TRUNAV: a Low-Cost Guidance/Navigation Unit Integrating a SAASM-Based GPS and MEMS IMU in a Deeply Coupled Mechanization

This paper addresses the need within the Projectile and Precision Guided Munitions marketplace for a high- precision guidance and navigation unit with a size, weight, power and cost suitable for ogive and eventually fusewell integration. Rather than being driven by separate GPS and IMU performance specifications, the TRUNAVTM navigation performance should be considered at the system level by enhancing navigation output of the combined GPS/IMU system. In order to optimize performance when subjected to jamming and/or high dynamics, the TRUNAVTM includes ultra-tightly (also known as deeply) coupled GPS/Inertial Measurement Unit (IMU) integration as well as hardware appliques. As part of the size, weight, power, and cost reduction, the IMU digital signal processing, guidance function, control function and fuzing function will be gradually re-hosted on the GPS receiver processor(s), thereby reducing the overall number of boards. IEC is the world's first company to house both advanced GPS receiver technology and MEMS inertial sensor technology under one roof. The TRUNAVTM includes IEC's GRAM/SAASM GPS receiver that is Wide Area GPS Enhancement (WAGE) capable, IEC's MicroSCIRAS (Micromachined Silicon Coriolis Inertial Rate and Acceleration Sensor) MEMS inertial technology, together with navigation, guidance, autopilot fuzing, and built-in-test (BIT) functionality, into a single product. The TRUNAVTM design includes a “customer area” to accommodate customer needs for a specific platform integration. TRUNAVTM deep coupling implementation utilizes the federated approach in which a bank of fast Kalman filters process the I and Q correlator outputs and generate measurements for a larger, slower filter to compute a navigation solution, calibrate IMU errors, and close the tracking loops. Between filter updates, the IMU, corrected for errors estimated by the navigation filter, is driving the NCOs. A high-fidelity simulation including IEC’s tracking and navigation software was developed. In addition to a complete GPS environment and a motion generator, the simulation includes a user-selectable IMU error model, a well as IEC’s tracking and navigation software for both the standard tracking loops and deep coupling mechanizations. This, coupled with the ability to simulate a user-selected J/S, make for a powerful design and analysis tool. The simulation was baselined by comparing simulated tracking loop performance versus actual receiver performance under various jamming and dynamics scenarios. It was then utilized to develop the deep coupling mechanization and determine the performance improvement as a function of dynamics, IMU quality and J/S environment. TRUNAVTM architecture will be described and simulation results will be presented.