1998 - Draper Laboratory

clock and ephemeris errors, as well as several of the inertialsystem errors, and when properly implemented, Circular ErrorProbables (CEPs) better than 8 m have been observed. 2 In thenear term, accuracies in the integrated navigation solution willimprove, first to the 3-m level (for precision-strike-typeapplications), and then possibly to the 1-m level. Theseaccuracies will need to be available in the face of hostile jammingof GPS, and the inertial system will provide autonomousnavigation information during periods of GPS outage.The following sections describe:• The expected technology trends for inertial sensors thatcan support autonomous operation at low cost.• Expected accuracy improvements and implementationsfor satellite navigation.• Issues and benefits of INS/GPS integration, particularlyin a jamming environment.upsets this symmetry, which is then photoelectronically detectedand processed to provide an indication of rotation rate. The FOGis implemented using an integrated optics chip constructed inlithium niobate, a fiber-optic sensing coil (meters to a kilometerlong), a broadband light source, and photodetectors (see Figure1). This configuration is expected to be supplanted eventually byquantum well technology, such as gallium arsenide, which willthen allow integration of most of the above components into asingle substrate attached to the fiber-optic coil, thus increasingreliability and reducing costs. FOG sensors have no gas ormirrors and do not exhibit lock-in at low rate, which aredisadvantages associated with some Ring Laser Gyros (RLGs).Therefore, they should be an economical replacement for theRLG, providing the same or better level of gyro bias driftperformance.The combination of a robust, antijam GPS receiver and anaccurate, low-cost inertial system will provide the global precisionnavigation system of the future.Inertial Sensor TrendsMajor changes are currently underway in technologies associatedwith inertial sensors used for stabilization, control, andnavigation. These changes are enabling the proliferation ofinertial sensors into a wide variety of new military andcommercial applications. Inertial sensor manufacturers havebegun to adopt many of the fabrication techniques that have beendeveloped by the solid-state electronics industry over the lastdecade. Inertial sensors are being fabricated in silicon and quartz,and with electro-optic materials such as lithium niobate byemploying low-labor-intensive batch processing techniques. Theuse of these techniques is resulting in low-cost, high-reliability,small size, and lightweight inertial sensors and the systems intowhich they are integrated. Some inertial sensors have alreadybeen fabricated with dimensions so small that they are barelyvisible to the naked eye. Some of the more trend-settingemerging sensor technologies are described next. They arefiber-optic gyros, micromechanical gyros, resonating beamaccelerometers, and micromechanical accelerometers (Refs. [1],[2]).Fiber-Optic Gyros (FOG)Sagnac effect rotation rate sensors result from thecounterpropagation of light beams in a fiber-optic waveguide thatexhibits optical reciprocity between its clockwise andcounterclockwise paths. Rotation normal to the waveguide planeFigure 1. Fiber-optic gyro.Silicon Micromechanical GyrosMicromechanical gyros are usually designed as anelectronically-driven resonator, often fabricated out of a singlepiece of quartz or silicon. Such gyros operate in accordance withthe dynamic theory that when an angular rate is applied to atranslating body, a Coriolis force is generated. When this angularrate is applied to the axis of a resonating tuning fork, its prongsreceive a Coriolis force, which then produces torsional forcesabout the sensor’s axis. These forces, which are proportional tothe applied angular rate, cause displacements that can bemeasured capacitively in a silicon instrument or piezoelectricallyin a quartz instrument. The output is then demodulated,amplified, and digitized to form the device output. Siliconmicromechanical instruments can be made by bulkmicromachining (chemical etching) single-crystal silicon or by................................................................................2The probability of the actual horizontal position lying inside a circle with a radius CEP is 50 percent.INS/GPS Technology Trends for Military Systems3