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Inertial navigation systems
Published in Mike Tooley, David Wyatt, Aircraft Communications and Navigation Systems, 2017
The original inertial navigation systems used electromechanical gyros; these were subsequently replaced by a more reliable and accurate technology: the ring laser gyro (RLG). Ring laser gyros use interference of a laser beam within an optic path, or ring, to detect rotational displacement. An IRU contains three such devices (see Figure 17.8) for measuring changes in pitch, roll and azimuth. (Note that laser gyros are not actually gyroscopes in the strict sense of the word—they are in fact sensors of angular rate of rotation about an axis.) Two laser beams are transmitted in opposite directions (contrarotating) around a cavity within a triangular block of cervit glass; mirrors are located in two of the corners. The cervit glass (ceramic) material is very hard and has an ultra-low thermal expansion coefficient. The two laser beams travel the same distance, but in opposite directions; with a stationary RLG, they arrive at the detector at the same time.
Fiber-Optic Sensors
Published in Rajpal S. Sirohi, Mahendra P. Kothiyal, Optical Components, Systems, and Measurement Techniques, 2017
Rajpal S. Sirohi, Mahendra P. Kothiyal
The FO gyroscope is a low cost, all solid-state device that has a very wide application range. It does not exhibit the lock-in effect that prevents the ring laser gyroscope from performing below a certain rotation rate.
Autonomous underwater vehicles - challenging developments and technological maturity towards strategic swarm robotics systems
Published in Marine Georesources & Geotechnology, 2019
N. Vedachalam, R. Ramesh, V. Bala Naga Jyothi, V. Doss Prakash, G. A. Ramadass
Precision gyroscopes for measuring the vehicle attitude changes in the angular DOF include the ring laser gyroscopes (RLG) and the interferometeric fibre optic gyroscope (I-FOG). Their technologies are in a highly advanced stage with extremely low bias stability, low angular random walk and capable of determining the true north using the true north seeking algorithms (Lefevre 2014a; Zhang and Liu 2017).These gyroscopes are immune to earth’s magnetic field and hence capable of providing effective measurements in the Polar Regions where magnetic field lines are near-vertical impairing use of traditional magnetic compasses (Lefevre 2014b). The RLG has attained full technological maturity with the ultimate challenge of “lock-in effect” experienced at very low rotation rates being overcome by the mechanical dithering technique. But RLG needs a larger volume to increase the length of the optical cavity, which makes it expensive and bulky (Lefevre 2013). The I-FOG characterized by their smaller footprint, light weight, wide dynamic range and faster response have attracted significant interest. The residual limit of the I-FOG bias stability is due to the ambient temperature time-transient, which when controlled could result in a long term bias stability of about 10−5°/h and capable of meeting the strategic grade requirements for underwater navigation (Ramadass et al. 2017). The advancements in the navigation grade accelerometer technologies and vehicle velocity measurements based on Doppler velocity has given a significant confidence to realize precise Doppler velocity-aided NS (Ellingsen 2008).