Shuttle Navigation System for Entry and Landing Mission Phases

An on-board filter is designed to efficiently process inertial-measurement-unit data in combination with data from other external navaids such as TAG AN, one-way Doppler, baro-altimeter, radar altimeter, and MLS. During the approach phase (altitude above 12,000 ft) a 13-state minimum-variance filter is recommended, with position (3), velocity (3), IMU alignment (3), and various navaid correlated errors (4) as state elements. The navaid errors are modeled as first-order Markov processes, with state (process) noise employed in the extrapolation procedure to account for unmodeled errors. A simple underweighting technique is employed to insure good navigation-system performance in the presence of large initial-condition errors. During the terminal phase (altitude below 12,000 ft) a 6-state filter is used, including as state elements only position (3) and velocity (3). Prestored weighting vectors are used here, based on the last computed values for the preceding approach phase.