The detection of low-frequency gravitational radiation requires a space-borne detector, principally to escape the effects of seismic noise which plague ground-based detectors at low frequencies. Although the seismic disturbance is eliminated, the space environment introduces its own set of problems. For example, solar radiation pressure. This can be shielded by the technique of 'drag-free control', whereby the spacecraft encompassing the test mass provides protection from the external distrubances. A feedback control system ensures that the spacecraft remains 'centered' on the internal mass, while at the same time being physically de-coupled from it. As well as attenuating the external disturbances, the drag-free controls system must minimize the relative motion between the spacecraft and test mass, in order to reduce forces arising from dynamic coupling between the two. Missions like LISA which will perform ultra-precise interferometric measurements require drag-free thruster and sensor technologies with extremely low noise levels. An overveiw of the requirements and the technologies currently being pursued for the LISA drag-free control system is presented.

The Disturbance Reduction System (DRS) is designed to demonstrate technology required for future gravity missions, including the planned LISA gravitational-wave observatory, and for precision formation-flying missions. The DRS is based on a freely floating test mass contained within a spacecraft that shields the test mass from external forces. The spacecraft position will be continuously adjusted to stay centered about the test mass, essentially flying in formation with the test mass. Any departure of the test mass from a gravitational trajectory is characterized as acceleration noise, resulting from unwanted forces acting on the test mass. The DRS goal is to demonstrate a level of acceleration noise more than four orders of magnitude lower than previously demonstrated in space. The DRS will consist of an instrument package and a set of microthrusters, which will be attached to a suitable spacecraft. The instrument package will include two Gravitational Reference Sensors comprised of a test mass within a reference housing. The spacecraft position will be adjusted using colloidal microthrusters, which are miniature ion engines that provide continuous thrust with a range of 1-20 mN with resolution of 0.1 mN. The DRS will be launched in 2007 as part of the ESA SMART-2 spacecraft. The DRS is a project within NASA's New Millennium Program.

Space-borne gravitational wave missions like LISA require Drag-Free Contorl (DFC) systems to control the motion of a constellation of spaceraft to high positional accuracy so that Michelson interferometers of vast scales can be implemented and used to detect gravitational waves. The spacecraft will continually experience forces and torques due to external disturbances, resulting in positional perturbations. Therefore the development of a DFC system is essential to stabilize the spacecraft to a specified tolerance. Prior to such space-borne gravitational wave missions, a technology demonstrator mission, such as the proposed ODIE, ELITE or SMART-2, is needed to test the feasibility of drag-free spacecraft technology. This paper discusses the requirements of the hardware needed to implement a DFC system. Contorl-loop models ahve been develoepd to model the DFC system's dynamic behavior, which enabled quantification of its performance. Results show that if an accelerometer noise level of 1×10^-12 m Hz^-0.5 and thruster noise level of 1×10^-8 N Hz^-0.5 can be realized then the ODIE acceleration budget can easily be met.

Space-based detection of gravitational radiation is limited at low frequencies by stray force contamination of the orbits of ideally 'free-falling' test masses. Shielding a test mass from external forces with a satellite requires precision position sensing that does not itself introduce excessive stray force or springlike coupling to satellite motion. The LISA (Laser Interferometer Space Antenna) mission to observe gravitational waves between 0.1 and 1000 mHz must limit stray acceleration to the fm/s² square root Hz level, with satellite position control at the nm\rthz\ level. We present here a design, and a discussion of electrostatic disturbances, for a capacitive position sensor for LISA 'drag-free' control, which will be flight-tested with the LISA Technology Package (LTP).

