Sensitive SKA1 Design Baseline (AA4)

• Sensitive SKA1 Design Baseline (AA4) is a reference design defining array configurations, advanced antenna performance, and calibration regimes for SKA Phase 1.
• It optimizes the effective collecting area to system temperature ratio using SKALA4 antennas, digital beamforming, and apodization techniques.
• The design enables high survey speeds and imaging fidelity for key science applications like 21 cm cosmology, pulsar timing, and high-resolution mapping.

The Sensitive SKA1 Design Baseline, designated AA4, is the principal sensitivity and configuration reference for SKA1-LOW and SKA1-MID arrays in Phase 1 of the Square Kilometre Array (SKA) project. AA4 defines the physical layout, antenna elements, electromagnetic performance, signal processing, and calibration regimes required to achieve stringent science goals in low-frequency radio astronomy, including 21 cm cosmology, pulsar timing, high-resolution imaging, and surveys of galactic and extragalactic sources. Central to AA4 is the optimization of effective collecting area to system temperature ratio ($A_{\rm eff}/T_{\rm sys}$), maximized over the designated frequency bands and field of view, by means of advanced antenna, array, and station design.

1. Array Architecture and Configuration

AA4 establishes the station and core layout for both SKA1-LOW and SKA1-MID arrays. For SKA1-LOW, the canonical AA4 configuration comprises 512 stations, each with 256 dual-polarized SKALA4 log-periodic dipole elements, densely distributed in a pseudo-random fashion over a circular area with diameter $D_s \approx 35\,\mathrm{m}$ (Acedo et al., 2020, Acedo et al., 2020, Huynh et al., 2013). The inner core—typically of radius 500–1000 meters—contains $\sim70\%$ of stations to maximize the instantaneous filling factor ($f_{\rm fill}$), while remaining stations are deployed along spiral arms or radially tapered outliers to achieve baselines up to 65–100 km for SKA1-LOW and up to 150–3000 km for the SKA1-MID array (Garrett et al., 2010, Godfrey et al., 2011). SKA1-MID AA4 comprises 133 new 15 m dishes plus 64 MeerKAT dishes, total 197, with similar split between a dense inner core and progressively sparser remote elements. The array configuration directly determines $uv$-coverage, synthesized beam properties, and survey speed.

2. Antenna and Station Design

The SKALA4 element, subject of the AA4 baseline, is a fourth-generation, broadband log-periodic dipole designed to optimize both impedance matching and radiation pattern smoothness over 50–350 MHz (Acedo et al., 2020). SKALA4 features 16 stages, longest dipole 1.60 m, shortest 0.22 m, overall height 2.10 m, and is mounted over a hexagonal wire mesh to provide a modeled infinite ground-plane. Element spacing within stations is close-packed while maintaining maintenance access. The key electromagnetic metrics are directivity $G(f,\theta,\phi)$, polarization purity (IXR $>$20 dB), matching ($S_{11}\!<-10$ dB across the band), and low receiver noise contribution ($T_{\rm rec}$ below 35 K in the upper band) (Acedo et al., 2020). Station-level beamforming is achieved by digital summation, employing possible apodization tapers (e.g., Hanning) to suppress sidelobes and mutual coupling ripples, with embedded pattern errors maintained below ±0.3 dB (Grainge, 2014).

3. Sensitivity Metrics and Performance

The central sensitivity metric is $A_{\rm eff}/T_{\rm sys}$, evaluated for each frequency band and array sub-component. For SKA1-LOW AA4, measured $A_{\rm eff}/T_{\rm sys}$ reaches $>$1000 m$^2$/K at 110 MHz and remains above 600 m$^2$/K at 350 MHz (Huynh et al., 2013). SKA1-MID AA4 achieves $A_{\rm eff}/T_{\rm sys} \approx 1600$ m$^2$/K at 1.4 GHz, with corresponding SEFD $\sim$1.1 Jy (Keane et al., 18 Dec 2025). System temperature $T_{\rm sys}$ includes sky ($$T_{\rm sky} \sim 60\,\mathrm{K}\cdot(300\,\mathrm{MHz}/\nu)^{2.55}$$), receiver ($T_{\rm rec}$), and spillover/ground ($T_{\rm ground}$) components (Garrett et al., 2010). The radiometer equation governs imaging or survey rms:

$$\sigma_S = \frac{2\,k_B\,T_{\rm sys}(\nu)}{A_{\rm eff}(\nu)\,\sqrt{2\,\Delta\nu\,\tau}}$$

For representative integration time ($\tau=1$ hr, $\Delta\nu=100$ MHz), continuum rms sensitivities are 2–3.5 $\mu$Jy for SKA1-LOW in the 50–350 MHz range and $\sim$1 $\mu$Jy for SKA1-MID at 1.4 GHz (Huynh et al., 2013). For pulsar timing, the minimum detectable flux density $S_{\min}(\nu)$ incorporates effects of duty cycle, pulse broadening, and bandwidth.

