GPS Approach Question

[FL530]

Why on some approaches, the heading information displayed on the GPS is slightly different that the heading shown on the chart, why does this happen?

 

[Clocks]

My guess, and this is only a guess, is that the GPS in the airplane doesn't have the procedure programmed in like it is on the chart, but rather has the precise location of the waypoints programmed in. Then the unit (or the computer that programs the unit) has the waypoints connected together in the correct order and calculates its own heading information. The difference is either a rounding error in the computer or an error with the paper chart.

But that could be completely wrong.

 

[tgrayson]

Why on some approaches, the heading information displayed on the GPS is slightly different that the heading shown on the chart, why does this happen?

From the AIM:

l. Conventional Versus GPS Navigation Data
There may be slight differences between the course information portrayed on navigational charts and a GPS navigation display when flying authorized GPS instrument procedures or along an airway. All magnetic tracks defined by any conventional navigation aids are determined by the application of the station magnetic variation. In contrast, GPS RNAV systems may use an algorithm, which applies the local magnetic variation and may produce small differences in the displayed course. However, both methods of navigation should produce the same desired ground track when using approved, IFR navigation system. Should significant differences between the approach chart and the GPS avionics' application of the navigation database arise, the published approach chart, supplemented by NOTAMs, holds precedence.

Due to the GPS avionics' computation of great circle courses, and the variations in magnetic variation, the bearing to the next waypoint and the course from the last waypoint (if available) may not be exactly 180 degrees apart when long distances are involved. Variations in distances will occur since GPS distance-to-waypoint values are along-track distances (ATD) computed to the next waypoint and the DME values published on underlying procedures are slant-range distances measured to the station. This difference increases with aircraft altitude and proximity to the NAVAID.