35 research outputs found
Algorithms for FFT Beamforming Radio Interferometers
Radio interferometers consisting of identical antennas arranged on a regular
lattice permit fast Fourier transform beamforming, which reduces the
correlation cost from in the number of antennas to
. We develop a formalism for describing this process and
apply this formalism to derive a number of algorithms with a range of
observational applications. These include algorithms for forming arbitrarily
pointed tied-array beams from the regularly spaced Fourier-transform formed
beams, sculpting the beams to suppress sidelobes while only losing
percent-level sensitivity, and optimally estimating the position of a detected
source from its observed brightness in the set of beams. We also discuss the
effect that correlations in the visibility-space noise, due to cross-talk and
sky contributions, have on the optimality of Fourier transform beamforming,
showing that it does not strictly preserve the sky information of the
correlation, even for an idealized array. Our results have applications to a
number of upcoming interferometers, in particular the Canadian Hydrogen
Intensity Mapping Experiment--Fast Radio Burst (CHIME/FRB) project.Comment: 17 pages, 4 figures, accepted to Ap
A GPU Spatial Processing System for CHIME
We present an overview of the Graphics Processing Unit (GPU) based spatial
processing system created for the Canadian Hydrogen Intensity Mapping
Experiment (CHIME). The design employs AMD S9300x2 GPUs and readily-available
commercial hardware in its processing nodes to provide a cost- and
power-efficient processing substrate. These nodes are supported by a
liquid-cooling system which allows continuous operation with modest power
consumption and in all but the most adverse conditions. Capable of continuously
correlating 2048 receiver-polarizations across 400\,MHz of bandwidth, the CHIME
X-engine constitutes the most powerful radio correlator currently in existence.
It receives \,Tb/s of channelized data from CHIME's FPGA-based F-engine,
and the primary correlation task requires complex
multiply-and-accumulate operations per second. The same system also provides
formed-beam data products to commensal FRB and Pulsar experiments; it
constitutes a general spatial-processing system of unprecedented scale and
capability, with correspondingly great challenges in computation, data
transport, heat dissipation, and interference shielding
RXTE All-Sky Monitor Detection of the Orbital Period of Scorpius X-1
The orbital period of Scorpius X-1 has been accepted as 0.787313 d since its
discovery in archival optical photometric data by Gottlieb, Wright, & Liller
(1975). This period has been confirmed in both photometric and spectroscopic
optical observations, though to date only marginal evidence has been reported
for modulation of the X-ray intensity at that period. We have used data taken
with the RXTE All Sky Monitor to search for such a modulation. A major
difficulty in detecting the orbit in X-ray data is presented by the flaring
behavior of Sco X-1, which contributes white noise to Fourier transforms of the
intensity time series, and tends to obscure weak modulations. We present a new
technique for substantially reducing the effects of the flaring behavior while
retaining much of any periodic orbital modulation, provided only that the two
temporal behaviors exhibit different spectral signatures. Through such a
search, we have found evidence for orbital modulation at about the 1% level
with a period of 0.78893 d, equal within our accuracy to a period which differs
by 1 cycle per year from the accepted value. If we compare our results with the
period of the 1 year sideband cited by Gottlieb et al. we conclude that the
actual period may be 0.78901 d.Comment: AASTeX, 20 pages, 5 Postscript figure
A GPU-based Correlator X-engine Implemented on the CHIME Pathfinder
We present the design and implementation of a custom GPU-based compute
cluster that provides the correlation X-engine of the CHIME Pathfinder radio
telescope. It is among the largest such systems in operation, correlating
32,896 baselines (256 inputs) over 400MHz of radio bandwidth. Making heavy use
of consumer-grade parts and a custom software stack, the system was developed
at a small fraction of the cost of comparable installations. Unlike existing
GPU backends, this system is built around OpenCL kernels running on
consumer-level AMD GPUs, taking advantage of low-cost hardware and leveraging
packed integer operations to double algorithmic efficiency. The system achieves
the required 105TOPS in a 10kW power envelope, making it among the most
power-efficient X-engines in use today.Comment: 6 pages, 5 figures. Accepted by IEEE ASAP 201