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Authors: John D. Bunton Carole A. Jackson Elaine M. Sadler

CSIRO Telecommunications and Industrial Physics RSAA, Australian National University School of Physics, University of Sydney

Submitted to the SKA Engineering and Management Team by The Executive Secretary Australian SKA Consortium Committee PO Box 76, Epping, NSW 1710, Australia June 2002
Updated 11 July 2002

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Table of contents
Table of contents ................................................................................................................. 2 Executive summary ............................................................................................................. 5 1 Introduction ..................................................................................................................6 1.1 Relative Costs: the best of both worlds ................................................................. 7 1.2 Instrument Overview............................................................................................. 7 2 Science Drivers .......................................................................................................... 10 2.1 Milky Way and local galaxies ............................................................................. 10 2.2 Transient phenomena .......................................................................................... 10 2.3 Early Universe and large-scale structure ............................................................ 11 2.4 Galaxy formation ................................................................................................ 11 2.5 Active Galactic Nuclei and supermassive black holes........................................ 12 2.6 Life cycles of stars .............................................................................................. 12 2.7 Solar system and planetary science ..................................................................... 12 2.8 Intergalactic medium........................................................................................... 12 2.9 Spacecraft tracking .............................................................................................. 12 2.10Multibeaming ...................................................................................................... 13 3 Preferred Array Configuration ................................................................................... 13 4 Antenna solution ........................................................................................................ 16 4.1 Offset versus centre feed ..................................................................................... 17 4.2 Antenna station design ........................................................................................ 19 5 RF systems ................................................................................................................. 19 6 Signal encoding/transport and beamforming ............................................................. 20 7 Signal processing ....................................................................................................... 21 8 Data Management ...................................................................................................... 22 9 Array Control, Diagnostics and Monitoring .............................................................. 22 10 Pivotal technologies ................................................................................................... 22 11 Proposed SKA location .............................................................................................. 23 12 Representative system performance and cost estimates............................................. 23 12.1Cost ..................................................................................................................... 24 13 SKA Molonglo Prototype (SKAMP) Concept demonstrator..................................... 26 14 Synergies with other SKA concepts ........................................................................... 27 15 Upgrade paths for the SKA ........................................................................................ 27 16 Acknowledgments ...................................................................................................... 27 17 Update History ........................................................................................................... 28 18 References .................................................................................................................. 28

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Appendix A ....................................................................................................................... Costing of Existing Telescopes .................................................................................. Appendix B ....................................................................................................................... Historical Perspective................................................................................................. B.1 SKA Perspective.................................................................................................. B.2 Conclusion ........................................................................................................... Appendix C ....................................................................................................................... Cost of an Imaging Correlator for the SKA ............................................................... C.1 Introduction ......................................................................................................... C.2 Reference correlator ............................................................................................ C.3 Filled aperture antenna stations ........................................................................... C.3 Antenna station arrays ......................................................................................... C.4 Conclusion ........................................................................................................... C.5 Beam width and average beam area of an array ................................................. Appendix D ....................................................................................................................... Horizontal Axis Cylindrical Reflector ....................................................................... D.1 Mechanical construction ..................................................................................... D.1.1 Molonglo Maintenance Issues ­ Multiple separate units ............................ D.1.2 Drive options ............................................................................................. D.2 Surface................................................................................................................ D.3 Fan beam rotation with longitude........................................................................ Appendix E........................................................................................................................ Vertical Axis Cylindrical Reflector ........................................................................... E.1 Selecting the Basic Antenna Topology ............................................................... E.2 Construction......................................................................................................... E.2.1 Foundation, track and bogies ....................................................................... E.2.2 Supporting structure ..................................................................................... E.2.3 Reflecting surface and line feed ................................................................... Appendix F ........................................................................................................................ Fixed Cylindrical Reflector ........................................................................................ Appendix G ....................................................................................................................... Surface construction ................................................................................................... G.1 Purlin spacing ...................................................................................................... G.2 Surface material options ...................................................................................... G.3 Surface alignment ................................................................................................ Appendix H ....................................................................................................................... Line feed..................................................................................................................... H.1 Feed elements ...................................................................................................... H.2 LNAs ................................................................................................................... H.3 System Temperature............................................................................................ H.4 RF Beamformer ................................................................................................... H.5 Digital beamformer ............................................................................................. Appendix I ......................................................................................................................... System cost estimates ................................................................................................. I.1 Cost against time................................................................................................... Appendix J......................................................................................................................... 17 Jul 02 Release CSIRO

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SKA Siting and Array Configuration ......................................................................... Appendix K ....................................................................................................................... Aperture Efficiency and Spillover ............................................................................. K.1 End effects ........................................................................................................... K.2 References ........................................................................................................... Appendix L........................................................................................................................ Compliance Table ...................................................................................................... Appendix M....................................................................................................................... Array Configuration to 31km .....................................................................................

