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Calibration Targets
 











Fundamental to the reduction and subsequent use of the raw SLR measurements obtained by the 
system is accurately
refering them to the fiducial point of the SLR telescope (intersection of alt-az axes).
This is acheived at an accuracy level of a few mm by regularly making range measurements
to a set of calibration targets distributed around the site. The distances between the
targets and the system fiducial point have been measured using classical and GPS-based
surveying
techniques. 












So the differences between the SLR-measured distances and the survey results
allow us to determine an internal calibration, which must be applied to each satellite
range measurement.



The picture shows a calibration target mounted on the outside of the orginal dome of
the Isaac Newton Telescope, which is some 400m East of the SLR Facility. The fiducial point
of this precision target is at the centre of the line of intersection of the two
mutually perpendicular plates. Its distance from the fiducial point of the SLR system
has been surveyed to an accuracy of a few mm.













The second calibration target close to that mounted on the INT is fixed on the top of an
Ordnance Survey pillar, which used to be the UK prime pillar (the prime pillar is now 
the one very close
to the Space Geodesy Facility). The trees are kept clear of
this target to permit occasional use by survey groups using GPS receivers.













A third target, plus an experimental, movable one, is mounted on the top of a building some
100m to the West of the Facility

 
Time series of the calibration measurements also allow us to monitor short and long-term 
system stability.












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