page_title: Laser Interferometer Space Antenna
section_title: Detection principle
hierarchical_section_title: Laser Interferometer Space Antenna / Detection principle
caption_attribution_description: Français&#160;: Vue stylisés des déformations induites au passage d'une onde gravitationnelle polarisée + sur les bras laser de l'interféromètre LISA. English: View of amplified effects of a + polarized gravitationnal wave (stylized) on LISA laser beams / arms paths.
context_page_description: The Laser Interferometer Space Antenna is a mission led by the European Space Agency to detect and accurately measure gravitational waves—tiny ripples in the fabric of space-time—from astronomical sources. LISA would be the first dedicated space-based gravitational wave detector. It aims to measure gravitational waves directly by using laser interferometry. The LISA concept has a constellation of three spacecraft arranged in an equilateral triangle with sides 2.5 million km long, flying along an Earth-like heliocentric orbit. The distance between the satellites is precisely monitored to detect a passing gravitational wave.
  The LISA project started out as a joint effort between the United States space agency NASA and the European Space Agency ESA. However, in 2011, NASA announced that it would be unable to continue its LISA partnership with the European Space Agency due to funding limitations. The project is a recognized CERN experiment. A scaled down design initially known as the New Gravitational-wave Observatory was proposed for ESA's Cosmic Vision L1 mission selection.
context_section_description: Like most modern gravitational wave observatories, LISA is based on laser interferometry technique. Its three satellites form a giant Michelson interferometer in which two "slave" satellites play the role of reflectors and one "master" satellite the one of source and observer. While a gravitational wave is passing through the interferometer, lengths of the two eLISA arms are varying due to space-time distortions resulting from the wave. Practically, it measures a relative phase shift between one local laser and one distant laser by light interference. Comparison between the observed laser beam frequency (in return beam) and the local laser beam frequency (sent beam) encodes the wave parameters.
  Unlike terrestrial gravitational wave observatories, LISA cannot keep its arms "locked" in position at a fixed length.  Instead, the distances between satellites varies significantly over each year's orbit, and the detector must keep track of the constantly changing distance, counting the millions of wavelengths by which the distance changes each second.  Then, the signals are separated in the frequency domain: changes with periods of less than a day are signals of interest, while changes with periods of a month or more are irrelevant.
  This difference means that LISA cannot use high-finesse Fabry–Pérot resonant arm cavities and signal recycling systems like terrestrial detectors, limiting its length-measurement accuracy.  But with arms almost a million times longer, the motions to be detected are correspondingly larger.
caption: View of amplified effects of a + polarized gravitational wave (stylized) on LISA laser beams / arms paths.