Welcome to the 10th International LISA Symposium

Before a link can be established, spaceborne laser interferometer based inter-satellite links must perform an intermediate signal acquisition phase. The complexity of signal acquisition can vary depending on the design of the interferometer and the environmental constrains. eLISA (the evolved Laser Interferometer Space Antenna) for example has to most likely scan an uncertainty area in search of the far satellite before the interferometric signal can be acquired. The signal acquisition algorithms hereby designed and presented are capable of autonomously performing such spatial scans together with (if necessary) laser frequency tuning. These algorithms are designed to fully cover an uncertainty area using randomly generated patterns and/or continuous curve patterns. Together with these acquisition strategies, an interferometer simulator has been developed in order to test the signal acquisition through Monte Carlo Simulations. The specific acquisition algorithms which have been developed to date are tailored to the GRACE Follow-On interferometer layout and external environment. Nevertheless, the formulation is sufficiently generic that the algorithms can easily be adapted to simulate signal acquisition for other laser interferometer based missions such as eLISA.

To improve the testing of drag-free proof mass systems for gravitational physics, a ground test system that simulates space as closely as possible is needed. A magnetic levitation system has been built that can levitate, hold and spin a hollow sphere in vacuum, simulating zero-g operation. The ball position sensor is a simple optical shadow sensor, consisting of an infrared emitter and a pair of phototransistors. An analog control system was built and the system debugged to achieve reliable operation. A next generation electronics board has been built and assembled that converts the analog signal to digital. The board has a switch to reverse the current direction through the coil in case the sphere becomes magnetized and sticks to the coil in vacuum. A digital control code has been developed to provide a stable feedback control system to drive the coil and hold the ball in a stable levitated position. The ball position can be adjusted digitally. The system is designed to restore the ball to the levitated position if it drops out of lock, and to perform initial levitation while it is running in vacuum. The heat generated by the coil is an issue in vacuum, so it was carefully designed to run continually for at least 3 hours without overheating. COMSOL is used for thermal and magnetic simulation and analysis of the system. The ball can also be spun using four coils that are driven by digitally generated sine and cosine functions to produce a rotating magnetic field around the equator of the sphere. Operation to 10 Hz is easily achieved. The system could be used to test the three-axis optical sensor system, DOSS, that is being developed separately, by rapidly switching to the DOSS sensor while the ball is levitated. Small magnetic pulses could also be used to simulate thruster perturbations in the plane orthogonal to the support axis.

Black hole merger waveforms, as predicted by general relativity, are striking for their remarkably simple features. This is advantageous both because it provides clear encoding of system properties on the observable waveforms, and it aids in the development of empirical analytical waveform models which may accurate encode information from costly simulations. Represented in spherical harmonic components aligned to the orbital plane, the waveform phase is strongly circularly polarized through inspiral merger and ring-down with a smooth transition from orbital frequency to rotational ring-down frequency. We describe recent developments in "Implicit Rotating Source" (IRS) modeling, an empirical approach exploiting these simple waveform characteristics for a compact waveform description. We discuss results providing parameters of late-time IRS merger description over a broad parameter space, and recent results on extending the model to make analytic contact with PN inspiral waveforms.

Space-based gravitational wave detectors based on the Laser Interferometer Space Antenna (LISA) design operate by synthesizing one or more interferometers from fringe velocity measurements generated by changes in the light travel time between three spacecraft in a special set of drag-free heliocentric orbits. Once these spacecraft are placed in their orbits, the orientation of the interferometers at any future time is fixed by Kepler’s Laws based on the initial orientation of the spacecraft constellation. Over the course of a full solar orbit, the initial orientation determines those locations in the sky were the detector has greatest sensitivity to gravitational waves as well as those locations where nulls in the detector response fall. By artful choice of the initial orientation, we can choose to optimize sensitivity to sources whose location may be known in advance (e.g., the Galactic center or globular clusters).

