SKA DISH ELEMENT TECHNICAL SOLUTION...Technical Solution the underlying philosophy has been to...

SKA‐TEL.DSH.MGT‐CSIRO‐TS‐004 Revision: 1

Thursday, 06 June 2013 of 48

Name Designation Affiliation Date Signature

Submitted by:

C. Jackson

On behalf of SKADC

consortium

CSIRO 7 June 2013

Accepted by:

Approved by:

SKA DISH ELEMENT TECHNICAL SOLUTION

Document number ....................................................... SKA‐TEL.DSH.MGT‐CSIRO‐TS‐004Revision ........................................................................................................................... 1Author ................................................................................................ .SKADC ConsortiumDate .............................................................................................................. 06 June 2013Status ................................................................................................................... Released



DOCUMENT HISTORY

Revision Date Of Issue Engineering Change

Number

Comments

A 2013‐05‐24 ‐ Full draft release for SKADC review.

1 2013‐06‐06 ‐ Released for RFP submission

DOCUMENT SOFTWARE

Package Version Filename

Wordprocessor MS Word Word 2010 SKA‐TEL.DSH.MGT‐CSIRO‐TS‐004_DishTechSol

Block diagrams

Other

ORGANISATION DETAILS

Name SKA Organisation

Registered Address Jodrell Bank Centre for Astrophysics

Room 3.116

Alan Turing Building

The University of Manchester

Oxford Road

Manchester, UK

M13 9PL

Registered in England & Wales

Company Number: 07881918

Fax +44 (0)161 275 4049

Website www.skatelescope.org



TABLE OF CONTENTS

DOCUMENT HISTORY ..................................................................................... 2

DOCUMENT SOFTWARE ................................................................................. 2

ORGANISATION DETAILS ................................................................................ 2

TABLE OF CONTENTS ...................................................................................... 3

LIST OF FIGURES ............................................................................................. 5

LIST OF TABLES ............................................................................................... 5

GLOSSARY ...................................................................................................... 6

1 INTRODUCTION ............................................................................................. 7 1.1 Purpose and Scope of this document ..................................................................................... 7 1.2 Review of SKA RfP Baseline & this Technical Solution ............................................................ 8 1.3 Validity and basis of the Technical Solution as derived in the SKADC Work Plans ................. 8

2 SKADC TECHNICAL SOLUTION FOR DISH ‐ SPECIFICATION ....................................... 9 2.1 DISH ‐ Context ......................................................................................................................... 9 2.2 DISH ‐ Optics ........................................................................................................................... 9 2.3 DISH Structure & Performance ............................................................................................. 11 2.4 SINGLE PIXEL FEEDS – SKA_mid ............................................................................................ 13 2.4.1 Band 1 Feed Package .................................................................................................... 15 2.4.2 Band 2 Feed Package .................................................................................................... 16 2.4.3 Band 3, 4 and 5 Feed Package ....................................................................................... 16

2.5 FEED PACKAGE – SKA1_survey ............................................................................................. 18 2.6 RECEIVER PACKAGE – SKA_mid ............................................................................................. 20 2.7 RECEIVER PACKAGE – SKA1_survey ...................................................................................... 23 2.8 POWER .................................................................................................................................. 25 2.9 Local monitor and control ..................................................................................................... 25 2.9.1 LMC implementation .................................................................................................... 26

3 IMPLEMENTATION ESTIMATES ........................................................................ 27 3.1 FEED PACKAGE ‐ SKA1_mid SKADC Technical Solution Estimates ........................................ 27 3.2 SKA1_Survey SKADC Technical Solution ‐ Estimates ............................................................. 27 3.2.1 SKADC Technical Solution ‐ Antenna Sub‐System(s) ..................................................... 28

3.3 RECEIVER PACKAGE ‐ SKA1_mid SKADC Technical Solution Estimates ................................. 28 3.3.1 SKADC Technical Solution ‐ Antenna Sub‐System(s) ..................................................... 29

3.4 RECEIVER PACKAGE ‐ SKA1_mid SKADC Technical Solution Antenna Based Support Systems Estimates ........................................................................................................................................... 30



4 REFERENCES .............................................................................................. 31

APPENDIX A1 .................................................................................................. 32 A1.1. Introduction – Designing the SKA optics ........................................................................... 32 A1.2. Optics parameters and options ......................................................................................... 32 A1.3. Initial reduction of the parameters ................................................................................... 33 A1.4. Selection of the dish design sets ....................................................................................... 35 A1.5. Restrictions ....................................................................................................................... 35 A1.6. Determine the evaluation criteria .................................................................................... 35 A1.7. Optics design candidates .................................................................................................. 37 A1.8. References for Appendix A1 – Designing the SKA optics .................................................. 48



LIST OF FIGURES Figure 1 CAD model of Eleven Feed for 0.35‐2GHz band. ......................................................... 15 Figure 2 MeerKAT L‐Band Receiver (left) with cryogenic waveguide load. .............................. 16 Figure 3 1‐4 GHz LNA MMIC and assembled demonstrator module. ....................................... 17 Figure 4 Measured gain and noise temperature of the demonstrator module. ...................... 17 Figure 5 Block diagram of the SKA_mid receiver package. ....................................................... 22 Figure 6 Block diagram of the SKA1_survey receiver package. ................................................ 24 Figure 7 Outline of LMC system. ............................................................................................... 25 Figure 8 Outline of SKA1_mid Feed/LNA and receiver system. ................................................ 28 Figure A1‐1: Definition of the parameters describing the offset Gregorian dishes ........................ 33 Figure A1‐2: Schematics of the selected optics design sets. ........................................................... 47

LIST OF TABLES Table 1 Summary of the most important Dish Performance Requirements. ............................... 10 Table 2 Summary of the most important Dish Performance Requirements. ............................... 12 Table 3 Summary of the most important SKA1‐Mid Array requirements. ................................... 13 Table 4 Summary of specifications for the LNA module for the Band 3 receiver. ........................ 18 Table 5 Summary of the most important SKA1_Survey Array requirements ............................... 19 Table 6 Summary of the most important SKA1_Mid Array requirements ................................... 20 Table 7 SKA1 Receivers – Technical Solution Bands 1 – 5a. .......................................................... 21 Table 8 SKA1 Receivers – Technical Solution Bands 5b – 5d. ....................................................... 21 Table 9 Summary of the most important SKA1_Survey Array requirements ............................... 23 Table 10 SKADC Feed package – Estimated requirements ............................................................. 27 Table 11 SKADC PAF Feed package – Estimated requirements ...................................................... 28 Table 12 SKADC Receiver antenna sub‐systems – Estimated requirements .................................. 29 Table 13 SKADC Receiver technical solution antenna based support systems .............................. 30 Table A1‐1 Dish design options with target reflector sizes for each design. ...................................... 38



GLOSSARY Definition of terms: SKA Baseline The SKA specification (baseline) as issued with the RfP and as set out by the SKAO. SKADC The SKA Dish Consortium comprising institutes, industry and other stakeholders. SKADC Technical Solution The SKA Dish Consortium’s response to the SKA Baseline, as submitted as part of the SKADC response, based on reasoned discussion of technologies able to at least meet the SKA Baseline. Feeds & LNAs Feed system and low‐noise amplifier (LNA). Feed package The item responsible for converting incident radiation to signals in co‐axial cables. This includes, feeds, LNAs as well as packaging and control and monitoring. Receiver Everything post‐LNA (except possibly a second stage amplification in the feed package) including analog‐to‐digital conversion (A/D), gain, RF and (any supporting) digital systems ahead of the correlator. SKA Element Level 3 item in the WBS e.g. Aperture Array, Dish, Science Data Processing. SKA sub‐system Level 4 item in the WBS, e.g. a major part of the dish element – e.g. Dish Structure



1 Introduction

This Technical Solution captures the SKADC’s response to the SKAO Baseline issued at RfP. As requested in the SKA Request for Proposals [1], this technical proposal for Dishes summarises:

The functional descriptions of the Sub‐systems that comprise the Dish Element

Key component characteristics

Performance analyses against the requirements

Key Element Level requirements

1.1 Purpose and Scope of this document

The SKADC Technical Solution is a major part of our RfP response and has formed the framework from which we have defined our work plans. In considering system optimizations during the work of the pre‐construction phase, we aim to deliver a superior, well‐engineered system. SKADC Technical Solution described in this document is aligned to the baseline design and is achievable on the SKA1 project timeline. This Technical Solution will be used by SKADC from the start of pre‐construction. This Technical Solution, plus the concept of operations (ConOps) and further requirements definitions (including interface control document (ICD) clarifications) will be reviewed by SKADC (with SKAO) as we develop a full set of Requirements Specifications. Development of the full Requirements Specifications is planned to occur in September 2013, and will be led by the SKADC Systems Engineering team. The Technical Solution will be updated and we suggest it be used as an informal description of the SKADC (dish) implementation that reflects the Requirements Specifications. The Technical Solution addresses all aspects of the baseline and sets out procedures to refine the selection of e.g. feeds, dish (optics), cryogenics systems, etc. to meet the intended SKA specifications. In deriving the Technical Solution the underlying philosophy has been to design for the production of thousands of units. The SKADC will consider options and alternatives, particularly during Stage 1, ahead of the Preliminary Design Review (PDR). The imperative to deliver the Critical Design Review (CDR) will mean that the focus will be on a well‐engineered, robust system covering all necessary aspects. In terms of pre‐construction planning, the SKADC has made some early estimates of the physical implementation (e.g. mass, size of feeds and their combinations, location of equipment, etc), which are captured in Section 3 of the Technical Solution.