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Executive summary
One key to a cost effective SKA, at frequencies above one GigaHertz, is a low-cost reflector or lens. Historical comparisons show that the elimination of one axis of rotation and one degree of curvature of the surface can reduce the cost of cylindrical reflectors by a factor of six when compared to comparable parabolic dish reflectors. However, the feed structure for a cylindrical reflector is costlier, reducing this advantage somewhat. The cylindrical antenna is in effect a marriage between reflector and phased array technology -- focusing is due to the reflector in one direction, while in the orthogonal direction it is due to phasing and beamforming of the feed structure. A cylindrical reflector has some of the advantages of both reflector and phased-array technologies. It provides another degree of freedom compared to pure reflector solutions while still minimising costs. This extra degree of freedom allows a field-of-view of up to 200 square degrees at 1.4 GHz, which is about two orders of magnitude greater than parabolic dishes. Compared to the phased array, the loss of one degree of freedom reduces the field-of-view by the same amount. This reduces the cost of feeds and beamformers by a large factor when compared to a phased array. This allows a cylindrical reflector based SKA to operate at much higher frequencies than would be possible with a phased array based SKA. In this proposal, we envisage a maximum initial operating frequency of 9GHz using 15m by 111m antennas, with the possibility of future upgrades reaching 20-24GHz at reduced sensitivity. The wide reflector sets the lowest operating frequency at 100 MHz, providing an instrument that is a sensitive probe into the Early Universe. High dynamic range is achieved by configuring the antenna as eight separate contiguous units, each of which provides a one-degree beam at 1.4GHz. With an SKA made of 600 antennas a 4800 input correlator is needed. The large number of inputs result in a snapshot dynamic range of ~100,000:1. Multi-frequency synthesis and longer integration times will increase the dynamic range to well over a million to one. For high-speed survey work, the full line feed of each antenna can be beamformed to generate fan beams. The correlator, without any increase in size, can process the 64 fan beams generated in this way, and image eight square degrees in a single snapshot. To keep within cost constraints, the instantaneously accessible sky is limited to 8 onesquare-degree beams lying in a 40 by 1 degree area of the sky at 1.4GHz. For each frequency band that the line feed array covers, a separate pointing is allowed within a 120-degree arc. For the antennas within 10 km of the central core of the array, the signal for all eight beams is brought to the correlator. As distance increases the number is progressively reduced so that at 3000km only a single one-degree beam is processed, again this reduction is necessary to contain costs. The array design itself follows the principle of equal effective area at baseline scales from 1 to 10,000km. For baselines scaling by a factor of three, the resulting effective area at each baseline is 0.47 of the total area. The cost of this concept in current dollars is estimated at: Cost of SKA = 549 + 90*BW + 9.6*BW*b US$M where BW is the bandwidth processed and b is the number of one degree beams. Thus, a 2.4GHz-bandwidth instrument with up to 8 one-degree beams operating at up to 9GHz is about one billion US dollars (2002 prices). 17 Jul 02 Release CSIRO 5


