Optimal time-domain combination of the two calibrated output quadratures of GEO 600

GEO 600 is an interferometric gravitational wave detector with a 600 m arm-length and which uses a dual-recycled optical configuration to give enhanced sensitivity over certain frequencies in the detection band. Due to the dual-recycling, GEO 600 has two main output signals, both of which potentially contain gravitational wave signals. These two outputs are calibrated to strain using a time-domain method. In order to simplify the analysis of the GEO 600 data set, it is desirable to combine these two calibrated outputs to form a single strain signal that has optimal signal-to-noise ratio across the detection band. This paper describes a time-domain method for doing this combination. The method presented is similar to one developed for optimally combining the outputs of two colocated gravitational wave detectors. In the scheme presented in this paper, some simplifications are made to allow its implementation using time-domain methods

Null-stream veto for two co-located detectors: Implementation issues

Time-series data from multiple gravitational wave (GW) detectors can be linearly combined to form a null-stream, in which all GW information will be cancelled out. This null-stream can be used to distinguish between actual GW triggers and spurious noise transients in a search for GW bursts using a network of detectors. The biggest source of error in the null-stream analysis comes from the fact that the detector data are not perfectly calibrated. In this paper, we present an implementation of the null-stream veto in the simplest network of two co-located detectors. The detectors are assumed to have calibration uncertainties and correlated noise components. We estimate the effect of calibration uncertainties in the null-stream veto analysis and propose a new formulation to overcome this. This new formulation is demonstrated by doing software injections in Gaussian noise.Comment: Minor changes; To appear in Class. Quantum Grav. (Proc. GWDAW10

New LISA dynamics feedback control scheme: Common-mode isolation of test mass control and probes of test-mass acceleration

The Drag-Free and Attitude Control System is a central element of LISA technology, ensuring the very high dynamic stability of spacecraft and test masses required in order to reach the sensitivity that gravitational wave astronomy in space requires. Applying electrostatic forces on test-masses is unavoidable but should be restricted to the minimum necessary to keep the spacecraft-test masses system in place, while granting the optimal quality of test-mass free-fall. To realise this, we propose a new test-mass suspension scheme that applies forces and torques only in proportion to any differential test mass motion observed, and we demonstrate that the new scheme significantly mitigates the amount of suspension forces and torques needed to control the whole system. The mathematical method involved allows us to derive a new observable measuring the differential acceleration of test masses projected on the relevant sensitive axes, which will have important consequences for LISA data calibration, processing and analysis

A statistical veto method employing an amplitude consistency check

Statistical veto methods are commonly used to reduce the list of candidate gravitational wave (GW) events which are detected as transient (burst) signals in the main output of GW detectors. If a burst event in the GW channel is coincident with an event in a veto channel (where the veto channel does not contain any GW signal), it is possible to veto the event from the GW channel with a low 'false-veto' rate. Unfortunately, many promising veto channels are interferometer channels which can, at some level, contain traces of any detected GW signal. In this case, the application of a 'standard statistical veto' could have a high false-veto rate. We will present an extension to the standard statistical veto method that includes an 'amplitude consistency check'. This method allows the application of statistical vetoes derived from interferometer channels containing GW information with a low false-veto rate. By applying a statistical veto with an amplitude consistency check to data from the GEO 600 detector, veto efficiencies between 5 and 20%, together with a use-percentage of up to 80%, were obtained. The robustness of this veto method was also confirmed by hardware injections. The burst triggers were generated using the mHACR detection algorithm

Novel O-linked methylated glycan antigens decorate secreted immunodominant glycoproteins from the intestinal nematode Heligmosomoides polygyrus

