Unmanned Aerial Vehicles (UAVs) are becoming an increasingly indispensable part of modern warfare, since they allow baThlefield surveillance, reconnaissance, and communications without imminent danger to an operator or "pilot" who is maneuvering the aircraftoften from a considerable distance. Of course, these are not the simple radio-controlled planes of hobbyist fame, but are sophisticated aircraftwith advanced computing, sensors, avionics, radar, and communications systems on board. Because of the need to precisely control these vehicles across long distances, UAVs rely heavily on their electronic systems and components for successful missions with safe returns.

The United States Army has developed a long-term plan for its development of advanced UAV technology in its Unmanned AircraftSystems (UAS) Roadmap. The document is conceptual in nature, but outlines developmental and organizational needs for UAVs from 2010 to 2035. The UAS program builds upon the legacy of the Pioneer UAV, with more than 300 successful combat missions in 1991 in Iraq and Afghanistan during Operation Desert Shield/Storm. The Army currently has over 1000 UAVs fielded with more than one million flight hours in support of combat operations. In the near term, the plan calls for further integration of UAVs into combat operations mainly for intelligence, surveillance, and reconnaissance. The Army also hopes to apply reduced size, weight, and power (SWaP) goals as part of the development of new UAVs, with an eye toward exploring new propulsion systems and even fully baThery-powered motor and control systems.

This past July, various Army technology development offices conducted an exercise at White Sands Missile Range, NM to evaluate the effectiveness of communications for many different battlefield elements, including soldiers and UAVs, as part of a wide-reaching wireless military communications and baThle-control network. The exercise brought together technology for the infantryman on the ground, the mounted soldier using the latest in armored vehicles, the aviator, the UAV operator, and the commander back at base. The site, with its abandoned mines and tunnels, provided similar conditions to those found in Afghanistan. Systems under evaluation included the Land Warrior system as part of an ad hoc network. The use of UAVs, including a new, longer-flight-time version of the Shadow (Fig. 1), and their remote sensors and communications equipment allowed the network to be extended beyond mountains that would normally obstruct communications.

This past May, the Army also announced that it had achieved one million hours of flight for its unmanned aerial systems. The hours were logged by a variety of systems, including the MQ-1C Extended Range Multi-Purpose UAS, the RQ-7B Shadow, and the RQ-11B Raven. About 88 percent of the total hours were flown in Iraq and Afghanistan, with the RQ-7B Shadow accounting for more than 478,000 hours. Colonel Gregory B. Gonzalez, Army Project Manager for unmanned aircraft systems, said that since the Army began really experimenting with UAS, a lot has changed: "Acceptance of unmanned aircraftsystems was not immediate. Upon their introduction into the Army inventory, unmanned aircraftwere met with some levels of skepticism and doubt. But after initial inefficiencies were overcome and improvements were made, these doubts turned to acceptance." With the increased demand for UAS support, the Army now flies more than 220,000 unmanned aircrafthours each year. The use of unmanned vehicles by the armed forces is certainly not an overnight phenomenon, and it wasn't even a US innovation. Inspired by the success of Israeli troops in using unmanned vehicles in the early 1980s for intelligence gathering, reconnaissance, and surveillance, one of the first US UAV procurements was made by the US Navy from a company then known as Pioneer UAV, Inc. (now AAI Corp.) working with Israeli AircraftIndustries. The Pioneer RQ-2B UAV was delivered to the US Navy in July 1986 and deployed on the USS Iowa battleship in December 1986. Designed to perform reconnaissance, surveillance, target acquisition, and battle damage assessment, additional models of the low-infrared (IR) signature, low radar-cross-section (RCS) aircraft were also delivered to the US Marines for use in the Persian Gulf, Bosnia, Yugoslavia, and Somalia.

The Pioneer UAV is launched by means of rocket-assisted takeoffor pneumatic rails, and recovered by net, ground landing, or sea landing. The aircraft has a length of 14 ft. with a 16.9-ft. wingspan. It has a ceiling of about 15,000 ft. and a range of about 100 nautical miles with an air speed of about 110 knots. With a payload of 65 to 100 lbs., it can patrol for more than five hours and provide imaging to a soldier in the field carrying a manpack receiving station.

Today, the number and types of UAVs have grown considerably since those humble beginnings. They can be categorized by size or tier, with Tier I vehicles representing low-altitude UAVs, Tier II comprising mediumaltitude, long-endurance vehicles, and Tier III including high altitude, longendurance aircraft. Different branches of the Armed Forces rely on different aircraft, with different models falling into different tiers. The RQ-7 Shadow UAV, for example, developed by AAI Corp. is used by both the US Army and US Marine Corps. It is launched from a trailer-mounted pneumatic catapult and features a liquid-nitrogen-cooled electrooptical and infrared (IR) camera and C-band LOS link to send high-resolution images and real-time video to a control station. Similarly, the US Army and Air Force both operate MQ-1 Predator and MQ-9 UAVs.