LISA employs a capacitive sensing and positioning system to maintain the drag free environment of the test masses acting as interferometer mirror elements. The need for detailed electrostatic modelling of the test mass environment arises because any electric field gradient or variation associated with test mass motion can couple the test mass to its housing, and ultimately the spacecraft. Cross-couplings between components in the system can introduce direct couplings between sensing signals, sensing axes and the drive signal. A variation in cross-couplings or asymmetry in the system can introduce capacitance gradients and second derivatives, giving rise to unwanted forces and spring constant modifications. These effects will vary dependent on the precise geometry of the system and will also tend to increase the sensitivity to accumulated charge on the test-mass. Presented are the results of a systematic study of the effect of the principal geometry elements (e.g. machining imperfections, the caging mechanism) on the test mass electrostatic environment, using the finite element code ANSYS. This work is part of an ongoing ESA study into drag-free control for LISA and the LTP on SMART 2 and ultimately aims to eliminate geometries that introduce too large a disturbance and optimise the electrostatic design.

We report on a Monte Carlo simulation of electrostatic charging of the LISA proof masses by cosmic-ray protons and alpha particles, developed using the Geant4 toolkit. A positive charging rate of 58+/-17 +e/s (proton charges per second) was obtained with the minimum Geant4 energy threshold for the production of secondary particles by electromagnetic processes. This charging rate does not seem to depend strongly on the tracking of low-energy secondary electrons, and is some 5 times larger than that found in previous simulations. The difference is only partly explained by the slightly larger proof mass considered in this study. This figure is used to place new limits on the required discharge time of the LISA test masses.

The LISA mission will form the equivalent of a Michelson interferometric beat signal from the 1064nm laser beams traversing the inter-spacecraft arms. The design gravitational wave sensitivity requires measurement of round trip path differences for each detector to be about 10 pm/square rootHz over a frequency range from 10-4 to 10-1 Hz. Thus LISA's phasemeter must measure the beat signal phase to 10-5 cycle/square rootHz at 1mHz. Doppler shifts between spacecraft in their orbits are expected to range from 1 to 15 MHz. Phase measurement using the digital phase-locked loop approach incorporated in a modified TurboRogue GPS receiver has been investigated in this study. It is found that only resolution in the range of 10-4 to 10-3 cycle/square rootHz at 1 mHz is achievable with this hardware. Abrupt fluctuations in the phase measurement at the millicycle level are responsible for the limitation.

The Laser-Interferometer-Space-Antenna (LISA) is a space-based interferometer with arm lengths of 5*10 9 m. Its design goal is to measure gravitational waves with a strain sensitivity of 10-23 at 10 mHz. Unlike in earth-based interferometers the arm lengths can differ by up to 2% or 108 m. For that reason frequency noise in the λ ~ 1 μm laser will not cancel in the direct interference signal. A laser locked to a ULE reference cavity in a 1°μK/square root Hz environment will have about 10 Hz/square root Hz frequency noise. The LISA sensitivity goal requires for the laser noise of less than 10-5 Hz/square root Hz, about a factor 10^-6 below what has been achieved (1). Cancellation of laser frequency noise can be achieved by time-delayed-interferometry (TDI) (2,3). We describe a laboratory test of TDI with an unequal arm interferometer. The intent is to ascertain the performance limitations and proof-of-concept for 6 orders of magnitude frequency noise suppression.

The optical paths on the LISA bench must have a length instability of less than 10~pm/square root Hz over time scales of 1s to 1000s. A small rigid interferometer has been constructed to measure the optical path length changes using various bonding techniques. The interferometer was constructed entirely from ultra-low expansion (ULE) glass by optically contacting ULE beamsplitters to a ULE bench. Preliminary results taken with the interferometer operating in air indicate optical path length fluctuations of approximately 100 pm/ sqaure root Hz or less for frequencies between 1 mHz and 1 Hz.

We are developing a pointing sensor as part of the technlogy development effort for the Laser Interferometer Space Antenna (LISA) mission. The sensor will measure the angle between two beams, by measuring the phase difference in the heterodyne frequency on different sides of the pupil plane. In LISA, one beam would be from the local laser, while the other beam comes from a different spaceraft. The beam coming from the other space ccraft will have a Doppler shift due to changes in the orbits of the satellites. The phase difference across the aperture will be measured to align the incoming and outgoing beams. We have characterized our pointing noise levels due to electronics over bandwidths of 0.001 to 1 Hz with a heterodyne frequency of 5 MHz. The LISA pointing requirements is on the order of 10 nrad/square root Hz stability on the sky, with a worst case scenario of 1 nrad/square root Hz. We present our first results, in which we have reached 4 micro-radians/square root Hz on the detector. This is equivalent to 70 nrad/square root Hz for LISA.