Airbus Defence and Space (former Astrium Satellites, Friedrichshafen) has been awarded by the German Space Agency (DLR) a LISA Technology Consolidation Study to specifically investigate a modified instrument concept that is based on previous developments in the frame of ESA´s LISA Mission Formulation study. This alternative instrument concept is assuming fixed telescopes with in-field pointing of laser arms and a single active test mass (SATM). The special focus of this poster is on the GRS architecture for its application in SATM configuration with two sensitive axes. The prime goal of this activity is to explore the potential of such an approach for a simpler, more robust, and affordable LISA-type mission, while maintaining science objectives. Independently, further optimization potential of the LISA GRS – exploiting its LISA Pathfinder (LPF) development heritage – is assessed. Although the LPF GRS is originally designed for compliance with the LISA specifications, its architecture is not entirely consolidated regarding the LPF lessons-learned and the application within the SATM concept. This lead to the following main tasks: wrap-up of the lessons-learned during the LPF GRS development, optimization of the GRS architecture for SATM application, assessment of performance drivers and robustness margins, and identification of the necessary way-forward for the eLISA GRS baseline (technology roadmap). These tasks were focused on three main technical areas: the residual acceleration performance, the charge management system, and the test mass release process. The residual acceleration performance aspects are assessed making use of a specifically developed parametric performance model of the GRS in SATM configuration, including also the test mass closed-loop control. The achievable performances and margins of the SATM concept are clearly identified, showing possible areas of requirements relaxation and hardware simplifications. Regarding the charge management system a fundamentally different UV light injection and actuation concept is proposed, providing substantial robustness margins compared to the current baseline. For the test mass release, the allowable initial release velocity could be increased by a factor three compared to the LPF baseline when adapting the actuation algorithm and the control concept.

Within the eLISA Mission, orbital dynamics cause the shape of the constellation to change over a period of one year. As a result, the angle between the interferometer arms varies by a few degrees on an annual timescale and must be actively compensated for. Most studies looking at eLISA type missions typically feature the “Telescope Pointing” concept - articulating the two telescopes with a mechanism and adjusting the entire payload to compensate. One possible alternative concept which has been studied in the LISA Mission Formulation study carried out by Astrium Satellites Germany (now Airbus DS) is to utilise "In-Field Pointing" (IFP). With IFP, a small mechanism would tilt a mirror positioned at an internal intermediate pupil of a (wide field) telescope, thus providing the required pointing corrections. IFP possesses inherent advantages over telescope pointing in that it removes the need to articulate large parts of the payload, does not require a backlink fibre and can result in smaller payload sizes and payload architectures with only one test mass. Demanding requirements are, however, placed on the optical properties of the telescope and the stability of the mechanism. To demonstrate the feasibility of IFP, we are developing an experiment - co-funded with DLR - to perform an end-to-end experimental validation of the IFP concept. The experiment will feature a wide-field off-axis telescope with a prototype In-Field Pointing Mechanism (developed by TNO) located at an intermediate pupil. A heterodyne interferometer will be used to measure the stability of the system and probe the performance under operation of the mechanism. We will give a short overview of this project, its goals and the current status.

While the upcoming LISA Pathfinder mission will be able to test LISA technology in space, ground-testing of the Gravitational Reference Sensor (GRS) and interferometry is still important. Currently the best way to do this is with a torsion pendulum, as any experiment directly involving the test masses must decouple from the local gravitational field. The University of Florida is currently building such a torsion pendulum. Though superficially similar to its counterpart at the University of Trento, the UF Torsion Pendulum is not tied to risk reduction for the LTP GRS. Its primary purpose is therefore to be a testbed for new technologies relevant to LISA or any LISA-like mission. Here we describe the sensing and actuation system for the UF Torsion Pendulum along with an outline of the experimental program to come, and report on initial results from the pendulum.

Photoreceivers (Photodiodes with matched preamplifiers) are a key component of space-based laser interferometers like LISA and GRACE Follow-On, since they need to convert the weak optical signal into an electrical signal and thus are at the beginning of the signal chain with the most stringent noise requirements. They also need to cope with the 2-20 MHz Doppler induced frequency variation of the optical beat note due to armlength changes. We report about the development of several photoreceivers with InGaAs quadrant photodiodes for a signal bandwidth between 1 and 25 MHz, and equivalent input current noise of a few pA/sqrt(Hz). Furthermore we present the design and first results of an automated test stand that scans the surface of a photodiode to measure the homogeneity of its responsivity.

