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					            Overview of Calibration and
            Dynamic Range Challenges

                   Paul Alexander




AAVP 2010              Dynamic Range      Paul Alexander
                     Required Dynamic Range
     •      Sensitivity: 2000 m2/K at 150 MHz; 300 MHz BW; station beam ~ 1 degree

             •   In 24 hrs integration s = 0.2 mJy

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

                   2.5 × 106

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

                   1 × 106

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


                                      Suggests 107 : 1

AAVP 2010                                  Dynamic Range                     Paul Alexander
                     Required Dynamic Range
     •      Note do need to think about source confusion

             •   In 24 hrs integration s = 0.2 mJy

             •   Source density ~ 1.5 x 105 sources per sq degree

                   Required baseline ~ 100km at 450 MHz




AAVP 2010                                  Dynamic Range            Paul Alexander
                Achieving high dynamic range now
     What do we know we have to include in an analysis:
     Include                     Discussion                               Maturity
     Antenna-based complex       Standard calibration and self             
     gains                       calibration – iterative
     Removing sources in         Removing bright sources from UV            
     global sky model            data even with local phase solution is
                                 relatively robust
     RFI and “bad data”          Can be critically important:
     excision                    • 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


AAVP 2010                            Dynamic Range                        Paul Alexander
                 Achieving high dynamic range now
     Include                      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


AAVP 2010                             Dynamic Range                     Paul Alexander
                     Achieving 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).




AAVP 2010                                  Dynamic Range                       Paul Alexander
                                                 AA Pros and Cons
      • 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

AAVP 2010                        Dynamic Range                 Paul Alexander
                   Designing 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
AAVP 2010                                 Dynamic Range                      Paul Alexander
                     Designing 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?


AAVP 2010                                   Dynamic Range                        Paul Alexander
                  Pathfinders
     • 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




AAVP 2010                           Dynamic Range                   Paul Alexander
                     Other 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




AAVP 2010                                  Dynamic Range              Paul Alexander
                     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


AAVP 2010                                  Dynamic Range                         Paul Alexander
                                           Magnitude of the task
                                                                                                                      Grid science reduction and
                                                                                                                             visualisation
                    Science                              Observation
                    proposal                              definition                       M&C
                                                                                         database
                                                                                                           Global
                                                                                                          and local
                                                                                                         sky model
                  Monitor and Control system
                                                                                                                               Cloud store




                                                                                                                                Data product
     Collectors




                                                                                                                                 distribution
                           Data excision




                                                           Data excision
                                            Correlator




                                                                                           Calibration
                                                                                              loop

                                                                                                          Imaging                               Science
                                                                                                         processor                              product
                                                                                                                                                archive

                                                                            Data    Visibility
                                                                           routing processors


                                                                                                                                  Local
                                                                                                                                 science
                                                                                                                                reduction


AAVP 2010                                                                      Dynamic Range                                         Paul Alexander
                SKA1 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 (W)
               • Line vs continuum requirements
            • What is the longest baseline
            • Single or multi-pass algorithms  increase data rate and
              buffering


AAVP 2010                            Dynamic Range                  Paul Alexander
                  Reducing 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

AAVP 2010                                 Dynamic Range                       Paul Alexander
AAVP 2010   Dynamic Range   Paul Alexander

				
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