Doppler tracking experiments using the earth and a distant spacecraft as separated test masses have been used for gravitational wave (GW) searches in the low-frequency band(~0.0001-0.1 Hz). The precision microwave tracking link continuously measures the relative dimensionless velocity, Δv/c, between the earth and the spacecraft. A GW incident of the systems produces a characteristic signature in the data, different from the signatures of the principal noises. For 40 days centered about its solar opposition in December 2001, the Cassini spacecraft was tracked in a search for low-frequncy GWs. Here we describe the GW experiment, including transfer functions of the signals and noises to the Doppler observable, and present noise statistics and compare them with the pre-experiment noise budget.

We propose here an optical displacement sensor (ODS) as a supplemental or backup sensor for the LISA inertial reference sensor concept. This simple ODS consists of a laser diode and a quad-cell photodiode (both commercially available). The inertial mass' reflective surface directs the laser beam onto the quad-cell photodiode. Changes in the inertial mass' position and orientation are then extracted from ratios of the differences and sums of the quad-cell photodiode outputs. A simpler proto-type using a 200 microns wide slit has demonstrated a resolution of 10 nm/square root Hz at 1 mHz and 1 nm/square root Hz above 5 mHz. The electronics noise was 1 nm/square root Hz at and above 1 mHz with simple and off the shelf electronics components. Although this ODS' current performance does not meet the LISA's system requirement1 of 1 nm/rtHz at 1 mHz, we think that is achievable in the near future.

The results of preliminary studies using a spherical proof mass in the LISA mission are given. These include the acceleration disturbance performance, methods for controlling and calibrating a spinning sphere, system considerations, the possibility of using the spinning reference as a precision gyroscope to improve the attitude control, and a preliminary design with a spherical proof mass.

For many of the proposed LIGO sources, we are currently unable to produce reliable theoretical waveforms or event rates. Usually the difficulty is in performing numerical simulations. These problems seriously undermine our ability to extract science from LIGO, or in some cases even to detect sources at all. We describe two sources where these problems are present: the merger of a binary black hole system, and a newborn neutron star unstable to r-modes. We explain the origin of the difficulties, and how they might be overcome.

We discuss the emission of gravitational waves from stellar collapse,with particular emphasis on the emission arising from convection above the surface of the proto-neutron star. We analytically estimate an upper limit to the gravitational wave emission from this convection. We also present results from the first 3-dimensional core-collapse simulations including realistic equations of state and neutrino physics and calculate the resultant gravitational wave signal from these collapse simulations. Convective overturns do not produce observable gravitational wave emission nor does this emission remove enough energy to effect the convective motions. But asymmetries in supernova are an almost certainty, and these asymmetries (possibly caused by rotation) will produce strong gravitational wave emission.

Estimates of the Galactic coalescence rate (R) of close binaries with two neutron stars (NS-NS) are known to be uncertain by large factors (about two orders of magnitude) mainly due to the small number of systems detected as binary radio pulsars. We present an analysis method that allows us to estimate the Galactic NS-NS coalescence using the current observed sample and, importantly, to assign a statistical significance to these estimates and to calculate the allowed ranges of values at various confidence levels. The method involves the simulation of selection effects inherent in all relevant radio pulsar surveys and a Bayesian statistical analysis for the probability distribution of R. The most likely values for the total Galactic coalescence rate (R_tot) lie in the range 2-60 Myr^-1 depending on different pulsar population models. For our reference model 1, where the most likely estimates of pulsar population properties are adopted, we obtain R_tot = 8_-5⁺⁹ Myr^-1 at a 68% statistical confidence level. The corresponding range of expected detection rates of NS-NS inspiral are 3_-2⁺⁴ × 10^-3 yr^-1 for the initial LIGO and 18_-11⁺²¹ yr^-1 for the advanced LIGO.