Major changes to the SKADC Dish Technical Solution, which are expected to be improvements over the first outline, will be agreed in conjunction with the SKAO Engineering team with suitable justification materials (analyses, performance measures, etc.). The SKAO RfP Baseline does not detail the optical design of the dish. As this is a critical component that is required ahead of the full mechanical design of the dish, feeds etc., Appendix A1 sets out the analyses that will be used to derive the SKA dish optics design. The outcome of this process will be both the dish optics and optimised feeds (the illuminating part only, not fully packaged feed and LNA systems) for each of the respective bands. The material in the Appendix is treated as a stand‐alone, self‐contained document with its own references. The way in which this process is to be carried out by SKADC is described in the Work Plan.



1.2 Review of SKA RfP Baseline & this Technical Solution

For each SKADC sub‐system, the Technical Solution begins by reproducing the SKA RfP Baseline extracted from [5], re‐stated in boxed text. In most cases, this will be a verbatim quote from [5]; however, in some instances, the expertise of the SKADC has been used to interpret the spirit and intent of the RfP Baseline. The following text, in each section, then describes the requirements and specifications that the SKADC considers are most appropriate for a practical dish system that will go the closest to achieving the SKA RfP Baseline.

1.3 Validity and basis of the Technical Solution as derived in the SKADC Work

Plans

Given all the above, and a number of face‐to‐face discussions at organisational and Work Package level, the SKADC has given due consideration to the Technological Readiness Levels (TRLs) and Risks of the Technical Solution proposed herein. The TRLs and Risks are described in detail in the SKADC Work Plans.



2 SKADC Technical Solution for Dish ‐ Specification

2.1 DISH ‐ Context

The DISH Technical Solution has been developed during a series of meetings and teleconferences commencing with the “DISH Consortium” discussion held on 11‐14 February 2013. The SKA Dish Array Consortium considers the following technical solution is a practicable implementation that satisfies the SKA baseline design. .

Design philosophy:

design for thousands of units

single dish design to accommodate both single pixel feeds for SKA_mid and phased array feeds for SKA1_survey.

The design of both SKA_mid and SKA1_survey will allow for the build‐out to SKA2 including the interchange of single pixel feeds and Wide‐Band SPF (WBSPF)/ Phased Array Feeds (PAFs) at some time in the future.

2.2 DISH ‐ Optics

RfP Baseline Reflector antennas are used for both SKA1_survey and SKA_mid. [5, Sect. 7] Ideally the same antenna will be used for both single‐pixel and phased‐array feeds. [5, Sect. 7] Some features of the SKA1 dishes may have to be altered to accommodate PAFs efficiently. It is conceivable that slightly different optical surfaces from the SKA1_mid dishes can be used for SKA1_survey, while sharing the mechanical structure and the local dish infrastructure (mount, foundation, control, etc.). Significant investigative effort is still needed to determine whether PAFs can be combined efficiently with offset Gregorian dishes. [5, Sect. 9.2] The selected antennas are 15‐m projected diameter, offset Gregorian optics. [5, Sect. 7] Designed for very high Ae/Tsys per unit of currency. The following are important qualitative characteristics required for SKA1 antennas [5, Sect. 7]:

Smoothness of response in spatial and spectral dimensions, as limited by fundamental physics (e.g. edge diffraction). Scattering objects tend to generate low‐level resonances, which will have relatively fine frequency structure and/or chromatic sidelobes.

Space at the focus for five independent receivers.

Very low sidelobes beyond the first one.

Excellent polarisation performance.

Circular beam.

Excellent performance down to ~450 MHz, good performance to 350 MHz.

Excellent performance to 15 GHz, good performance to 20 GHz.



Equivalent physical aperture diameter

15 m

Low Frequency 350 MHz

High Frequency 20 GHz

Optics Clear aperture

Efficiency >77 % 65% at 400 MHz and 20 GHz

Total spillover noise 3 K L‐band

Other losses <2 K L‐band

1st sidelobe ‐21 dB

Far‐out sidelobe level <‐50 dB

Polarization purity ‐30 dB Within HPBW

Beam symmetry TBD

Table 1 Summary of the most important Dish Performance Requirements – Extracted from [5, Sect. 7, Table 5].

SKADC Technical Solution The following were agreed as an appropriate baseline SKADC Technical Solution:

15 m diameter, offset Gregorian dish with clear unobstructed optics; [5, Sect. 7]

See the list of parameter sets in Appendix A1.

f/D (eff) =0.5

f/D (eff) =0.5 is a compromise between a shorter f/D that is optimum for smaller feed sizes and smaller phased array feeds, and a longer f/D that is optimum for ease of dish optics design. This parameter will be investigated in the first phase leading to the optical down select.

Support bore sight and off‐axis 6x6 beams;

The dish optics needs to accommodate the baseline phased array feeds, which have 36 beams – especially to achieve good performance for all 36 beams at the centre of the SKA1_survey Band 1, that is, ~600 MHz. This may require a sub‐reflector that extends beyond the standard geometrical optics boundaries.

6 m secondary;

The sub‐reflector must be large enough to be a reasonable reflector at the low frequency (350 MHz) end. The initial estimate is that it would need to be of the order of 6 m in diameter. Any extensions for spill‐over shielding or supporting off‐axis beams will be included in this dimension (i.e. the effective sub‐reflector will be slightly smaller).

Unshaped reflectors

The baseline optics will be unshaped, primarily because shaping the optics deteriorates off‐axis PAF beams and unshaped optics are less feed pattern specific. This will, however, be investigated during the initial optics selection process – once a set of optimal feed patterns for the same unshaped geometry is available. 1st sidelobe: ‐21 dB; Far‐out sidelobe level <‐50 dB [5, Sect. 7]

This only has an impact on the dish structure when shaping the reflectors.

Surface rms (both primary and secondary reflectors) less than 0.5mm



With the same feed art that achieves an efficiency of 0.77 in L‐band, the surface error efficiency has to be less than 0.84 to achieve a 0.65 overall efficiency. At 20 GHz, this translates to an effective surface accuracy of 0.5 mm RMS. Such surfaces are routinely achievable for a 15 metre dish.

Feed mount ‘high’;

The DVA‐1 investigation resulted in a severe cost penalty for the ‘feed low’ configuration. Thus the ‘feed high’ configuration will be the baseline proposal. It should be noted that the ‘feed low’ configuration can be better shielded for spill‐over and allows much easier feed access for maintenance purposes. Hence both options will be considered during the optics selection phase.

Reflector construction ‐ carbon fibre composite

It was agreed that the reflector construction, for the baseline specification, would be carbon fibre composite. However, studies of steel/aluminium options should be undertaken as the baseline specification of ‘composite’ still needs to be substantiated as the optimum solution (and this is the only case where the SKA Baseline appears to specify the material, which is strictly an implementation choice). The more appropriate high level requirement would consider the implications of reflectors made from panels (typical with aluminium and steel constructions) as opposed to single piece reflectors.

All feeds shall be located at the secondary focus;

The baseline SKADC Technical Solution specifically excludes the option of feeds being swung into the primary focal region of the primary reflector.

Single feed indexer design: minimum of 5 feed positions selectable; Rotary indexer

Same feed indexer design is to be used for both the single pixel feeds for SKA_mid and for the phased array feeds for SKA1_survey. The feed indexer design shall be rotary ‐ not a linear feed translator.

2.3 DISH Structure & Performance

RfP Baseline Reflector antennas are used for both SKA‐survey and SKA‐mid. [5, Sect. 7] Ideally the same antenna will be used for both single‐pixel and phased‐array feeds. [5, Sect. 7] Some features of the SKA1 dishes may have to be altered to accommodate PAFs efficiently. It is conceivable that slightly different optical surfaces from the SKA1_mid dishes can be used for SKA1_survey, while sharing the mechanical structure and the local dish infrastructure (mount, foundation, control, etc.). Significant investigative effort is still needed to determine whether PAFs can be combined efficiently with offset Gregorian dishes. [5, Sect. 9.2] For SKA1 it is assumed that between 3 and 5 feeds are available and can be fitted for each antenna. [5, Sect. 8.3] The selected antennas are 15 m projected diameter, offset Gregorian optics. [5, Sect. 7] Designed for very high Ae/Tsys per unit of currency. The following are important qualitative characteristics required for SKA1 antennas [5, Sect. 7]:

Excellent pointing.

Excellent stability of key parameters (beam shape, pointing, etc.).

Smoothness of response in spatial and spectral dimensions, as limited by fundamental physics (e.g. edge diffraction). Scattering objects tend to generate low‐level resonances, which will have relatively fine frequency structure and/or chromatic sidelobes.

Space at the focus for five independent receivers.



Very low sidelobes beyond the first one.

Excellent polarisation performance.

Circular beam.

Excellent performance down to ~450 MHz, good performance to 350 MHz.

Excellent performance to 15 GHz, good performance to 20 GHz.

