Improving Length and Scale Traceability in Local Geodynamical Measurements

Abstract

Traceability is a feature that is required more frequently in local geodetic high-precision measurements. This basic term of metrology, a measurement science, describes the property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty (BIPM International vocabulary of metrology – basic and general concepts and associated terms (VIM). JCGM 200:2008. Joint Committee for Guides in Metrology, 2008b).

GPS measurements are widely used in local geodynamical research. From the viewpoint of metrology, their traceability is uncontrollable because the scale cannot be unambiguously conducted based on the definition of the metre. In particular, atmospheric effects on a GPS signal cannot be modelled or calibrated along the path of the signal.

We are testing a method to bring the traceable scale to small GPS networks using high-precision electronic distance measurement (EDM) instruments, the scales of which have been corrected and validated in calibrations at the Nummela Standard Baseline. The traceable scale of EDM is expected to explain the annual scale variations that have been found in GPS time series and to improve results of episodic GPS campaigns.

The scale of a standard baseline is validated and maintained through regular interference measurements with the Väisälä interference comparator, in which a quartz gauge conveys the traceable scale. The results from the interference measurements in 2005 and 2007 in Nummela are presented here together with a brief description of the present state of the renowned measurement standard. A standard uncertainty of ±0.08 ppm was obtained again for the baseline length of 864 m, and the results confirm the good long-term stability of the baseline. The scale is transferred further to geodetic and geophysical applications by using calibrated high-precision EDM instruments as transfer standards.

Local geodynamical measurements will profit from the reduced and accurately estimated uncertainty of the measurement, and therefore we seek further innovations to improve their traceability. We present here a topical example of calibrations and scale transfer for a baseline and monitoring network around a nuclear power plant. We compare simultaneously measured EDM and GPS results and show a scale bias of approximately 1 ppm between them. By using a traceable length in the network, the bias could be reduced, e.g. by improving the processing strategy of GPS observations. This paper focuses on the metrological part of EDM. Some related results and analysis of GPS measurements are discussed in another paper in this volume (Koivula et al. GPS metrology – bringing traceable scale to local crustal deformation GPS network. IAG Scientific Assembly “Geodesy for Planet Earth”, Buenos Aires, Argentina, 2010).