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Artifact

In imaging, an artifact is the perception of a feature which isn't actually there. Artifacts can be caused by noise, motion, instrumentation error, and a range of other problems.

Atomic clock

An atomic clock uses microwaves emitted by or absorbed by atoms as a timing signal. The frequency of the emission/absorption is a fundamental physical property of the type of atom used, and so has a precise and constant value. This allows immensely high accuracies to be achieved, with a typical error being one second per million years. An overview of how atomic clocks work is available at How Stuff Works. A history of atomic clocks can be seen on the London Science Museum website.

Capacitance

Capacitance is a measure of object's ability to hold an electrical charge. If two metal plates are separated by an insulator, and a voltage is applied across them, a negative electric charge will build up on one plate, and a positive charge will build up on the other. The greater these charges are (for a given voltage) the greater the capacitance is. The capacitance is dependent on both the physical dimensions of the setup, and an intrinsic property of the material itself, known as dielectric permittivity.

CAT

CAT stands for Computerised Axial Tomography. See the tomography page for details.

CT

CT stands for Computerised Tomography. See the tomography page for details.

GNSS

A GNSS (Global Navigation Satellite System) uses timing signals from multiple satellites in order to calculate position. See the positioning, navigation and timing page for details.

Inverse problem

An inverse problem is the task of using the system's observed behaviour to calculate its internal parameters. See the inverse problem page for details.

Ionosphere

The ionosphere is a region in the uppermost part of the Earth's atmosphere. See the space weather page for more information.

LORAN

LORAN (LOng RAnge Navigation) is a marine navigation system using longwave radio signals. See the positioning, navigation and timing page for details.

MRI

MRI (Magnetic Resonance Imaging) is a 3D imaging technique which uses the resonances of atomic nuclei aligned in a magnetic field to gain data. It is the basis of many modern medical scanners. An overview of MRI is available at How Stuff Works.

Muon

A muon is a subatomic particle. It is similar to an electron, the main difference being that a muon is 207 times heavier. Muons are unstable, and will eventually decay (amongst other things) into electrons.

Nonlinear

A system is nonlinear if its output isn't proportional to its input. For example, if a spring is linear, its extension under force will be directly proportional to the force applied, but if it is nonlinear, the relation between extension and force will be more complicated. Problems involving nonlinearity are usually much more difficult to analyse than linear problems, as many useful mathematical techniques become inapplicable. In fact, the analysis of nonlinear problems is a major field in modern mathematics.

Phantom

A Phantom is a three-dimensional model of known composition, which is used to develop and test tomographic equipment and software.

Plasmasphere

The plasmasphere is a doughnut-shaped region of plasma (ionised gas) extending from the top of the equatorial ionosphere, up to a distance of about 30,000km. It is much less dense than the ionosphere, but also much larger.

Resistance

Electrical resistance is a measure of how difficult it is to pass an electrical current through on object. A copper wire will have a very low resistance, whilst an insulating object (such as a sheet of glass) will have a very high resistance. The resistance of an object depends on both its physical dimensions, and an intrinsic property of the material itself, known as resistivity.

Scintillation

Scintillation is the rapid variation in the apparent position and brightness of an object, as perceived by a distant observer. An example is the twinkling of stars, which is caused by turbulence in the atmosphere distorting the light reaching an observer on the ground. Scintillation is also seen in radio communication, where variations in the ionosphere can cause variations in the strength and apparent source of a signal. A scintillation receiver is a GPS receiver which takes rapid measurements in order to determine the effect of scintillation on GPS signals.

SPECT

SPECT (standing for Single Photon Emission Computed Tomography) is a medical imaging technique, in which the patient is injected with a short-lived radioactive substance, and the emissions from the substance are used to create an internal image. See the SPECT.net website for more information.

Tomography

Tomography is the process is which an object is viewed at multiple angles, and the results processed in a computer to calculate the object's internal structure. See the tomography page for details.