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albar965

Why the GPS does not give the right course to a VOR

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First of all: It has nothing to do with magnetic variation and nothing with old navdata.

 

Normally when navigating the great circle route (or orthodrome)  is used since it gives the shortest distance on the earth surface. But when you fly along this you have to change your heading constantly. Normally it is done by the autopilot and the GPS or the FMC, so you won't notice.

 

When looking at a VOR it is obvious that the radiowaves don't change heading but travel along a constant course. This is the rhumb line or loxodrome. It results in a slightly longer travelling distance between two points.

 

The GPS gives you the course of the great circle route which is not the right one to approach a VOR. Depending on heading and distance it can be degrees off.

 

The "Direct °M" column in Little Navmap's flight plan table and also the course at the flight plan lines show the heading of the rhumb line and this should be used when using VOR or NDB navigation.

 

The picture below shows the difference. The upper black line is the great circle route (the shortest route) that has the course at departure and destination attached. When flying along the great circle route you have to change your heading constantly between 85° and 95°. The GPS will show 85° at departure and dialling that value into the CDI will put you far off (provided there is a VOR at the end of the line).

 

The lower gray line has a constant 90 degree course and follows exactly the 70° north latitude. But it has a slightly longer distance between the two points (not noticeable here, both show 205 nm).

 

great_circle_and_rhumb_line.jpg

 

Alex

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