The NPL provides a measurement service for the multistation field baselines which are used by the surveying profession to calibrate electronic distance meters. This activity employs the Mekometer ME5000, a precision laser distance meter whose predecessor was originally developed at the laboratory. To confirm traceability, the Mekometer can be compared directly with a fringe-counting laser interferometer by mounting the retroreflectors for both instruments on a moving carriage, but only up to 50 m. However, the range can be increased greatly by multiply reflecting the Mekometer laser beam between two large plane mirrors, one of which is mounted on the moving carriage. A change in the mirror separation as measured by the interferometer, when multiplied by the number of passes of the Mekometer beam, should agree with the change in the Mekometer reading. By this means, the claimed accuracy of 0.2 mm has been confirmed up to 550 m. The corrections needed for beam obliquity have a comparable order of magnitude.
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