Equivalent physical aperture diameter

15 m

Low Frequency 350 MHz

High Frequency 20 GHz

Optics Clear aperture

1st sidelobe ‐21 dB

Far‐out sidelobe level <‐50 dB

Polarization purity ‐30 dB Within HPBW

Beam symmetry TBD

Receivers 5 Cryo‐cooled, spanning frequency range

Elevation limit <15 deg

Azimuth range ±270 deg

Pointing repeatability 10, 17,180 arcsec P, S, D respectively arcsec, rms

Receiver noise temperature & Feed Losses

<15 K Assumed for performance estimates

Classes of Environmental Operating Conditions

Precision Wind <7 m/s; night

Standard Wind <7 m/s; day

Degraded Wind <20 m/s

Operation continuous Except for extreme weather.

Table 2 Summary of the most important Dish Performance Requirements – Extracted from [5, Sect. 7, Table 5].

SKADC Technical Solution The following were agreed as appropriate baseline SKADC Technical Solution:

2‐axis (az‐el) – no 3rd axis

Mount etc; steel;

El range 15 – 95 deg; Az range +/‐ 270 deg [5, Sect. 7]

Wind survival 45 m/s (~160 km/hr); normal full‐spec operations 8 m/s (~30 km/hr).

Slew speeds 2 deg/s (Az), 1 deg/s (El)

Design lifetime 50 years



2.4 SINGLE PIXEL FEEDS – SKA_mid

RfP Baseline

Band #. Band (GHz)

RF BW IF BW (GHz)

Antenna & Feed Efficiency

Band 1 0.35 – 1.05 3:1 1 0.65 @ 400 MHz

0.78 above 600 MHz

Band 2 0.95 – 1.76 1.85:1 1 0.78

Band 3 1.65 – 3.05 1.85:1 2.5 0.78

Band 4 2.8 – 5.18 1.85:1 2.5 0.78

Band 5 4.6 – 13.8 3:1 2.5 0.78 @ 8 GHz

0.7 above 8 GHz

Table 3 Summary of the most important SKA1‐Mid Array requirements – Extracted from [5, Sect. 7, Table 6].

Polarisation: Dual (2 orthogonal) [5, Sect. 8.4.1]

Only the two lower‐frequency feeds need be populated for SKA1 science [5, Sect. 8.3]

Efficiency

At the lowest frequency it is important to provide a relatively compact feed (although it will still be larger than any other feed). At the frequencies below ~400 MHz, the optics design will limit the efficiency and spillover noise, even with a large feed. For example, with a small sacrifice in efficiency, a quad‐ridge feed design is both fairly compact and has the potential for wide bandwidth. A 3:1 feed design has been selected here. Other feed designs, such as an Eleven feed [15] may also be suitable. [5, Sect. 8.3]

Optimize for Ae/Tsys; 1st sidelobe: ‐21 dB; Far‐out sidelobe level <‐50 dB [5, Sect. 7]

Excellent performance down to ~450 MHz, good performance to 350 MHz. [5, Sect. 7]

Feed system flexibility:

Once the dish designs are fixed, the main avenue for improvement of the system is by replacing feeds and receivers with better models. A critical design requirement for SKA1 dishes is the capability to allow this to be done with as little interference with other parts of the system, especially other feed bands. [5, Sect. 8.3] Cryogenic cooling An initial cryogenics study was carried out in 2012 to examine options and to develop a cost model for the SKA [16]. The main conclusion of this study is for a given requirement for Ae/Tsys performance, that cooling of receivers to physical temperatures between 20 and 100 K is cost effective by a large factor. The optimum temperature depends on the contributions of operations costs (electricity and maintenance), dish capital cost, and the fraction of total system cost attributable to the dishes. Various forms of Stirling and more traditional Gifford‐McMahon coolers were evaluated. As would be expected, the higher the system cost per dish, the more cost‐effective cryo‐cooled receivers become. Or for a given Ae/Tsys, fewer dishes are needed when the receivers are cooled. [5, Sect. 7.1]

Only one receiver can operate on any antenna at one time. [5, Sect. 8.4.2]




SKA 1 shall be outfitted with Bands 1–2; [5, Sect. 8.3]

The design phase will consider all 5 bands as this may have an impact on the dish selection. The higher frequency feed package will be developed to the same level as bands 1 and 2, but may not be rolled out in the initial stages.

Optimise design for good performance in the 400‐500 MHz band; dish efficiency should be very good for observing frequencies above 600MHz

It should be pointed out here that the efficiency specified in the RfP baseline requires shaped optics – especially at the lower bands where physical size constraints limit the possibility of shaping feed patterns. Even with shaping, diffraction may limit the actual performance at 600 MHz. It is proposed to achieve the required sensitivity by reducing the system temperature as much as possible.

All 5 single pixel feeds shall be accommodated on the one feed indexer.

All feeds shall be located at the secondary focus;

The SKADC Technical Solution specifically excludes the option of feeds being swung into the primary focal region of the primary reflector.

The SKA_mid dish will receive in two linear polarisations.

All orthomode transducers (OMTs), apart from Band 1, shall be cooled

The Band 1 feed aperture would most likely be too large to put the entire feed inside a vacuum vessel and the feed design may make it impossible to have a thermal break inside the feed. It is thus likely that only the LNAs will be cooled. For Band 2 the lossy parts of the OMT will be cooled to between 50 K and 70 K in a cryostat whose outer wall is also the vacuum boundary.

All LNAs shall be cooled to < 20 Kelvin;

A cost/benefit analysis has shown that cryogenically cooling the LNAs is cost effective. The proposed baseline design requires cooling all LNAs to < 20 K. The detailed cooling study may suggest cooling some LNAs to 30 K.

There will be three (3) integrated cryostats;

Baseline design: There will be three dewars that will accommodate Band 1, Band 2 and Bands 3, 4 & 5 respectively. It is impractical to combine the lower frequency bands into a single cryostat as the space between feeds would lead to a very large cryostat, unless all feeds are mounted as spokes protruding outward from a central cryostat. This would severely restrict the length (and the possibility of future change) of all the feeds. The cryostats will share a single helium compressor with provision for reduced output during the roll‐out phase when not all feeds are installed. Each dish will be fitted with an automated vacuum system to simplify installation (e.g. after servicing the cryostat) and allow vacuum regeneration if a cryostat has been installed long enough for cryo‐pumping to become ineffective. The individual single pixel feed package will have as little control as possible and will communicate via slow serial optical fibre links with a central Feeds Controller in the Antenna Pedestal. This controller will also control the helium and vacuum services.



Gifford‐McMahon cryogenic coolers;

The SKADC Technical Solution specifies Gifford‐McMahon cryogenic coolers [5, Sect. 7.1]. There are alternative technologies which may be applicable which promise a reduction in both procurement and operating costs with increased reliability, e.g. Sterling Cycle coolers. However these technologies do not have the same level of maturity as current Gifford‐McMahon systems. Furthermore, experience thus far is that currently fielded systems have only been able to demonstrate an improvement in one of these areas. As such, Gifford‐McMahon coolers offer the only viable solution on the SKA1 timescale. The improvements promised by these alternative technologies are significant and therefore work should be carried out to develop them to the level of maturity necessary for their inclusion in SKA2. If the promised improvements were demonstrated on a suitable timescale for inclusion in SKA1 then this would be contemplated. However this work will not form part of the critical path for SKA1 feed package development.

Dewar will be designed to survive at least 3 minutes without power and then restart without requiring maintenance.

2.4.1 Band 1 Feed Package

The proposal for the Band 1 feed package is to consider both the Eleven Feed and the Quad‐Ridge Feed Horn (QRFH) and evaluate their performance at the same time as the optical down select. In both cases, only the LNAs will be cryogenically cooled. This results in a small cryostat that needs to be very close to the feed output.

Figure 1 CAD model of Eleven Feed for 0.35‐2GHz band.



The proposed Band 1 LNA design is based on the technology developed for the ALMA Band 3 IF cryogenic LNAs. After prototyping at NRC‐Herzberg, these were manufactured in large volume (350) by Nanowave Inc. in Canada. The design uses discreet InP transistors and allows optimum performance while cooled at 15 K. Noise of the 4‐8 GHz ALMA LNAs is below 4 K. Following on this design NRC‐Herzberg prototyped and manufactured LNAs for the MeerKAT L‐Band, 900‐1670 MHz. Noise of the LNA, when cooled at 15 K, is 3 K. To date, 8 units have been delivered to EMSS, for the pre‐production phase. NRC‐Herzberg is currently prototyping a Band 1 SKA LNA using the same technology, with similar expected performance.

2.4.2 Band 2 Feed Package

The Band 2 feed package will be a scaled (factor 900/950) version of the MeerKAT L‐Band feed (shown in Figure 2) that is currently at TR level 7. It will consist of an axially corrugated horn optimised to give a flat pattern with sharper drop‐off at the edge angle than a typical Gaussian beam. The feed horn will be optimised for an optimum Ae/Tsys over the band, probably resulting in a spill‐over of around 4 to 5 K. This is connected to a compact OMT constructed with dipoles in the back of the waveguide which also form the cryostat wall. The LNAs will use 3 stages with InP HEMT transistors similar to the MeerKAT ones. Additional RF gain on a temperature stabilised platform inside the cryostat will allow conditioning the RF output for a simple interface to the Receiver. The LNAs will be cooled to 15 K and the OMT to between 50 and 70 K. This should result in a receiver noise temperature of below 10 K and a total system temperature about 20 K.

Figure 2 MeerKAT L‐Band Receiver (left) with cryogenic waveguide load for noise calibration.

2.4.3 Band 3, 4 and 5 Feed Package

The Band 3 feed and OMT will be a scaled version of the Band 2 feed.

