The VOR uses a dated variation in that the variation is established when the VOR was installed or when it was last updated. The GPS can use the local variation at the position the aircraft is at.
One difference is how the magnetic variation is calculated. The AIM provides some words on the differences between a VOR radial or Localizer course and a GPS course over the same route. Up for a challenge? AOPA’s Air Safety Foundation has this great GPS for IFR Operations Quiz you can take here. I would keep your GPS needle centered as long as you are able to get those intersections on the airway plugged in. During approach mode, the course width is only 0.3 nm on either side of the course centerline. 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. 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. 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. 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. 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). My question is which instrument should I use for navigation? Sometimes, I will split the difference between the two instruments.
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. However, when I do this my GPS and VOR indicator often do not agree on the correct course. If I use my GPS to navigate I have to fly using the VOR waypoints. My GPS does not have the capability to fly these routes. Hopefully they can be convinced that this was an incorrect design choice.When I file an IFR flight plan using VOR waypoints ATC will be often give me clearance to fly a Victor route instead. This is not a true “bug” because they deliberately designed ILS systems to work this way. It remains to be seen if they will change it at some point. They have been made aware of the problem through their Zendesk bug reporting system. Navigraph can fix the nav data, but only Asobo can remove the localizer/runway lock in the airport scenery files.
Unfortunately, because of the other value contained in the airport scenery file, the localizer is still forced to the runway heading, even with correct nav data. The Navigraph data does at least give you the correct published course for every ILS localizer, including those which are offset. With default nav data, that course will always be the same as the runway heading, even when it should actually be offset. That course value comes from the nav data files. When you tune an ILS on the Nav receiver in an MSFS airplane, the localizer course will be automatically set on the PFD display when the ILS is being received. The nav data file controls the localizer identifier, frequency, width of the localizer beam, glideslope angle, and most important, the localizer course. Probably because of this, the default NavBlue nav data has been modified by Asobo to also set every localizer to the runway heading, even though the original source data NavBlue provides probably has the correct value. All localizers in MSFS are forced to the runway heading by a byte value contained in the airport scenery file.
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