This dissertation investigates the feasibility of completing an aircraft precision approach using two GNSS satellites in combination with a Stable Frequency Reference (SFR) and various altimeters. Two different sensor combinations are implemented for altimetry. The first combination uses both barometric and radar altimeters to provide height estimates, which are integrated with Global Navigation Satellite Systems (GNSS) satellites from different constellations with a SFR for positioning. Before the start of the approach, a full GNSS solution is used to calibrate the SFR and the vertical solution relative to the aircraft touchdown point (ATP). The theoretical clock and position error covariance is derived as a function of measurement error, satellite geometry, SFR stability, barometric height and radar altimeter performance. Detailed error models for each of the navigation sensors are developed for a covariance analysis. This is followed by both simulations and evaluations using flight test data to verify the positioning accuracy and the feasibility of completing an aircraft precision approach with only two satellites from different constellations. With respect to Category I precision approach requirements of 16 m (95%) horizontal and 4 m (95%) vertical, the horizontal radial 2-σ positioning performance is approximately 6 m, while the vertical 2-σ positioning performance is approximately 4 m.
The second sensor combination uses GPS reflection measurements from a software defined receiver (SDR) for aircraft passive bistatic altimetry, and a SFR to continue navigation when only two GPS satellites are available. The scenario for this combination is focused on flights over water, which provides strong reflected signals while alternate terrestrial radio navigation signals are generally not available. Theoretical clock and position error covariance are derived as a function of measurement error, satellite geometry, SFR stability, and GPS bistatic altimetry performance. This is followed by computer simulations, and evaluations using flight test data to characterize the positioning performance. The positioning performance in east and north are 13.6 m (2-σ) and 9.0 m (2-σ), respectively, and 5.6 m (2-σ) in vertical over 170 seconds of 2-satellite positioning.