1 Introduction
It is almost universally accepted that two-dimensional phased arrays, such as LOFAR Bregman [1.1], are the optimum technology at low frequencies. At high frequencies, such as those covered by ALMA, fully steerable parabolic reflectors are used. The SKA will observe at frequencies in between these: 100 MHz to 20 GHz. In this range of frequencies, there is no ready consensus as to the best technology. The number of feed elements in two-dimensional phased arrays grows quadratically with frequency. In comparison, the field-of-view of a parabolic reflector diminishes quadratically with frequency. Moreover, reflectors are not cost effective at low frequencies especially if designed with the precision needed at high frequencies. It is unlikely that any single technology can cover the possibly 200:1 or greater frequency range of the SKA and provide the lowest cost solution at all frequencies. Inevitably, there will be some frequency range where an alternative technology would have provided a cheaper solution. Thus, the final SKA implementations will be a complex optimisation in which the tradeoff between planar arrays and steerable dishes is potentially a central issue. This leads to the consideration of cylindrical reflectors1 that exploit the best of the technologies that are optimum at the extremes of the SKA frequency range. Using reflector technology to form the beam in one direction and phased array technology for the orthogonal direction. The field-of-view is restricted when compared to a phased array, but is larger than that of a parabolic reflector. At 1.4GHz the instantaneous fieldof-view of a small parabolic reflector for the SKA is 1 to 4 square degrees. For a cylindrical reflector it can be two orders of magnitude greater: 120 to 240 square degrees. A phased array would be two orders of magnitude greater still, at about 10,000 square degrees. Cylindrical reflectors fall mid-way between parabolic reflectors and phased arrays in terms of field-of-view. The cylindrical reflector provides an antenna with enhanced multibeaming capabilities when compared to a parabolic reflector and is economical at frequencies much higher than those practical for a phased array. The proposed design is for a cylindrical reflector of width 15m with a surface good to 12GHz. The initial line feed would set the upper frequency limit to 9GHz. Upgrades to 12GHz and beyond would be attractive especially if future developments provide low-noise uncooled LNAs at these frequencies. Aperture efficiency beyond 12 GHz will be lower, but operation at frequencies as high as 24GHz may nonetheless be possible. With cylindrical reflectors, antenna stations can be built as a single long reflector. This is no costlier than multiple reflectors and allows the beam area to be maximised for a given antenna station effective area. Maximising the beam area reduces the number of beams needed to image a given area and this results in reduced signal transport and correlator costs.

For brevity cylindrical antenna or reflector will refer to an antenna or reflector whose surface is in the form of a cylindrical paraboloid. 17 Jul 02 Release CSIRO 6

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1.1 Relative Costs: the best of both worlds The cost of a planar phased array is driven by the number of elements, which is proportional to the square of the maximum operating frequency. For the line feed of a cylindrical reflector the technology is similar but the costs scale directly with frequency. Thus when compared to a phased array SKA the line feed costs of a cylindrical reflector are small. The cost of the reflector must then be added to this. The reflector component of a cylindrical antenna is less costly than a parabolic reflector. An estimate of the cost reduction can be found by comparing the cost of existing radio telescopes. For antennas of similar dimensions and maximum frequency, a Molonglostyle cylindrical reflector is six times cheaper per unit area than one which uses the technology adopted in the Lovell, Parkes, Effelsberg or GBT reflectors (see Appendix A for details of this calculation). This arises because one axis of rotation is eliminated, construction takes place at or near ground level and the surface is curved in only one direction. For the SKA, we estimate that cylindrical reflectors based on Molonglo construction techniques would cost about US$300M, excluding site acquisition, with a surface that is good for frequencies up to 12GHz. The line feed, uncooled LNAs and initial RF beamformers will approximate double this cost. As cylindrical reflectors have not been popular for the last 35 years, some have considered it not to be a viable technology. The historical reasons behind this are explored in [Appendix B]. In summary, it is found that beamforming was a major problem that resulted in narrow band systems with high system noise and limited upgradeabilty. But in the SKA beamforming is a problem for most designs. Advances in digital beamforming overcome this problem. This, together with broadband line feeds and high-performance uncooled LNAs, make cylindrical reflectors an increasingly viable option. In the sense that it can provide a wide field-of-view at frequencies up to 10GHz and beyond, it is not only viable but a close to optimal solution to the SKA problem over the 1 to 10GHz frequency range. The major challenge in achieving this goal is the design of line feeds to illuminate the reflector. The line feed can be thought of a one-dimensional phased array and it will be difficult to achieve very wide bandwidths. To cover the required frequency range a number of scaled line feeds will be needed each covering at least a 2:1 frequency range. This will be a major challenge in terms of achieving the required bandwidth, polarisation purity, calibration, noise performance and fabrication cost. Associated with the line feed is the challenge of developing suitable downconverters and digitisers. Here radio-on-achip concepts are the way forward. 1.2 Instrument Overview The SKA implementation proposed here consists of 600 antenna stations, each comprising a single 15m x 111m cylindrical reflector, illustrated below, operating over the frequency range 100MHz to 9GHz. The East-West horizontal axis reflector has the advantage of full declination coverage at transit even though the sky coverage is limited to about 70% of the visible sky. It is proposed that the array configurations cover baselines of 1, 3, 10, 30, 100, 300, 1000, 3000 and 10,000 km. For the whole array the 17 Jul 02 Release CSIRO 7


