INS GPS technology trends

RTO-EN-SET-116(2010) 1 - 1 INS/GPS Technology Trends George T. Schmidt Massachusetts Institute of Technology 10 Goffe Road Lexington, MA 02421 USA ABSTRACT 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. 1.0 INTRODUCTION Inertial navigation systems have progressed from the crude electromechanical devices that guided the early V-2 rockets (Figure 1a) to the current solid-state devices that are in many modern vehicles. The impetus for this significant progress came during the ballistic missile programs of the 1960s, in which the need for high accuracy at ranges of thousands of kilometers using autonomous navigation systems was apparent. By “autonomous” it is meant that no man-made signals from outside the vehicle are required to perform navigation. If no external man-made signals are required, then an enemy cannot jam them. 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 all- inertial 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. INS/GPS Technology Trends 1 - 2 RTO-EN-SET-116(2010) Figure 1a: V-2 Rocket. Figure 1b: SPIRE System. 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, a Circular Error Probable (CEP) better than 5m has been observed. In the very 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. The hope is for strapdown INS/GPS systems that are smaller than 3 in3 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. INS/GPS Technology Trends RTO-EN-SET-116(2010) 1 - 3 The combination of a 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. START HERE GPS SPACE AND GROUND SEGMENT IMPROVEMENTS RECEIVER IMPROVEMENTS AND ALL IN VIEW TRACKING FINISH ANTI-JAM ENHANCEMENTS HIGH A/J, HIGHLY INTEGRATED, LOW COST PRECISION NAVIGATION LOW COST, ACCURATE INERTIAL SYSTEMS TOLL Figure 2: Roadmap to precision navigation for multiple applications. 2.0 INERTIAL SENSOR TRENDS The major error sources in the inertial navigation system are due to gyro and accelerometer inertial sensor imperfections, incorrect navigation system initialization, and imperfections in the gravity model used in the computations. But, in nearly all inertial navigation systems, the largest errors are due to the inertial sensors. Whether the inertial sensor error is caused by internal mechanical imperfections, electronics errors, or other sources, the effect is to cause errors in the indicated outputs of these devices. For the gyros, the major errors are in measuring angular rates. For the accelerometers, the major errors are in measuring acceleration. For both instruments, the largest errors are usually a bias instability (measured in deg/hr for gyro bias drift, or micro g (µg) for the accelerometer bias), and scale-factor stability (which is usually measured in parts per million (ppm) of the sensed inertial quantity). The smaller the inertial sensor errors, the better the quality of the instruments, the improved accuracy of the resulting navigation solution, and the higher the cost of the system. As a “rule-of-thumb,” an inertial navigation system equipped with gyros whose bias stability is 0.01 deg/hr will see its navigation error grow at a rate of 1 nmi/hr of operation. The navigation performance requirements placed on the navigation system lead directly to the selection of specific inertial instruments in order to meet the mission requirements. Figure 3, “Current Gyro Technology Applications,” gives a comprehensive view of the gyro bias and scale- factor stability requirements for various mission applications and what type of gyro is likely to be used in current applications (Figures 3 – 9 are revised versions of the figures in Ref. [1]). INS/GPS Technology Trends 1 - 4 RTO-EN-SET-116(2010) RLG & IFOG Rate & Integrating Gyros MEMS Self- Aligning Strategic Missile Consumer RLG & IFOG Autonomous Submarine Navigation Tactical Missile Midcourse Guidance Cruise Missile Air/Land/Sea Navigation Surveying AHRS Torpedoes Flight Control, Smart Munitions, Robotics IFOG & Quartz DTG Bias Stability (deg/hr) Sc al e Fa ct or S ta bi lit y (p pm ) 1,000 100 10 1 0.1 1,000 100 10 1 0.1 0.000150.000015 0.0015 0.015 0.15 1.5 15 1500150 36000.000150.000015 0.0015 0.015 0.15 1.5 15 1500150 3600 Mechanical RLG = Ring Laser Gyro DTG = Dry Tuned Gyro IFOG = Interferometric Fiber Optic Gyro Quartz = Coriolis Sensor Mechanical = Spinning Mass AHRS = Attitude Heading Reference System MEMS = Micro-Electro-Mechanical Sensors (silicon) Interceptor Figure 3: Current gyro technology applications. Solid-state inertial sensors, such as Microelectromechanical System (MEMS) devices, have potentially significant cost, size, and weight advantages, which has resulted in a proliferation of the applications where such devices can be used in systems. While there are many conventional military applications, there are also many newer applications that will emerge with the low cost and very small size inherent in such sensors, particularly at the lower performance end of the spectrum. A vision of the gyro inertial instrument field for relevant military applications for the near-term is shown in Figure 4. Strapdown systems will also dominate. 