I find it disappointing and frankly astonishing that there is such confusion in some peoples' minds between track (TRK) and heading (HDG). A similar confusion exists in some peoples' heads between TRK and desired track (DTK) - the notion being that if DTK = TRK, then they are on the planned track..... Of course that isn't true, they're simply parallel to the planned track.
DTK - the planned track between waypoints.
TRK - the actual track at the moment of observation
BRG - the track between position at the moment of observation and the next planned waypoint.
HDG - the direction in which the aircraft's nose is pointing. CANNOT be measured directly by GPS.
XTK - the off-track distance at the moment of observation
TKE - the track angle error between TRK and DTK
If DTK=BRG, you're on your planned track to the next waypoint (assuming you've passed the previous waypoint).
If TRK=BRG, you're on a the direct track from where you are to the next waypoint
If TRK=DTK, you're flying parallel to your planned track and will not necessarily reach the next desired waypoint.
Is it any wonder there are airspace busts resulting from GPS users ignorance of their systems?
GPS is a superb, reliable and flexible system. But it cannot be used correctly by people who don't understand the capabilities of the system and the meaning of the displayed values.