Glycan molecules from helminth parasites have been associated with diverse biological functions ranging from interactions with neighbouring host cell populations to down-modulation of specific host immunity. Glycoproteins secreted by the intestinal nematode Heligmosomoides polygyrus are of particular interest as the excretory-secretory products (termed HES) of this parasite contain both heat-labile and heat-stable components with immunomodulatory effects. We used MALDI-TOF-MS and LC-MS/MS to analyse the repertoire of N- and O-linked glycans released from Heligmosomoides polygyrus excretory-secretory products by PNGase A and F, β-elimination and hydrazinolysis revealing a broad range of structures including novel methylhexose- and methylfucose-containing glycans. Monoclonal antibodies to two immunodominant glycans of H. polygyrus, previously designated Glycans A and B, were found to react by glycan array analysis to a methyl-hexose-rich fraction and to a sulphated LacDiNAc (LDN; GalNAcβ1-4GlcNAc) structure, respectively. We also analysed the glycan repertoire of a major glycoprotein in Heligmosomoides polygyrus excretory-secretory products, VAL-2, which contains many glycan structures present in Heligmosomoides polygyrus excretory-secretory products including Glycan A. However, it was found that this set of glycans is not responsible for the heat-stable immunomodulatory properties of Heligmosomoides polygyrus excretory-secretory products, as revealed by the inability of VAL-2 to inhibit allergic lung inflammation. Taken together, these studies reveal that H. polygyrus secretes a diverse range of antigenic glycoconjugates, and provides a framework to explore the biological and immunomodulatory roles they may play within the mammalian host

Robust vetoes for gravitational-wave burst triggers using known instrumental couplings

The search for signatures of transient, unmodelled gravitational-wave (GW) bursts in the data of ground-based interferometric detectors typically uses `excess-power' search methods. One of the most challenging problems in the burst-data-analysis is to distinguish between actual GW bursts and spurious noise transients that trigger the detection algorithms. In this paper, we present a unique and robust strategy to `veto' the instrumental glitches. This method makes use of the phenomenological understanding of the coupling of different detector sub-systems to the main detector output. The main idea behind this method is that the noise at the detector output (channel H) can be projected into two orthogonal directions in the Fourier space -- along, and orthogonal to, the direction in which the noise in an instrumental channel X would couple into H. If a noise transient in the detector output originates from channel X, it leaves the statistics of the noise-component of H orthogonal to X unchanged, which can be verified by a statistical hypothesis testing. This strategy is demonstrated by doing software injections in simulated Gaussian noise. We also formulate a less-rigorous, but computationally inexpensive alternative to the above method. Here, the parameters of the triggers in channel X are compared to the parameters of the triggers in channel H to see whether a trigger in channel H can be `explained' by a trigger in channel X and the measured transfer function.Comment: 14 Pages, 8 Figures, To appear in Class. Quantum Gra

Results from the first burst hardware injections performed on GEO600

GEO 600 is a 600 m arm-length, laser interferometric gravitational wave detector, located about 25 km from Hannover, Germany. Starting in November 2003, GEO 600 took part in a coincident data taking period with other detectors around the world. During this time, GEO 600 acquired three weeks of 'science quality' data. These data are currently being processed to search for gravitational wave signatures. One such search is that for burst gravitational waves. To investigate the performance of any burst-search codes used to analyse the data, burst-like signals were injected into the detector after the data taking run; this was done by differentially driving the end mirrors of the interferometer. In this paper, we report on how the first burst hardware injections were performed on GEO 600, the type of signals injected and the analysis of the data from the period of injections. Results from the analysis are also presented and discussed

Investigating the response of leaf area index to droughts in southern African vegetation using observations and model-simulations

