Introduction to UAV Systems. Mohammad H. Sadraey

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Figure 1.4 Global Hawk

      (Source: Bobbi Zapka / Wikimedia Commons / Public Domain)

Table 1.1 RQ‐4B Global Hawk data and performance

The prime navigation and control system consists of two systems, the Inertial Navigation System and the Global Positioning System (INS/GPS). The aircraft is flown by entering specific way points into the mission plan. The GCS consists of two elements, the Launch and Recovery Element (LRE) and the Mission Control Element (MCE). The LRE is located at the air vehicle base. It launches and recovers the air vehicle and verifies the health and status of the various onboard systems. The MCE is employed to conduct the entire flight from after takeoff until before landing.

      Many changes have been applied in the design of Northrop Grumman RQ‐4B Global Hawk as compared with RQ‐4A. For instance, the RQ‐4B Global Hawk has a 50% payload increase, larger wingspan (130.9 ft) and longer fuselage (47.6 ft), and a new generator to provide 150% more electrical output. Although RQ‐4B carries more fuel than RQ‐4A, it has a slightly shorter range and endurance, due to a heavier maximum takeoff weight.

      1.5.3 Payloads

      Originally RQ‐4A had three sensors (as payload): an Electro‐Optical/Infrared sensor and two Synthetic Aperture Radar Sensors – which are located under the fuselage belly in the integrated sensor suit – have been enhanced for RQ‐4B. The main thrust of the air vehicle changes over time has involved the sensors. The enhancement improves the range of both the SAR and infrared system by approximately 50%.

      1.5.4 Communications System

      The Global Hawk has a wide‐band satellite data link and a line‐of‐sight data link. Data is transferred by: (1) Ku‐band satellite communications, (2) X‐band line‐of‐sight links, and (3) both Satcom and line‐of‐sight links at the UHF‐band. The synthetic aperture radar and ground moving target indicator operates at the X‐band with a 600 MHz bandwidth.

The air traffic control (ATC) and command‐and‐control (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 СКАЧАТЬ