Dave,

A GPS-only unit has no way of knowing what your current heading is in the
first place.

I agree with this statement.

I haven't heard of a GPS receiver that will tell you what heading to fly or
sail to compensate for wind or current.

Neither have I.

It can tell you how much to turn to align your heading with the desired
course, but that assumes no wind or current. In practice, you'll have to
make
adjustments to that heading, while watching the cross-track error.

I agree that a GPS will not tell you an absolute heading to fly, but it will
tell you a relative heading. The GPS will give you a value of TRACK and it
will give you a value for BEARING. The idea is to adjust your heading so
that TRACK matches BEARING. Many GPS receivers have a data field called
TURN --this is just the difference between TRACK and BEARING. If the GPS
indicates a TURN of 5 degrees left, adjust your heading 5 degrees left. If
you follow TURN or match TRACK and BEARING, you should track directly to the
active waypoint.

I agree that the GPS could care less what your heading is. Example: If you
have a TRACK of 85 and a BEARING of 80, the TURN would be L5. Turn 5 left
so that TRACK matches BEARING. Let's say that you have done that and the
TRACK and BEARING are now both 80. The GPS has no idea if you are heading
065 for a left crosswind, 95 for a right crosswind, or 080 for no wind.

If you were to follow a heading to match the GPS value of BEARING, a curved
path would result from a crosswind or cross current. This is often refereed
to as homing and is considered bad technique.

For large turns the change in heading does not always match the change in
TRACK. For example, if you were flying north into a northerly wind and
turned to 090, the change in track would be more than the change in heading.
The headwind would change to a crosswind. However for small adjustments,
the change in heading is proportional to the change in track.

There is a slight variation in this technique for panel mounted aviation
receivers. The difference is that they usually do not offer TURN, but do
offer a field called Track Angle Error, TKE. TKE is the difference between
TRACK and DTK or COURSE (synonyms). TKE gets you parallel to the course as
opposed to directly to the active waypoint. In this case, cross-track error
is necessary. However by turning so that TKE is zero will result in a
constant cross-track error. It may then be necesary to adjust the heading a
little more to reduce the cross-track error to zero.

The method that you describe of adjusting your heading to manage cross track
is not incorrect. However, I thing that the ability to get an exact heading
that compensates for wind or currents by comparing the TRACK to BEARING is
one of the most powerful features that GPS has to offer aviators and
boaters.

I have more details in the navigation section of either of my online books
at www.cockpitgps.com.

--John Bell