Aeff/Tsys is 2x104 m2/K and the proposed configuration gives an effective Aeff/Tsys of ~104 m2/K at each resolution. As the antenna station consists of a single filled aperture antenna, there is no trade-off needed between sky coverage and minimum observable elevation, except in the central compact core where the trade-off is between surface brightness sensitivity and minimum elevation. In addition, a filled aperture antenna station maximises the beam area, which in turn minimises correlator and signal transport cost.

Figure 1 Cylindrical reflector antenna A distinguishing feature of the cylindrical reflector are the multiple dual-polarisation line feeds which illuminate the cylindrical reflector. Their cost, in part, offsets savings that are made in the price of the reflector. The line feeds have a 2:1 to 3:1 frequency coverage and a number are needed to cover the full frequency range. Up to three will be active at any one time. The beamforming, Figure 2, for the linefeed is done across ~12m sections of the line feed giving approximately circular one-degree beams at 1.4 GHz.
Dual Pol. Line feed Delay line beam former, 0.3 m section Downconversion and digitisation three 800Mhz bands
LO

12 m section of line feed

Dual Pol. Line feed Delay line beam former, 0.3 m section Downconversion and digitisation three 800Mhz bands
LO

Eight Output per band Digital Beamformer

Digital To Correlator Filter banks

Figure 2 Line feed for a single antenna section showing analogue and digital beamforming. 17 Jul 02 Release CSIRO 8


The outputs from the feed elements of the line feed are amplified with uncooled LNAs and RF beamformed over 0.3m sections using a delay line beamformer. This RF beamforming reduces the arc scanned by the line feed 40o at 1.4 GHz. Over a 12m section of line feed 40 RF signals are generated which are then downconverted and digitised. These 40 signals are then digitally beamformed to generate up to eight beams in each of three 800 MHz bands. The various levels of beamforming are illustrated below, at 1.4GHz the final beam size is one square degree. Digital beamforming, Eight independent onedegree fields within the RF beam

Feed element 120o by 1o FOV

RF beamformed over 0.3m of line feed 40o by 1o FOV (1.4GHz)

Figure 3 Beamforming hierarchy for one 12 m antenna section The beamforming done by the reflector and RF beamformer limit the area of sky that the eight independent one-degree beams can be placed. To fully utilise the available beams automatic queue scheduling of observations is needed. This requires as few as 4,000 targets of interest at 1.4 GHz. The eight independent beams also greatly enhance the surveying capability of the instrument. For high dynamic range imaging in a single one-degree field, the signals from the eight section of each of the 600 antennas in the SKA are correlated. The correlator is a 2.3GHz bandwidth full-Stokes correlator with 4800 inputs giving 11 million correlations. For snapshot imaging, this high correlation count gives a dirty beam with sidelobes levels of about 0.1%. Longer observations together with self calibration and the high redundancy of the array allow dynamic ranges exceeding 106 to be achieved. For other observing modes the one-degree beams from all antenna sections are beamformed and, as the reflector is contiguous, a grating lobe free fan beam can be produced. To cover the area of the eight independent beams 64 fan beams are sufficient. The correlator can form all correlations between antennas for these 64 fan beams to give a survey mode imaging capability of eight square degrees at 1.4GHz. The fan beams arrayed with other antennas are used to target compact sources.

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2 Science Drivers
The wide range of science priorities identified by the SKA Science Working Groups requires that the final telescope provide both a wide frequency range and high sensitivity over a range of angular resolutions. A concept based on cylindrical reflectors addresses the following specifications: · · · · · · · · · · Frequency coverage from 100 MHz to 9 GHz (up to 12 GHz with reduced multibeaming) High spatial resolution at low frequencies Continuous u-v coverage on scales of 10m to 3km Sensitivity independent of resolution (constant effective area) (Very) compact core configuration (specification is 0.4 km2 within 1 km). Electronic beamforming giving rapid access (within milliseconds) to 120 x 1.4/f degrees of sky (f=frequency in GHz), or 8.5% of the accessible sky at 100 MHz Large field-of-view 56/f x 1.4/f degrees of sky independently over a 120-degree arc for each of line feed. Three independent frequency bands each steerable over 120 degrees Multibeaming capabilities: Can observe/image eight fields within each frequency band.

