VOR’s, GPS, and Magnetic Variation
Why is there a difference between the magnetic variation for the airport and the VOR located at the same airport?
When a navaid is first constructed, the antenna is physically oriented to True North. Then a potentiometer adjustment is made to slave the navaid with Magnetic North. This action matches the isogonic line making it agree with a magnetic compass. Initially these two values are the same, but the magnetic variation of the earth changes at differing rates depending upon location and time. Navaids are commissioned and remain online 24 hours per day 365 days per year. Although periodic maintenance is performed as needed, a re-slaving to match the isogonic value requires a total navaid shut down, re-alignment and a re-certification flight check. Only when the navaid is out of tolerance by at least +/-6 degrees will a re-slaving procedure be initiated allowing the navaid and airport magnetic variation to match again.
Note: GPS databases use a MAGVAR model to calculate the most up-to-date magnetic variation.
FAA Aviation System Standards Link
AIM 1-1-19. Global Positioning System (GPS)
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° 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.