1,000 100 10 1 0.1 0.0000015 0.000015 0.00015 0.0015 0.015 0.15 1.5 15 150 1500 SC A LE F A C TO R S TA B IL IT Y (p pm ) MECHANICAL RLG IFOG QUARTZ SILICON MICRO- MECHANICAL 1 nautical mile/hour earth rate BIAS STABILITY (°/hr) RLG IFOG = Ring Laser Gyro = Interferometric Fiber Optic Gyro RLG IFOG = Ring Laser Gyro = Interferometric Fiber Optic Gyro Figure 4: Near-term gyro technology applications. INS/GPS Technology Trends RTO-EN-SET-116(2010) 1 - 5 The MEMS and Interferometric Fiber-Optic (IFOG) technologies are expected to replace many of the current systems using Ring Laser Gyros (RLGs) and mechanical instruments. However, one particular area where the RLG is expected to retain its superiority over the IFOG is in applications requiring extremely high scale-factor stability. The change to all-MEMS technology hinges primarily on MEMS gyro development. The performance of MEMS instruments is continually improving, and they are currently being developed for many applications. This low cost can only be attained by leveraging off the consumer industry, which will provide the infrastructure for supplying the MEMS sensors in extremely large quantities (millions). The use of these techniques will result in low-cost, high-reliability, small-size, and lightweight inertial sensors and the systems into which they are integrated. The tactical (lower) performance end of the application spectrum will likely be dominated by micromechanical inertial sensors. The military market will push the development of these sensors for applications such as “competent” and “smart” munitions, aircraft and missile autopilots, short- time-of-flight tactical missile guidance, fire control systems, radar antenna motion compensation, “smart skins” using embedded inertial sensors, multiple intelligent small projectiles such as flechettes or even “bullets,” and wafer-scale INS/GPS systems. Figure 5 shows how the gyro technology may possibly be applied to new applications in the far term. The figure shows that the MEMS and integrated-optics (IO) systems technology may dominate the entire low- and medium- performance range. The rationale behind this projection is based on two premises. The first is that gains in performance in the MEMS devices will continue with similar progression to the orders-of-magnitude improvement that has already been accomplished in the last decades. That further improvements are likely is not unreasonable since the designers are beginning to understand the effects of geometry, size, electronics, and packaging on performance and reliability. Second, efforts have already demonstrated how to put all six sensors on one (or two) chips, which is the only way to reach a possible cost goal of less than $1000 per INS/GPS system. In addition, since many of the MEMS devices are vibrating structures with a capacitive readout, this may restrict the performance gains. It is in this area that the integrated optics technology is most likely to be required to provide a true solid-state micromechanical gyro with optical readout. At this time, the technology to make a very small, accurate gyro does not exist, but advances in integrated optics are already under development in the communications industry. For the strategic application, the IFOG could become the dominant gyro. Work is underway now to develop radiation-hard IFOGs as well as super-high-performance IFOGs. 1,000 100 10 1 0.1 0.0000015 0.000015 0.00015 0.0015 0.015 0.15 1.5 15 150 1500 Sc al e Fa ct or S ta bi lit y (p pm ) IFOG Bias Stability (deg/hr) IFOG MEMS IO = Interferometric Fiber Optic Gyro = Micro-Electro-Mechanical Systems = Integrated Optics IFOG MEMS IO = Interferometric Fiber Optic Gyro = Micro-Electro-Mechanical Systems = Integrated Optics MEMS/IO 1 nautical mile/hr earth rate Atom Figure 5: Far-term gyro technology applications. INS/GPS Technology Trends 1 - 6 RTO-EN-SET-116(2010) A potentially promising technology, which is in its infancy stages, is inertial sensing based upon cold atom interferometry. (Refs. [16], [18]) A typical atom de Broglie wavelength is 10-11 times smaller than an optical wavelength, and because atoms have mass and internal structure, cold atom interferometers are extremely sensitive. Accelerations, rotations, electromagnetic fields, and interactions with other atoms change the atom interferometric fringes. This means that atom interferometers could make the most accurate gyroscopes, accelerometers, gravity gradiometers, and precision clocks, by orders of magnitude. If this far-term technology can be developed, then it could result in a 2 to 5-meter/hour navigation system without GPS, in which the accelerometers are also measuring gravity gradients. Figure 6, “Current Accelerometer Technology Applications,” gives a comprehensive view of the accelerometer bias and scale-factor stability requirements for various mission applications and what type of accelerometer is likely to be used in current applications. “Mechanical Instruments” refers to the use of a Pendulous Integrating Gyro Assembly (PIGA) which is a mass unbalanced spinning gyroscope used to measure specific force. Mechanical Floated Instruments Self-Aligning Strategic Missile Stellar-Aided Strategic Missile Autonomous Submarine Navigation Cruise Missile Land Navigation Aircraft Navigation Stellar-Aided Interceptor Tactical Missile Midcourse Guidance Quartz Mechanical