152 research outputs found
Towards Dynamic Catalogues
The International LOFAR Telescope is designed to carry out unique science
in the spatial, spectral, polarisation and temporal domains.
The Transients Key Science Project aims to study all transient and variable sources
detected by LOFAR. One of its products will be an up-to-date catalogue of all sources
detected by LOFAR, i.e. a spectral light-curve database, with real-time capabilities,
and expected to grow gradually with 50â100 TB/yr. The response time to transient and
variable events depends strongly on the query execution plans of the algorithms that
search the LOFAR light-curve database for previous (non-)detections in the spatial,
spectral, polarisation and temporal domains.
Here we show how the Transients Key Science Project of LOFAR approaches
these challenges by using column-stores, sharded databases and implementing the new
array query language SciQL (pronounced as âcycleâ)
GroundâPenetrating Radar for Closeâin Mine Detection
In this chapter, two of the major challenges in the application of groundâpenetrating radar in humanitarian demining operations are addressed: (i) development and testing of affordable and practical ground penetrating radar (GPR)âbased systems, which can be used offâground and (ii) development of robust signal processing techniques for landmines detection and identification. Different approaches developed at the Royal Military Academy in order to demonstrate the possibility of enhancing closeârange landmine detection and identification using groundâpenetrating radar under laboratory and outdoor conditions are summarized here. Data acquired using different affordable and practical GPRâbased systems are used to validate a number of promising developments in signal processing techniques for target detection and identification. The proposed approaches have been validated with success in laboratory and outdoor conditions and for different scenarios, including antipersonnel, lowâmetal content landmines, improvised explosive devices and real mineâaffected soils
Observing pulsars and fast transients with LOFAR
Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is
a new radio interferometer operating in the lowest 4 octaves of the ionospheric âradio windowâ: 10â240 MHz, that will greatly facilitate observing
pulsars at low radio frequencies. Through the huge collecting area, long baselines, and flexible digital hardware, it is expected that LOFAR will
revolutionize radio astronomy at the lowest frequencies visible from Earth. LOFAR is a next-generation radio telescope and a pathfinder to the
Square Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual elements. We discuss
the motivation for low-frequency pulsar observations in general and the potential of LOFAR in addressing these science goals.We present LOFAR
as it is designed to perform high-time-resolution observations of pulsars and other fast transients, and outline the various relevant observing modes
and data reduction pipelines that are already or will soon be implemented to facilitate these observations. A number of results obtained from
commissioning observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low frequency
pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science papers with LOFAR
Developments on an IEEE 802.15.4-based wireless sensor network, Journal of Telekommunications and Information Technology, 2008, nr 2
In this paper a summary is given of the ongoing research at the Belgian Royal Military Academy in the field of mobile ad hoc networks in general and wireless sensor networks (WSNs) in particular. In this study, all wireless sensor networks are based on the physical and the medium access layer of the IEEE 802.15.4 low rate wireless personal area networks standard. The paper gives a short overview of the IEEE 802.15.4 standard in the beaconless mode together with a description of the sensor nodes and the software used throughout this work. The paper also reports on the development of a packet sniffer for IEEE 802.15.4 integrated in wireshark. This packet sniffer turns out to be indispensable for debugging purposes. In view of future applications on the wireless network, we made a theoretical study of the effective data capacity and compared this with measurements performed on a real sensor network. The differences between measurements and theory are explained. In case of geograph- ically meaningful sensor data, it is important to have a knowledge of the relative position of each node. In the last part of the paper we present some experimental results of positioning based on the received signal strength indicators (RSSI). As one could expect, the accuracy of such a method is poor, even in a well controlled environment. But the method has some potential
