From: "Richard Hertz"


I have yet to try this myself, but two pilots I know suggested I try it.
The airways are defined by VOR radials. GPSs do not seem to navigate them
correctly. In fact, one of the pilots mentioned flew with an
"instructor"
who was relying on a GPS for lateral guidance on an IFR flight plan. The
pilot told the instructor to knock it off and use the vors as he was
seeing
CDI needle deflection. The instructor said not to worry.

After some time ATC informed them that they were off the airway (though
the
gps showed dead-on).

You should be able to navigate an airway more accurately using a GPS than
you can with a VOR.

The problem with VORs is that they are not aligned with magnetic north.
Usually, they are aligned with magnetic north when the VOR station is first
installed, but they are not kept in alignment as the earth's magnetic field
shifts. Have you ever noticed how runways are occasionally recharted with
new magnetic headings? Also check he
http://www.geolab.nrcan.gc.ca/geomag...vt_nmp_e.shtml.

Many GPS receivers use a model of the earth's magnetic field that results in
a magnetic correction similar to the isogonic lines on the chart. If the
instructor executed a direct to the VOR waypoint in the GPS and then used
the OBS feature to select the inbound course as depicted by the VOR radial
on the chart, he may be several degrees off.

If you want to look at an example, go to www.airnav.com and pull up SWL VOR
and KOXB (Ocean City). SWL has a variation of 8W (1965) and KOXB has a
variation of 12W (2000). There might be a close airport, but SWL is 22 nm
from KOXB.

Some GPS receivers such as the Garmin 196, 295, 296, 430, and 530 use the
slaved value of the VOR for the OBS mode. Thus, setting the OBS mode to a
value results in the same path over the ground as if you selected the VOR
OBS to the same number. On some GPS receivers, such as the Garmin GPS III
Pilot the GPS does not compensate for this VOR misalignment.

What I don't know is whether there is any requirement for an IFR certified
GPS to use the VOR slaved value for magnetic variation. There is also the
possibility that the instructor in your story was using a non-IFR GPS.

This is not to say the OBS mode is the best way to navigate an airway, just
that this might be a cause for error. A better way is to set up a route in
the GPS to reflect points on the airway. Such as from VOR A to VOR B or
INTERSECTION to VOR if there is a bend in the airway.

John Bell
www.cockpitgps.com