Analysis of an Ultra-Tightly Coupled GPS/INS system in jamming

The performance of an Ultra-Tightly Coupled (UTC) Global Positioning System/Inertial Navigation System (GPS/INS) is evaluated using a system simulation of the GPS receiver and navigation processing. The UTC system being analyzed uses a bank of pre-filters to estimate code delay error and Doppler frequency error for each satellite. These outputs are sent to a central Kalman navigation filter. This central processor generates estimates of inertial navigation position, velocity, and attitude errors; IMU biases; and user clock errors, which are used to correct the navigation solution. In the UTC approach, outputs from the central navigation processor, after projection into satellite line-of-sight coordinates, are used to control the code and carrier replica signals for each satellite channel. In contrast, a conventional tightly coupled GPS/INS system uses separate tracking loops for each satellite channel, which operate autonomously. As a result, the UTC design is considered more robust to jamming and vehicle dynamics. Closed-loop UTC navigation performance is evaluated over a realistic precision guided munition (PGM) scenario in the presence of broadband jamming directed against the GPS system. A realistic IMU error model is assumed, and UTC performance is evaluated over a range of jamming levels. Some preliminary conclusions are then formed about the effectiveness of the UTC approach.