In the coming months the LISA Gravity Reference Sensor (GRS) Front End Electronics (FEE) test campaign will be performed at ETH Zurich. The aim of the test campaign is to characterize the behavior of the LISA GRS FEE. Sensing noise and actuation stability performances of the FEE will be characterized down to 30 uHz, such low frequency together with the challenging LISA requirements make the design and realization of the experiments a difficult task especially from the point of view of the thermal stability of the system and the Test Mass simulator noise. We will discuss the approaches we have adopted to mitigate such effects. Together with low frequency noise measurements we will test sensing electronics cross-talk, actuation electronics cross-talk and actuation-to-sensing cross talk. We will report the details of the test-plan from the point of view of the test preparation, test hardware and data analysis. Moreover, if data will be available at the time of the symposium, we will report and discuss preliminary results of the test campaign.

Many electronics devices onboard eLISA will require long term, low noise characterization. In the mHz band, the main perturbations are due to temperature fluctuations of the device under tests and/or the characterization instruments. A low noise, low frequency test facility is currently being developed at the APC (Paris), especially designed for a precise control and measurement of electronics devices on long term (hours and days) acquisitions. The current design of this facility will be described, as well as the on-going project of adding ultra-stable frequency references (up to 1 GHz carrier frequency with 10^(-15) stability level at 1 s) generated with an optical frequency comb. First measurements have also been performed and we report here the characterization (temperature coefficients and noise level) of selected voltage references in the frequency range from $10^{-5}$ to 10 Hz. The goal of this work was to update previous studies, with a characterization at lower frequencies, and find voltage references that may be suitable for the space-based interferometry mission eLISA. The requirements of relative output stability of 1 ppm/sqrt(Hz) down to 0.1 mHz were not met by any of the tested devices, but 4 references approaches the objective : the AD587UQ, the MAX6126AASA50, the LT1021-BCN8-5 and the LT6655BHM. While the first three were already identified as potential devices in previous studies, the later is a new promising candidate using a different technology (bandgap).

Stellar mass black hole binaries may have individual masses between 10 and 80 solar masses. Some of these systems may emit gravitational waves at frequencies detectable at Megaparsec distances by space-based gravitational wave observatories. We used a signal-to-noise-ratio (SNR) threshold limit of observing these systems with detectors similar to the Laser Interferometer Space Antenna (LISA) to determine detectability. Detectable systems were found from a generated population of binary black holes (BBHs) that cover a reasonable parameter space and calculate their signal-to-noise ratio. We populate the galaxies in our local universe, less than (30 Mpc) with BBH systems drawn from a distribution found in the Synthetic Universe to determine the likely event rate of detectable binaries from galaxies in the local universe.

Two current and successful Earth geodesy space missions, Gravity Recovery And Climate Experiment (GRACE) and Gravity and Ocean Circulation Explorer (GOCE) demonstrated that the geoid heights (or spherical harmonic geopotential coefficients) can be estimated with high accuracy. The GRACE mission uses a pair of polar orbiting satellites and measures the range rate between them to estimate the geoid height. On the other hand, the GOCE mission uses a single drag free satellite and a gravity gradiometer to avoid all other perturbations but the one caused by the gravity field. This research analyzes a space mission that is formed by combining the ideas of the two missions stated above, which leads to a pair of drag free satellites The goal for this research is to determine how well the spherical harmonic geopotential coefficients can be recovered from the simulated data which consists of the range rate between the two satellites and the positions of each satellite. The measurement noise and the residual acceleration noise are designed and embedded within the simulated measurement and during the estimation process. Noise models are not restricted to be just white, but colored noise models were used in the simulation based on the expected performance of current technology. The two sets of different coefficients that were used, one to create the measurement data and another one as a priori information, were acquired from the Center of Space Research at the University of Texas at Austin. Similar to the GRACE mission, a single set of coefficients were estimated with the period of 30 days. The estimation process was broken down into small batches, one batch containing 3 hours of data to save the computation time and to decrease the memory space associated with the simulation. Therefore the Kalman batch estimation process derived from the least squares estimation was applied for the simulation. The resulting geoid height error and the covariance are compared to the actual result from the GRACE mission.

