The OBS mode uses magnetic track – as do all things in an IFR GPS.
From RTCA DO 229D which is the specification for the WAAS GPS
2.2.1.3.12 Magnetic Course
The source of the magnetic variation used for paths that are defined using magnetic course shall be in accordance with the following:
a) If the leg is part of a database terminal area procedure and the magnetic variation is specified by the State for that procedure, the magnetic variation to be used is
the value specified.
b) If the leg is not part of a procedure and the active fix is a VOR or TACAN, the magnetic variation to be used is the published station declination for the VOR or TACAN.
c) If the leg is not part of a procedure and the terminating fix is not a VOR or TACAN, the magnetic variation to be used shall be defined by the equipment using an internal model. The navigation equipment shall have the capability of assigning a magnetic variation at any location within the region that flight operations may be conducted using Magnetic North reference. The assigned magnetic variation shall be within two degrees of the value determined at the same location and time by an internationally recognized magnetic model that is valid for the time of computation (e.g. USGS, IGRF).
What I find curious is that American VORs tend to be left unaligned for years, whereas here (in the UK, for sure) VORs are periodically re-aligned so they generate the right bearing. I believe the realignment is done remotely, over a telephone line. Every VOR seems to have a phone line going to it.
It is a massive and labor intensive job to change all of the airways, approaches, SIDs, STARs, and their associated charts for a single VOR. Before they can be used they all need new flight tests and airways and the associated waypoints can be NOTAMed out of service. The re-alignment of most of the US VOR’s deployed are of older generation equipment and use mechanical means of generating the radials with a rotating antenna, From Wikipedia “A VOR ground station sends out an omnidirectional master signal, and a highly directional second signal is propagated by a phased antenna array and rotates clockwise in space 30 times a second.” The entire antenna array is spun at 1800 RPM.
I would have thought that if you do a DCT [VOR] and set the OBS to say 270, then you will fly a mag track to the VOR on a magnetic bearing of 270 and this will be regardless of how well or badly the VOR is aligned. After all, the VOR is just a waypoint, so why would the GPS give you anything other than a correct magnetic bearing?
No, the VOR is fixed in space and the radial remains in the same position that it was installed in. The magnetic north pole is continuously moving and over a period of years, the magnetic variation can change. The OBS setting to track the course does not change over time, but the zero wind correction magnetic course to keep the needle centered does change with the variation change. So set the CDI to the same radial as the airway and you should fly over the same dirt.
It would be bizzare that if you did this with a VOR as the waypoint, versus doing it with say the airport as the waypoint (in which case the waypoint ought to be the middle of the principal instrument runway), you would end up flying along different radials.
Bizzare but true.
Bizzare but true.
There’s some logic to it.
But it really has gotten too complex and error prone, it would be time to switch to true everywhere (except for the bubble compass), even in moderate latitudes.
