Figure 1a V-2 Rocket Figure 1b. Spire System

This paper focuses on accuracy and other technology trends for inertial sensors, Global Positioning Systems (GPS), and integrated Inertial Navigation System (INS)/GPS systems, including considerations of interference, that will lead to better than 1 meter accuracy navigation systems of the future. For inertial sensors, trend-setting sensor technologies will be described. A vision of the inertial sensor instrument field and strapdown inertial systems for the future is given. Planned accuracy improvements for GPS are described. The trend towards deep integration of INS/GPS is described, and the synergistic benefits are explored. Some examples of the effects of interference are described, and expected technology trends to improve system robustness are presented. One of the early leaders in inertial navigation was the Massachusetts Institute of Technology (MIT) Instrumentation Laboratory (now Draper Laboratory), which was asked by the Air Force to develop inertial systems for the Thor and Titan missiles and by the Navy to develop an inertial system for the Polaris missile. This request was made after the Laboratory had demonstrated in 1953 the feasibility of autonomous allinertial navigation for aircraft in a series of flight tests with a system called SPIRE (Space Inertial Reference Equipment), Figure 1b. This system had gimbals, was 5 feet in diameter and weighed 2700 pounds. The notable success of those early programs led to further application in aircraft, ships, missiles, and spacecraft such that inertial systems are now almost standard equipment in military and civilian navigation applications. RTO-EN-SET-116(2009) 1 1 Inertial navigation systems do not indicate position perfectly because of errors in components (the gyroscopes and accelerometers) and errors in the model of the gravity field that the INS implements. Those errors cause the error in indicated position to grow with time. For vehicles with short flight times, such errors might be acceptable. For longer-duration missions, it is usually necessary to provide periodic updates to the navigation system such that the errors caused by the inertial system are reset as close to zero as possible. Because GPS offers world-wide, highly accurate position information at very low cost, it has rapidly become the primary aid to be used in updating inertial systems, at the penalty of using an aid that is vulnerable to interference. Clearly, the ideal situation would be low-cost but highly accurate INS that can do all, or almost all, of the mission without using GPS. The military has had access to a specified accuracy of 21 m (95-percent probability) from the GPS Precise Positioning Service (PPS). This capability provides impressive worldwide navigation performance, especially when multiple GPS measurements are combined in a Kalman filter to update an INS on a military platform or a weapon. The Kalman filter provides an opportunity to calibrate some of the GPS errors, such as satellite clock and ephemeris errors, as well as several of the inertial system errors, and when properly implemented, Circular Error Probables (CEPs) better than 5m have been observed. In the near term, accuracies in the integrated navigation solution are predicted to improve to the 1 meter level. These accuracies will need to be available in the face of intentional interference of GPS, and the inertial system will provide autonomous navigation information during periods of GPS outage. The following sections describe: • The expected technology trends for inertial sensors and strapdown (no gimbals) systems that can support autonomous operation at low cost. Expectations are for strapdown INS/GPS systems that are smaller than 3 in 3 and weigh less than a pound, and possibly cost under $1000. • Expected accuracy improvements and implementations for GPS. • Issues and benefits of INS/GPS integration, particularly in an environment with interference. The combination of a robust, antijam GPS receiver and an accurate, low-cost inertial system will provide the global precision navigation system of the future. Figure 2 depicts the “roadmap” to meeting this objective. S T A R T H E R E GPS SPACE AND GROUND SEGMENT IMPROVEMENTS RECEIVER IMPROVEMENTS AND ALL IN VIEW TRACKING IN IS H ANTI-JAM ENHANCEMENTS HIGH A/J, HIGHLY INTEGRATED, LOW COST PRECISION NAVIGATION LOW COST, ACCURATE INERTIAL SYSTEMS