Murchison Widefield Array - System Overview

System Overview

An MWA antenna comprises four by four regular grid of dual-polarisation dipole elements arranged on a 4m x 4m steel mesh ground plane. Each antenna (with its 16 dipoles) is known as a “tile”. Signals from each dipole pass though a low noise amplifier (LNA) and are combined in an analogue beamformer to produce tile beams on the sky. Beamformers sit next to the tiles in the field. The radio frequency (RF) signals for the tile-beams are transmitted to a receiver, each receiver being able to process the signals from a group of eight tiles. Receivers therefore sit in the field, close to groups of eight tiles; cables between receivers and beamformers carry data, power and control signals. Power for the receivers is provided from a central generator. The receiver contains analogue elements in order to condition the signals in preparation for sampling and digitisation. The frequency range 80–300 MHz is Nyquist-sampled at high precision. Digital elements in the receiver (after the digitiser) are used to transform the time-series data to the frequency domain with a 1.28 MHz resolution – 5 bits real and 5 bits imaginary for each resolution element. Sets of 1.28 MHz coarse frequency channels are transmitted via an optical fibre connection to the correlator subsystem, located in the CSIRO Data Processing Facility near the MWA site. MWA shares the CSIRO facility with the ASKAP program.

The majority of the tiles (112) will be scattered across a roughly 1.5 km core region, forming an array with very high imaging quality, and a field of view of several hundred square degrees at a resolution of several arcminutes. The remaining 16 tiles will be placed at locations outside the core, yielding baseline distances of about 3 km to allow higher angular resolution for solar burst measurements.

The correlator subsystem comprises Poly-phase Filter Bank (PFB) boards that convert the 1.28 MHz coarse frequency channels into channels with 10 kHz frequency resolution in preparation for cross-correlation. Correlator boards then cross-multiply signals from all tiles to form visibility data. A distributed clock signal drives the coherence of receivers in the field and maintains timing for the correlator.

Data from the correlator subsystem are transmitted to a RealTime Computer Processing Array (RTC), which is also located in the CSIRO Data Processing Faciltity. The primary function of the RTC is to run the RealTime Software (RTS); a software suite that performs realtime calibration and imaging of the correlator output. The output information within the RTC/RTS is then further processed, depending on the science mode in operation at a given time. The RTS also writes out calibration data, including bright source measurements, tile gain solutions, and parameters for the properties of the ionosphere above the MWA site. The science output files and calibration data are written to an off-site archive for further analyses.

The MWA will be operated remotely through an interface to a Monitor and Control (M&C) software package resident on a dedicated computer located within the CSIRO Data Processing Facility at the MWA site. The M&C software maintains a state-based description of the hardware and an event-driven database describing the observation scheduling of the Instrument. M&C software commands several elements of the system including pointing and tracking of the beamformers, frequency selection of the receivers, correlation parameters for the correlator, and RTC/RTS functions, amongst others. The M&C system contributes to the MWA archive by storing instrument “metadata” into an external database. This includes both the instrument configurations for each observation and also housekeeping information collected from various hardware components.

Data will be transferred from the RTC storage disks to the MWA archive located at the end of a high-bandwidth network connection. The primary MWA data archive(s) will likely be located in Perth, with copies in other locations in Australia and the US. The resultant qualified data will then be provided to, and stored by, the various scientific databases for subsequent distribution to the respective scientific communities for analysis and interpretation.