In general, however, the smaller Tier I UAVs include the Desert Hawk, developed by Lockheed Martin and the Raven, from AeroVironment. Larger UAVs in the second tier include the Scan Eagle from Insitu, the Shadow from AAI Corp., and the Sentry from DRS Technologies. The largest UAVs in Tier III are the Global Hawk from Northrop Grumman, the rotary-wing FireScout from Boeing Co., and the Predator from General Atomics.

The electronic technology aboard a UAV varies with the design and mission capability of the vehicle. For example, the Predator (Fig. 2) as used by the US Air Force has two models, the RQ-1 and the MQ-1. The RQ-1 Predator is designed for surveillance missions and captures images behind enemy lines from synthetic aperture radar (SAR), video cameras, and a forward-looking-infrared (FLIR) system that can be distributed in real time to front line soldiers and to the operational commander via line-of-sight and satellite communications links. The MQ-1 version of the Predator is armed with a multispectral targeting system (MTS), sensors to detect wind speed and direction and other targeting information, and a pair of AGM-114 Hellfire missiles for assault missions.

The MQ-1 Predator UAV uses two fuel tanks to carry as much as 600 lbs. of jet fuel; the RQ-1 has a 100-gallon fuel tank and maximum payload of 450 lbs. Both rely on a C-band line-of-sight (LOS) communications links when in range and Ku-band Common Data Link (CDL) satellite communications (satcom) for distances beyond the range of the LOS link. (In addition to C-band links, some medium-sized UAVs have also employed LOS links operating at S-band and L-band frequencies.) Navigation is aided by on on-board Global Positioning System (GPS) receiver. Image data is gathered with the aid of a variable-aperture video camera, a variable-aperture IR camera for night viewing, and a SAR system for cuffing through cloud and storm cover. The UAV includes two 8-lb. nickelcadmium (NiCd) rechargeable battery packs for backup power in addition to a dedicated power-generation system (somewhat similar to a conventional commercial automotive alternator).

The size of each platform will dictate the amount of electronics that can be carried. The larger UAVs, for example, can carry the weight of a Ku-band antenna to use satcom as a means of sending critical data to a control station as well as to soldiers in the field. Smaller vehicles may only be equipped with C-band LOS links because of the need to reduce payload weight as much as possible. For example, among the smallest of UAVs, the Dragon Eye is used by Marines in Iraq by force-protection troops to search the waters ahead of a submarine for threats. The compact vehicle weighs only about five pounds and can be disassembled into five pieces and stored in a small suitcase. Developed by Naval Research Labs (NRL) as a stealth information-gathering tool, the Dragon Eye flies with a quiet electric motor that runs for about 50 minutes flying time on a single battery charge and can achieve a range of 40 km and altitudes to 10,000 ft .

Although the long-term vision of many military studies points to a fully networked, remote-controlled air space filled with UAVs from different service branches, the reality of current platforms is that interoperability is limited by available bandwidth. While many larger UAVs have access to satcom links, the smaller UAVs are essentially gathering information and sending data back by means of a short-range LOS microwave link. And many of these microwave links are based on spectrally inefficient analog modulation techniques, such as frequency modulation (FM). If too many of these UAV systems are operating at the same time, not only are forward (control of the UAV) signals lost, but return data is lost as well.

For that reason, many providers of microwave data links for UAVs are working on ways to either replace or adapt the existing analog links. Israeli company Elisra, for example, markets Starlink, which is a C-band data link for tactical UAVs. It uses time-divisionduplex (TDD) techniques to achieve spectral efficiency of 4 MHz/channel and can work in single-channel or frequencyhopping modes. The data link connects UAVs to operators and control stations at distances to 60 miles.

As another example, EnerLinks II full-duplex data links from EnerDyne Technologies provide digital video over analog FM links for a number of UAV platforms. They work with the existing transmitter, power amplifier, filter, and antenna (at L- and S-band frequencies) to provide highquality compressed H.264 video and network data traffic with as much as 12 dB improvement in link margin over the existing analog link system.

Although UAVs may be considered mainly for tactical applications, government agencies such as NASA are finding other uses for the remotecontrolled vehicles. The space agency has been working with General Atomics Aeronautical Systems as part of the Environmental Research Aircraft and Sensor Technology (ERAST) program at NASA's Dryden Flight Research Center at Edwards Air Force Base, CA to develop a modified version of the General Atomics' Predator B UAV for high-altitude missions. The extended-wingspan NASA version of the aircraft (Fig. 3) is designed for high-altitude earth research missions to 65,000 ft . Even military UAVs can be equipped for emergency applications, outfit with food and medical supplies when needed. The usefulness of remotely piloted vehicles is reaching beyond the battlefield and to such applications as disaster relief and for border patrols as part of expanded Department of Homeland Security (DHS) operations.