Ultra-Stable Optical Bench is one of the key technical elements used in the laser ranging system for spaceborne gravitational waves detection. In order to achieve a picometer-level displacement measurement, various effects, e.g., the thermal drift of optical bench, the satellite’s attitude jitter, the wavefront distortion coupling with misalignment of laser beam pointing control, must be taken into account. We are developing a simulation software based on MATLAB development platform for analyzing and evaluating the performance of the ultra-stable optical bench of laser interferometer. With this software, we can build a 3-D model of the optical measurement system, and deal with the design of the optical configuration of laser interferometer system. We had manufactured a new silicate-bonded optical bench by using this simulation software for preliminary analysis and design. This software is still under further developing and extending functions. The updated progress will be reported in the symposium.

Katharina-Sophie Isleif, Jan-Simon Hennig, Roland Fledermann, Christian Diekmann, Michael Troebs, Gerhard Heinzel, Karsten Danzmann The backlink is the optical connection between two optical benches within one satellite which slowly rotate against each other (a few degree per year). For this, a connection is required in which the optical path length in both directions differs by less than a few pm/sqrtHz in the frequency range of mHz - Hz. So far, experiments were conducted at the AEI that demonstrated a reciprocal fibre backlink in the pm/sqrtHz range but also showed that Rayleigh scattering in the backlink fiber degrades the phase readout. A new experiment is currently in the planning stage. It will allow to compare three alternative methods in only one optical set-up consisting of two movable optical benches in a vacuum chamber. Those methods are the stabilised fibre backlink that was used until now, a frequency separated fibre backlink and a free-beam backlink. The frequency separated fibre backlink uses an additional laser per optical bench and separates the backscatter signal from the measurement signal in frequency. This talk presents the results of the stabilised fibre backlink experiment. The influence of stray light caused by fibres could be sufficiently minimised. Afterwards it gives an overview of the three backlink solutions and the current design of the optical set-up.

In the alternative eLISA payload concept with In-Field Pointing, proposed by Astrium several years ago, the use of a single active test mass on each spacecraft was considered a possible solution which could potentially increase the performance of the eLISA instrument. This is an interesting alternative especially in combination with a spherically shaped test mass which requires no guiding effort in its translational or rotational degrees of freedom during science runs compared to a cubically shaped test mass. A few years ago Astrium started theoretical and experimental work on this topic. We will present the results achieved so far and introduce the experimental design of our inertial reference sensor setup which is currently under construction.

The science interferometer on the optical bench is measuring the distance between two remote spacecraft in a LISA mission. Due to spacecraft motion the received beam will tilt what leads to a pathlength change. For minimizing this effect there will be an imaging system on the optical bench in front of the photodiodes. To characterize this effect and the imaging system in an experiment we are building a minimal optical bench and a telescope simulator together with the European Space Agency (ESA), Airbus Defence and Space and the University of Glasgow. Here we will report on the design and construction of the Telescope Simulator. This device is simulating a laser beam from a remote spacecraft for the science interferometer. It is implemented on an ultra-stable baseplate and also holds a reference interferometer and elements for the testing procedure.

In space laser interferometer gravitational wave (G.W.) detection missions, the stability of laser beam pointing direction has to be kept at several nrad/Hz. Otherwise beam pointing jitter noise will dominate the noise budged and make detection of G.W. impossible. Disturbed by residue non-conservative forces, the fluctuation of laser beam pointing direction could be as large as 10rad/Hz at frequencies between 0.1mHz to 10Hz. Therefore laser beam pointing control system is an essential requirement for those space G.W. detection missions. An on-ground test of such beam pointing control system is performed; where the Differential Wave-front Sensing (DWS) technique is use to sense the beams pointing jitter. An active controlled steering mirror is employed to adjust beam pointing direction to compensate the jitter. The experimental result shows that the pointing control system can be used for very large dynamic range up to 50rad. At the interested frequencies of space G.W. detection missions, between 1mHz to 1Hz, beam pointing stability of 100nrad/Hz is achieved.

One way of testing general relativity and cosmic censorship is by mapping the strong gravitational field around an astrophysical black hole and testing whether it is well described by the famous Kerr solution. The current state of the art for quantitative measurements in the strong gravitational field regime are X-ray observations of accretion disk spectra (in particular the Iron K-alpha line profile). Looking into the future, a LISA-like mission observing extreme-mass-ratio-inspirals offer an exciting opportunity to dramatically improve on this current situation. I will compare, both quantitatively and qualitatively, the constraints it is possible to place on a wide class of non-Kerr black holes solutions using existing X-ray observations against those from future gravitational wave observations.

