e `ref' at time `et'. Then
               `srfvec' can be transformed to the result `refvec' as
               follows:

                     xform  = <a href="cspice_pxfrm2.html">cspice_pxfrm2</a> ( fixref, ref, trgepc, et )
                     refvec = xform * srfvec

      phase    is the phase angle at `spoint', as seen from `obsrvr' at
               time `et'. This is the angle between the spoint-obsrvr
               vector and the spoint-illumination source vector. Units
               are radians.