3,573 research outputs found
Receiver architecture of the thousand-element array (THEA)
As part of the development of a new international radio-telescope SKA (Square Kilometre Array), an outdoor phasedarray prototype, the THousand Element Array (THEA), is being developed at NFRA. THEA is a phased array with 1024 active elements distributed on a regular grid over a surface of approximately 16 m2. The array is organised into 16 units denoted as tiles. THEA operates in the frequency band from 750 to 1500 MHz.\ud
On a tile the signals from 64 antenna elements are converted into two independent RF beams. Two times 16 beams can be made simultaneously with full sensitivity by the real-time digital beam former of the THEA system. At the output of each tile the analog RF signal from a beam is converted into a 2 × 12-bit digital quadrature representation by a receiver system.\ud
A double super-heterodyne architecture is used to mix the signal band of interest to an intermediate frequency of 210 MHz. The IF-signal is shifted to baseband by means of a partly digitally implemented I/Q mixer scheme. After a quadrature mixer stage, the I and Q signals are digitised by means of 12 bit A/D converters at 40 MS/s. Implementing a part of the mixing scheme digitally offers the flexibility to use different I/Q architectures, e.g. Hartley and Weaver mixer setups. This way the effect of RFI in different mixing architectures can be analyzed. After the digital processing, the samples are multiplexed, serialised and transported over fibres to the central adaptive digital beam former unit where the signals from all tiles are combined giving 32 beams.\ud
This paper focuses on the design choices and the final implementation of the THEA system. In particular, the receiver architecture is addressed. A digital solution is presented, which enables switching between a Hartley and a Weaver based mixer scheme
Radio Frequency Interference Mitigation
Radio astronomy observational facilities are under constant upgradation and
development to achieve better capabilities including increasing the time and
frequency resolutions of the recorded data, and increasing the receiving and
recording bandwidth. As only a limited spectrum resource has been allocated to
radio astronomy by the International Telecommunication Union, this results in
the radio observational instrumentation being inevitably exposed to undesirable
radio frequency interference (RFI) signals which originate mainly from
terrestrial human activity and are becoming stronger with time. RFIs degrade
the quality of astronomical data and even lead to data loss. The impact of RFIs
on scientific outcome is becoming progressively difficult to manage. In this
article, we motivate the requirement for RFI mitigation, and review the RFI
characteristics, mitigation techniques and strategies. Mitigation strategies
adopted at some representative observatories, telescopes and arrays are also
introduced. We also discuss and present advantages and shortcomings of the four
classes of RFI mitigation strategies, applicable at the connected causal
stages: preventive, pre-detection, pre-correlation and post-correlation. The
proper identification and flagging of RFI is key to the reduction of data loss
and improvement in data quality, and is also the ultimate goal of developing
RFI mitigation techniques. This can be achieved through a strategy involving a
combination of the discussed techniques in stages. Recent advances in high
speed digital signal processing and high performance computing allow for
performing RFI excision of large data volumes generated from large telescopes
or arrays in both real time and offline modes, aiding the proposed strategy.Comment: 26 pages, 10 figures, Chinese version accepted for publication in
Acta Astronomica Sinica; English version to appear in Chinese Astronomy and
Astrophysic
SARAS 2: A Spectral Radiometer for probing Cosmic Dawn and the Epoch of Reionization through detection of the global 21 cm signal
The global 21 cm signal from Cosmic Dawn (CD) and the Epoch of Reionization
(EoR), at redshifts , probes the nature of first sources of
radiation as well as physics of the Inter-Galactic Medium (IGM). Given that the
signal is predicted to be extremely weak, of wide fractional bandwidth, and
lies in a frequency range that is dominated by Galactic and Extragalactic
foregrounds as well as Radio Frequency Interference, detection of the signal is
a daunting task. Critical to the experiment is the manner in which the sky
signal is represented through the instrument. It is of utmost importance to
design a system whose spectral bandpass and additive spurious can be well
calibrated and any calibration residual does not mimic the signal. SARAS is an
ongoing experiment that aims to detect the global 21 cm signal. Here we present
the design philosophy of the SARAS 2 system and discuss its performance and
limitations based on laboratory and field measurements. Laboratory tests with
the antenna replaced with a variety of terminations, including a network model
