Bluefin Inertial Navigation

 

     Inertial Navigation Systems (INS) use the acceleration detected by sensors like lasers, accelerometers, gyroscopes, etc. and algorithmic equations to calculate their position relative to the frame of the Earth by measuring the centrifugal force from the rotation of the Earth.  The underwater environment increases the complexity of calibrating an INS when compare to systems used in aviation since aviation INSs drift errors are typically updated by the onboard Global Positioning System (GPS).  However, INS is not dependent on GPS in order to function proper.  GPS merely helps eliminate INS drift error which is inherent to modern systems.  In contrast, INSs that operate in underwater environments are only able to utilize GPS corrections when the vehicle is at the surface.  The article Achieving High Navigation Accuracy Using Inertial Navigation Systems in Autonomous Underwater Vehicles by Robert Panish and Mikell Taylor for the Bluefin Robotics Corporations demonstrates the calibration methods of two different INS systems, the T-24 Ring Laser Gyro (RGL) and PHINS III Fiber Optic Gyro (FOG), which are both used on their BlueFin Autonomous Underwater Vehicle.  The operational advantages of each system is minimal in comparison through the evaluation of each INS calibration method presented in this article.

     The T-24 uses RLGs, which means the beam path is created by a set of mirrors redirecting the laser into a loop (Panish and Taylor), while the PHINS III, the laser beam travels through a long optical fiber to create the beam path (Parnish).  Both systems measure the change of the laser’s frequency as a result of the laser being bent by acceleration forces outside of the system.   The measured change in frequency allows the INS to calculate its linear acceleration.  When this linear acceleration is referenced with the Earth’s rotation the INS is able to calculate the systems location by the corresponding it to a unique acceleration vector on each point of the Earth.  Each of these systems uses of a Dopper Velocity Log (DVL), depth sensor and sound speed log (Panish) with the INS in order to calculate its position over time and eliminate drift error.  And both systems use GPS corrections during calibration in order to minimize the initial drift error.  However, the actual methods of each is calibration is unique. 

    During the calibration of the PHINS III INS, inertial sensors and GPS are used to determine its motion.  The DVL velocities are recorded.  The difference in known motion from the INS and GPS in comparison to the DVL motion determines its calibration parameters.  This is done by sending the vehicle on a 5km track line with continuous GPS contact and monitoring the convergence of roll, pitch, and heading misalignment angles (Panish).  Dissimilarly, during the calibration procedure of the T-24 INS the vehicle is submerged and follows a box shaped pattern with surfacing at each of the corners.    Each side takes no less than fifteen minutes.  Using the GPS fixes at each corner it determines its internal biases, scale factors, and misalignment angles (Panish).  Roll and pitch are determined from a simple measurement of the direction of the gravity vector from the accelerometers measurements.  While heading is determined by using the time derivative of the gravity vector, easterly, and the Gravity vector in order to calculate north.  While the alignment of the PHINS III took less time to calibrate than the T-24, the PHINS III was more susceptible to sea state and required more monitoring because of the possibility of collision with other surface ships during calibration. 

     Although both of these methods measure accelerations differently, both of these methods calibrated the two INS systems within a drift error less 0.1% of distance traveled.  The drift error of these two systems far exceeds the design specifications for the system.  The minimization of drift error during calibration is important when evaluating these systems since it will not be able to utilize the GPS to correct for drift while being submerged for long durations and distances.  Since the most notable difference in an operational point of view is the calibration method, each of these systems provides exceptional navigation accuracy that can be used to collect high quality oceanographic data (Panish).      

Robert Panish and Mikell Taylor.  Achieving High Navigation Accuracy Using Inertial Navigation Systems in Autonomous Underwater Vehicles. (2011) Retrieved January 18, 2015, from http://www.bluefinrobotics.com/news-and-downloads/papers-and-articles/