2.4.3.1 Band 3 receiver – LNA MMIC

The low noise amplifier MMIC will be designed and fabricated by Fraunhofer IAF. For a joint project (IAF, MPIfR, IRAM) aimed to exploit the potential of Fraunhofer IAF’s metamorphic high‐electron‐mobility‐transistor (mHEMT) technology for cryogenic applications several demonstrator MMIC LNAs were designed, one of them a 3 stage LNA in the frequency range 1‐4GHz employing external input matching of the first stage. The HEMT technology used at that time featured gate‐lengths of 100 nm, achieving fT of more than 220 GHz. Fig. 1 shows the MMIC used in a demonstrator LNA built and characterized in 2009 by MPIfR.



Figure 3 1‐4 GHz LNA MMIC and assembled demonstrator module.

Performance of this initial design is very promising. Fig. 2 shows measured performance at ambient and cryogenic temperatures. Noise temperature in the band of interest here was ~6K for a physical temperature of 18K of the module.

Figure 4 Measured gain and noise temperature of the demonstrator module at a physical temperature of the module of 300K (left) and 18K (right), frequency range 1‐4GHz.

By reducing the gate‐lengths to 50 nm even higher transit frequencies (fT) of more than 375 GHz and consequently even lower noise figures and higher gain are achieved by Fraunhofer IAF. Since Fraunhofer IAFs 50 nm mHEMT technology offers best cryogenic noise performance and has been validated at cryogenic temperatures in several national and international projects in close cooperation with the MPIfR, any MMICs developed for Band 3 of SKADC will be fabricated in this well established 50 nm mHEMT technology, thus combining a cryogenic noise performance competitive to the InP process with the big technological advantage of fabrication on cheaper, less brittle GaAs substrates which makes the mHEMT process an ideal candidate for large scale production for the SKA. Based on the demonstrated results and the improvements guaranteed by the advanced technology together with the reduced requirements on bandwidth, the main design goals and challenges are to deliver a noise performance of less than 5 K average noise temperature and to increase the gain above 30 dB while not jeopardizing the input return loss (IRL) requirements of 15 dB on module level.



2.4.3.2 Band 3 receiver – LNA module

Module will employ a LNA MMIC fabricated using Fraunhofer IAF’s 50nm cryogenic mHEMT process. Baseline chip is a redesign of an existing 1‐4GHz LNA MMIC. Due to the low frequency of the band, and since use of a cryogenic isolator on the input of the LNA has to be avoided, an additional input‐matching circuit external to the MMIC might be necessary to simultaneously meet noise and IRL specifications. RF input and output of the LNA module will have DC‐blocks and will therefore not carry DC voltages. RF input and output connections of the module will be standard SMA connectors. Mechanical interface specifications must be provided by OMT‐package subtask. Interface specifications for DC‐bias connector and for mechanical attachment including cryogenic interface will be provided by the cryostat‐package subtask. The LNA module will provide basic lowpass filters for DC‐bias connections to protect the chip from harmful ESD. Further RFI filtering may be necessary and this will need to be determined as part of pre‐construction.

Frequency range 1.6 – 3.1GHz

Gain ≥ 30dB

Noise temperature ≤ 6K

Input return loss ≤ 15dB

Output return loss ≤ 10dB

DC‐power dissipation ≤ 40mW

Operating temperature range 300 ‐15 K, all specifications given @ Tamb=15K

No. of voltages for DC‐bias ≤ 4

Reference impedance 50Ω

Test & verification of specs As outlined in the work plan

Table 4 Summary of specifications for the LNA module for the Band 3 receiver. The band 4 feed may use a more conventional quad‐ridge OMT as the dimensions of the dipoles and the coaxial feed lines extending from the back short become more critical at higher frequency.

2.4.3.3 Band 4

The feed will be a corrugated horn that will cover 2.8 – 5.18 GHz. The feed will be optimised for Ae/Tsys on the final selected optics (in addition to the feed optimisations during the optics down select process). The entire feed and OMT would be cooled for best sensitivity.

2.4.3.4 Band 5

The feed will be a corrugated horn or quad‐ridge horn that will cover 4.6 – 13.8 GHz. The feed will be optimised for Ae/Tsys on the final selected optics (in addition to the feed optimisations during the optics down select process). The entire feed and OMT would be cooled for best sensitivity.

2.5 FEED PACKAGE – SKA1_survey

RfP Baseline

It is assumed that the dishes can easily accommodate three PAFs for different frequency ranges, but only one would be populated for SKA1 [5, Sect. 9.1]

Only one feed available at a time [5, Sect. 9.5.1]

Upgrade Path:

It is not anticipated that SKA1_survey will be expanded to SKA2. SKA1_survey could be enhanced by the addition of more PAF arrays to cover a greater frequency range. In principle, these could be added



in such a way as to share the beamformers. [5, Sect. 9.1]

Band #. Band (GHz)

RF BW IF BW (MHz)

PAF dia‐meter (m)

Average Tsys (K)

Band 1 0.35 – 0.9 2.57:1 500 1.82 50

Band 2 0.65 – 1.67 2.57:1 500 1 30

Band 3 1.5 – 4.0 2.67:1 500 0.41 40

Table 5 Summary of the most important SKA1_Survey Array requirements – Extracted from [5, Sect. 9.5.1, Table 15].


Number of PAF elements: 188 (94 each polarisation) [5, Sect. 9.5.1]

Number of PAF beams: 36 [5, Sect. 9.5.1]


Phased array feeds may be located at the primary focus (where there is no secondary reflector) or at the secondary focus.

This baseline SKADC Technical Solution includes designs where:

the PAF is located at the secondary focus of the Gregorian optics, or

the PAF is located at the focus of the primary reflector, there being no secondary reflector in this case.

The baseline SKADC Technical Solution specifically excludes the option of dual reflector optics with PAF(s) being swung into the focal region of the primary reflector.

PAF sized to accommodate 6x6 beams at band centre [5, Sect. 9.5.1]

Each PAF will have ~200 RF receiver elements that are digitised before being combined in a beamformer to form 36 dual‐polarised beams (72 outputs in total).

All 3 phased array feeds shall be accommodated on the one feed indexer

Band 1 will be a chequerboard‐based design because it is believed that a Vivaldi array would be too large. Band 2 could be either Vivaldi or chequerboard feed type. Both feed types to be tested/compared on like‐for‐like basis (testbed) before PDR to allow evaluation in time for Dishes PDR. The CSIRO ASKAP phased array feed would be option zero if the chequerboard feed type is chosen; the value of reworking this design to exact SKA specification is minimal.



2.6 RECEIVER PACKAGE – SKA_mid

RfP Baseline

Band #. Band (GHz)

RF BW (MHz)

IF BW (GHz)

# of Ifs

# of bits

Data Rate Gb/s

1* 0.35 – 1.05 700 1 2 8 48

2* 0.95 – 1.76 808 1 2 8 48

3 1.65 – 3.05 1403 2.5 2 6 90

4 2.8 – 5.18 2380 2.5 2 4 60

5 4.6 – 13.8 9200 2.5 4 3 90

* Bands 1 and 2 are a priority for SKA1 [5, Sect. 8.4.2]

Table 6 Summary of the most important SKA1_Mid Array requirements – Extracted from [5, Sect. 7, Table 6].

Only one receiver can operate on any antenna at one time

Only one receiver can operate on any antenna at one time. Some receiver bands may be divided into

sub‐bands, only one of which may overlap a MeerKAT band. In that case the other band can be correlated as one or more sub‐arrays of SKA1 antennas, [5, Sect. 8.4.2]. At least bands 1 and 2 will be available for SKA1

To cover the key SKA1 science, only the two lower‐frequency feeds need be populated [5, Sect. 8.3].

At least SKA1 bands 1 and 2 will be available. Depending on funds, not all of the other SKA1 receivers will be initially available. It is assumed that the correlator, non‐imaging processor and similar equipment will be sized to fit the maximum requirements, or will be specifically designed to be expandable to such [5, Sect. 8.4.2].

Digitise the RF band at the antenna

Signals from the dishes will be transported to a central signal processing building via the digital data backhaul system. The SKA1_mid array will transmit data from the output of the digitising stage at the receiver to the input of the signal processing system. The transmission will use optical fibre to carry digitised data. It will be a point‐to‐point deterministic network in which the data flows in a uni‐directional fashion from the antennas to the central processor for SKA1_mid. [5, Sect. 8.5] Synchronisation will be provided at each antenna

The synchronisation sub‐system within the SKA to provide the frequency reference signals required. Coherence can be maintained through the use of accurate independent clocks or by a frequency reference distribution from a central reference clock. At every antenna or station, the synchronisation system will provide a standard reference sine wave from which clocks for digitisation and/or local oscillator signals can be derived and a pulse‐per‐second (1‐PPS) signal from which time‐tags can be derived [5, Sect. 11.0].

