499 research outputs found
Software correlators as testbeds for RFI algorithms
In-correlator techniques offer the possibility of identifying and/or excising
radio frequency interference (RFI) from interferometric observations at much
higher time and/or frequency resolution than is generally possible with the
final visibility dataset. Due to the considerable computational requirements of
the correlation procedure, cross-correlators have most commonly been
implemented using high-speed digital signal processing boards, which typically
require long development times and are difficult to alter once complete.
"Software" correlators, on the other hand, make use of commodity server
machines and a correlation algorithm coded in a high-level language. They are
inherently much more flexible and can be developed - and modified - much more
rapidly than purpose-built "hardware" correlators. Software correlators are
thus a natural choice for testing new RFI detection and mitigation techniques
for interferometers. The ease with which software correlators can be adapted to
test RFI detection algorithms is demonstrated by the addition of kurtosis
detection and plotting to the widely used DiFX software correlator, which
highlights previously unknown short -duration RFI at the Hancock VLBA station.Comment: 6 pages, 1 figure, accepted for publication in Proceedings of Science
[PoS(RFI2010)035]. Presented at RFI2010, the Third Workshop on RFI Mitigation
in Radio Astronomy, 29-31 March 2010, Groningen, The Netherland
RFI mitigation with phase-only adaptive beamforming
Connected radio interferometers are sometimes used in the tied-array mode:
signals from antenna elements are coherently added and the sum signal applied
to a VLBI backend or pulsar processing machine. Usually there is no
computer-controlled amplitude weighting in the existing radio interferometer
facilities. Radio frequency interference (RFI) mitigation with phase-only
adaptive beamforming is proposed for this mode of observation. Small phase
perturbations are introduced in each of the antenna's signal. The values of
these perturbations are optimized in such a way that the signal from a radio
source of interest is preserved and RFI signals suppressed. An evolutionary
programming algorithm is used for this task. Computer simulations, made for
both one-dimensional and two-dimensional array set-ups, show considerable
suppression of RFI and acceptable changes to the main array beam in the radio
source direction.Comment: 7 pages, 11 figure
Statistically Stable Estimates of Variance in Radioastronomical Observations as Tools for RFI Mitigation
A selection of statistically stable (robust) algorithms for data variance
calculating has been made. Their properties have been analyzed via computer
simulation. These algorithms would be useful if adopted in radio astronomy
observations in the presence of strong sporadic radio frequency interference
(RFI). Several observational results have been presented here to demonstrate
the effectiveness of these algorithms in RFI mitigation
SERPent: Automated reduction and RFI-mitigation software for e-MERLIN
The Scripted E-merlin Rfi-mitigation PipelinE for iNTerferometry (SERPent) is an automated reduction and RFI-mitigation procedure utilising the SumThreshold methodology (Offringa et al., 2010a), originally developed for the LOFAR pipeline. SERPent is written in the Parseltongue language enabling interaction with the Astronomical Image Processing Software (AIPS) program. Moreover, SERPent is a simple ‘out of the box’ Python script, which is easy to set up and is free of compilers. In addition to the flagging of RFI affected visibilities, the script also flags antenna zero-amplitude dropouts and Lovell telescope phase calibrator stationary scans inherent to the e-MERLIN system.
Both the flagging and computational performances of SERPent are presented here, for e-MERLIN commissioning datasets for both L-band (1.3–1.8 GHz) and C-band (4–8 GHz) observations. RFI typically amounts to <20%–25% for the more problematic L-band observations and <5% for the generally RFI quieter C-band. The level of RFI detection and flagging is more accurate and delicate than visual manual flagging, with the output immediately ready for AIPS calibration. SERPent is fully parallelised and has been tested on a range of computing systems. The current flagging rate is at 110 GB day−1 on a ‘high-end’ computer (16 CPUs, 100 GB memory) which amounts to ∼6.9 GB CPU−1 day−1, with an expected increase in performance when e-MERLIN has completed its commissioning.
The refining of automated reduction and calibration procedures is essential for the e-MERLIN legacy projects and future interferometers such as the SKA and the associated pathfinders (MeerKAT and ASKAP), where the vast data sizes (>TB) make traditional astronomer interactions unfeasible
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