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Using DIFX with ASKAP
Chris Phillips, eVLBI Project Scientist November 2011


Murchison Radio Observatory


Murchison Radio Observatory
gazetted towns: 0 population: "up to 160"

population: >16 million


ASKAP Design Goals High-dynamic range, wide field-of-view imaging
Number of dishes Dish diameter Max baseline Resolution Sensitivity Speed Observing frequency Field of View Processed Bandwidth Channels Integration time Focal Plane Phased Array 36 12 m 6 km (30 dishes inside 2 km) 10 65 m2/K 1.3x105 m4/K2.deg2 700 ­ 1800 MHz 30 deg2 300 MHz 16384 5 seconds 192 elements

+ Infrastructure for new SKA-ready observatory Murchison Radio Observatory (MRO)


Phased Array Feed


Signal Processing Overview ·192 elements per PAF ·384 MHz band with "coarse" polyphase filterbank to 1 MHz spectral resolution
· 8 bit/sample · 16 bits/complex sample from filterbank

·Beam forming on 1 MHz channels
· 36 dual pol beams

·Fine filterbank (with fringe rotation) ·Correlation and accumulation

CSIRO. eVLBI Oz 2011


Current Status ·14 antenna on-site
· 2 more in Australia

· · · · ·



Control building being assembled 3 PAF being installed onsite this week 9 dualpol beams available No correlator firmware Can capture up to 64 MHz bandwidth baseband data

DIFX to the rescue
CSIRO. eVLBI Oz 2011


Data Capture ·Firmware NOT designed for streaming of baseband data ·Onboard memory enough to capture 1 second of data then download beams individually ·~2 seconds overhead for capture ·1 second/beam/16 MHz to readout
· Poor duty cycle

·Written in VDIF format

CSIRO. eVLBI Oz 2011


Oversampled? ·Front end element filters slow turn off only inner 304 MHz processed
· Outer channels discarded

·Engineers claim this is oversampled.... ·Polyphase filterbank is oversampled
· 1 MHz channels oversampled by factor 32/27 · Oversampling needed to ensure consistent data over full spectrum

·Implemented as overlapped FFTs
· 60 sample overlap
CSIRO. eVLBI Oz 2011


CSIRO. eVLBI Oz 2011


DIFX Implications ·Not at integral number of samples/second
· 1.185[185]... Msamples/sec

·Only every 27th sample on nanosec boundary ·Samples only aligned on second boundary every 27 seconds ·NOT supported by VDIF
· VDIF extension proposed to allow define cycle period for integral number of frames
· 27 seconds for ASKAP · Count frames from start of period

CSIRO. The high resolution component of ASKAP


DIFX processing ·Initial approach ignore problem entirely
· Short baselines, low frequency · Geometric delay only couple samples per day · Fringe rotation < 20sec

·Small model error (couple degrees) over 1 second dump ·Times should resync on each new data ·Will investigate internal tweak for model application to account for wrong times
· Also bandwidths, times in files etc
CSIRO. eVLBI Oz 2011


High Resolution Spectrum

CSIRO. eVLBI Oz 2011


What's going on ·Artifact introduced by oversampling filterbank ·Can be corrected by applying phase ramp to time series data
· Correction proportional to channel number modulo 32

CSIRO. The high resolution component of ASKAP




Solution ·Initially ignore! ·Same on all antenna, can average data to "continuum" channel ·Should be possible to treat as large LO offset
· Additional phase term in fringe rotation · Can probably implement in generic was within DIFX

CSIRO.. The high resolution component of ASKAP


Is this relevant? ·More antennas are using non-traditional sampling rates
· GMRT 16.6.. MHz · KAT would like to use 56.25 MHz

·Should DIFX attempt to cope with this or force hardware engineers to be "sensible"?

CSIRO. eVLBI Oz 2011


Astronomy and Space Science Chris Phillips e-VLBI Project Scientist Phone: 02 93724608 Email: Chris.Phillips@csiro.au Web: www.atnf.csiro.au

Thank you
Contact Us Phone: 1300 363 400 or +61 3 9545 2176 Email: enquiries@csiro.au Web: www.csiro.au