In many regions of the world, frequent and continual dry spells are exacerbating drought conditions, which have severe impacts on vegetation biomes. Vegetation in southern Africa is among the most affected by drought. Here, we assessed the spatiotemporal characteristics of meteorological drought in southern Africa using the standardized precipitation evapotranspiration index (SPEI) over a 30-year period (1982–2011). The severity and the effects of droughts on vegetation productiveness were examined at different drought timescales (1- to 24-month timescales). In this study, we characterized vegetation using the leaf area index (LAI) after evaluating its relationship with the normalized difference vegetation index (NDVI). Correlating the LAI with the SPEI, we found that the LAI responds strongly (r=0.6) to drought over the central and southeastern parts of the region, with weaker impacts (r<0.4) over parts of Madagascar, Angola, and the western parts of South Africa. Furthermore, the latitudinal distribution of LAI responses to drought indicates a similar temporal pattern but different magnitudes across timescales. The results of the study also showed that the seasonal response across different southern African biomes varies in magnitude and occurs mostly at shorter to intermediate timescales. The semi-desert biome strongly correlates (r=0.95) to drought as characterized by the SPEI at a 6-month timescale in the MAM (March–May; summer) season, while the tropical forest biome shows the weakest response (r=0.35) at a 6-month timescale in the DJF (December–February; hot and rainy) season. In addition, we found that the spatial pattern of change of LAI and SPEI are mostly similar during extremely dry and wet years, with the highest anomaly observed in the dry year of 1991, and we found different temporal variability in global and regional responses across different biomes. We also examined how well an ensemble of state-of-the-art dynamic global vegetation models (DGVMs) simulate the LAI and its response to drought. The spatial and seasonal response of the LAI to drought is mostly overestimated in the DGVM multimodel ensemble compared to the response calculated for the observation-based data. The correlation coefficient values for the multimodel ensemble are as high as 0.76 (annual) over South Africa and 0.98 in the MAM season over the temperate grassland biome. Furthermore, the DGVM model ensemble shows positive biases (3 months or longer) in the simulation of spatial distribution of drought timescales and overestimates the seasonal distribution timescales. The results of this study highlight the areas to target for further development of DGVMs and can be used to improve the models' capability in simulating the drought–vegetation relationship

Physical instrumental vetoes for gravitational-wave burst triggers

We present a robust strategy to \emph{veto} certain classes of instrumental glitches that appear at the output of interferometric gravitational-wave (GW) detectors.This veto method is `physical' in the sense that, in order to veto a burst trigger, we make use of our knowledge of the coupling of different detector subsystems to the main detector output. The main idea behind this method is that the noise in an instrumental channel X can be \emph{transferred} to the detector output (channel H) using the \emph{transfer function} from X to H, provided the noise coupling is \emph{linear} and the transfer function is \emph{unique}. If a non-stationarity in channel H is causally related to one in channel X, the two have to be consistent with the transfer function. We formulate two methods for testing the consistency between the burst triggers in channel X and channel H. One method makes use of the \emph{null-stream} constructed from channel H and the \emph{transferred} channel X, and the second involves cross-correlating the two. We demonstrate the efficiency of the veto by `injecting' instrumental glitches in the hardware of the GEO 600 detector. The \emph{veto safety} is demonstrated by performing GW-like hardware injections. We also show an example application of this method using 5 days of data from the fifth science run of GEO 600. The method is found to have very high veto efficiency with a very low accidental veto rate.Comment: Minor changes, To appear in Phys. Rev.

A closed-loop EKF and multi-failure diagnosis approach for cooperative GNSS positioning

Current cooperative positioning with Global Navigation Satellite System (GNSS) for connected vehicle application mainly uses pseudorange measurements. However the positioning accuracy offered cannot meet the requirements for lane-level positioning, collision avoidance and future automatic driving, which needs real-time positioning accuracy of better than 0.5m. Furthermore, there is an apparent lack of research into the integrity issue for these new applications under emerging driverless vehicle applications. In order to overcome those problems, a new Extended Kalman Filter (EKF) and a multi-failure diagnosis algorithm are developed to process both GNSS pseudorange and carrier phase measurements. We first introduce a new closed-loop EKF with partial ambiguity resolution (PAR) as feedback to address the low accuracy issue. Then a multi-failure diagnosis algorithm is proposed to improve integrity and reliability. The core of this new algorithm includes using Carrier phase based Receiver Autonomous Integrity Monitoring (CRAIM) method for failure detection, and the double extended w-test detectors to identify failure. A cooperative positioning experiment was carried out to validate the proposed method. The results show that the proposed closed-loop EKF can provide highly accurate positioning, and the multi-failure diagnosis method is effective in detecting and identifying failures for both code and carrier phase measurements