TABLE OF CONTENTS Most conventional aircraft automatic landing systems use the instrument or microwave landing systems (ILS or MLS) in their terminal approach phases. These systems supply the autopilot with the aircraft's angular deviation from a desired fight path, which essentially corresponds to the measurement of vertical and lateral positions. Basic ILS can only satisfy the FAA's nonprecision and Category I landing requirements; and aircraft using the MLS or an improved ILS system can land with Category III required accuracy. In these autopilots, velocity is typically calculated by filtering and differentiating position, or by integrating the acceleration outputs of an Inertial Measurement Unit. Some disadvantages of these integrated systems include their high user costs and dependence o expensive ground equipment. Also, the noisy ILS and MLS signals are typically processed with a smoothing filter. This causes lags that must be compensated for by the landing autopilot.
Unlike most other navigation aids, a GPS receiver directly measures three-dimensional velocity with extreme accuracy (for DGPS and CDGPS, [ ] better than 5 cm/s). For the autopilot designer, this is of great value. A direct measurement of true ground speed not only assists in normal landings, it gives important advanced knowledge of wind gusts and shears.
