LISA consists of three satellites in a triangle formation, following the Earth along its orbit around the Sun. Every satellite transmits a laser beam to each of the others; the result is an equilateral triangle of light whose sides are five million kilometres long. Positioning the satellites correctly, and aiming properly the laser beams over such a large distance is only the beginning of the technical challenges associated with LISA.
Each satellite contains two test masses, freely floating in space. The laser beams are used to measure the distances between the test masses with great precision. Gravitational waves traversing the LISA triangle lead to systematic changes in these distances.