Timing signals will be provided at each antenna

Sufficiently accurate Coordinated Universal Time (UTC), converted to sidereal time using regularly published Earth‐rotation data (UT1 – UTC and higher order corrections will be provided [5, Sect. 11.1]. MeerKAT will be incorporated into the joint array The key parts of the MeerKAT system will be incorporated into the joint array. This includes the receivers, synchronisation and time distribution, and data transport system [5, Sect. 8.4.2]



In Band 5, two 2.5 GHz bands can be observed simultaneously [5, Table 6]

SKADC Technical Solution The following were agreed as appropriate baseline SKADC Technical Solution: Only one receiver can operate on any antenna at one time [5, Sect. 8.4.2]

At least bands 1 and 2 will be available for SKA1 [5, Sect. 8.4.2]

The proposed implementation is shown in Figure 5. A common sampler is used for bands 1 ‐4. Bands 1 – 4 and the low‐frequency part of Band 5, designated Band 5a would be directly sampled in the first or second Nyquist zones, as outlined in Table 7. The higher frequencies in Band 5, designated Band 5b – Band 5d would be down‐converted with a high‐side local oscillator and the IF would be sampled in the first Nyquist zone, as outlined in Table 8.

Band Frequency

Range (GHz)

No. of Ifs

No. of bits

Sample clock (Gs/S)

Total(Gb/S)

Comment

1* 0.35 – 1.05 2 8 8 128 Direct Sample in 1st Nyquist Zone

2* 0.95 – 1.76 2 8 8 128 Direct Sample in 1st Nyquist Zone

3 1.65 – 3.05 2 8 8 128 Direct Sample in 1st Nyquist Zone

4 2.8 – 5.18 2 4 12.5 100 Direct Sample in 1st Nyquist Zone

5a 4.6 – 7.6 4 3 8 96 Direct Sample in 2nd Nyquist Zone

* Bands 1 ad 2 are a priority for SKA1 [5, Sect. 8.4.2] but the receiver system for band 3 is also implemented as part of the technical solution.

Table 7 SKA1 Receivers – Technical Solution Bands 1 – 5a.

Band

Frequency Range (GHz)

No. of Ifs

No. of bits

Sample clock (Gs/S)

Total (Gb/S)

LOFrequency

(GHz)

Sampled IF Band (GHz)

5b 7.2 – 9.7 4 3 8 96 10.8 1.1 – 3.6

5c 9.2 – 11.8 4 3 8 96 12.8 1.0 – 3.6

5d 11.3 – 13.8 4 3 8 96 14.9 1.1 – 3.6

Table 8 SKA1 Receivers – Technical Solution Bands 5b – 5d.

In Band 5, any two 2.5 GHz bands can be observed simultaneously [5, Table 6]

Note that the sampler clock frequencies and the local oscillator frequencies are within Band 5. These may cause interference when two, 2.5 GHz bands, in Band 5 are observed simultaneously. Synchronisation will be provided at each antenna [15, Sect. 11.0].

Band 5 implementation will require a local oscillator (LO) derived from the reference signal at each antenna. Round trip phase measurement and tracking is likely to be required.



Digitise the RF band at the antenna [15, Sect. 8.5]

Timing signals will be provided at each antenna [15, Sect. 11.1]

Components of the system will be located in both the antenna and at the central site

The baseline design assumes digitisation at the antenna. This may occur at the focus or in the pedestal. Terminal equipment and/or digital signal processing hardware will also be required at the central site. The exact requirements and locations of the various sub‐system components will be implementation specific. Sampler clocks and other signals will be required.

Both polarisations will be observed at the same frequency

Figure 5 Block diagram of the SKA_mid receiver package.



2.7 RECEIVER PACKAGE – SKA1_survey

RfP Baseline

Band#.Band(GHz)

RFBWIFBW(MHz)

Band1 0.35–0.9 2.57:1 500

Band2 0.65–1.67 2.57:1 500

Band3 1.5–4.0 2.67:1 500

Table 9 Summary of the most important SKA1_Survey Array requirements – Extracted from [5, Sect. 9.5.1, Table 15].


Number of elements per PAF : 188 (94 each polarisation) [5, Sect. 9.5.1]

Average PAF efficiency (Band 2): 0.8 – Area‐weighted average over beams and frequencies [5, Sect. 9.5.1]

Maximum available bandwidth: 500 MHz [5, Sect. 9.5.1]

Number of sample streams per PAF element: 2 x 8 bits (both pol’ns) [5, Sect. 9.5.1]

Number of PAF beams: 36 [5, Sect. 9.5.1]

PAF signal/data transport

The following two possibilities result in slightly different approaches to interfacing with PAFs and ASKAP equipment to the combined telescope system: [5, Sect. 9.5.1]

1. The ASKAP Mark III PAFs are likely to utilise RF‐over‐fibre to transport the RF in analogue form from each element of the PAF array to a processing centre, where digitisation takes place. In the case of ASKAP, the processing centre is the central correlator building. However, the maximum reach of RF‐over‐fibre is about 10 km. Thus if this model is adopted for more distant SKA1 antennas, then enclosures supplied with power will be needed at strategic points in the array configuration. Digital data would then be transported from these enclosures to the central building. Optionally, beamforming will also take place in these enclosures to reduce the amount of data transmitted.

2. If the Mark III PAFs are equipped with internal digitisers (or upgraded to such), then digital data will be transmitted all the way to the central correlator building. Optionally, beamforming will take place in or near the antennas to reduce the amount of data transmitted.

RF‐over‐fibre (analog) to Digitiser location: 188 fibres; one switchable RF‐over‐fibre subsystem per antenna. [5, Sect. 9.5.1]




All SKA1_survey bands shall be sampled at full bandwidth. Whilst full bandwidth is the baseline, options for lower sample rates will also be investigated.

Processed (IF) bandwidth = 500 MHz. [5, Sect. 9.5.1]

The polyphase filterbank shall be sized to process SKA1_survey Band 2. SKA1_survey Bands 1 and 2 will require less processing power than SKA1_survey Band 3. Select 500 MHz of bandwidth to output from the polyphase filterbank to the beamformer. SKA1_survey Band 3 will have a higher processing requirement and the polyphase filterbank for Band 3 will be supplied when the SKA1_survey Band 3 receiver system is installed.

The SKA1_survey receiver system will be split so that part will be located at the focus and the remainder will be located off the dish. On‐dish sampling and beamforming is the SKA baseline design. However, a first review of this option (as scoped, with ASICs) looks to be too large or heavy to be located at the focus. The analog (RF) portion of the receiver system will be split so that part will be located at the focus and the remainder will be located off the dish – together with the sampler and beamformer. RF‐over‐fibre links will be used to send the RF signal to the off‐dish portion of the receiver system.

Figure 6 Block diagram of the SKA1_survey receiver package.



2.8 POWER

The power requirements of the Dish systems will be significant. The major contributions to the Dishes power budget will be from:

Antenna drives

Cryogenics systems

Electronics – including the digital processing associated with the PAF systems

Environmental cooling of the electronics systems. The power requirements will be estimated for the RfP response to set a baseline.

2.9 Local monitor and control

Comprehensive monitor and control shall be designed to interface the Telescope Manager (TM) and the dishes (e.g. antenna drives, feed and receivers and other systems), to permit the scheduled observations and feedbacks to the TM. Interface Telescope Manager ‐ dish

Interface TM and simulator

Interface with Dish and simulator The Local monitor and control (LMC) system shall permit the dish operations, monitor the status, provide alarms and request for maintenance. Dish operation

Pointing, tracking, receiver setup, sensors...

Determination of pointing corrections

Logging

Alarms, safety, request for maintenance

Figure 7 Outline of LMC system.



2.9.1 LMC implementation

Hardware ‐ Rack with computer, interface boards for interfaces, data archiving ‐ Size of the system about 50x50x50 cm ‐ Additional space for 2 computers (simulators) Software ‐ Operating System: Open source (Linux real‐time), ‐ Programming language: C/C++ (or python, Java) Other ‐ Need for power (~2 kW), cool air (operative temperature range 10 ‐ 25 C, probably OK inside the basement of the dish without air conditioner)



3 Implementation Estimates

The information in this section is to allow the commencement of e.g. Optics, Structure, etc, studies and to provide the Feed and Receiver design groups with upper bounds on the systems to design for PDR.

3.1 FEED PACKAGE ‐ SKA1_mid SKADC Technical Solution Estimates

Assuming an effective f/D = 0.5, the feed suite to be accommodated

Bands Feed Type

Area on Feed

Indexer (dia m)

VolumeFeed Package

(m3)

MassFeed

Package(kg)

Power(W)

Cryogenic Systems

Comment

1(1)(2)

Eleven

1.0

100 2kW(4,5) Cooled LNA

Feed Rotation required

1(1)(2)

Quadridge

1.0

200 2kW(4) Cooled LNA

Feed Rotation Required

2(1)

OBSPF

0.7

0.5 m3 (Length ~1m)

60 2kW(4,5) Cooled OMT and

LNA

3

OBSPF

0.4

0.1 m3 (Length ~1m)

Cooled OMT and

LNA

Common Cyrostat(3)

4

OBSPF

0.2

0.1 m3 (Length ~1m)

100 5kW(4) Cooled OMT and

LNA

5

Horn

0.2

0.1 m3

(Length ~0.5m)

Cooled OMT and

LNA

(1) Bands 1 and 2 are a priority for SKA1 [5, Sect. 8.4.2].

(2) Only one of these feed package types will be part of the final technical solution.

(3) OMT and LNAs for Bands 3, 4 and 5 will be housed in a common cryostat. Feed horns will be external to the common cryostat. The final dimensions will depend on the feed indexer layout.

(4) Estimate includes power consumption of compressor necessary for cryogenic cooling.

(5) When sharing a compressor between multiple cryostats. Compressor availability may require that the compressor rating be slightly larger than the sum of the components.

