Murchison Widefield Array

The Murchison Widefield Array ( MWA ) is a radio telescope in Australia. It is characterized by a large number of very simple and low-cost antennas, which are distributed over a large area in the Australian outback. Unlike many other radio telescopes, the MWA has no moving parts. However, the telescope can receive radio signals from a specific direction. The directional reception is done electronically.

Specifications

The telescope can receive optimized in the range of 80 MHz to 300 MHz signals. In the adjacent spectral regions reception is even possible, but due to the decreasing sensitivity antenna no longer profitable.

The telescope produces 50 TB per day of raw data that is processed over a HPC cluster. The 2048 dipole antennas are distributed in 128 groups of 16 antennas in a square array on a surface of 2000 m².

The analog -to-digital conversion of all signals is performed in the reception band, the reason for the large amount of data. The MWA is located in the Mid- West Radio Quiet Zone, a radio protective zone at 70 km radius within which strictly regulates any radio operation, when it is not even banned. The nearest major town, Geraldton (Australia) with 27 000 inhabitants, is located about 300 km away, which leads to a corresponding attenuation of noise, which usually come from residential and commercial areas.

Design principle

The telescope consists of a plurality of discrete ultra-wideband antennas. They are distributed in a random grid Pseudeo on a large area (1.5 km in diameter ). The antennas are mounted rigidly, such that a single antenna having no variable directivity. The antennas themselves each contain two dipoles, with which two linear polarizations can be detected.

Beamformer

The directivity of the telescope is achieved by beamformer devices that generate by runtime adjustments in the measurement lines of the individual antennas a preferred direction in the detection of radio signals. For each antenna tile with 16 antennas, a beamformer is used. The beam former to process the signals of the antennas 16 may vary independently of each other for the two polarizations. For directionality, the signals are passed through a selectable combination of five delay lines. By controlling the delay beam forming is achieved, which has no significant frequency dependence, as otherwise would be the case at a phase control. The beamformer are located directly at the associated antenna tiles and send her amplified analog output signal on the polarization separately to the receiver node. The beamformer is dimensioned so that an antenna lobe per tile ( main reception direction ) is formed. A formation of different clubs would certainly have been possible, but was not considered to be economically viable.

Receiver

The two signals from a total of 8 tiles are transmitted via coaxial cable to a receiver node. There are therefore 16 receiver node. The signals that have already passed through band-pass filters in the antenna tiles are filtered at the inputs of the receiver node again for the suppression of aliasing and DC components. These and other functions for signal processing and noise suppression are included in an assembly known as analog signal conditioning. The obtained therefrom signals that were previously not subject to mixing (multiplication in the time domain ), then go into a digitizer ( ATMEL AT84AD001B ) which scans with 655.36 Msample / s and quantized with 8 bit resolution. The digitized signals are immediately filtered and divided into 1.28 MHz wide frequency bands, which are up to 24 selected and prepared for further transmission. Therefore transmitted via optical fibers, the data of all devices connected to the receiver node tiles, but this is reduced to one-third of the entire frequency band. The amount of data transmitted, the receiver node adds up to roughly a little less than 100 Gbit / s

Correlators

The correlators operate on the 'FX' principle, that is, it first finds a transformation of the signals into the frequency domain by filtering and then instead of a correlation. Filtering operates with a minimum of 10 kHz bandwidth, which in turn means a high computational effort. The plurality of the narrowband data streams of the respective antenna tiles are then correlated to the phase and thus to obtain the direction information. This fall CMAC (complex multiply and accumulate ) operations on the order of 1012 per second. This challenging task is handled by FPGA boards. The resulting data set is similar extensive as at the input of the correlator and is passed to the server with conventional technology for further processing. This GPU clusters are used, which have proven in the past for similar calculations as particularly powerful.

Development and Future

A test device with 32 tiles was built and tested in the period from 2007 to 2011. 2010 were asked to build the MWA available a total of 4.6 million Australian dollars. Construction began in 2011 and after a test phase, the telescope took on 9 June 2013, official Beobachungsbetrieb on.

The infrastructure permits expansion of the telescope at 256 pages. This is planned but not yet scheduled.

Applications and skills

With each antenna tile, a separate reception lobe are generated, which means that basically as many different goals as antenna tiles can be observed simultaneously. However, since the antenna surface of a tile is comparatively low, this application, however, the exception will be of. Due to the large expansion of the tiles over a field of 1.5 km in diameter to baseline for synthetic apertures can be created with this size. This allows, as VLBI facsimile, only on a smaller scale, observations with very high signal to noise ratio and angular resolution.

The beam shaping by controlled delay lines allows alignment of the telescope with no moving parts, which has the advantage of being able to eliminate expensive, wear-prone mechanics, but has the disadvantage that the angular resolution of the beam alignment of a tile by the gradations of the delay lines is fixed.

The large number of antenna tiles, as well as their distribution is to be particularly favorable for the Störvermeidung and interference suppression. Interference from terrestrial and orbital sources can be isolated, locate and remove from the useful signal.

Specific monitoring objectives

The Murchison Widefield Array is to provide, among other findings on the reionization of the universe.

The scientist Steven Tingay from Curtin University in Perth has proposed in the scientific journal Astronomical Journal, to use the MWA for the localization of space junk. The radio waves of commercial broadcasters out of the room Perth are reflected according Tangay of the debris in space, so that these signals are received and conclusions about the situation in low Earth orbit ( 400-2000 km) could be drawn.