for the antenna impedance, show that the gain calibration and modeling of
internal additives leave no residuals with Fourier amplitudes exceeding 2~mK,
or residual Gaussians of 25 MHz width with amplitudes exceeding 2~mK. Thus,
even accounting for reflection and radiation efficiency losses in the antenna,
the SARAS~2 system is capable of detection of complex 21-cm profiles at the
level predicted by currently favoured models for thermal baryon evolution.Comment: 44 pages, 17 figures; comments and suggestions are welcom
Daris, a low-frequency distributed aperture array for radio astronomy in space
DARIS (Distributed Aperture Array for Radio Astronomy in Space) is a radio astronomy space mission concept aimed at observing the low-frequency radio sky in the range 1-10 MHz. Because of the Earth's ionospheric disturbances and opaqueness, this frequency range can only be observed from space. The astronomical science cases include sensitive extragalactic surveys, radio transients such as Jupiter-like burst and Crab-like pulses, and coronal mass ejection tracking. The focus of the DARIS concept study is on feasibility aspects of a distributed aperture synthesis array in space, consisting of small satellite nodes and a mother-ship. The study selected suitable science cases, antenna concepts, communications, signal processing, orbital design, and mission analysis. With current-day technologies a satellite cluster can be built consisting of at least eight satellite nodes and a mother-ship, which could be launched with a Soyuz rocket from Kourou. Such a satellite cluster would open up the last unexplored frequency range for astronomy
Multichannel interference mitigation methods in radio astronomy
Radio-astronomical observations are increasingly corrupted by RF
interference, and online detection and filtering algorithms are becoming
essential. To facilitate the introduction of such techniques into radio
astronomy, we formulate the astronomical problem in an array signal processing
language, and give an introduction to some elementary algorithms from that
field. We consider two topics in detail: interference detection by rank
estimation of short-term covariance matrices, and spatial filtering by subspace
estimation and projection. We discuss experimental data collected at the
Westerbork radio telescope, and illustrate the effectiveness of the space-time
detection and blanking process on the recovery of a 3C48 absorption line in the
presence of GSM mobile telephony interference.Comment: 39 pages, 18 figures.Enhanced figures can be downloaded from
http://cas.et.tudelft.nl/~leshem/postscripts/leshem/figs34567.ps.gz To appear
in Astrophysical Journal Supplements serie
The Murchison Widefield Array: Design Overview
The Murchison Widefield Array (MWA) is a dipole-based aperture array
synthesis telescope designed to operate in the 80-300 MHz frequency range. It
is capable of a wide range of science investigations, but is initially focused
on three key science projects. These are detection and characterization of
3-dimensional brightness temperature fluctuations in the 21cm line of neutral
hydrogen during the Epoch of Reionization (EoR) at redshifts from 6 to 10,
solar imaging and remote sensing of the inner heliosphere via propagation
effects on signals from distant background sources,and high-sensitivity
exploration of the variable radio sky. The array design features 8192
dual-polarization broad-band active dipoles, arranged into 512 tiles comprising
16 dipoles each. The tiles are quasi-randomly distributed over an aperture
1.5km in diameter, with a small number of outliers extending to 3km. All
tile-tile baselines are correlated in custom FPGA-based hardware, yielding a
Nyquist-sampled instantaneous monochromatic uv coverage and unprecedented point
spread function (PSF) quality. The correlated data are calibrated in real time
using novel position-dependent self-calibration algorithms. The array is
located in the Murchison region of outback Western Australia. This region is
characterized by extremely low population density and a superbly radio-quiet
environment,allowing full exploitation of the instrumental capabilities.Comment: 9 pages, 5 figures, 1 table. Accepted for publication in Proceedings
of the IEE
A review of RFI mitigation techniques in microwave radiometry
Radio frequency interference (RFI) is a well-known problem in microwave radiometry (MWR). Any undesired signal overlapping the MWR protected frequency bands introduces a bias in the measurements, which can corrupt the retrieved geophysical parameters. This paper presents a literature review of RFI detection and mitigation techniques for microwave radiometry from space. The reviewed techniques are divided between real aperture and aperture synthesis. A discussion and assessment of the application of RFI mitigation techniques is presented for each type of radiometer.Peer ReviewedPostprint (published version
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