Lateral navigation (LNAV) refers to navigating over a ground track with guidance from an electronic device which gives the pilot (or autopilot) error indications in the lateral direction only and not in the vertical direction. In aviation lateral navigation is one of two guidance types: linear guidance and angular guidance. Linear means that the left and right deviations of the aircraft are available as a distance of the aircraft from the desired ground track to its actual position on either side of the desired track. In angular guidance, the error indication is given in degrees of angle from the desired line relative to a ground-based navigation device. To provide an illustration, as the aircraft approaches the ground device with a constant angular error, its distance to the desired ground line decreases. In the context of aviation instrument approaches, an LNAV approach (one that uses lateral navigation) is implied to be a GPS-based approach and to have linear lateral guidance. A VOR-based approach will have angular lateral guidance.
Using LNAV on instrument approaches: the non-precision approach
The approach minimums for LNAV approaches are higher than that of ILS approaches, and RNAV approaches that incorporate vertical guidance. Aircraft executing an LNAV instrument approach must descend incrementally rather than follow a fixed glide slope. This is called a 'non-precision' approach to distinguish it from a precision approach in which there is electronic vertical (slope) guidance down to a decision altitude (DA). In the case of the non-precision approach, the aircraft can descend only to what is referred to as a minimum descent altitude (MDA). An MDA segment is flown until the airport is in sight and the pilot can land. If the airport is not in sight by the time the pilot reaches a missed approach point (MAP) on the MDA, the aircraft must execute a missed approach.
The GPS implementation of the non-precision LNAV approach can only be flown if satellite configuration at the time of the approach will accurately support a full scale course deviation indication of 0.3 nautical miles (about 1800 feet to the left and right or 3600 feet total) starting at the final approach fix and extending all the way to the missed approach point. If this sensitivity does not occur, the pilot will be notified by the on-board receiver (via RAIM checking) and must not make the descent onto the final leg.