Table 10 SKADC Feed package – Estimated requirements

3.2 SKA1_Survey SKADC Technical Solution ‐ Estimates

The SKA1 Survey PAF system will be architecturally and physically separated into at least two major parts (or sub‐systems). A portion of the system will be located at the focus of the antenna. Assuming an effective f/D = 0.5, the feed suite to be accommodated



3.2.1 SKADC Technical Solution ‐ Antenna Sub‐System(s)

The estimated requirements of the antenna based sub‐system(s) of the SKA receiver system are detailed in Table 11. The estimate provided is for the implementation detailed in Section 2.6 and are indicative only.

Bands PAF Beams Area on Feed

Indexer (m2)

Package Depth (m)

Massat Focus (kg)

Power(W)

Support Systems

Comment

1

36

(critically sampled at 0.9GHz)

3.0m dia

0.5

Dry Air

2(1)

36

(critically sampled at 1.7GHz)

1.5m dia

0.5 200 Dry Air

3

36

(critically sampled at ~4GHz)

0.7m dia

0.1 100 Dry Air

(1) Band 2 is a priority for SKA1.

Table 11 SKADC PAF Feed package – Estimated requirements

3.3 RECEIVER PACKAGE ‐ SKA1_mid SKADC Technical Solution Estimates

The SKA receiver system will be architecturally and physically separated into at least two major parts (or sub‐systems). A portion of the system will be located as close as practicable to the feed system, i.e. at the focus of the antenna.

Figure 8 Outline of SKA1_mid Feed/LNA and receiver system.

The SKA dish array receiver system will have overlap with and will be influenced by:

• Signal Transport, and • Timing and Synchronisation.



3.3.1 SKADC Technical Solution ‐ Antenna Sub‐System(s)

The estimated requirements of the antenna based sub‐system(s) of the SKA receiver system are detailed in Table 12. The estimate provided is for the implementation detailed in Section 2.6 and is indicative only. Band Frequency

Range (GHz)

Volume (litres)

Mass(kg)

Power(W)

Mount Options

Support Systems

Comment

1(1) 0.35 – 1.05

2(1) 0.95 – 1.76

30 (0.3m x 0.3m x 0.2m)

30(3) 200(3) Focus Dish

Pedestal

Cooling Temp Control Dry Air Supply

RFI Shielding required

3 1.65 – 3.05

Same enclosure as Band 1‐3

4(2) 2.8 – 5.18 10(3) 50(3) FocusDish

Pedestal

Same enclosure as Band 1‐3


5a 4.6 – 7.6 30

(0.3m x 0.3m x 0.2m)

5b 7.2 – 9.7

5c 9.2 – 11.8 30(3) 150(3) FocusDish

Pedestal

CoolingTemp Control Dry Air Supply


5d 11.3 – 13.8

(1) Bands 1 and 2 are a priority for SKA1 [5, Sect. 8.4.2] but the receiver system for band 3 is also implemented as part of the technical solution.

(2) Although these bands are not a priority the technical solution proposed is capable of supporting Band 3 and with Band 4.

(3) Mass and power consumption are estimates only. They do not include elements of synchronisation and timing (e.g. clock generation, terminal equipment), reference distribution (e.g. LO cleanup loop or LO source), or power regulation (e.g. DC supplies and regulators). In some implementations these components may be integrated into the receiver package; if so, the receiver package will have an additional weight and power requirement.

Table 12 SKADC Receiver antenna sub‐systems – Estimated requirements



3.4 RECEIVER PACKAGE ‐ SKA1_mid SKADC Technical Solution Antenna Based

Support Systems Estimates

There are a number of sub‐systems that support the feed packages and the receiver systems that will need to be physically located on the antenna. These may be co‐located with the receiver systems, in a separate enclosure, in a different location on the antenna, or integrated into the same enclosure as the receiver system.

System(s) Volume

(litres) Mass(kg)

Power(W)

Mount Options

Support Systems

Comment

Mains Distribution and Control

30 50 Pedestal RFI Shielding required

Cooling 50 1000 Pedestal RFI Shieldingrequired

DC Power Supplies(1)

200 (0.5m x 0.5m x 0.8m)

50 500 Focus Dish

Pedestal Cooling


Local Oscillator(1)

15 (0.5m x 0.05m x .6m)

10 60 Focus Dish

Pedestal



Sampler Clock(1)

15 (0.5m x 0.05m x 0.6m)

10 100 Focus Dish

Pedestal



Control and Monitor

Hardware(1)

3 (0.5m x 0.03m x 0.2m)

1 10 Focus

Pedestal Cooling


(1) Support equipment such as reference distribution, clock generation or power supplies may be integrated into the receiver package. In this case there will be an additional weight and power requirement for the receiver package.

Table 13 SKADC Receiver technical solution antenna based support systems – Estimated requirements



4 References

Doc # Title Official SKAO DMS reference

1) SKA Request for Proposals (‘header’ document) SKA‐TEL.OFF.RFP‐SKO‐RFP‐001

2) Statement of Work for the Study, Prototyping and Preliminary Design of an SKA Element SKA‐TEL.OFF.SOW‐SKO‐SOW‐001

3) Statement of Work for the Study, Prototyping and Preliminary Design of an SKA Advanced Instrumentation Programme Technology SKA‐TEL.OFF.AIP‐SKO‐SOW‐001

4) SKA Pre‐Construction Top Level WBS SKA‐TEL.OFF.WBS‐SKO‐WBS‐001

5) SKA‐1 System Baseline Design SKA‐TEL.SKO‐DD‐001

6) The Square Kilometre Array Design Reference Mission: SKA Phase 1 SCI‐002.010.020‐DRM‐002

7) SKA System Engineering Management Plan SKA‐TEL.SE.SKAO‐MP‐001

8) The Square Kilometre Array Intellectual Property Policy n/a

9) Draft Consortium Agreement n/a

10) Document Requirements Descriptions SKA‐TEL.SE.SKO‐DRD‐001

11) SKA Document Management Plan SKA‐TEL.OFF.MGT‐SKO‐MP‐001

12) SKA Product Assurance & Safety Plan SKA‐OFF.PAQA‐SKO‐QP‐001

13) Change Management Procedure SKA‐TEL.SE.CONF‐SKO‐PR‐001

14) SKA Interface Management Plan SKA‐TEL.SE.INTERF‐SKO‐MP‐001

15) M. Pantaleev, J. Yin, M. Ivashina, J. Conway, “Final Report of the Eleven Feed Project: Development of Broadband Cryogenic Frontend Prototype for the SKA”, SKA Memo 144, May 2012.

16) N. Roddis, R. Rayet, “Design approaches for cryogenically cooling receiver front ends for the SKA Dish Array”, REP/1304/2774, Callisto Limited, Mar 7, 2012.



Appendix A1

The SKAO RfP Baseline does not detail the optical design of the dish. As this is a critical component that is required ahead of the full mechanical design of the dish, feeds etc., Appendix A1 sets out the analyses that will be used by SKADC to derive the SKA dish optics design. The process described in this Appendix was written by Isak Theron, Marianna Ivashina and Robert Lehmensiek.

A1.1. Introduction – Designing the SKA optics

This section describes the process to derive the SKA optics. The outcome of this process will be both the dish optics and optimised feeds (the illuminating part only, not fully packaged feed and LNA systems) for each of the respective bands.

A1.2. Optics parameters and options

The SKA baseline design [A1.1] uses offset Gregorian dishes and this proposal limits itself to such dishes. The basic offset Gregorian dual reflector system can be described in terms of six parameters. If the Mizugutch condition for optimal cross‐polarisation [A1.2] is satisfied, this reduces to five. This document uses one of the sets defined by Granet [A1.3] (illustrated in Figure A1‐1):

Parameter Description

Main reflector diameter, projected in direction of main beam.

Offset angle of the centre of the main reflector with respect to a symmetrical paraboloid. For

the orientation in Figure A1‐1, is negative.

Distance from the angular centre of the sub‐reflector (relative to the feed point) to the

secondary focus.

Half the angular width of the sub‐reflector as measured from the feed, i.e. the feed half‐angle.

Angle between the optical axis and the line passing through the two foci of the sub‐reflector

ellipsoid.



Figure A1‐1: Definition of the parameters describing the offset Gregorian dishes, shown on the symmetry plane cut.

The global axes are defined such that the z‐axis is parallel to the optical axis and the plane of symmetry is the xz‐plane.

The projected main reflector diameter, , is specified in [A1.1], leaving four open parameters.

In addition to the design parameters, the reflectors can be shaped to improve the illumination efficiency; and spill‐over shielding can be added to reduce spill‐over noise. The extension of the sub‐reflector to shield spill‐over is defined by the addition from P2 to P3 in Figure A1‐1. The typical (geometrical optics) sub‐reflector is derived by cutting the sub‐reflector ellipsoid on a plane normal to the symmetry plane and through the points P1 and P2. The extension is obtained by increasing the feed edge angle with an angle to intersect the ellipse at point P3. The sub‐reflector with extension is now derived by cutting the ellipsoid on a plane normal to the symmetry plane and through the points P1 and P3. Finally, the beam can be tipped from zenith in the plane of symmetry such that the feed moves closer to the ground (feed down) or higher up (feed up); or it can be tipped normal to the plane of symmetry (the so‐called cradle mount).

