Dual site SKA1 and SKA2 implementations

There has been a lot of nonsense talked about what the dual-site SKA will look like. I thought it was worth setting the record straight. Note that this is my personal view of where we are now - it need not be the view of the SKA Organisation.

Mike Garrett (garrett@astron.nl)

Last Friday the SKA Members met and adopted a dual-site implementation for SKA1 and SKA2.

The SKA1 implementation will use existing (or currently being built) infrastructure at both sites, including the precursor telescopes - MeerKAT in RSA and ASKAP in ANZ. It will also include a low-frequency aperture array (SKA1_LOW). SKA1_Mid will be a dish array incorporating MeerKAT. SKA1_SURVEY will be a dish array equipped with PAFs incorporating ASKAP.

So SKA1 is comprised of the following elements and will be located at these sites:

SKA1_Mid (RSA),
SKA1_LOW (ANZ),
SKA1_SURVEY (ANZ)

This split makes excellent use of the current infrastructure at both sites (e.g. network, power provision etc) and the precursors MeerKAT and ASKAP. It also enhances the science case of SKA1 (see memo 125) better addressing the main phase 1 themes: HI near and far + fundamental physics (pulsar timing/surveys).

SKA2 is comprised of the following elements and will be located at these sites:

SKA2_LOW (ANZ),
SKA2_Mid (RSA),
SKA2_AA (RSA)

Comments:

Since MeerKAT is very close to the original SKA1_Mid specification, adding MeerKAT to SKA1_Mid only requires 180 SKA1 spec dishes to be constructed. This permits 60 dishes to be added to ASKAP (the baseline SKA1 called for a 240 dish array). Note also that for SKA1 we are not talking about a simple extension of MeerKAT or ASKAP - we are talking about the incorporation of these telescopes within SKA1 - SKA1 will have a higher spec dish design common to both sites. SKA1_LOW will not incorporate the MWA as the scales and technologies are quite different. Some of the MWA (non-telescope) infrastructure may however be relevant.

The major additional cost differential between the baseline SKA1 and SKA1 as now envisaged is in the form of 60+ PAFs required for SKA1_SURVEY. N.B. the adoption of PAFs as part of the Adv. Instrumentation Programme (AIP) on SKA1 still needs to be ratified via current ASKAP and WSRT-APERTIF demonstrators within the context of the SKA engineering process. A decision on PAF readiness is expected to be made in 2014.

The dual site SKA implementation takes into account the characteristics and advantages of the sites - as detailed in the SSAC report. The SOWG relied heavily on the work conducted by the SSAC and their report.

There are many uninformed comments about the doubling of costs via a dual site implementation - this is not the case - all the SKA1 and SKA2 components have their own independent cores where most of the antennas are located - duplication only becomes apparent on the longer baselines - not an issue for SKA1 and a smaller issue than expected for SKA2 via ANZ & RSA cost predictors. Power is the dominant operational cost.

Some handy figures to know - SKA1_LOW is ~ 5 x LOFAR (A/T) but has a different freq range. MeerKAT has an A/T of 260; SKA1_Mid has an A/T ~1000 - MeerKAT has more aggressive front-end cooling than that currently proposed for SKA1_Mid. ASKAP has an A/T of ~ 65, SKA1_Survey ~ three times ASKAP A/T - ASKAP PAFs may be upgraded, if project costs allow.

SKA2_LOW and SKA2_Mid might be expected to build out from their SKA1 precursors (though the scale is so different this need not be the case). SKA2_AA is a dense aperture array (survey instrument) operating at mid-frequencies < 1500 MHz. It is likely to form a substantial component of Phase 2 but also has its own AIP demonstration process to go through w.r.t engineering readiness a la PAFs in SKA1.

Abbreviations

RSA=Rep. of South Africa
ANZ=Australia/New Zealand
AIP = Adv. Instrumentation Programme

References

SKA Site Advisory Committee (SSAC) Report

SKA Board of Directors Commentary on the SSAC Report

SKA Siting Options Working Group (SOWG) Report