Introduction to UAV Systems. Mohammad H. Sadraey

(C2) of the NASA Global Hawk from the Dryden Flight Research Center is applied in two distinct regions: (1) The line‐of‐sight (LOS) and (2) The beyond line‐of‐sight (BLOS). The communications link used for LOS are through UHF/VHF links. The primary communications links used for BLOS are two Iridium Satcom links. However, an Inmarsat Satcom link provides a backup communications capability.

      The NASA Global Hawk payload communications architecture is independent of the communications links utilized to control the aircraft. Four dedicated Iridium SatCom communication links are used for continuous narrow band communications between the ground station and the UAV payloads. Moreover, two additional Iridium links are used to monitor power consumption by individual payloads, and to control features such as lasers and dropsonde. The use of the Iridium system provides a complete global coverage, including the Polar regions.

      1.5.5 Development Setbacks

      During Global Hawk flight tests programs and long operations, there were a number of setbacks [4], where a few resulted in the loss of the air vehicle and one caused damage to the sensor suite of another air vehicle.

      The major setback during flight testing was the destruction of air vehicle 2 on March 29, 1999. The aircraft experienced an uneventful liftoff from the runway at Edwards Air Force Base (AFB). As it was climbing, the air vehicle unexpectedly flipped over on its back, shut down its engine, and locked the flight controls into a death spin. The aircraft executed the termination command and crashed. The crash was due to a lack of proper frequency coordination between the Nellis AFB and Edwards AFB flight test ranges.

      In December 1999, a software problem caused another Global Hawk to accelerate to an excessive taxi speed after a successful, full stop landing on Edwards’ main runway. An error in software code to coordinate between the mission planning system and the aircraft commanded the vehicle to taxi at 155 knots. The nose gear collapsed causing $5.3 million worth of damage to the electro‐optical/infra‐red (EO/IR) sensors. The primary cause of this mishap was the execution of a commanded ground speed of 155 knots for a taxi on the contingency mission plan.

      During the deployment phase, two of the prototype air vehicles were lost and sustained. The first loss occurred on December 30, 2001, when the Global Hawk was returning from a truncated operational mission in support of Operation Enduring Freedom. To help a descent at 54,000 ft, four spoilers were raised to the maximum deflection (45 degrees), which caused a turbulent air‐induced flutter. The subsequent energy of the resultant flutter was absorbed by the right V‐tail main spar. The right outboard ruddervator actuator control rod failed, allowing the ruddervator to travel unrestrained beyond its normal range. Then, the vehicle departed controlled flight, entered a right spin, and crashed. The loss was attributed to a structural failure of the right ruddervator assembly of the V‐tail (massive delamination of the main spar).

      The second loss occurred on July 10, 2002, when a Global Hawk was flying an operational mission in Operation Enduring Freedom. The mishap vehicle experienced a catastrophic engine failure and glided for about half an hour. The vehicle impacted the ground during the attempted emergency landing. The mishap was attributed to a fuel nozzle failure in the high flow position that eventually led to the engine internal failure.

      Another loss was when on June 20, 2019, Iran shot down a Global Hawk with a surface‐to‐air missile over the Strait of Hormuz. Iran said that the UAV violated its airspace, while US officials responded that the air vehicle was flying in international airspace.

      These real stories provided valuable lessons and presented expensive experiences for young UAV designers. As typical of any development program, the Global Hawk design changed as the result of flight tests.

1.6 Predator Family

      1.6.1 Predator Development

      RQ‐1 Predator is a long‐endurance, medium‐altitude unmanned aircraft system for surveillance, reconnaissance, and attack missions, designed and manufactured by General Atomics Aeronautical Systems.

      The Predator had an unconventional development cycle with origins going back to a project by Abraham E. Karem. He is a pioneer in innovative fixed and rotary‐wing unmanned vehicles and is regarded as one of the founding fathers of UAV technology. Initially, by 1983, a small long‐endurance tactical reconnaissance UAV prototype was developed called the Albatross for the DARPA. Then, by 1988, further development resulted in a more advanced design, the Amber, which was followed by the GNAT 750. Karem’s company (Karem Aircraft, Inc.) and its UAV were soon acquired by General Atomics.

      The CIA utilized the GNAT 750 in military operations over Bosnia in 1993 and 1994. The program suffered from a few weaknesses, but it held enough promise that the DOD expressed interest in a larger, more advanced version of the GNAT 750 for medium‐altitude reconnaissance, then designated RQ‐1 Predator. By 1995, it became operational over Bosnia. In parallel, the Air Force saw the Predator as a new tool in tactical reconnaissance with the added benefit of a live satellite data link.

      In the late 1990s, Predator’s capability was expanded to include a laser designator to illuminate targets and guide weapons dropped from other aircraft. In 1999, the UAV had its first significant test during Operation Allied Force in Kosovo. By 2000, due to concern over the rising threat of al Qaeda, the Predator was scheduled for arming with the Hellfire laser‐guided missile.

      After the September 11, 2001 attacks, the armed Predator become fully operational, and by January 2003, flew 164 missions over Afghanistan. The armed Predator – capable of both reconnaissance and attack missions – has continued to have a pivotal role in combat operations. In 2002, the Air Force adapted a Predator to carry Stinger missiles and attempted an air‐to‐air engagement with an Iraqi MiG‐25, but resulted in the loss of the Predator.

      Predator UAVs have been operational since 1995 in support of NATO, UN, and US operations, and as part of Operation Enduring Freedom in Afghanistan and Operation Iraqi Freedom, flying more than 500,000 flight hours. The US Air Force Predator production ended in 2011 with 268 air vehicles manufactured. Hundreds of Predators have been sold to a number of countries including Italy, Spain, France, UK, Australia, Netherlands, Canada, and Germany.

      This military UAV has been used in the Balkans, Afghanistan, Iraq, and other global locations. By 2011, the US military had nearly 11,000 UAVs on their inventory, including hundreds of Predators. The Predator was retired in 2018. The Predator‐series family encompasses MQ‐1 Predator, MQ‐1C Gray Eagle, MQ‐9 Reaper (Predator B), MQ‐9B SkyGuardian, and Predator C Avenger.

      1.6.2 Reaper

      After about 10 years of Predator operations, and when some weaknesses were identified, new challenges arose in employing Predator. DOD decided to have a new version of Predator with enhanced performance features and an advanced design. The operation requirements included such performance items as a faster cruising speed and higher flight altitude, and also heavier and more advanced payloads. The conceptual design and the air vehicle configuration were almost kept. The only major configuration change was to have a V‐tail instead of an inverted V‐tail.

The US Air Force first deployed the MQ‐9 Reaper (developed by General Atomics, then called Predator B) to Afghanistan in October 2007 for precision airstrikes and it slowly began replacing the Predator. The General Atomics MQ‐9 Reaper (Figure 1.5) flew its first operational mission in Iraq in July 2008. In the meantime, the Army began development of a refined derivative, the MQ‐1C Gray Eagle, which began operations in 2012. The US Air Force retired the Predator in 2018, replacing it with the Reaper.

Figure 1.5 General Atomics MQ‐9 Reaper

      Left: A British