Space-Time Metrology for the LISA Gravitational Wave Observatory, and Its Demonstration on LISA Pathfinder

Abstract

The paper describes the key instrumental features of LISA, and discusses its limiting sources of uncertainty. It is shown how the basic element of LISA is a "Doppler link" where the frequency of a laser beam is compared by two far apart, free falling "observers". These observers are two of the six LISA free falling test-masses. Each of the three LISA arms carries two such Doppler links, propagating along opposite directions. It is shown that a link like this acts as a time-delayed, differential accelerometer. Its performance is limited by three main sources of disturbance. Parasitic forces accelerate the test masses relative to their geodetic trajectories. Interferometry measurement noise is interpreted as relative motion of test-masses. Finally, because of intervening optical components fixed to spacecraft, the measurement is also corrupted by the relative acceleration of spacecraft and the test mass. We analyze these sources within a unifying picture, and show that LISA has a large "science margin". Even with a sensitivity substantially poorer of what has been calculated, LISA remains a powerful astrophysics observatory.

From laboratory experiments to LISA Pathfinder: achieving LISA geodesic motion

“…This is the same as the LISA requirement, though in LISA the TM-to-TM measurements is obtained by combining two local TM-to-SC displacement measurements, as those in LPF, with one SC-to-SC displacement measurement over a distance of 5 million kilometers [3]. The 10 pm/√Hz requirement is alleviated below 3 mHz, as, at low frequency, TMs motion due to parasitic forces is much larger than this measurement error.…”

The diagnostics subsystem on board LISA Pathfinder and LISA

“…Likewise, magnetic susceptibility, χ , will be present. These are required to comply with the following constraints [2]:…”

The LISA Pathfinder DMU and Radiation Monitor

“…Magnetic noise can be readily inferred from the above formulae. There is no specific requirement on either magnetic field or gradient fluctuations; there is only one on the total contribution of magnetic disturbances to the overall mission acceleration noise budget; magnetic is required to stay below 40% of that budget [1]. On the other hand, there are specific requirements on the dc values of magnetic field and gradient, which should be kept below 10 μTesla and 5 μTesla m −1 , respectively.…”

“…LISA Pathfinder (LPF) has a reduced acceleration noise budget, both in magnitude and in frequency band with respect to LISA [1]: …”

“…This noise can be apportioned to different components, each of them individually interfering with the mission performance. Requirements are set on each of those so that the total sum is compatible with equation (1) [1]. Thermal, magnetic and charged particle flux are specifically monitored by dedicated hardware and software, forming what is known as the data and diagnostics subsystem (DDS), which also includes the DMU (Data Management Unit), and the LTP (LISA Technology Package, or mission payload) computer.…”