The Difference Between VOR Radials and GPS Directions
VOR radial does not match GPS course
Many pilots have noticed that the GPS DTK given by the GPS does not match the published radial on the chart. I mention this in Cockpit GPS, which is available for download on the main page at www.cockpitgps.com. However, I thought that I would throw together a couple of examples.
Look at V139 from CBEAV to SWL. CBEAV is on the 034 radial. Thus, it is 214 inbound.
In both cases, the aircraft is at CBEAV
We put CBEAV to SWL into a route on a GPS and the DTK indicates 218 rather than 214. Which is correct?
It turns out that both are correct. It turns out that the worlds magnetic variation has changed since the SWL VOR was built. Go to www.airnav.com and look up SWL. You will find that it was oriented to a variation of 8W in 1965. Look at the isogonic lines on a chart and you will see that the magnetic variation around SWL is closer to 12.
The magnetic variation of the earth actually changes over time. This is why you will often see runways recharted by a couple of degrees -- the pavement has not moved. Below is a cool .gif which should show the movement of the earth's magnetic field over time. Notice the movement of the magnetic North Pole, which is at the convergence of the lines in the upper left corner.
Before we get into the discussion and examples, a little perspective is important. For the most part, it is better to define a route using waypoints. In our example, it is better to describe this leg of V319 by inputing CBEAV -> SWL into a flight plan. You should also know how to use the intercept leg feature on those GPS receivers that support it. When the GPS is used in this manner, this discussion has no operational implications other than explaining why the DTK and VOR radial don't match.
OBS mode
Most GPS receivers have an OBS feature which allows you to set a bearing to the waypoint as you would select a VOR radial. The CDI, XTK (Cross Track or Off Course on some models), and line on the map display will now reference this OBS setting. What was a point of trivia as to why the VOR radial does not match the XTK now becomes the question of what should you dial into the OBS. In our example, the problem would you dial in -- 218 or 214?
The answer is- it depends on your model of GPS. The Garmin GPS 196, 295, 296, 400, 500, 1000 all reference the magnetic variation of the VOR for setting the OBS. The 400, 500, and 1000 additionally use this variation for data fields such as DTK, TRK, etc. Thus, on any of these units, you would dial in the charted radial -- 214 rather than 214.
Below is the GPS 530 simulator in the OBS mode with the DTK set to 214. The GPS 500 is programmed to make the GPS give you the same line over the ground as tuning the VOR radial.
No movement - still over CBEAV
Neglecting great circle effects and the fact that magnetic variation may change over a distance. Within the distances involved and for all practical purposes, the following are essentially the same path over the ground:
CBEAV -> SWL
218 magnetic course from CBEAV to SWL
The 034 radial from SWL
So far we have discussed how the GPS will treat the VOR waypoint like a VOR with respect to setting the OBS. Just for kicks, let's see what would happen if we set the OBS to 218 because we pulled the number off a computer flight plan rather than looking at the VOR radial on the chart:
Over CBEAV
Notice that in the above screenshot, setting the OBS to 218 creates a different line over the ground. The aircraft is still over CBEAV, but the GPS is indicating that we are 1.35 miles west of course.
Note that the GPS will only use the VOR variation if the active waypoint is the VOR. The screenshot below demonstrates how setting the OBS to 034, the published radial, into CBEAV will not work. The simulated location has been changed below to being at SWL. The OBS has been set to 034 as is indicated by the DTK. However, the BRG is 038 and the GPS is showing that we are 1.54 miles off course even though the aircraft is directly over SWL.
Located over SWL
There are differences in how the use of the VOR variation is implemented. I have not tested the 296 or 96, but the Garmin 196 (below) will use the VOR variation for setting the OBS, but not for the COURSE (Garmin handheld equivalent of DTK). See below:
CBEAV to SWL at CBEAV
Located at CBEAV in OBS mode
Located at CBEAV with OBS mode and true directions
I prefer the map mode over the HSI for using the 196, but that is another subject. The point of the above screenshots is that the OBS is set to the 034 radial (214 inbound) and the COURSE and BEARING both indicate the magnetic bearing without compensating. This is not a problem, as long as the COURSE, BEARING, and TRACK all use the same variation. Notice that the COURSE and BEARING match and the CROSS TRACK is essentially zero. The important thing to know is whether or not the OBS mode compensates for the mis-alignment of the VOR. In the case of the 196 it does.
Handheld GPS receivers can be set to use true rather than magnetic values for directions. When the GPS is set to use true values, no magnetic correction is used and this whole discussion is irrelevant as shown in the bottom Garmin 196 screenshot.
Some GPS receivers do not compensate for the VOR mis-alignment at all. Although I think that it is good for the GPS to use the variation of the VOR when in OBS mode, I don't think that this would be a significant factor in choosing a handheld GPS. I know that the Lowrance 500 and 1000 do not have this feature and the Garmin GPS III Pilot does not have this feature.
Lowrance 1000 at CBEAV in OBS mode
Garmin GPS III Pilot at CBEAV in OBS
G1000 screenshots
G1000 CBEAV to SWL (click for full size image)
GPS 1000 OBS mode over CBEAV (Click for full size image)
The G1000 works similarly to the Garmin 500 series. The aircraft positions are exactly the same in the two shots. The wind is set to zero for these screenshots. Notice that in the OBS mode, it looks our heading is 4 degrees off even though we are actually heading directly to SWL.
Summary
If this is a little confusing, it really comes down to this: Some GPS receivers allow you to set the OBS just like it were a VOR when the active waypoint is a VOR and some don't. If you use the OBS mode, it is a good idea to know how the GPS receiver handles this.
A little perspective is also in order that usually the magnitude of error that you would make is probably within the permissible error range for the VOR anyway. For all but the most mis-aligned VORs, your error is generally only going to be a couple of degrees. For things like an overlay of a VOR approach, you should not be using the OBS mode anyway.