Victor’s Vectors: GPS and VOR radials

Yesterday, I spoke with a very confused pilot regarding the operation of the VOR receiver in our GNS units.  His confusion was based on a similar situation to the one presented in the picture.  Notice on the Nearest (NRST) page that the ORL VOR has a bearing of 005M degrees.  This Nearest page information is based entirely on GPS and the magnetic variation – using the aircraft lat/long/mag-var compared to the VOR ground transmitter lat/long/mag-var.  On the Nearest pages, all data is based on waypoint data from the database and not on any received (VOR, NDB, etc.) signal.  Now notice the inset window on the GNS 530.  It shows a radial of 180 degrees.  This information is based on the actual received VOR signal from the VOR ground transmitter.  (If the VOR signal is noisy or out of specification, the GNS unit will not be able to decode the signal and this inset window will be blank.)  Now normally, the bearing-to/radial-from a VOR should be +/-180 degrees.  However, in this instance, the radial is 180 while the bearing is 005. 

How is this possible?

I’ll explain.  While the VOR ground transmitter should be aligned with magnetic north, this (ORL) VOR is misaligned by 5 degrees from magnetic north.  Every VOR signal output (and thus every received radial) from this ORL VOR is off by 5 degrees compared to the magnetic bearing to the waypoint.  While our GPS systems can determine the magnetic bearing to a waypoint (airport, waypoint, VOR, NDB, Airport, etc.), this bearing information may not match the “radial” signal from a VOR ground transmitter due to the transmitter not being aligned with magnetic north.  You can find information on a VOR ground transmitter magnetic-north alignment in the A/FD (Airport/Facility Directory).  Hopefully this information helped to clear the air and keep a few pilots from similar confusion.

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