Prasad,

Chris (who wrote the package) might be able to tell you more, but my
understanding is that it is more of a utility package than anything
else. It is meant to sit between the robot's own odometry and a
subscriber to that odometry. It provides a service that allows you to
"correct" the robot's odometry.

It's probably easiest to understand with a concrete example:

Let's say you had a dead-reckoning robot that could interact with a
number of docking stations at set locations. The robot's odometry
would drift hither and yon as it moves about, but the position of the
docking stations is known. When the robot is at a docking station, you
could use the position_tracker's service to force the robot's position
and orientation. The odometry would then be correct and the drifting
process would start over.

The behavior the service gives is "snapping", so depending on what you
want to do you might want to use another form of sensor fusion. I'm
not sure of its compatibility with other services in the nav stack.
We've mostly used it in teleoperation situations where the user is
driving the robot through a pre-mapped course that is sparsely
populated with fiducials.

Hopefully that clears things up, let me know if you have any more questions.

_Trevor