A drag-free spacecraft utilizes a Gravitational Reference Sensor (GRS) to shield an internal free-floating test mass (TM) from (a) external disturbances and (b) from disturbances caused by the spacecraft itself. The GRS measures the position of the spacecraft with respect to the TM and a feedback control system commands thrusters to maintain that position. In principle, the test mass is then completely freed from non-gravitational disturbances so that it and its “tender” spacecraft follow a pure geodesic. Drag-free satellites are used to measure the changing mass distribution over the surface of the Earth, upper atmospheric density and winds, effects of general relativity, and eventually gravitational waves. The first drag-free spacecraft, TRIAD I, was sponsored by the U. S. Navy and launched in 1972 to improve ocean navigation by satellite. The drag-free system on the NASA Gravity Probe B spacecraft reduced electrostatic suspension forces acting on its ultra-precise gyroscopes, and the European Steady-State Ocean Circulation Explorer (GOCE) flew drag-free to reduce the dynamic range and thus improve the performance of its gravity gradiometer. Recently a scaled down gravitational reference sensor has been proposed for a 3U CubeSat primarily for Earth aeronomy and geodesy applications. The GRS consists of a 25 cm diameter sphere housed inside a 50 cm cubic cavity. The sphere's position is sensed with a LED-based differential optical shadow sensor, it is spun-up using magnetic coils, its electric charge is controlled by photoemission using UV LEDs, and the spacecraft position is maintained with respect to the sphere using a microfabricated electric propulsion system. This poster will focus on the simulated dynamics and the design of the control system for the Drag-free CubeSat mission.

Deep phase modulation interferometry was proposed as a method to enhance homodyne interferometers to work over many fringes, allowing for instance continuous real-time tracking. In this scheme, a sinusoidal phase modulation is applied in one arm while the demodulation takes place as a post-processing step. In this contribution we report on the implementation of this scheme in a fiber coupled interferometer controlled by means of a FPGA, which includes a LEON3 soft-core processor. The latter acts as a CPU and executes a custom made application to communicate with a host PC. In contrast to usual FPGA-based designs, this implementation allows a real-time fine tuning of the parameters involved in the setup, from the control to the post-processing parameters.

UV-LED-based charge control for LISA Taiwo Olatunde, Ryan Shelley, Andrew Chilton, Giacomo Ciani, Guido Mueller, John Conklin University of Florida Test masses inside the LISA gravitational reference sensor (GRS) must maintain almost pure geodesic motion for gravitational waves to be successfully detected. The residual accelerations have to stay below 3 fm/s^2/rtHz at all frequencies between 0.1 and 3 mHz. One of the well-known noise sources is associated with the charges on the test masses which couple to stray electrical potentials and external electro-magnetic fields. The LISA Pathfinder (LPF) will use Hg-discharge lamps emitting mostly around 254 nm to discharge the test masses via photoemission in its 2015/16 flight. A future LISA mission launched around 2030 will likely replace the lamps with newer UV-LEDs. Their lower mass, better power efficiency and small size make them an ideal replacement for Hg lamps. Without contamination, which generally causes a reduction in the work function of Au, UV photons with energy below 5.1eV would not have sufficient energy to liberate electrons from pure Au. Presented here is a preliminary design for effective charge control through photoelectric effect by using latest generation UV-LEDs which produce light at 240 nm with work function above that of pure Au.

Within the frame of Einstein's General Relativity, gravitational waves are expected to possess two tensorial polarizations, namely the well-known h_+ and h_x modes. Other theories of gravitation however allow the existence of additional modes (at most two vector and two scalar modes), and the (non-)observation of those additional polarizations could put constraints on the validity of all existing theories, and thus provide a test for General Relativity. In its 2-arm-planned-configuration, eLISA only consists of one detector orbiting around the Sun, and we therefore investigate if there is a possibility to still detect and separate additional modes of a given signal.