Denoising and migration techniques for target identification from ground penetrating radar 2d data; a case study
Le radar de pĂ©nĂ©tration du sol (GPR) est une technique de tĂ©lĂ©dĂ©tection employĂ©e pour obtenir l'endroit et la rĂ©flectivitĂ© spatiaux des objets enterrĂ©s. Puisque la plupart des antennes de GPR ne sont pas directives, les signaux dispersĂ©s enregistrĂ©s par le radar se prolongent au-dessus d'une grande ouverture latĂ©rale [1, 2]. Dans cette Ă©tude, des algorithmes de dĂ©bruitage et de migration sont employĂ©s pour refocaliser les signaux dispersĂ©s de nouveau Ă leur point d'origine. Les donnĂ©es ont Ă©tĂ© prises pour diffĂ©rents scĂ©narios. Afin de rĂ©aliser la sĂ©paration optimale de la signature de la cible de la rĂ©ponse du soil, techniques de dĂ©bruitage son utilisĂ©es sur les donnĂ©es 2D. La transformĂ©e de Hough randomisĂ© est employĂ© pour extraire des informations importantes [3]. Ces informations sont incluses dans un algorithme de migration [4], et la largeur aproximĂ©e de l'objet dans la direction du balayage est trouvĂ©e. Bien que les rĂ©sultas sont pormetteurs, les algorithmes doivent toujours ĂȘtre validĂ©s dans diffĂ©rentes conditions
Astronomical Data Processing Using SciQL, an SQL Based Query Language for Array Data
SciQL (pronounced as âcycleâ) is a novel SQL-based array query language for scientific applications with both tables and arrays as first class citizens. SciQL lowers the entrance fee of adopting relational DBMS (RDBMS) in scienti
Fast in-database cross-matching of high-cadence, high-density source lists with an up-to-date sky model
Coming high-cadence wide-field optical telescopes will image hundreds of thousands of sources per minute. Besides inspecting the near real-time data streams for transient and variability events, the accumulated data archive is a wealthy laboratory for making complementary scientific discoveries. The goal of this work is to optimise column-oriented database techniques to enable the construction of a full-source and light-curve database for large-scale surveys, that is accessible by the astronomical community. We adopted LOFAR's Transients Pipeline as the baseline and modified it to enable the processing of optical images that have much higher source densities. The pipeline adds new source lists to the archive database, while cross-matching them with the known cataloguedsources in order to build a full light-curve archive. We investigated several techniques of indexing and partitioning the largest tables, allowing for faster positional source look-ups in the cross matching algorithms. We monitored all query run times in long-term pipeline runs where we processed a subset of IPHAS data that have image source density peaks over 170,000 per field of view (500,000 degâ2). Our analysis demonstrates that horizontal table partitions of declination widths of one-degree control the query run times. Usage of an index strategy where the partitions are densely sorted according to source declination yields ano
An automated archival VLA transients survey
In this paper we present the results of a survey for radio transients using data obtained from the Very Large Array archive. We have reduced, using a pipeline procedure, 5037 observations of the most common pointings - i.e. the calibrator fields. These fields typically contain a relatively bright point source and are used to calibrate 'target' observations: they are therefore rarely imaged themselves. The observations used span a time range Ë1984-2008 and consist of eight different pointings, three different frequencies (8.4, 4.8 and 1.4 GHz) and have a total observing time of 435 h. We have searched for transient and variable radio sources within these observations using components from the prototype LOFAR transient detection system. In this paper we present the methodology for reducing large volumes of Very Large Array data; and we also present a brief overview of the prototype LOFAR transient detection algorithms. No radio transients were detected in this survey, therefore we place an upper limit on the snapshot rate of GHz frequency transients >8.0 mJy to φ0.032 deg-2 that have typical time-scales 4.3 to 45.3 d. We compare and contrast our upper limit with the snapshot rates - derived from either detections or non-detections of transient and variable radio sources - reported in the literature. When compared with the current Log N-Log S distribution formed from previous surveys, we show that our upper limit is consistent with the observed population. Current and future radio transient surveys will hopefully further constrain these statistics, and potentially discover dominant transient source populations. In this paper we also briefly explore the current transient commissioning observations with LOFAR, and the impact they will make on the field
- âŠ