4 research outputs found
Finite precision arithmetic in Polyphase Filterbank implementations
The MeerKAT is the most sensitive radio telescope in its class, and it is important that systematic effects do not limit the dynamic range of the instrument, preventing this sensitivity from being harnessed for deep integrations. During commissioning, spurious artefacts were noted in the MeerKAT passband and the root cause was attributed to systematic errors in the digital signal path. Finite precision arithmetic used by the Polyphase Filterbank (PFB) was one of the main factors contributing to the spurious responses, together with bugs in the firmware. This thesis describes a software PFB simulator that was built to mimic the MeerKAT PFB and allow investigation into the origin and mitigation of the effects seen on the telescope. This simulator was used to investigate the effects in signal integrity of various rounding techniques, overflow strategies and dual polarisation processing in the PFB. Using the simulator to investigate a number of different signal levels, bit-width and algorithmic scenarios, it gave insight into how the periodic dips occurring in the MeerKAT passband were the result of the implementation using an inappropriate rounding strategy. It further indicated how to select the best strategy for preventing overflow while maintaining high quantization effciency in the FFT. This practice of simulating the design behaviour in the PFB independently of the tools used to design the DSP firmware, is a step towards an end-to-end simulation of the MeerKAT system (or any radio telescope using nite precision digital signal processing systems). This would be useful for design, diagnostics, signal analysis and prototyping of the overall instrument
COSMIC: An Ethernet-based Commensal, Multimode Digital Backend on the Karl G. Jansky Very Large Array for the Search for Extraterrestrial Intelligence
The primary goal of the search for extraterrestrial intelligence (SETI) is to
gain an understanding of the prevalence of technologically advanced beings
(organic or inorganic) in the Galaxy. One way to approach this is to look for
technosignatures: remotely detectable indicators of technology, such as
temporal or spectral electromagnetic emissions consistent with an artificial
source. With the new Commensal Open-Source Multimode Interferometer Cluster
(COSMIC) digital backend on the Karl G. Jansky Very Large Array (VLA), we aim
to conduct a search for technosignatures that is significantly more
comprehensive, more sensitive, and more efficient than previously attempted.
The COSMIC system is currently operational on the VLA, recording data, and
designed with the flexibility to provide user-requested modes. This paper
describes the hardware system design, the current software pipeline, and plans
for future development.Comment: 30 pages, 17 figures. Accepted for publication in A
MeerKAT-16 H I observation of the dIrr galaxy WLM
We present observations and models of the kinematics and the distribution of
the neutral hydrogen (HI) in the isolated dwarf irregular galaxy,
Wolf-Lundmark-Melotte (WLM). We observed WLM with the Green Bank Telescope
(GBT) and as part of the MeerKAT Early Science Programme, where 16 dishes were
available. The HI disc of WLM extends out to a major axis diameter of 30 arcmin
(8.5 kpc), and a minor axis diameter of 20 arcmin (5.6 kpc) as measured by the
GBT. We use the MeerKAT data to model WLM using the TiRiFiC software suite,
allowing us to fit different tilted-ring models and select the one that best
matches the observation. Our final best-fitting model is a flat disc with a
vertical thickness, a constant inclination and dispersion, and a
radially-varying surface brightness with harmonic distortions. To simulate
bar-like motions, we include second-order harmonic distortions in velocity in
the tangential and the vertical directions. We present a model with only
circular motions included and a model with non-circular motions. The latter
describes the data better. Overall, the models reproduce the global
distribution and the kinematics of the gas, except for some faint emission at
the 2-sigma level. We model the mass distribution of WLM with a
pseudo-isothermal (ISO) and a Navarro-Frenk-White (NFW) dark matter halo
models. The NFW and the ISO models fit the derived rotation curves within the
formal errors, but with the ISO model giving better reduced chi-square values.
The mass distribution in WLM is dominated by dark matter at all radii.Comment: Accepted for publication in MNRAS, 25 pages, 21 figures, 5 table
MeerKAT-16 HI observation of the dIrr galaxy WLM
We present observations and models of the kinematics and the distribution of the
neutral hydrogen (Hi) in the isolated dwarf irregular galaxy, Wolf-Lundmark-Melotte
(WLM). We observed WLM with the Green Bank Telescope (GBT) and as part of
the MeerKAT Early Science Programme, where 16 dishes were available. The Hi disc
of WLM extends out to a major axis diameter of 300 (8.5 kpc), and a minor axis
diameter of 200 (5.6 kpc) as measured by the GBT. We use the MeerKAT data to
model WLM using the TiRiFiC software suite, allowing us to t di erent tilted-ring
models and select the one that best matches the observation. Our nal best- tting
model is a
at disc with a vertical thickness, a constant inclination and dispersion,
and a radially-varying surface brightness with harmonic distortions. To simulate bar-
like motions, we include second-order harmonic distortions in velocity in the tangential
and the vertical directions. We present a model with only circular motions included
and a model with non-circular motions. The latter describes the data better. Overall,
the models reproduce the global distribution and the kinematics of the gas, except for
some faint emission at the 2 level. We model the mass distribution of WLM with a
pseudo-isothermal (ISO) and a Navarro-Frenk-White (NFW) dark matter halo models.
The NFW and the ISO models t the derived rotation curves within the formal errors,
but with the ISO model giving better reduced chi-square values. The mass distribution
in WLM is dominated by dark matter at all radii.The MeerKAT telescope is operated by the South African
Radio Astronomy Observatory, which is a facility of the Na-
tional Research Foundation, an agency of the Department
of Science and Innovation.
This work is based upon research supported by the South
African Research Chairs Initiative of the Department of Sci-
ence and Technology and National Research Foundation.
The nancial assistance of the South African Radio Astron-
omy Observatory (SARAO) towards this research is hereby
acknowledged (www.sarao.ac.za).
PK is partially supported by the BMBF project 05A17PC2
for D-MeerKAT.
AS acknowledges the Russian Science Foundation grant
19-12-00281 and the Program of development of M.V.http://mnras.oxfordjournals.orgam2021Physic