Space-based gravitational wave observatories will enable the systematic study of the low-frequency band of gravitational waves from astrophysical sources. All current observatory designs require propagation of a monochromatic laser beam from one spacecraft to another over immense distances. To achieve this optical link over immense distances, the spacecraft need to be equipped with optical telescopes which serve to simultaneously send light to the distant spacecraft and collect light from that distant spacecraft. An optical telescope is needed for efficient power delivery but is directly in the beam path and therefore its design properties are driven by the requirements for the displacement sensitivity of the space-based gravitational wave observatory. Here we describe the design for a catoptric telescope that meets those requirements with emphasis on differences from the usual specifications for high quality image formation found in traditional telescopes. We also discuss various design trade-offs as well as early results from research into scattered light suppression techniques that may enable alternative designs.

One crucial noise source in precision interferometry is the coupling from beam tilt to the longitudinal pathlength readout. For example a residual angular jitter of a LISA test mass with respect to the optical bench leads to an error in its position measurement. In order to decouple such a tilt from the pathlength readout, imaging systems can profitably be used to project the pivot of rotation onto the detector. At the pm level, secondary effects such as lens aberrations, Gaussian beam shapes and alignment tolerances all need to be taken into account. We performed an experimental test of such an imaging system, with an equal arm-length homodyne Mach-Zehnder interferometer locked to mid-fringe, to prove its ability to reduce cross coupling between beam tilt and length measurements to below 20 pm/urad.

The Kozai mechanism for a hierarchical triple system could reduce the merger time of inner eccentric binary emitting gravitational waves (GWs), and has been qualitatively explained with the secular theory that is derived by averaging short-term orbital revolutions. However, with the secular theory, the minimum value of the inner pericenter distance could be excessively limited by the averaging operation. Compared with traditional predictions, the actual evolution of an eccentric inner binary could be accompanied by (i) a higher characteristic frequency of the pulse-like GWs around its pericenter passages, and (ii) a larger residual eccentricity at its final inspiral phase. These findings would be important for GW astronomy with the forthcoming advanced detectors.

The Laser Interferometer Space Antenna (LISA) requires free falling test masses, whose acceleration must be below 3 fm/s2/Hz in the lower part of LISA’s frequency band ranging from 0.1 to 100 mHz. Gravitational reference sensors (GRS) house the test masses, shield them from external disturbances, control their orientation, and sense their position at the nm/Hz level. The UF Torsion Pendulum is a laboratory test bed for GRS technology. By decoupling the system of test masses from the gravity of the Earth, it is possible to identify and quantify many sources of noise in the sensor. The mechanical design of the pendulum is critical to the study of the noise sources and the development of new technologies that can improve performance and reduce cost. The suspended test mass is a hollow, gold-coated, aluminum cube which rests inside a gold-coated, aluminum housing with electrodes for sensing and actuating all six degrees of freedom. This poster describes the design, analysis, and results to date of the mechanical subsystems of the UF Torsion Pendulum.

The GravitoMagnetism (GM) or the frame-dragging effect is one of the important predictions of Einstein's general theory of relativity, which, by means of direct measurements, is poorly tested till today. The precise measurements of the Earth GM field have important significances in both fundamental physics and the future developments of space science, such as the determinations of inertial frames and the synchronizations of clocks in space. Driven by the need of the pathfinder mission for future Chinese space-borne gravitational wave antenna and also the plan of the joint scientific space mission of European Space Agency and Chinese Academy of Sciences, we study the detections of the GM field by measuring the relative motions between two proof masses orbiting around Earth, which are separated in the along-track direction by a distance of 50cm. GM effects are of “Global” effects, therefore the characteristic signal produced by the Earth GM field in the relative motions between the two masses is measured relative to a “globally” fixed direction. We found that for medium Earth orbital experiments, the magnitude of the GM signal will accumulate to a few nanometers in 2~5 days. Therefore, with the help of the onboard laser interferometer of displacement measurement resolution 20~50pm/Hz^(1/2) around milliHertz, one would expect the 0.1%~1% measurement of the Earth GM field within ten days operation. If the effective data can add up to about one hundred days long, with the help of proper data analysis methods, the accuracy could be further improved with another one order of magnitude. This talk focuses on the theoretical studies of the GM signal, the principles of the measurement and the separation of the GM signal from that generated by Earth J_2 gravity field. Some preliminary error analysis will also be discussed.