This invention relates to area navigation systems, and more particularly to an area navigation system having improved accuracy in the presentation of course deviation information. The invention is described in the particular context of the area navigation systems described and claimed in Assignee's Abnett et al U.S. Pat. No. 3,796,867 entitled, "ANGLE MODE AREA NAVIGATION COMPUTER", and in Assignee's Bean U.S. Pat. No. 3,750,942 entitled, "DISTANCE MODE AREA NAVIGATION COMPUTER", the disclosures of which patents are incorporated herein to the extent pertinent.
An extended description of the concepts of area navigation and its many advantages is set forth in the Abnett et al and Bean patents, and the reader is referred to these patents for more information. Briefly, however, there exists a network of navigation aids called VORTAC's which provide ratio information from which range and bearing information may be derived. In area navigation, the VORTAC range and bearing data are used in conjunction with an arbitrarily selected range and bearing from the VORTAC to a so-called waypoint. From these, an on-board computer generates range and bearing data from the aircraft to the waypoint. Because the location of a waypoint is arbitrary within the range of a particular VORTAC, the pilot is no longer constrained to fly along or near the established airways (defined with respect to a series of VORTAC stations). Instead, one may fly along any convenient course simply by appropriate choice of waypoints. In broad terms, area navigation systems such as those of the Bean and Abnett et al patents facilitate the above by solving a vector triangle defined by the aircraft, the VORTAC, and the waypoint, given the selected waypoint address vector and the measured aircraft to VORTAC vector.
In the Abnett et al patent, there is shown an area navigation system in which the aircraft to waypoint vector is used to generate a display of range from the aircraft to the waypoint, and a track angle deviation display, the latter is based on the difference between the actual aircraft to waypoint bearing (the angle component of the aircraft to waypoint vector) and the bearing at which it is desired that the aircraft intercepts the waypoint, i.e. the selected track angle. The angle display is advantageously accomplished by means of a "left-right" needle included in the course deviation indicator (or CDI) of the standard VORTAC navigation systems.
In the Bean patent, track deviation is displayed on a left-right meter as distance from the selected track.
Each of the aforementioned types of systems has particular advantages, as described in the respective patents. In both instances, however, one of the most important factors in assuring accurate and useful system operation (apart from good system design) is overcoming a track deviation display instability due to radio propagation defects associated with the VORTAC station, and corresponding errors inherent in the mathematical formulation of the data, and the display mode required for the track error.