Sorry if these ideas have been mentioned before.

#1  Send an ultrasonic pulse ( USP ) from the base station.  At the same ti=
me send an electromagnetic pulse ( RF or optical ).  Measure the time of fl=
ight of the USP.  You can calibrate the speed of the USP by setting up a fi=
xed receiver that receives the USP.  You can send this info to the moving d=
evice.
Dm / Tm =3D Df / Tf  or Dm =3D Tm x Df / Tf   where Dm is moving distance ,=
 Df is fixed distance , Tm is time of flight ( TOF ) for the moving device =
and Tf is the TOF for the fixed distance.

#2 Use a collimated laser with a constant rotation period.  Put the laser o=
n the mobile unit or at the base station.  At the receiving end are two sen=
sors that measure the time it takes for the laser beam to travel from one s=
ensor to another.  Based on trigonometry , you can calculate distance.  I r=
ealized JIT that the receiving sensors would have to point directly at the =
transmitter.  This may prove to be too difficult.

Gus in Denver
--=20
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