Pendulous Rebalance Accelerometer MEMS 1,000 100 10 1 0.1 1,000 100 10 1 0.1 0.1 1 10 100 1,000 10,0000.1 1 10 100 1,000 10,000 Bias Stability (µg) Sc al e Fa ct or S ta bi lit y (p pm ) Consumer Figure 6: Current accelerometer technology applications. Current applications are still dominated by electromechanical sensors, not only because they are generally low-cost for the performance required, but also because no challenging alternative technology has succeeded, except for quartz resonators, which are used in the lower-grade tactical and commercial applications. MEMS inertial sensors have not yet seriously broached the market, although they are on the verge of so doing, especially in consumer applications. In the near-term (Figure 7), it is expected that the tactical (lower) performance end of the accelerometer application spectrum will be dominated by micromechanical accelerometers. As in the case for gyros, the military market will push the development of these sensors for applications such as “competent” and “smart” munitions, aircraft and missile autopilots, short-time-of-flight tactical missile guidance, fire control systems, radar antenna motion compensation, “smart skins” using embedded inertial sensors, multiple intelligent small projectiles such as flechettes or even “bullets,” and wafer-scale INS/GPS systems. Higher performance applications will continue to use mechanical accelerometers and possibly resonant accelerometers based on INS/GPS Technology Trends RTO-EN-SET-116(2010) 1 - 7 quartz or silicon. Quartz resonant accelerometers have proliferated widely into tactical and commercial (e.g., factory automation) applications. Silicon micromechanical resonator accelerometers are also being developed. Both of these technologies have possible performance improvements. 1,000 100 10 1 0.1 0.1 1 10 100 1,000 10,000 SC A LE F A C TO R S TA B IL IT Y (p pm ) BIAS STABILITY (µg) PRECISION RESONATORS MECHANICAL QUARTZ SILICON MICROMECHANICAL Figure 7: Near-term accelerometer technology applications. Figure 8 shows how the accelerometer technology may be applied to new applications in the far term. As in the case of gyro projections for the future, the figure shows that the MEMS and integrated optics technology will dominate the entire low- and medium-performance range. The rationale behind this projection is based on exactly the same two premises as for the gyros. However, it is likely that the far-term accelerometer technology projections will be realized years sooner than the gyro. 1,000 100 10 1 0.1 0.1 1 10 100 1,000 10,000 Sc al e Fa ct or S ta bi lit y (p pm ) Bias Stability (µg) MECH. SILICONQUARTZ MEMS/IO MEMS IO = Micro-Electro-Mechanical Systems = Integrated Optics MEMS IO = Micro-Electro-Mechanical Systems = Integrated Optics Atom Figure 8: Far-term accelerometer technology applications. INS/GPS Technology Trends 1 - 8 RTO-EN-SET-116(2010) Figure 9 shows INS or INS/GPS relative strapdown system cost “projections” as a function of inertial instrument technology and performance. The cost of a GPS receiver is likely to be so small that it will be insignificant. The systems are classified as: laser gyro or IFOG systems containing various types of accelerometer technologies; quartz systems with both quartz gyros and quartz accelerometers; and MEMS/integrated optics systems. The solid line indicates the range of approximate costs expected. Clearly, the quantity of systems produced affects the cost; large production quantities would be at the lower end of the cost range. The IFOG systems have the potential for lower cost than laser gyro systems because the IFOG should be well below the cost of an RLG. However, this has not happened to date, primarily because the RLG is in relatively large-volume production in well-facilitated factories and the IFOG is not yet manufactured in similar production quantities. Clearly, the MEMS/integrated optics INS/GPS systems offer the lowest cost. The ultimate low cost only becomes feasible in quantities of millions. This can be achieved only with multi- axis instrument clusters and on-chip or adjacent-chip electronics and batch packaging. C os t $ 100,000 50,000 10,000 1,000 100 10 100,000 50,000 10,000 1,000 100 10 .001°/hr .01°/hr 0.1°/hr 1°/hr 10°/hr 100°/hr 1000°/hr 1 µg 25 µg 500 µg 1 mg 10 mg 100 mg 1000 mg Performance MEMS/IO IFOG LASER QUARTZ IFOG MEMS IO QUARTZ = Interferometric Fiber Optic Gyro = Micro-Electro-Mechanical Systems = Integrated Optics = Coriolis Sensor IFOG MEMS IO QUARTZ = Interferometric Fiber Optic Gyro = Micro-Electro-Mechanical Systems = Integrated Optics = Coriolis Sensor Figure 9: Strapdown INS cost as a function of instrument technology. The ability of silicon-based MEMS devices to withstand high “g” forces has been demonstrated recently in a series of firings in artillery shells where the g forces reached over 6500 g. These small MEMS-based systems, illustrated in Figure 10, have provided proof-of-principal that highly integrated INS/GPS systems can be developed and led to a recent program where the goal was a system on the order of 3 in3, or 2 in3 for the INS alone (Ref. [2]). Unfortunately, the goals were not met. The current status of a typical MEMS INS is represented by the Honeywell HG1900