Laser distance meters/ laser trackers

I'm researching laser trackers at the moment with a view to building a
low resolution one.

If you want to know what one is check out:

http://www.faro.com/content.aspx?ct=uk&content=pro&item=3
http://www.leica-geosystems.com/corporate/en/products/laser_tracker/lgs_35317.htm
http://www.apisensor.com/

Basically the head tracks a retro reflector and using the angle and
the distance to the reflector works out position. The returned beam
from the retro reflector is off-set unless the beam strikes the centre
and this is used for tracking control (servos try to keep zero offset).

Distance is determined by interferometry (incremental encoding) or by
time-of-flight phase shift methods that provide absolute distance
measurement (ADM). Neither of this methods involve triangulation.
These methods give increadible positioning accuracy (tens of microns)
at 10's of meters range.

Leica do hand held laser distance meters accurate to +/-1.5mm, I need
something that can hold 1mm resolution accurately. I'm looking for
distance measuring modules that can do that but am not having much
look. I thought I'd cast a line to the group and ask. I'd prefer ADM
but an interferometer would not be ruled out, Renishaw do them but
they seem to have two beams, it must be single beam.

Cheers,

Graham