found more accurately by adding frequencies, for example, frequencies at which propagation is sensitive to the presence of water vapor. This way non-ideal contributions to the refractive index can be measured and corrected for at another frequency using established theoretical models. It may be noted again, by way of contrast, that the transit-time measurement of length is independent of any knowledge of the source frequency, except for a possible dependence of the correction relating the measurement medium to the reference medium of classical vacuum, which may indeed depend on the frequency of the source. Where a pulse train or some other wave-shaping is used, a range of frequencies may be involved. Diffraction measurements For small objects, different methods are used that also depend upon determining size in units of wavelengths. For instance, in the case of a crystal, atomic spacings can be determined using X-ray diffraction. The present best value for the lattice parameter of silicon, denoted a, is: a = 543.102 0504(89) × 10−12 m, corresponding to a resolution of ΔL/L ≈ Similar techniques can provide the dimensions of small structures repeated in large periodic arrays like a diffraction grating. Such measurements allow the calibration of electron microscopes, extending measurement capabilities. For non-relativistic electrons in an electron microscope, the de Broglie wavelength is: with V the electrical voltage drop traversed by the electron, me the electron mass, e the elementary charge, and h the Planck constant. This wavelength can be measured in terms of inter-atomic spacing using a crystal diffraction pattern, and related to the metre through an optical measurement of the lattice spacing on the same crystal. This process of extending calibration is called metrological traceability. The use of metrological traceability to connect different regimes of measurement is similar to the idea behind the cosmic distance ladder for different ranges of astronomical length. Both calibrate different methods for length measurement using overlapping ranges of applicability. Far and moving targets Ranging is technique that measures distance or slant range from the observer to a target, especially a far and moving target. Active methods use unilateral transmission and passive reflection. Active rangefinding methods include laser (lidar), radar, sonar, and ultrasonic rangefinding. Other devices which measure distance using trigonometry are stadiametric, coincidence and stereoscopic rangefinders. Older methodologies that use a set of known information (usually distance or target sizes) to make the measurement, have been in regular use since the 18th