Laser trackers are becoming the tool of choice for large volume dimensional metrology
applications such as the measurement of aerospace assemblies, power plant structures,
civil engineering structures and terrestrial transportation vehicles. A laser tracker is a
portable coordinate measuring system that tracks a moving target reflector and measures
the position of the target in spherical coordinates (r,
θ, φ). The metrological performance of a laser tracker
is influenced by many factors including: compensation for atmospheric effects, thermal
expansion of the instrument and its mount, thermal distortion of the workpiece or artefact
being measured, the wavelength of the laser radiation, the internal alignment of the
gimbal axes and the linearity and alignment of the internal angular measuring scales. The
most important of these potential error sources, which fundamentally limit the achievable
uncertainty, are the internal mechanical and optical alignments and the quality and
alignment of the angular scales. Several national and international standards exist or are
in the process of being developed for performance verification of laser trackers. ASME
B89.4.19-2006 is one of the established standards used to verify the performance of laser
trackers. The main test relies on measuring a known reference length in a variety of
configurations and ranges and comparison of the observed error (laser tracker measured
length minus reference length) with the specified maximum permissible error (MPE) defined
by the manufacturer. The establishment of an ASME B89.4.19 laser tracker verification
facility at NPL is introduced. We highlight the importance of tracker verification and
discuss the error sources, which contribute to the tracker measurement uncertainty. Some
initial results obtained using this new facility are presented.