SLA_RVEROT - RV Corrn to Earth Centre

ACTION:
     Velocity component in a given direction due to Earth rotation.
CALL:
     R = sla_RVEROT (PHI, RA, DA, ST)

GIVEN:

      PHI   R geodetic latitude of observing station (radians)

      RA,DA R apparent alpha, delta (radians)

      ST    R local apparent sidereal time (radians)

RETURNED:

      sla_RVEROT R Component of Earth rotation in direction RA,DA (km s-1)

NOTES:
     1.   Sign convention: the result is positive when the observatory is
          receding from the given point on the sky.
     2.   Accuracy: the simple algorithm used assumes a spherical Earth and
          an observing station at sea level; for actual observing sites, the
          error is unlikely to be greater than 0.0005 km s-1. For
          applications requiring greater precision, use the routine
          sla_PVOBS.