LISA will be the first dedicated gravitational-wave observatory in space. So far, the only space searches for gravitational waves have been performed using measurements of radio signals from spacecraft on their way to other planets; these missions were not optimized for gravitational wave searches. By contrast, LISA will employ an advanced system of laser interferometry and some of the most sensitive measuring instruments ever flown.

LISA detects gravitational waves by measuring the change in separation between freely floating test masses, so sources of both external and internal disturbance need to be eliminated or damped down to extremely low levels. By minimizing such disturbances, motions that would imitate or mask the effect of gravitational waves are less likely to occur. To accomplish this, the LISA mission relies on two core technologies: drag-free operation and laser interferometry.

Drag-free operation

Examples of external disturbances are the pressure from the light of the Sun and its very small variations, the variable solar magnetic field, and distortion of the LISA array by the gravitational effects of the Earth and Moon. Examples of internal disturbances are the interaction of the electrical field generated by the spacecraft computer acting on the test masses, effects from residual gas pressure near the test masses, and thermal radiation by the electrodes used to measure the spacecraft position.

To counteract the solar disturbances, the spacecraft structure will act as a shield to protect the test masses. LISA's orbit also helps, by placing the spacecraft away from the Earth to minimize the effects of the Earth's gravity.

In order to minimize the effect of internal disturbances, the spacecraft must be controlled to follow the test masses, effectively flying around them, with an accuracy of 10 nanometers, or about 1/100th of the wavelength of light! This way of controlling the position of spacecraft, called drag free, is similar to the operation of low-Earth orbiting satellites that need to correct for the force due to the drag (i.e., the friction) of the Earth's atmosphere. To keep the test masses floating freely in space, the separation between the test masses and the surrounding spacecraft is constantly monitored: when the separation changes, specially developed microthrusters fire to move the spacecraft back into position, away from the undisturbed test masses.

Laser interferometry

LISA's implementation of interferometric measurements resembles the technique known as spacecraft Doppler tracking, but it is realized with infrared laser light instead of radio waves. The laser light going out from one spacecraft to the other corners is not reflected back directly, because diffraction losses over such long distances would be too great. Instead, the phase of the incoming laser is measured, and used to set the phase of the outgoing laser, which is transmitted back at full intensity: this process is known as transponding. When the transponded laser light arrives back at the original spacecraft, it is superposed with a portion of the original laser beam, and their phases compared.

This relative phase measurement, which is referenced to the position of the two test masses, gives information about the separation between the spacecraft. The difference between the phase measurements for the two arms gives information about the relative changes in the two arms, which are induced by gravitational waves.

Such a two-arm interferometer can be prone to phase errors due to the fluctuations of laser frequency. If the arms were exactly equal in length, then laser frequency fluctuations would cancel perfectly when the two phase measurements are subtracted. Unfortunately, the freely evolving LISA orbits cause slowly changing differences between the arm lengths, so the phase errors must be removed in a different way. First, the lasers are frequency stabilized—first to an optical cavity, and then to the 5-million-km interferometer arm. Any residual laser frequency noise in the LISA measurements is then removed by post-processing on the ground using a technique called Time Delay Interferometry.