This concept matches nearly all the SKA science specifications and has exceptional advantages for studies of the Early Universe because of its high sensitivity, complete wavelength coverage and excellent spectral response in the frequency range vital to studies of cosmic HI, i.e. 100 MHz to 1.4 GHz. We now discuss the links between the cylindrical reflector concept and the individual science drivers identified in a series of memos from the January 2002 Bologna SKA Workshop (http://www.skatelescope.org/ska_memos.shtml ). 2.1 Milky Way and local galaxies The key science drivers in this area are studies of the ISM, magnetic fields and relativistic electrons. The cylindrical reflector concept is well matched to the specifications laid down by this working group (frequency range 1 to 10 GHz, imaging field-of-view at least 1 deg2, angular resolution 0.1 arcsec at 20cm, surface brightness sensitivity 10 mK/arcsec2). 2.2 Transient phenomena The key science drivers here are surveys of radio transients, Galactic pulsars and SETI. The cylindrical reflector concept meets all the requirements except for frequency coverage in the range 12-15 GHz which the working group states is needed to overcome interstellar scattering of pulsars near the Galactic centre. Galactic centre pulsars will not be observable with an SKA based on the cylindrical reflector concept. However, these observations are high-risk even for an SKA which reaches to 15 or 20 GHz. The steep radio spectrum of pulsars means that their observed 17 Jul 02 Release CSIRO 10


flux density decreases as ~ 1/frequency2, and it is not yet clear that 15 GHz is a high enough frequency to overcome interstellar scattering near the Galactic centre. It may be necessary to go to 20 GHz or even higher, with a correspondingly large increase in the required collecting area. The cylindrical reflector concept has the ability to image up to 8 square degrees simultaneously on shorter baselines, falling to one square deg at VLBI resolutions, making it possible to carry out all-sky surveys at arcsecond resolutions. This survey mode could provide multi-epoch radio sky maps of very large areas of sky, which would be particularly useful in studies of radio transients. For time-critical transient events, the cylindrical reflector concept has the advantage of instantaneous access to 8.5% of the available sky at 100 MHz with electronic beam steering. With a mechanical slew rate of 20 degree/min in either direction the further part of the visible sky that can be accessed, at all frequencies, is 24% after one minute. As a result, this telescope would be able to respond very rapidly to GRB and other transient events. 2.3 Early Universe and large-scale structure The main science driver here is observation of the Epoch of Reionisation (EoR), which requires sensitive spectral-line observations in the range 140-180 MHz. The cylindrical reflector concept meets the specifications of this working group, though we note that it may be possible to provide a substantial increase in the effective collecting area at frequencies below 200 MHz at modest cost (see section 15) if required by the science goals for EoR and high-redshift HI. 2.4 Galaxy formation This working group has identified two key science drivers, sensitive wide-field HI surveys and deep high-resolution radio continuum surveys. These drivers have quite different configuration requirements. The former requires most of the collecting area to be on baselines less than 60 km, while the latter requires significant collecting area on VLBI baselines (~ 3000 km). The cylindrical reflector concept can meet the specifications for the HI surveys outlined by the working group, which include both very deep surveys in redshift space and shallower wide-angle surveys. The concept is also well matched to the specifications for deep radio continuum surveys. The approximately constant 0.5-square-km geometric area at all operating frequencies sets the surface brightness sensitivity. This will allow both the detection of faint objects and high-precision measurement of their positions, which is vital for cross-identification with NGST, ALMA and next-generation optical telescopes. Observations of redshifted CO at 10 - 20 GHz, identified as a lower-priority science driver by the Galaxy Formation working group, would not be possible with the cylindrical reflector concept. However, observations of high-redshift CO emission will

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be possible with the upgraded VLA (eVLA, http://www.aoc.nrao.edu/evla/) and later with ALMA. 2.5 Active Galactic The key science driver in this area holes, which power active galactic well matched to the specifications starburst activity, and to studies of Nuclei and supermassive black holes is the cosmic evolution of the supermassive black nuclei (AGN). The cylindrical reflector concept is needed for detecting the first epoch of AGN and the radio luminosity function and its evolution.

However,