4. Field of View, Survey Speed, and Imaging Fidelity

AA4 design prioritizes large field of view (FoV) and survey speed by selecting moderate station diameters (35–38 m), yielding primary beam FoVs of $\sim$27 deg$^2$ for SKA1-LOW at 110 MHz, and up to 1.4 deg$^2$ for SKA1-MID at 1 GHz (Keane et al., 18 Dec 2025, Huynh et al., 2013). Survey speed is defined as $$S_{\rm survey} \propto \Omega_{\rm eff} \cdot (A_{\rm eff}/T_{\rm sys})^2$$, where $\Omega_{\rm eff}$ is the effective instantaneous footprint after tiling with tied-array beams (Keane et al., 18 Dec 2025). For AA4, survey speeds exceed AA* by 20–30%, with larger increases at mid frequencies. Imaging fidelity is ensured by minimizing mutual coupling and station beam ripple, employing apodized station weighting, and maintaining high filling factors (>0.12 for core SKA1-LOW AA4) (Grainge, 2014). Spiral arm and randomized station layouts provide robust $uv$-coverage, minimizing PSF artifacts.

5. Calibration, Synchronization, and Systematics Management

Phase-synchronous operations across SKA1-LOW AA4 are realized by the UWA stabilized frequency-transfer system, achieving coherence loss $<$0.01% at 1 s intervals and $<$0.01% at 60 s, with timing jitter $<$53 fs, well below AA4 requirements (Schediwy et al., 2018). Active servo control via acousto-optic modulators suppresses environmental fibre path noise over buried links up to 58 km (extendable to 175 km). Polarimetric calibration is facilitated by element intrinsic cross-polarization ratios (IXR) $>$20 dB and station-level digital beamforming. Mutual coupling and element-to-element variations are managed via embedded calibration models, with station layout designed to average out small-scale systematics (Acedo et al., 2020). Sidelobe suppression ($< -30$ dB for apodized stations) and dense $uv$-sampling yield high dynamic range and spectral purity (Grainge, 2014).

6. Scientific Applications and Key Performance Results

AA4 sensitivity enables leading science goals in HI cosmology, pulsar astrophysics, and high-angular-resolution mapping. For 21 cm power spectrum and tomography, the AA4 compact core maximizes low-$k$ sensitivity, with three-tier survey strategies (deep, medium-deep, shallow) providing percent-level constraints on EoR astrophysical parameters (ionizing efficiency $\zeta$, mean free path $R_{\mathrm{mfp}}$, minimum halo temperature $T_{\mathrm{vir}}$) (Greig et al., 2015). HII region imaging is achieved at sub-mK noise and arcminute angular resolution (core filling factor $\sim$0.5), fully resolving bubble morphology over several degrees (single pointing) (Wyithe et al., 2015). Pulsar census yields are increased by $>$20% compared to AA*, predicting discoveries of $>$13,!400 slow pulsars and $\sim$1,!030 MSPs in full-sky surveys (Keane et al., 18 Dec 2025, Xu et al., 18 Dec 2025). High angular resolution imaging with SKA1-MID AA4 reaches $\lesssim$50 nJy/beam at mas scales, supporting parallax, proper-motion, and ultra-compact source studies (Godfrey et al., 2011).

7. Configuration Trade-Offs and Evolution

AA4 incorporates trade-offs between physical collecting area, angular resolution, survey speed, cost, and calibration complexity. Reducing station diameter increases FoV and short-baseline density but raises correlator load. Element sharing and apodization improve core filling and sidelobe suppression but require discarding auto-correlated baselines and increase beamformer input count (Grainge, 2014). The AA4 cost cap, set at 350 M€, divides hardware, infrastructure, and contingency to allow flexible scaling for future upgrades (Garrett et al., 2010). The array geometry and sensitivity baseline are designed to accommodate evolving scientific requirements and hardware advances, with rapid assembly and maintenance by design (Acedo et al., 2020).

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Table: SKA1 AA4 Sensitivity and Survey Metrics | Subarray | $A_{\rm eff}/T_{\rm sys}$ (m²/K) | FoV (deg²) | RMS 1hr (μJy) | |-------------------|----------------------------|------------|-------------| | SKA1-LOW @ 110MHz | 1000 | 27 | 2.0 | | SKA1-LOW @ 350MHz | 600 | 27 | 3.4 | | SKA1-MID @ 1.4GHz | 1600 | 0.68 | 1.1 |

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In summary, the AA4 baseline provides a rigorously optimized, reference design for the SKA1-LOW and SKA1-MID arrays, integrating advanced antenna hardware, array configuration, signal transport, and calibration strategies to support the realization of key scientific goals in cosmology, galactic structure, and fundamental physics.