1998 - Draper Laboratory

INS/GPS Technology Trendsfor Military SystemsGeorge T. Schmidt The Charles Stark Draper Laboratory, Inc.Based on the paper published in the Proceedings of the AIAA Guidance, Navigation, and Control ConferenceAbstractThis paper focuses on accuracy and other technology trends forinertial sensors, Global Positioning Systems (GPS), and integratedInertial Navigation System (INS)/GPS systems, includingconsiderations of jamming for military platforms and weaponsthat will lead to 1-m accuracy navigation systems of the future.For inertial sensors, trend-setting sensor technologies applicableto military systems will be described. A vision of the inertialsensor instrument field and inertial systems for militaryapplications for the next few decades is given. GPS-specified andobserved current accuracies are described, as well as plannedaccuracy improvements, intersatellite ranging, and “all-in-view”tracking. Uses of Relative and Differential GPS (RGPS and DGPS,respectively) are discussed. The trend toward highly integratedINS/GPS is described and the synergistic benefits are explored.Some examples of the effects of jamming are described, andexpected technology trends to improve system antijam capabilityare presented.IntroductionDuring the last 50 years, Inertial Navigation Systems (INS) haveprogressed from the crude electromechanical devices that guidedthe early V-2 rockets to the current solid-state devices that are inmany modern vehicles. The impetus for this significant progresscame during the ballistic missile programs of the 1960s, in whichthe need for high accuracy at ranges of thousands of kilometersusing autonomous navigation systems was apparent. By“autonomous” it is meant that no man-made signals from outsidethe vehicle are required to perform navigation. If no externalman-made signals are required, then an enemy cannot jam them.One of the early leaders in inertial navigation was theMassachusetts Institute of Technology (MIT) InstrumentationLaboratory (now Draper Laboratory), which was asked by the AirForce to develop inertial systems for the Thor and Titan missilesand by the Navy to develop an inertial system for the Polarismissile. This request was made after the Laboratory haddemonstrated the feasibility of autonomous all-inertial navigationfor aircraft in a series of flight tests with a system called SPIRE(Space Inertial Reference Equipment). The notable success ofthose early missile programs led to further application in aircraft,ships, missiles, and spacecraft such that inertial systems are nowalmost standard equipment in military and civilian navigationapplications.Inertial navigation systems do not indicate position perfectlybecause of errors in components (the gyroscopes andaccelerometers) and errors in the model of the gravity field thatthe INS implements. Those errors cause the error in indicatedposition to grow with time. The best autonomous militaryaircraft INS have errors that grow at the rate of about 500 m/h.For vehicles with short flight times, such errors might beacceptable. For longer-duration missions, it is usually necessaryto provide periodic measurements to the navigation system suchthat the errors caused by the inertial system are reset as close tozero as possible. Because GPS offers world-wide, highly accuratenavigation information at very low cost, it has rapidly become theprimary aid to be used in updating inertial systems, at thepenalty of using an aid that is quite vulnerable to jamming.Clearly, the ideal situation would be low-cost but highly accurateINS that can do all, or almost all, of the mission without usingGPS. 1The military has access to a specified accuracy of 21 m (95-percent probability) from the GPS Precise Positioning Service(PPS). This capability provides impressive world-wide navigationperformance, especially when multiple GPS measurements arecombined in a Kalman filter to update an INS on a militaryplatform or a weapon. The Kalman filter provides anopportunity to calibrate some of the GPS errors, such as satellite................................................................................1An INS must be initialized with information concerning position and velocity. A GPS receiver can determine its ownposition and velocity given enough time to do so. Thus, inertial navigation is a relative process; GPS is absolutenavigation in a given reference frame.INS/GPS Technology Trends for Military Systems2