A project under the auspices of the National Solar Observatory of the USA for the continuous monitoring of solar oscillations by a network of six observing stations spaced around the world at approximately 60° intervals in longitude. The instrument used, called a Fourier tachometer, is based on the Michelson interferometer. It measures small Doppler shifts of the spectral line of nickel at 676.8 nanometres, which is isolated by means of a Lyot filter. The whole disc of the Sun is focused at the same time on to an electronic detector composed of a matrix of 256 by 256 pixels. The Doppler shift at each pixel can be measured independently as a function of time. See also: helioseismology.