century. Special ranging makes use of actively synchronized transmission and travel time measurements. The time difference between several received signals is used to determine exact distances (upon multiplication by the speed of light). This principle is used in satellite navigation. In conjunction with a standardized model of the Earth's surface, a location on that surface may be determined with high accuracy. Ranging methods without accurate time synchronization of the receiver are called pseudorange, used, for example, in GPS positioning. With other systems ranging is obtained from passive radiation measurements only: the noise or radiation signature of the object generates the signal that is used to determine range. This asynchronous method requires multiple measurements to obtain a range by taking multiple bearings instead of appropriate scaling of active pings, otherwise the system is just capable of providing a simple bearing from any single measurement. Combining several measurements in a time sequence leads to tracking and tracing. A commonly used term for residing terrestrial objects is surveying. Other techniques Measuring dimensions of localized structures (as opposed to large arrays of atoms like a crystal), as in modern integrated circuits, is done using the scanning electron microscope. This instrument bounces electrons off the object to be measured in a high vacuum enclosure, and the reflected electrons are collected as a photodetector image that is interpreted by a computer. These are not transit-time measurements, but are based upon comparison of Fourier transforms of images with theoretical results from computer modeling. Such elaborate methods are required because the image depends on the three-dimensional geometry of the measured feature, for example, the contour of an edge, and not just upon one- or two-dimensional properties. The underlying limitations are the beam width and the wavelength of the electron beam (determining diffraction), determined, as already discussed, by the electron beam energy. The calibration of these scanning electron microscope measurements is tricky, as results depend upon the material measured and its geometry. A typical wavelength is and a typical resolution is about Other small dimension techniques are the atomic force microscope, the focused ion beam and the helium ion microscope. Calibration is attempted using standard samples measured by transmission electron microscope (TEM). Nuclear Overhauser effect spectroscopy (NOESY) is a specialized type of nuclear magnetic resonance spectroscopy where distances between atoms can be measured. It is based on the effect where nuclear spin cross-relaxation after excitation by a radio pulse