The purpose of this report is to determine the four remaining dish parameters and to determine the best combination of the additional options given the restrictions in Section A1.5. The selection of these options will require a trade‐off between the system’s performance and the mechanical complexity / cost.

A1.3. Initial reduction of the parameters

The diverse nature of the SKA requirements results in a number of different classes of feeds. The dish parameter that primarily determines how well a given feed would perform on that dish is the feed half‐angle, . Since some classes of feeds can be tailored to a much wider range of feed angles than others, it is not

possible to start with a collection of feed patterns and to design the “best compromise” optics for all of these.



The proposal is thus to select a number of dish designs (combinations of the four open parameters) and optimise feeds for each of these.1 This step is done initially with unshaped optics. The performance is then determined for feed up without spill‐over shielding2 as well as feed down with spill‐over shielding3 (the sub‐reflector is extended with χ = 20° as shown in Figure A1‐1). It is proposed to ignore the cradle mount as none of demonstrators used such a mount – it would therefore be a higher risk option. Since the sub‐reflector extension effectively increases the sub‐reflector, each feed down option uses a slightly different parameter set so that the selected size applies to the extended sub‐reflector. This results in a doubling of the number of designs.

For Phased Array Feeds (PAFs) it may be necessary to extend the sub‐reflector to avoid vignetting and the subsequent loss in efficiency (for 6 by 6 critically sampled beams, the scan angle at the 650 MHz lowest frequency of the Band 2 PAF may be as much as 5°). Thus the PAF designers may elect to use the extended sub‐reflector models also when tipping feed up. (This would require the centre beam to be slightly offset from the optical axis to use the extension symmetrically.) The extended sub‐reflector does not work well with single pixel feeds, hence SKA1‐mid and SKA1‐survey will have slightly different dishes if feed up is selected and the PAF designers need the extension. The two dishes will tip in the same direction and use the same main reflector, but the sub‐reflector may be slightly different.

The different designs are then compared in terms of electromagnetic performance, mechanical complexity and cost. This process will be done by human intelligence, rather than a score‐card type system. Here larger weight will be given to the octave band feeds, but they are expected to perform well with most of the dish designs. The outcome of this will be an unshaped dish design and a set of optimal feeds for this optics design.

The next step is to investigate how much improvement can be achieved by shaping the reflectors of the selected design using the optimised feed patterns. The shaping may be compromised by trying to allow a wide range of feeds, but the feed patterns are expected to be relatively similar as they were optimised for the same geometry. Analysing the final performance of all the feeds on the shaped optics will then allow an intelligent decision regarding shaping the system or not.

If the system is shaped, the different feeds may be optimised one last time for the final geometry. It should also be noted that the “feed up” / “feed down” decision may have to be revisited if the system is shaped, as shaping will affect the ground spill‐over.

It is important to note that the modelling of the system at Band 1 (300 MHz – 1050 MHz) should be done through a rigorous full‐wave approach so as to take into account the near‐field and feed‐dish interaction effects. It is expected that these effects will play an important role in the prediction of the antenna cross‐polarisation and side‐lobe levels; also some minor reduction of the antenna efficiency with respect to that obtained with high‐frequency asymptotic approaches. For the modelling at Band 2 (950 MHz – 1760 MHz); a hybrid approach can be accurate enough, as the near‐field and interaction effects can be ignored at the main reflector–sub‐reflector level. It is suggested to use a multilevel fast multipole method (MLFMM) technique (such as in FEKO) for Band 1 and a combined method of moments–physical optics (MoM‐PO) approach or similar for Band 2 (for example, GRASP + MoM solver). Cross‐validation tests with different software tools that are currently in use by the partners within the Dish consortium should be planned for a small sub‐set of design options. The higher frequency bands can be modelled using high frequency approximations.

1 For this optimisation, only the feed pattern is important and the feed excitation should be as simple as possible. For example, horn feeds should be excited with a simple waveguide mode. 2 In the feed up configuration, spill‐over shielding has to be done on the main reflector. This requires large costly surfaces. In all probability this would result in an over‐sized under‐illuminated main reflector. 3 One of the advantages of the feed down configuration is that spill‐over can be efficiently reduced with a simple extension of the sub‐reflector.



A1.4. Selection of the dish design sets

The most important parameter determining the feed dish interaction is the feed half‐angle, , and it is proposed to cover a range of values by selecting = 58° (effective F/D approximately 0.45), = 53° (effective F/D just over 0.5) and = 49° (effective F/D just over 0.55)4. Note that = 49° will most likely require a considerably larger PAF for the same field of view. The PAF designers need to comment on the cost driving aspects of the dish design. The implications of shaping the dish optics with respect to PAF size will also be considered during the optics down‐select phase.

The sub‐reflector size determines the performance at the low frequency end. Analysis of the MeerKAT optics suggests that 6 m sub‐reflectors5 may allow operation down to 300 MHz with relatively little performance degradation. (This size limit allows road transport of the entire sub‐reflector – although not by standard transport.) Since a certain level of performance degradation can be accepted at the low frequency end, it is proposed to consider sub‐reflector sizes of 4 m, 5 m and 6 m. The sub‐reflector size is determined by Ls, but the relationship is influenced by the other parameters. Hence the exact values are only listed in the set of designs in Section A1.7. The feeds for bands 2 to 5 need only be optimised for the 5 m sub‐reflector, but the full performance analysis must be done for all three sizes. (If there is no difference between the 4 m and 5 m sub‐reflectors, the 6 m one may be skipped.) For Band 1, the electrically small separation between the feed and sub‐reflector may require different optimisations for the three sub‐reflector sizes. If the final performance of the low frequency feed does not meet the requirement, a larger sub‐reflector will be considered. This should, however, not influence the optimum configuration for any of the higher frequency feeds.

One parameter which is not expected to have a significant influence on the dish performance is the offset angle . (It will influence the performance on a fine frequency resolution, but the average performance will be the same.) This is therefore selected for each design such that there is approximately a 0.5 m clearance between the projections of the main reflector and sub‐reflector along the optical axis.

The last parameter controls the main dish size. From MeerKAT experience, this is expected to have very little influence on the optical performance. Hence only one size6 is considered for this analysis – 15 x 18.2 m (approximately as used by both the DVA‐1 and DVC designs; MeerKAT has a flatter main reflector). This parameter may be modified during the final mechanical design.

A1.5. Restrictions

The mechanical option designs should produce limits on the feed aperture diameter which will influence the achievable field‐of‐view (FoV) for PAFs. This value should be approximately 1 m to accommodate the single pixel feeds. For SKA1_survey, it may be larger. There may also be distinct limits on the physical size (including the cryostats / support hardware length) for each of the different frequency bands.

The polarisation requirement is currently assumed to be dual linear, but this will be revisited during the SKA.TEL.DSH.SE.REQ task.

A1.6. Determine the evaluation criteria

The final evaluation criteria will be determined in consultation with the SKA office during the SKA.TEL.DSH.SE.REQ task.

4 The geometry is defined in terms of the primary parameters to exactly the precision given here – working backwards from the equivalent focal ratios will result in slightly different values. 5 The sub‐reflector size is specified as the distance ǁP1P2ǁ in Figure A1‐1. 6 The main reflector is specified as Dm x ǁQ1Q2ǁ as shown in Figure A1‐1.



Validation of the feed designs at the SKA dish optics will be carried out through numerical simulations and modelling of the key performance parameters of the radio telescope for a set of test scenarios for observations (scientific cases). To develop the set of these standard test scenarios, with respect to which the different design options should be compared, we will use inputs from SKA Office during the SKA.TEL.DSH.SE.REQ task. The key performance parameters to be considered will include the following classes:

(i) ‘Engineering performance measures’ – such as receiving sensitivity (Aeff/Tsys), the side‐lobe levels and polarimetric figures of merit (e.g. cross‐polarization levels, conditioning of the Jones matrix or IXR) – over the entire frequency band and field of view coverage. To determine these measures, dedicated electromagnetic‐microwave software tools that can model the signal and noise performance of the integrated optics‐feed‐receiver system in their entirety will be used. To assure a uniform methodology for the analysis of the receiving sensitivity for all feed designs (including both single‐ and multi‐port antenna feeds), ‘standard’ definitions of generalized signal‐noise terms for active receiving (array) antennas [A1.4] to [A1.6] will be adopted (as detailed below). Analysis of the sensitivity ripple over frequency (that occurs due to electromagnetic interaction between the primary reflector and sub‐reflector) will require a dedicated effort in terms of numerical modelling and computation time, especially for the low frequency feeds.

(ii) Top‐level performance measures, as used by astronomers – such as dynamic range, image fidelity, survey speed – over the entire frequency band and field of view coverage. These measures will be derived from simulations in dedicated radio astronomic calibration‐imaging software tools (e.g. MeqTrees) that can model the image maps for different observation scenarios (including the important calibration effects), based on the given antenna beams (obtained with the electromagnetic tools), and models of the sky & interferometer configuration to be considered for the SKA. In addition to the numerical simulations of the maps, we will assess the quality of the beam shapes with the low‐level performance measures of the antenna calibratability – such as the beam roundness and the number of the beam model calibration parameters. For this analysis, we will adopt analytic and physics‐based beam calibration models [A1.9] to [A1.11]. These indirect measures of the imaging quality will allow for the fast analysis of the feed design options and their co‐optimization with the optics.

