VOR vs GPS
John writes in asking:
When I file an IFR flight plan using VOR waypoints ATC will be often give me clearance to fly a Victor route instead. My GPS does not have the capability to fly these routes. If I use my GPS to navigate I have to fly using the VOR waypoints. However, when I do this my GPS and VOR indicator often do not agree on the correct course. Sometimes, for example, the GPS will indicate I may be slightly off course but the VOR will indicate I am on course or vice versa. My question is which instrument should I use for navigation? Sometimes, I will split the difference between the two instruments.
What kind of GPS receiver are you using? I’m kind of surprised that your IFR certified GPS doesn’t have the victor airway intersections and waypoints in the GPS database? Using the Garmin GNS 430 or 530 I am able to manually enter each intersection on an airway when building the flight plan (although it can be a real pain). One of the things I love about the G1000 is being able to load an entire airway, just like I can in the “big box” FMS units. It is important to have those intersection inserted as there are often very subtle course changes at intersections in the airway; especially, on longer segments.
To get back to your question, when an IFR-certified GPS is operating in en route mode, the course width on either side of the centerline is 5 nm. A CDI reading from a VOR signal, on the other hand, will display course guidance in degrees, each of the five dots on either side representing a 2 degree course error. During approach mode, the course width is only 0.3 nm on either side of the course centerline. I would keep your GPS needle centered as long as you are able to get those intersections on the airway plugged in.
Up for a challenge? AOPA’s Air Safety Foundation has this great GPS for IFR Operations Quiz you can take here.
-Paul
John D. Collins on Jan 24, 2011
The AIM provides some words on the differences between a VOR radial or Localizer course and a GPS course over the same route. One difference is how the magnetic variation is calculated. The GPS can use the local variation at the position the aircraft is at. The VOR uses a dated variation in that the variation is established when the VOR was installed or when it was last updated. Updating the VOR variation as the magnetic variation changes over time is a complex operation and requires adjusting the VOR orientation to magnetic north and changing all airways, intersections, approaches, SIDs and STARs, etc. on which the VOR defines.a radial. Even though the variation may change, the VOR orientation remains the same as long as it is not updated, so over time,the actual magnetic course will differ more and more from the charted radial. However, since the VOR still points its radials in the same direction, the path over the ground doesn’t move. The GPS uses a current magnetic variation and if you enter an airway with the VOR waypoints and any course change waypoints into the flightplan, the path over the ground will be identical to the VOR path, even though with the VOR it will require a different magnetic radial setting on the OBS.
One additional update, on an IFR WAAS GPS, the Full Scale Deflection (FSD) of the CDI when enroute is +/- 2 NM and on an approach, it is generally angular (+/- 2 degrees).