Using the Star Track population synthesis code we compute the distribution of masses of merging compact object (black hole or neutron star) binaries. The shape of the mass distribution is sensitive to some parameters governing the stellar binary evolution. We discuss the possibility of constraining stellar evolution models using mass measurements abtained from detection of compact object inspiral with upcoming gravitational-wave observatories.

Coalescing binary neutron stars (NS) are expected to be an important source of gravitational waves detectable by laser interferometers. We discuss recent theoretical work on the hydrodynamics of NS binary mergers and possible methods for determining the NS compactness ratio M/R and constraining the equation of state of dense nuclear matter using gravitational wave signals. One particularly simple and promising method is based on the properties of quasi-equilibrium binary NS sequences and does no require full hydrodynamic merger calculations.

The detection of gravitational waves by the first generation of ground-based interferometric detectors, like LIGO, relies on sophisticated data analysis techniques. For the inspiral phase of binary compact objects, the optimal one is the so-called matched-filtering technique. The output of the detector is cross-correlated with a bank of templates. The closer the templates are to the real signal, the higher the S/N of the detection is. In this paper we quantify the loss of S/N that occurs when one tries to detect a precessing binary using non-precessing templates. To do so, we compute the fitting factor which is a measure of the mismatch between the signal and the templates. The precessing signal is obtained using a 1.5 PN analytical approximation of the real solution called simple precession. We found regions of the parameter space for which the detection could be jeopardized if precession is not accounted in the templates. The solution of this problem could be to use more complete templates, that could capture the main features of the precession. Specifically we examine such a family of 'mimic' templates, that requires only three additional parameters, first proposed by Apostolatos. However we find that this family does not recover the main part of the signal. We conclude that a more efficient template family will be needed in the near future.

Resonant detectors of gravitational waves (g.w) have been operating for many years. These detectors allow to investigate various classes of signals, such as bursts, continuous waves, stochastic background. We will describe here some of the data analysis tools we have developed. In particular we will outline the problem of optimal filtering of the data, given the fact that the sensitivity of the detector is time varying, and some of the problems that arise when performing a coincidence analysis; we will describe some tools developed to look for signals from periodic sources, with a particular emphasis on the all-sky search we are running on the data of Explorer; we will describe the technique used to perform coincidences with Gamma Ray Bursts.

A network of five cryogenic 'bar' gw detectors has been in operation in the years 1997-2000. A generic coincidence search has been performed with such a network, under the International Gravitational Events Collaboration, IGEC, for millisecond burst gw signals. A triple coincidence within the network has a false alarm rate below 10^-2 y^-1. No triple coincidence was found over the 173.2 days during which at least three detectors where on the air and improved upper limits have been established for the gw flux on earth. The typical search thresholds of the detectors correspond to a neutron star - neutron star coalescence at 10 kpc distance. The network is currently under upgrade and it is expected to usefully complement, in searches for ms gw signals, the interferometric detectors under completion.

The advent of kilometer-scale interferometers, such as those in LIGO, makes detection of gravitational waves from astrophysical sources a realistic goal in the near-term. Data collected simultaneously from the LIGO and GEO interferometers has started to flow during dedicated engineering runs and is being analyzed by working groups of the LIGO Scientific Collaboration. This paper presents a short review of the data analysis techniques that have been developed and are now being used in the effort to detect gravitational waves. In particular, enhancements of the basic methods to improve efficiency and to handle non-Gaussian noise are emphasized.

The advent of kilometer-scale interferometers, such as those in LIGO, makes detection of gravitational waves from astrophysical sources a realistic goal in the near-term. Data collected simultaneously from the LIGO and GEO interferometers has started to flow during dedicated engineering runs and is being anlyzed by working groups of the LIGO Scientific Collaboration. This paper presents a short review of the data analysis techniques that have been developed and are now being used in the effort to detect gravitational waves. In particular, enhancements of the basic methods to improve efficiency and to handle non-Gaussian noise are emphasized.

