[Proj] using proj4 for transformation from local polar coordinates to other local polar (with different center and height)

[Mgr. Martin Boka]

Hi List,
I am newbie in using proj and map projections in general.
I have this problem:

I have some sensor(s) which is placed at given WGS84 coordinates
(lat/long from GPS) and given height above the sea level (also from
GPS). I have some detections on some Rho (the distance from the sphere
center - slant range) and Theta (the azimutal angle, 0 deg is due north
approximately) from this given center. I have also heights of these
targets above the ground (using a pressure altimeter - so it is above
sea level).
Now i need some transformation and projection to the plane (map)
tangential to the Earth at the place of sensor or at another place with
given coordinates and height (for multi sensor coordination).
I think I could use the pj_transform and pj_datum_transform. But I dont
know how to use it with the given height which is not from the sensor
possition but against sea level and with local polar coordinates of
targets. Also I need to transform slant-range to  the range from the
sensor (at target tangential plane).

Because I am getting data (possitions of targets) continually I must do
transformation for each target or a small nuber of targets separately.
But there is the same center and therefore is not necessary to compute
The Earth radius or sin and cos of coordinates each time. Is there any
way to compute these constants at initialization of projection an use it
for computations?

Thanks for any help

Martin Boka
Czech republic