The SLALIB routine
sla_OBS
supplies details of groundbased observatories from an internal
list. This is useful when writing applications that apply to
more than one observatory; the user can enter a brief name,
or browse through a list, and be spared the trouble of typing
in the full latitude, longitude *etc*. The following
Fortran code returns the full name, longitude and latitude
of a specified observatory:

CHARACTER IDENT*10,NAME*40 DOUBLE PRECISION W,P,H : CALL sla_OBS(0,IDENT,NAME,W,P,H) IF (NAME.EQ.'?') ... (not recognized)

(Beware of the longitude sign convention, which is west +ve for historical reasons.) The following lists all the supported observatories:

: INTEGER N : N=1 NAME=' ' DO WHILE (NAME.NE.'?') CALL sla_OBS(N,IDENT,NAME,W,P,H) IF (NAME.NE.'?') THEN WRITE (*,'(1X,I3,4X,A,4X,A)') N,IDENT,NAME N=N+1 END IF END DO

The routine
sla_GEOC
converts a *geodetic latitude*
(one referred to the local horizon) to a geocentric position,
taking into account the Earth's oblateness and also the height
above sea level of the observer. The results are expressed in
vector form, namely as the distance of the observer from
the spin axis and equator respectively. The *geocentric
latitude* can be found be evaluating ATAN2 of the
two numbers. A full 3-D vector description of the position
and velocity of the observer is available through the routine
sla_PVOBS.
For a specified geodetic latitude, height above
sea level, and local sidereal time,
sla_PVOBS
generates a 6-element vector containing the position and
velocity with respect to the true equator and equinox of
date (*i.e.* compatible with apparent ). For
some applications it will be necessary to convert to a
mean frame (notably FK5, J2000) by multiplying
elements 1-3 and 4-6 respectively with the appropriate
precession matrix. (In theory an additional correction to the
velocity vector is needed to allow for differential precession,
but this correction is always negligible.)

See also the discussion of the routine sla_RVEROT, later.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk