Paul Alexander Dynamic RangeAAVP 2010 Overview of Calibration and Dynamic Range Challenges Paul Alexander

Paul AlexanderDynamic RangeAAVP 2010

Overview of Calibration and Overview of Calibration and Dynamic Range ChallengesDynamic Range Challenges

Paul Alexander


Required Dynamic RangeRequired Dynamic Range

Suggests 107 : 1

• Sensitivity: 2000 m2/K at 150 MHz; 300 MHz BW; station beam ~ 1 degree

• In 24 hrs integration = 0.2 Jy

• ~ 1 500 mJy source per sq degree at 150 MHz

2.5 × 106

• EoR signal ~ 10 mK in presence of ~1000K foreground. Image at > 10

1 × 106

Take this estimate with a “pinch of salt” – limited by

foreground subtraction


Required Dynamic RangeRequired Dynamic Range

• Note do need to think about source confusion

• In 24 hrs integration = 0.2 Jy

• Source density ~ 1.5 x 105 sources per sq degree

Required baseline ~ 100km at 450 MHz


Achieving high dynamic range nowAchieving high dynamic range now

What do we know we have to include in an analysis:

Include Discussion Maturity

Antenna-based complex gains

Standard calibration and self calibration – iterative

Removing sources in global sky model

Removing bright sources from UV data even with local phase solution is relatively robust

RFI and “bad data” excision

Can be critically important: •still largely done by hand for GMRT, eVLA and LOFAR•Expert algorithms not well developed

Bandpass calibration Well defined, but often more problematic than it should be – software limitation

?


Achieving high dynamic range nowAchieving high dynamic range nowInclude Discussion Maturity

Debugging the system We learn a great deal about our instruments over time and correct often 2nd order errors

Position-dependent effects Hugely Important relatively recent advance•Time dependent pointing errors – antenna models may be limit•Position-dependent phase screen – critically important for the ionosphere – modelling?•Many algorithms (peeling. A-projection ...)

Full stokes imaging A position-dependent effect – polarization response changes across FoV


Achieving high dynamic range nowAchieving high dynamic range now

Other known issues

• Algorithm approximations mean analysis has known problems and errors

which are not necessarily well dealt with Wide-field imaging approximations (faceting, w-projection)

Deconvolution errors and artefacts – still an art using human judgement to

drive non-linear algorithms

Time-averaging and bandwidth smearing poorly dealt with (but also useful

in very wide fields).


• AA is operating at low frequency Ionosphere!

• Physical stability (wind etc.) Good, study details

• Unblocked aperture Inherent

• Smaller beams are better >60m collectors

• Narrow band is important AA is Wide Band but many channels

• Calibration capability Excellent, by channel

• Trade DR for sensitivity AA v. flexible

AA Pros and ConsAA Pros and Cons


Designing for dynamic rangeDesigning for dynamic range

Stable, known antenna patterns are key

•AA advantages

• AA’s mechanically stable

• Unblocked aperture

• Direct measurement of field

•But

• Need to calibrate 105 elements per station

• What accuracy of element calibration is needed

• Model dependent calibration – how many parameters can we solve for?

• Station beam is time dependent – transit experiment for individual elements

• Multiple independent elements for AA-low


Designing for dynamic rangeDesigning for dynamic range

Element-level calibration options and issues

• Importance of phase versus amplitude – how accurate? How often?

• Where are the main errors introduced in the RF chain -

• If copper is used for signal transport – active measurement of cable lengths?

• Deployment issues – position, orientation, misalignment

• If digitisation at the element – accuracy of clock distribution

• Temperature variations – large ambient fluctuations – fibre better than copper?

• Expert health monitoring system at element level – flag failed or failing

elements

• Noise injection?


PathfindersPathfinders

• The design decision must be informed by the pathfinders and

precursors

• SKA community mu go beyond – “waiting to see what we will learn”

SKA team must pose the questions that we want to be answered

• Get answers either from experience of the pathfinders or doing explicit

experiments and measurements

Obvious area of immediate cooperation between all the experiments

and the AAVP team


Other design issuesOther design issues

• Sufficiently good ionospheric model

• Station size and UV coverage – competing issues

• Larger stations – smaller station beam easier

ionospheric model? Lower cost & less processing

• Smaller station size better – more stations better

UV coverage, better imaging capability

• Hierarchical beam former

• Hierarchical beamformer in which data decimated

reduced accuracy of station beam


Algorithm issues (AA emphasis)Algorithm issues (AA emphasis)

• Maturity of approaches is not there yet

• Much use of human intervention still required

• Transitioning to totally automated pipelines will be a major challenge

• Expert system for RFI excision?

• Are wide-field imaging approaches sufficiently accurate?

• Is our underpinning understanding of interferometry based too much on

“experience” rather than a formal understanding of the underpinning

formalism?

• Relying very much on a tiny group of real experts who have both the

“experience” and the formal analysis


Magnitude of the taskMagnitude of the task

Imaging processor

Visibility processors

Science product archive

Local science

reduction

Science proposal

Data product distribution

Data routing

Col

lect

ors

Grid science reduction and visualisation

Monitor and Control system

M&C database

Global and local sky model

Calibration loop

Observation definition


SKASKA11 Data Rates and Configuration Data Rates and Configuration

• AA Line experiment 50 AA-low stations

• 100 sq degrees, 10000 channels over 380 MHz bandwidth

3.3 GS/s

• Issues

• What data rate can we process?

• Trade UV coverage (Ns) for FoV and hence survey speed ()

• Line vs continuum requirements

• What is the longest baseline

• Single or multi-pass algorithms increase data rate and

buffering


Reducing the data rateReducing the data rate

• Relax criteria for dump rates and frequency resolution

– My criteria based on uniqueness in UV plane

– Can the criteria be relaxed and still achieve high dynamic range?

• Dump times and frequency resolution baseline dependent

Design correlator for worse case upgrade path


Documents

Paul Alexander Dynamic RangeAAVP 2010 Overview of Calibration and Dynamic Range Challenges Paul Alexander