircraft. Control systems engineer Vince Raska, a retired USAF officer with a wide operational background, says that "a common guiding principle is to solve DSA without the requirement for special handling by ATC or changes to the civil airspace infrastructure." Others have noted that the UAVs' unmanned status must be "transparent" to the controller and must appear like a normal manned aircraft.
AFRL's recent approach for large UAVs is the passive ranging and collision avoidance (PaRCA) concept, a control algorithm which computes and commands a deconfliction course for the UAV and then returns to its previous course once safe separation is achieved. PaRCA will include inputs from electro-optical sensors and ranging techniques, and will be evaluated later this year or early FY2007, when it will be installed in a Learjet-simulating a Global Hawk-and flown on test collision scenarios against an FAA Convair-simulating a commuter transport-and an FAA Beech King Air 200, simulating a general aviation twin.
But Raska stresses that AFRL is not developing "final solutions" to the problem, but rather aiming for proof of technical feasibility and assessment of operability. The laboratory's task is to demonstrate whether different concepts meet the requirement at sufficiently low risk to continue their further development into operational systems. Raska adds that FAA has been involved in USAF discussions on overall UAV requirements, and not just on DSA aspects. The agency has been a "vital participant in providing guidance on a number of other UAV issues," he points out.
But DSA protection is not restricted to UAV operations within civil airspace. Ideally, it should apply wherever manned and unmanned aircraft fly, and it is particularly important in military conflict areas such as Iraq, where both are often in close proximity to each other. But even then, the military's move to small, lightweight UAVs for local reconnaissance presents another challenge. These can be small enough to be extremely difficult to detect either by eye or by non-visual means, making avoidance difficult even at close range, yet at the same time they can cause damage if struck. One recent civil development speaks of installing "tiny LED lights" for night use in hopes of making UAVs more visible.
On the other hand, superminiaturized UAVs could possibly be harmless in a mid-air encounter, should the Naval Research Laboratory's CICADA concept-for Close-In Covert Autonomous Disposable Aircraft-come to pass. With 8-inch (20-cm) wingspans, weighing less than 1 ounce (28 g), and described as "flying circuit boards," these devices would glide to individual preprogrammed GPS locations inside enemy territory, where they would become electronic intelligence listening posts, sending reports from their built-in data links to overflying aircraft and even transmitting audible "psychological operations" messages to demoralize enemy troops. A single C-130 Hercules would be expected to carry up to 10,000 CICADAs, for high-altitude release as a "swarm" covering a 2,000 square mile area. Though flying through a swarm of these UAVs might not damage the airframe, the effect on aircraft windscreens and propulsion systems is hard to predict.
Civil researchers are equally busy investigating a wide variety of DSA techniques using video, laser, radar and other sensors. And investigators at the Physical Science Laboratory of New Mexico State University and NASA Dryden have jointly conducted a number of advanced, and successful, tests of these technologies involving a Scaled Composites Proteus, simulating a UAV, flying against aircraft types from a glider to an F/A-18.
Nevertheless, while tracking moving objects in the sky, particularly small ones, is technically challenging, recent research suggests that the objects to be concerned about are those which, paradoxically, appear not to be moving. This approach is based on the very old, but very true, pilot's rule of thumb-possibly derived from earlier marine tradition-that an aircraft seen through the windshield that does not move as it approaches is actually on a collision course, while those that do move across one's field of view are not. In practice, this suggests that a detection system concentrating solely on non-moving objects, while ignoring all others, could be a less demanding technical development.
Currently, all aspects of UAV operations, including military applications, but primarily civil ones, are being extensively examined by RTCA Special Committee SC-203. The committee's membership includes representatives from virtually all government and private aviation interests, who have drafted a comprehensive study of the UAV situation to date. A key group among SC-203 participants is the UAV National Industry Team (UNITE) which includes manufacturers Boeing, Northrop, Lockheed, General Atomics, AeroVironment and Aurora Flight Sciences. Until NASA canceled its funding late last year, UNITE formed the commercial element of the ACCESS-5 government/industry partnership aimed at achieving high-altitude NAS access by UAVs in 2009. While that program is no more, the core UNITE group remains very active.
SC-203 is understood to be finalizing its study document for submission to FAA later this year and, following approval, would forward it to government agencies with UAV interests, such as the departments of Defense, Homeland Security, Commerce, State and others. Detect, sense and avoid is a major component of the study. While the report does not mention specific technical solutions to the problem, it is understood to outline the issues in considerable detail. One of its recommendations is reported to be a relaxation of the very strict adherence to full airworthiness standards for smaller UAVs and their ground-based control stations-constraints which are seen by many as a real impediment to the vehicles' development and application.
Separately, FAA also has drawn up guidelines for UAV operations in the NAS, as have other national authorities such as the UK's National Air Traffic Service. Eurocontrol and the Joint Aviation Authority also assembled a high-level government/industry UAV Task Force in 2003, whose final report was published late last year.
UAVs have reached a high level of maturity in many areas, particularly within the military, and we can only expect this to increase. Already, Lockheed Martin has proposed to offer an unmanned version of its F-35 fighter, an airplane that was never initially planned to be other than a piloted aircraft. Military strategists now expect that almost half, and perhaps more, of the nation's future air strength could be in unmanned vehicles. Detect, sense and avoid will be the key to seeing that happen. And while it may take somewhat longer for FedEx founder Fred Smith to realize his long held dream of unmanned intercontinental air freighters, that too can no longer be seriously discounted.