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Aviation History
1985
1985 - 0688.PDF
The British Aerospace laser inertial navigation system, under development for RAE flight-test in January 1986, uses 30cm path-length ring laser gyros inspection techniques, as any inclusion in the substrate will not be "filled in" during deposition. BAe now produces three laser-gyro sizes: a low-accuracy, 2°/hr 12cm gyro for missiles; a medium-accuracy, 0-002°/hr 30cm RLG for aircraft; and a high- accuracy, 0 • 0005°/hr 70cm RLG for ships. In January the company completed interim expansion of its RLG production facility, enabling it to produce 12 laser gyros a month. The company is currently developing Triad, a three-axis "monoblock" laser gyro which it sees as the replacement for ring laser gyros in the 1990s. All three axes are measured by the same gyro, which there fore becomes an inertial measurement unit (IMU), replacing three single-axis RLGs. Fewer mirrors are required, and the Triad gyro promises to be cheaper to produce than a three-RLG IMU, says BAe. The first Triad application is likely to be in missiles, where high accuracy is not required because of the short flight times. This is because of a phenomenon exhibited by all laser gyros—frequency lock. When the gyro is stationary, scatter from the mirrors results in the opposing beams becoming "locked" together and remaining locked until the rotation rate exceeds a certain value. To aviod this the single-axis gyro is vibrated at a resonant frequency outside this dead zone by a piezo-electric "dither" mechanism. This dither must then be removed optically or electronically before rate can be measured. Dithering a three-axis gyro presents something of a challenge. A dither axis must be chosen which produces vibration components in all three axes sufficient to overcome frequency lock. This currently limits the accuracy of Triad gyros using mechanical dither. The three-axis gyro will come into its own, BAe believes, when the magnetic mirror is perfected. This uses the ferro-magnetic properties of an yttrium iron garnet layer deposited beneath the mirror surface to change the phase of the reflected beam relative to the incident beam, thereby avoiding frequency lock and the need for dithering. Magnetic mirrors should enable the prod uction of high-accuracy Triad gyros in the 1990s, BAe believes. Meanwhile, BAe's laser INS spearheads its entry into the integrated navigation system business. The company is proposing its laser INS for Air Staff Target 404, the RAF's Wessex/Puma support helicopter replacement, for EH.101, the Royal Navy's next anti submarine helicopter, for Tornado improvements, and for combat aircraft such as BAe's single-seat Hawk 200. BAe's EH.101 proposal includes its MAHRS-701 Microflex attitude and heading reference system, as a backup to the laser INS in the naval variant, and on its own in the commercial version. The Microflex AHRS is also on offer for the RAF's turboprop basic trainer, AST 412. This low-cost system uses two two-axis flex rate gyros and provides an accuracy of 4-5 n.m./hr. Ferranti's Edinburgh-based Navigation Systems Department is already a leading supplier of military inertial systems. The company expects to produce 1,800 to 2,000 26 of its FIN1000 series of digital nav/attack systems, and the backbone of current production is the FIN1010 INS for the Tornado, with more than 500 delivered. In operational service FIN1010 is achieving 0-8 n.m./hr against its 1 n.m./hr specifica tion, says Ferranti. FIN1010 has several derivatives, includ ing the FIN1012 installed in RAF Nimrod MR.2s and AEW.3s, and the export FIN1040 in Japan's Mitsubishi F.l. A major derivative is the FIN1064 nav/ attack system now replacing analogue equipment in RAF Jaguars, with some 160. systems required. FIN1064 derivatives include the FIN1063 INS for the RAF Buccaneer update, with 60 systems required; the databus-compatible FIN1070 INS for the British Aerospace Experimental Aircraft Programme (EAP); and the FIN1075, a reboxed 1064 proposed A ring laser gyro is, in fact, a rate sensor (see tone box on opposite page) for the RAF Harrier GR.5. As the Buccaneer is a single-role aircraft, intended only to carry the Sea Eagle anti-ship missile, FIN1064's weapon-aiming circuitry will be redun dant in its 1063 derivative. Similarly, weapon-aiming functions will not be used in the 1553B -compatible FIN1070 for the EAP. Ferranti is supplying three FIN1070s to BAe, one for the ground avionics rig, a flight unit, and a spare. Ferranti's FIN1100 series of low-cost strapdown systems uses floating gyros, as does the fully gimballed FIN 1000 series. First application for the FIN1100 is the attitude, heading, and navigation system for the Casa C.101DD, for which Ferranti is supplying the Fastac flexible avionics system for training and combat. FIN1110, whimsically named 2 Gins for two-gyro inertial navigation system, features one floated (azimuth) and one dry (attitude) gyro. Its first application is the Hughes 530MG combat helicopter. The system is undergoing flight trials in a Royal Navy airborne early warning Sea King helicopter as a means of improving antenna stabilisation for the Thorn-EMI Searchwater radar. Ferranti has proposed FIN1110 for the AST 404 support helicopter and for the EH.101. The Royal Navy's EH.101 speci fication calls for Navstar global posi tioning system (GPS) capability, and Ferranti says it has already studied GPS/ inertial mixing with FIN1110. In the FIN2000 series of inertial naviga tion systems two two-axis dry gyros replace three single-axis floated gyros. The result is half the size and two-thirds the cost of a FIN1000 series INS, yet FIN2000 provides 1 n.m./hr accuracy after 5min gyrocompassing alignment. Developed as a private-venture and intended for retrofit, FIN2000 has yet to find a major customer. Ferranti has FLIGHT International, 9 March 1985
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