### SLA_TP2V

Tangent Plane to Direction Cosines

ACTION:
Given the tangent-plane coordinates of a star and the direction cosines of the tangent point, determine the direction cosines of the star (single precision).
CALL:
CALL sla_TP2V (XI, ETA, V0, V)
##### GIVEN:
 XI,ETA R tangent plane coordinates of star (radians) V0 R(3) direction cosines of tangent point

##### RETURNED:
 V R(3) direction cosines of star

NOTES:
(1)
If vector V0 is not of unit length, the returned vector V will be wrong.
(2)
If vector V0 points at a pole, the returned vector V will be based on the arbitrary assumption that $\alpha =0$ at the tangent point.
(3)
The projection is called the gnomonic projection; the Cartesian coordinates $\left[\phantom{\rule{0.3em}{0ex}}\xi ,\eta \phantom{\rule{0.3em}{0ex}}\right]$ are called standard coordinates. The latter are in units of the distance from the tangent plane to the projection point, i.e. radians near the origin.
(4)
This routine is the Cartesian equivalent of the routine sla_TP2S.