The Global Positioning System (GPS) can deliver an exceptionally accurate frequency standard to any point in the world. When we use the GPS signal to control an optical frequency comb, the comb+GPS system provides laser light with well-known frequencies (or equivalently, vacuum wavelengths) over much of the optical spectrum between 0.53 (mu)m and 2 (mu)m. The comb vacuum wavelengths can serve as primary length standards for calibration of the wavelength of metrology lasers, and the uncertainties of the comb wavelengths are sufficiently low that it is suitable for almost any imaginable task associated with length metrology, The GPS signal is "traceable" in the sense that its uncertainty is continually assessed via measurements at NIST/Boulder, and results of the measurements (in effect, "calibration reports"), are published on the web. Thus it can potentially deliver a traceable standard of unprecedented accuracy to any laboratory, but how can the user be certain that the resulting laser calibrations have comparable accuracy? These calibrations depend not only on the GPS signal but also on much additional equipment (including a disciplined oscillator, optical frequency comb, and optics/electronics for beat frequency measurement), and any such system might contain additional sources of error if it is poorly designed or operated by inexperienced personnel. However, in this paper we argue that internal consistency checks can be effectively used to verify the proper operation of the measurement system. In many respects these internal consistency checks provide better confidence in the results than what is likely to be achieved by more traditional methods of establishing traceability, such as sending an instrument or artifact to NIST for calibration.
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