GPS not precise

[krazyj]

hey

when I set my autopilot on cruise control and set the HDG and all the DTK on the GPS doesnt match the course set on the panel usually is of by anywhere from 2-6 degress ?

any idea why

[Nexus]

Copied and pasted from the FS learning centre
"A GPS receiver cannot determine an aircraft's heading, only its track across the ground. Never assume that the TRK (track) on the GPS display is the same as your heading. If there's a crosswind it won't be"

DTK is desired track and it's the shortest course to the next waypoint, and again it's not rare that DTK value won't match the HDG. You may have to compensate winddrift by selecting lower or higher heading than the DTK in order to maintain the desired track.

However why don't you just fly the GPS course, so you don't have to worry about selecting correct headings?

[krazyj]

hey nexus

you are truly a wise person hehe

how do I do that ??

havent really been working with that yet just stuck to what I knew ??

thanx

[Liam]

I think he just means to switch the NAV/GPS switch to GPS, then flick on the NAV autopilot button.

[sdodsworth]

If you are interested in the technicalities, try an internet search for 'triangle of velocities' or 'wind triangle'.

It's the reason why, when your watching an a/c on approach, it will often appear to be 'crabbing', that is pointing in a slightly different (or very different, if there is a big crosswind) direction to that in which it is actually travelling.

'Track' is the course that the a/c travels over the ground, 'heading' is the direction in which the a/c is pointing, and as Nexus says, they ain't necessarily the same. Have fun!  :)

[deKoven]

I think he just means to switch the NAV/GPS switch to GPS, then flick on the NAV autopilot button.

Actually, you have that backwards. Switch on the NAV button first, then switch to GPS on the NAV/GPS switch. It doesn't work the other way around for all planes.

[Liam]

Works for me.

[Scottler]

Quick intro lesson on GPS.

GPS units use a receiver to read info from a satellite, thus reporting your precise location.  Since the satellite has no idea where you're going, there's no way it could plot your course.