Challenges in the Measurement and Data-Processing Chain of the LISA Mission

The LISA Mission (Laser Interferometer Space Antenna) is currently under mission formulation with a launch date planned in 2020. The purpose of the mission is the observation of gravitational waves at frequencies between 0.1 mHz and 1 Hz by measuring distance fluctuations between inertial reference points, represented by cubic proof masses. In order to provide a sufficient sensitivity of the instrument, distance fluctuations between two inertial reference points must be measured with a strain accuracy of around 10^?20 Hz^?1/2. This is achieved by setting up a laser interferometer with a base-length of 5?10⁶ km and a path-length measurement noise in the order of 10 pm?Hz^?1/2. For a correct evaluation of the data on the ground, it is essential that the science data telemetry preserves all required frequency domain information. That is, any on-board data-processing and down-sampling must be done with great care in order not to introduce aliasing or other artifacts into the data stream. As an additional complication, most of the optical metrology data is dominated by laser phase noise which is about eight orders of magnitude larger than the required instrument sensitivity. However, by applying a method called “time-delayed interferometry” during the ground data processing, this laser phase noise can be eliminated from the data. This method has already been demonstrated in a detailed simulation environment, but it requires a very careful filtering, synchronization, and interpolation of the individual data streams. Last but not least, a calibration of system parameters is necessary in many areas of the LISA measurement system. The system design must therefore ensure that all data required for these calibrations is available on-ground in a quality that allows a successful computation of the calibration coefficients within a reasonable time-frame. The data streams do not only include data from the optical metrology system, but also from the drag-free and attitude control system which are used to derive other information, such as the charge state of the proof mass. This yields a strong coupling between the different disciplines since data that is only used for housekeeping purposes in other missions becomes an essential part of the science data stream for the LISA mission. This paper gives an overview of the LISA measurement and data-processing chain. It highlights the most challenging areas that have been identified so far and describes the intended solution methods.     

Autonomous orbit maintenance system

Orbit maintenance is a major cost factor for Earth satellites in specialized orbits, such as a repeating ground track, or in formations. While autonomous attitude control is well established, the spacecraft's orbit is usually uncontrolled or maintained by ground station commands. For small, lower cost satellites, operations costs can be a dominant element of both cost and risk. This implies a need for low-cost autonomous orbit maintenance in order to allow such systems to be economically viable, particularly in today's constrained budget environment.

In addition, if the position of the spacecraft is controlled, it is therefore known in advance. Thus, mission planning can be done as far in advance as desired, without the need for replanning and frequent updating due to unpredictable orbit decay. An interesting characteristic of autonomous orbit maintenance is that it typically requires less software, and less complex software, than does orbit control from the ground. In many cases, an onboard orbit propagator is not needed.