Helicopters and small aircraft flying at low altitude in the visual flight rules often strike against obstacles. According to the aircraft accident reports in Japan, many collisions by small aircraft were caused by long, thin artificial objects as power lines because they are often very difficult to find by pilot eyes even when visibility is good for flight
Saying a March 2018 helicopter crash in Iraq that killed four members of a Long Island-based National Guard rescue team could have been avoided, Sen. Chuck Schumer has asked the Senate Armed Services Committee to press the Pentagon to ensure that military aerial rescue teams “are properly equipped to complete their missions safely and effectively.” “The tragic Iraq crash of ‘Jolly 51’ concerns me and a number of my colleagues, who believe insufficient progress has been made in equipping our aircrews with the best available technologies,” Schumer (D-N.Y.) said in a statement.
The crash occurred as the seven-member crew was part of a mission to preposition equipment at a landing zone closer to a military operation. All died. Four of the dead were members of the 106th Rescue Wing, based in Westhampton Beach.
An Investigation Board report released in October said “pilot error” led to the crew to misinterpret navigation displays, overshoot the target, and descend over an unplanned location. The investigation identified several errors leading up to the crash. The helicopter accidentally flew past its landing zone when it made a wrong turn, yet the aircraft’s crew was sure it was heading in the right direction even though their navigation equipment indicated that they had passed their destination.
Part of the confusion stems from the navigation route that the crew planned, according to the investigation. The route contained seven “navigational waypoints” to help orient the crew, but only the first three led to the landing zone. The remaining four were not part of the mission that night. They were intended to be used only if two helicopters needed to be on alert for rescue missions and were meant to be flown at higher altitudes.
On the way to the landing zone, the helicopter’s pilot was repeatedly interrupted by his wingman, his crew, and a Joint Terminal Attack Controller at the landing zone, the investigation found. These “non-navigation related tasks” ate up so much time for the crew of both helicopters that it “reduced their time available to identify their navigation error.”
The helicopter became entangled in a 3/8-inch diameter galvanized steel cable strung horizontally between two 341-foot high towers and tumbled into the desert. Once the cable became tangled around the main rotor, the helicopter “suffered catastrophic structural failures and was completely uncontrollable prior to impact with the ground,” the investigation found.
Because it was so dark that night, none of the crew could see their power lines with their night vision goggles, according to the investigation. None of the crew called out seeing the power lines before the helicopter flew into them.
In a letter Wednesday to the committee’s chairman and ranking Democrat, Schumer asked that Congress require the head of the U.S. Special Forces Command to provide a briefing on what the military is doing to “evaluate and procure systems that enable aircrews to detect uncharted wires and obstacles.”
Members of the senator’s staff said Schumer was not aware of any existing equipment that could have prevented the crash, but wants the Pentagon to press for the development of such detection technology.
In 2002, Japanese Researchers developed a 94 GHz experimental MMW radar to test the validity as the sensor for the obstacle detection and warning system for civil helicopters. Two types of modulation, pulse and FMCW for the radar have been investigated.
A lot of work has been done to avoid such collisions by developing obstacle sensors . These sensors are mainly designed to expand the operational conditions for military applications. In order to reduce such collisions, however, techniques to detect obstacles that are very difficult to find, must be developed. It is also important to develop an obstacle detection and warning system easily applicable to civil aircraft.
The Obstacle Detection and Warning System for civil aircraft flying in the visual flight rules must be low in cost, compact and light in weight.
Millimeter Wave Radar for the Obstacle Detection and Warning System for Helicopters
Japanese Researchers led by K. Yamamoto reported on development of an obstacle detection and warning system for civil helicopters. An infrared camera and a 94 GHz millimeter wave (MMW) radar have been used as its sensor. Recently, IR camera becomes small and light (approx 90×85×85 mm, 500g) and the price is about 1/8 compared to 5 years ago. MMW radars are also becoming cheaper and smaller.
The system consists of an IR camera, a MMW radar, Scanning Equipment, a Controller, a Data Processor and a Display. It has been demonstrated by Yamamoto and Yamada that a higher resolution IR sensor is suitable to detect a long and thin object as a power line . The author introduced an IR camera detecting 3−5μm IR with 500×800 IR sensing elements.
They chose 940Hz as the radar frequency because there are many components and high gain antennas in this frequency.
A Voltage Controlled Oscillator (VCO) driven by a triangular wave generator (Linearizer) produces 94GHz FMCW signal. The signal is divided into transmitting and reference signals by the directional coupler, The transmitting power of this radar is about +11dBm. The signal backscattered from objects in the radar field of view, enters the receiving antenna and is amplified by a low noise Mmw amplifier (Gain: 36dB, NF: 5dB). It is then mixed with the reference signal to obtain the IF (beat). The IF is amplified and then supplied to the Vector Signal Analyzer (Agilent Technology 89610A). The IF contains the frequency shift by the range to the target and the Doppler shift due to relative target, motion. The vector signal analyzer and PC calculate the beat frequency and the frequency shift to derive the range and relative speed. The MMW modulation bandwidth is 148MHz and the modulation cycle is 10KHz.
At present, two Cassegrain antennas are used for transmission and reception. The diameter of the major parabola is 31cm and the minor one 6cm to have the gain of 45dBi. The polarization is vertical. However, the Cassegrain antenna is rather large for small aircraft.
Vivaldi antenna is then studied because it is easy to constitute a printed array and is suitable for monopulse radar in the future. In order to make the radar compact and to facilitate production of active antennas, application of Vivaldi antenna to the MMW radar is proposed by Migliaccio et al.
Measured results demonstrated that the experimental FMCW radar has a satisfactory range and accuracy. Preliminary measurements showed that the range of the 94GHz pulse radar was about 120m and its accuracy was about 7%. It was also shown that the range and accuracy of the FMCW radar were about 500m and 5.5 to 15,5% respectively. It was also shown that the Vivaldi antenna worked well at A 90 to 100 GHz frequency range. The distance between the radar and the reflector was about 100m. The radar could separate the target and the background foliage well.
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