We present a data analysis technique for detecting a stochastic background of gravitational radiation. Current observational constraints and upper limits on the stochastic background signal strength, and methods for dealing with cross-correlated noise are also discussed.

Presently there are six interferometric gravitational wave detectors in the commissioning or construction phase in North America, Europe, and Japan. Once completed this worldwide network of detectors will be capable of detecting gravitational waves with unprecedented detail and sensitivity. Their ambition reaches well beyond the first direct detection of gravitational waves; they promise the dawn of a new field, the gravitational wave astronomy. One of the major goals of interferometric gravity wave detectors is to develop and exploit gravitational wave detection in conjunction with other conventional observational techniques, which are capable of observing the same astronomical process using different methods. The most promising areas are the optical, GRB and neutrino searches for energetic processes. Coincident observation of astronomical events shall revolutionize the way we understand energetic processes and will provide a new window on compact and difficult to study astronomical objects such as stellar cores. We will discuss the status, the potential future, and benefits of collaboration amongst gravitational wave detector networks and astronomical/GRB/neutrino networks and some of the practical experiences with the LIGO detectors.

Resonant gravitational wave detectors are described. Examples are given for signal improvement by combining signals from several independent detectors. The successful test run of Allegro in coincidence with LIGO Livingston during an engineering run (E7) is also described.

The GEO600 laser interferometric gravitational wave detector is approaching the end of its commissioning phase which started in 1995. During a test run in January 2002 the detector was operated for 15 days in a power-recycled michelson configuration. The detector and environmental data which were acquired during this test run were used to test the data analysis code. This paper describes the subsystems of GEO600, the status of the detector by August 2002 and the plans towards the first science run.

LIGO construction has been completed. The three interferometers at the two LIGO observatory sites (Livingston, Louisiana and Hanford, Washington) have been operated successfully as power-recycled Michelson interferometers with Fabry-Perot arm cavities. Commissioning of the interferometers has progressed to operating them simultaneously in this final optical configuration. The initial coincidence operation between the observatory sites has provided a full test of the detector hardware and software subsystems, and full operation of the data acquisition and data analysis systems. The LIGO Laboratory and the LIGO Scientific Collaboration are working together to exploit the early series of interleaved engineering and science runs to commission the detector and data systems, to provide a detailed characterization of the detector and to produce the first scientific results from LIGO. The operation of LIGO is also coordinated with operation of the GEO 600 detector and the ALLEGRO resonant mass detector. The status of this early operation and data study will be presented.

The Australian Consortium for Interferometric Gravitational wave Astronomy (ACIGA) is carrying out research on the detection of gravitational waves using laser interferometry. Here we discuss progress on each of the major sub systems: data analysis, lasers and optics, isolation suspension and thermal noise, and configurations, and report on the development of a high optical power test facility in Gingin, Western Australia.

We discuss optical methods which either enhance the signal response or reduce the quantum noise in a long baseline interferometric type detector of gravitational waves. We review current progress in the science and engineering of the different techniques and consider when they may be applicable to full scale interferometers.

The interferometers being planned for second generation LIGO promise an order of magnitude increase in broadband strain sensitivity-with the corresponding cubic increase in detection volume-and an extension of the observation band to lower frequencies. In addition, one of the interferometers may be designed for narrowband performance, giving further improved sensitivity over roughly an octave band above a few hundred Hertz. This article discusses the physics and technology of these new interferometer designs, and presents their projected sensitivity spectra.

To obtain improved sensitivities in future generations of interferometric graviational wave detectors, beyond those proposed as upgrades of current detectors, will require different approaches in different portions of the gravitational wave frequency band. However the use of silicon as an interferometer test mass substrate, along with all-reflective interferometer topologies, could prove to be a design enabling sensitivity improvements at both high and low frequencies. In this paper the thermo-mechanical properties of silicon are discussed and the potenial benefits from using silicon as a mirror substrate material in future gravitational wave detectors are outlined.