The primary figure of merit is the sensitivity of the radio telescope, generally defined as Ae/Tsys. It is suggested to adopt the Ae/Tsys definition in Eq. 16 of [A1.4]

7; as this definition is based on the generalized noise‐based terms for active receiving (array) antennas that have been recently approved by the IEEE Antenna Standards Working Group for their inclusion in the next update of the IEEE standard for antenna terms [A1.5].

1 / ,

where one needs to determine the aperture efficiency ap, radiation efficiency rad, physical temperature of the antenna Tp (Tp = 300 K for non‐cooled receivers), noise temperature due to the external noise sources Text

and the noise mismatch efficiency of the receiver mis.

For large aperture‐type antennas, the aperture efficiency ap can be derived from the directivity of the antenna in the direction of the signal of interest and the physical projected area of the antenna in a plane transverse to the signal arrival direction.

4

The radiation efficiency rad is a combined effect of the dish efficiency and antenna feed efficiency due to ohmic losses.

The Tmin/mis quantifies the noise contribution of the receiver due to its minimum noise temperature as well as the impedance noise mismatch at the ports of the antenna feed. If the receivers and antenna ports are ideally

7 It still needs to be determined if it is possible to split this sufficiently for all classes of feeds.



matched (that is when the optimal source reflection coefficient parameter for each LNA is equal to the active

reflection coefficient at the corresponding array element port) the noise matching efficiency mis is unity. In [A1.4], two equivalent formulations of this noise mismatch contribution are presented: one formulation is based on the active reflection coefficient concept and the other one uses the isotropic noise response. When a single port‐feed is considered this formulation reduces to the classical formula for two‐port receivers:

4

1 1 | |

where , , are the noise parameters of a two‐port, Tmin is the minimum noise temperature of the

LNA (if its optimal source reflection coefficient opt is equal to the active reflection coefficient at the antenna

port act).

The Text represents the combined noise temperature contribution due to the external noise sources (thermal ground noise and sky),. This is derived by integrating the antenna pattern multiplied with the ground / sky brightness. (Reference for the complete Tsky model can be found in [A1.7]). For the purpose of our analysis, we need to determine consistent brightness and integration techniques for all feed developers. Text must be calculated as a function of tipping angle for a number of frequencies; and as a function of frequency for a number of tipping angles.

For the evaluation of the polarisation performance, it is suggested to use the so‐called intrinsic cross‐polarization ratio (IXR) as proposed by Carozzi [A1.8],8

IXR11

where κ(J) is the condition number of the Jones matrix of the polarimeter. The IXR measures the orthogonality between channels and also accounts for differential channel gains. The IXR can be understood as the worst‐case cross‐polarization ratio of a given polarimeter before calibration, and it is closely related to the total relative error of the fully calibrated polarimeter.

It is still required to specify requirements / limits on the beam roundness and its calibratability; and the maximum allowable ripple of the sensitivity, antenna feed impedance as well as the scattering parameters of the LNAs due to the multiples scattering and interaction effects between the sub‐reflector, feed and primary reflector. The analysis frequencies should be spaced close enough to give a smooth response of the fine‐scale detail caused by the interaction of sub‐reflector diffraction with the main beam. (For MeerKAT in L‐band, this required a spacing of no more than a couple of MHz.)

More complex is the interaction with imaging and dynamic range – this may require some MeqTrees modelling which must still be defined.

This final performance qualification plan will be determined during the SKA.TEL.DSH.SE.REQ task. The consortium should also make some arrangement to allow comparing representative analysis of each type of feed at one or more single locations.

For PAFs, the calculations should be done for the centre beam as well as for the maximum scan angle in each offset direction.

A1.7. Optics design candidates

The different optics candidates are summarised in Table A1‐1 and the figures following the table. Note that the extended sub‐reflectors are deeper and smaller in width than the standard ones. (This is due to the fact

8 This is still under discussion as most antenna engineers are not familiar with this definition.



that they are reduced so that with the extension they will have the specified size. The reduction also reduces the width that is not extended.)

The top line in each of the figures is the parameters determining that geometry. These are specified with limited precision such that the reflectors may not have exactly the target sizes. Since any combination of these parameters will give a valid geometry, these exact parameters determine each of the optics designs (as opposed to values optimised to give the exact reflector target sizes). The name for each optics design is given at the start of the parameter line for that figure.

The mechanical description needs all 6 degrees of freedom, hence the figures also list some derived quantities, most notably the focal depth F of the main reflector (which should not be confused with the equivalent focal length of the system as determined from θe). These parameters are given with more precision as they need to match the design description.

The horn in the images has a 1 m diameter and is representative of the largest single pixel feed (the Band 1 quad‐ridge feed horn option).

The naming convention allocates an integer index to any target parameter, such as the main‐ or sub‐reflector sizes or the feed angle. This allows adding additional options that may be required. The U denotes unshaped optics. (Shaped models will later be demarcated with Sh where h will be an integer counter indicating the amount of shaping.) The feed up variations do not have an extension (demarcated “EX0”) while the feed down variations all have a 20° extension (demarcated “EX1”). The sub‐reflector is defined as an ellipsoid with the origin and feed point as its focal points and eccentricity e as indicated in the figures. This is then split in a plane normal to the plane of symmetry and through P1 and P2 or P3 (for feed up and feed down respectively).

i j m n θe [°] ǁP1P2ǁ [m] ǁQ1Q2ǁ [m] χ [°]

1 1 1 0 58 4 18.2 0

1 2 1 0 58 5 18.2 0

1 3 1 0 58 6 18.2 0

2 1 1 0 53 4 18.2 0

2 2 1 0 53 5 18.2 0

2 3 1 0 53 6 18.2 0

3 1 1 0 49 4 18.2 0

3 2 1 0 49 5 18.2 0

3 3 1 0 49 6 18.2 0

i j m n θe [°] ǁP1P3ǁ [m] ǁQ1Q2ǁ [m] χ [°]

1 1 1 1 58 4 18.2 20

1 2 1 1 58 5 18.2 20

1 3 1 1 58 6 18.2 20

2 1 1 1 53 4 18.2 20

2 2 1 1 53 5 18.2 20

2 3 1 1 53 6 18.2 20

3 1 1 1 49 4 18.2 20

3 2 1 1 49 5 18.2 20

3 3 1 1 49 6 18.2 20

Table A1‐1 Dish design options with target reflector sizes for each design.



File Name Convention: SKA_U_TEi_SRj_MRm_EXn.



Figure A1‐2: Schematics of the selected optics design sets.



A1.8. References for Appendix A1 – Designing the SKA optics

[A1.1] SKA‐1 System Baseline Design, SKA document number SKA‐TEL.SKO‐DD‐001, 2013‐03‐12.

[A1.2] Y. Mizugutch, M. Akagawa and H. Yokoi, “Offset dual reflector antenna,” IEEE APS International

Symposium Digest, October 1976, pp. 1–5.

[A1.3] C. Granet, “Designing classical offset Cassegrain or Gregorian dual‐reflector antennas from

combinations of prescribed geometric parameters,” IEEE Antennas and Propagation Magazine,

vol. 44, no. 3, June 2002, pp. 114–123.

[A1.4] K. F. Warnick, M. Ivashina, R. Maaskant, and B. Woestenburg, “Unified Definitions of Efficiencies and

System Noise Temperature for Receiving Antenna Arrays,” IEEE Trans. on Antennas and Propagation,

Vol.58, Issue 6, pp. 2121 – 2125, June, 2010.

[A1.5] K. Warnick, M. Ivashina, R. Maaskant, and B. Woestenburg, “Noise‐Based Antenna Terms for Active

Receiving Arrays,” Antennas and Propagation Symposium, (invited) July 8‐14, 2012, Chicago, July

2012.

[A1.6] M.V. Ivashina, R. Maaskant and B. Woestenburg, “Equivalent System Representation to Model the

Beam Sensitivity of Receiving Antenna Arrays,” IEEE Antennas Wireless Propagation Letter (AWPL), pp.

733‐737, Oct. 2008.

[A1.7] German Cortes. Medellin, Antenna Noise Temperature Calculation, SKA Memo 95.

[A1.8] T. D. Carozzi and G. Woan, “A Fundamental Figure of Merit for Radio Polarimeters,” IEEE Transactions

on Antennas and Propagation, vol. 59, no. 6, pp. 2058– 2065, Jun. 2011.

[A1.9] R. Maaskant, M. V. Ivashina, S. J. Wijnholds, and K. F. Warnick, “Efficient Prediction of Array Element

Patterns Using Physics‐Based Expansions and a Single Far‐Field Measurement, IEEE Trans. Antennas

and Propagation, vol. 60, no.8, pp. 3614—3621, Aug. 2012.

[A1.10] A. Young, R. Maaskant, M. Ivashina, and D. Davidson, “Performance Evaluation of Far‐Field Patterns

for Radio Astronomy Applications Through the Jacobi‐Bessel Series,” International Conference on

Electromagnetics in Advanced Applications, (invited) Cape Town, South Africa, 2‐7 Sept. 2012, pp. 884‐

887.

[A1.11] A. Young, R. Maaskant, M. V. Ivashina, D. I. L. de Villiers, and D. B. Davidson, “Accurate Beam

Prediction Through Characteristic Basis Function Patterns for the MeerKAT/SKA Radio Telescope

Antenna”, IEEE Trans. Antennas and Propagation, Vol.61, Issue 5, pp. 2466 – 2473, May, 2013.

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