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A.B. Smolders and M.P. van Haarlem (eds.)Perspectives on Radio Astronomy Technologies for Large Antenna ArraysNetherlands Foundation for Research in Astronomy - 1999

THE TECHNOLOGY CHALLENGE FOR THE NEXT GENERATION RADIO

TELESCOPES

ARNOLD VAN ARDENNENFRA/ASTRON

P.O. Box 27990 AA Dwingeloo

The NetherlandsE-mail: [email protected]

1 Introduction

At the occasion of NFRAs 50-th celebration, at the time the Westerbork upgrade is nearing itscompletion and the dawn of this century, it seemed appropriate to orient toward the instruments forradioastronomy that will lead the onset of the next century. The most ambitious and far reachinginstrument now generally become known as the Square Kilometre Array [1], will concentrate on the

science questions made possible by its orders of magnitude increase in sensitivity together with alarge field of view, three decades frequency coverage and large resolution range among others.Generally felt desirable to fully operate in the second decade, the period of up to ten years should beused for setting up the international framework, organise the science and the scientific community,addressing the R & D at the appropriate level and identify the funding schemes. The ScienceConference last week in Amsterdam aimed toward addressing outstanding science issues [2] requiringSKAs capabilities to unravel and the complementary role of radio astronomy in the larger frameworkof astrophysics and instrumental developments in shorter i.e. (sub) mm and optical/IR wavelengthsregimes that will precede SKA.

The Technology Conference held at NFRAs premises in Dwingeloo on the other hand, aim toconcentrate on aspects of immanent importance to the technical realisation of SKA. For this purpose,scientist and technologists from organisations like NFRA, from other knowledge institutions and fromindustries around the world, have been joining these three days while attending a fairly condensed andloaded technology program. Broadly speaking, their presentations covered introductionary talks aboutconcepts, enabling technologies ranging from antennas, (integrated) front-ends, photonics, signalprocessing and packaging trends in electronic Industry to calibration, interference mitigationstrategies and data processing. It not only shows the breadth of nowadays technology and advancedthinking, but aim to indicate the relevance for SKA and the gaps to be closed e.g. requirements andrelevant industrial developments, on the really important challenges in order to bring its realisation astep nearer. No doubt, this conference will be followed by others to keep track of current andemerging technologies vis a vis new insights in astronomy.

Traditionally, the technical evolution of radio astronomy comprised regular upgrades (retrofitting) ofmodular subsystem blocks being part of stable and long lasting observing platforms. Examples ofthese platforms at cm and longer wavelengths are the (recently upgraded) Westerbork and VLAsystems of the 70-th, MERLIN and the VLBA in the 80-th and the GMRT in the 90-th. In parallel,Very Long Baseline Interferometry evolved over several decades with baselines now reaching out asfar as space through an orbiting radiotelescope. Continuous incremental developments of subsystemsi.e. receiving, recording, data processing systems and software resulted in enhanced mapmakingcapabilities e.g. through model based closure relations [3,4]. This now makes possible unpolarizedmaps up to a 100000:1 dynamic range on a routine basis together with calibrated large field surveys[e.g. 5]. As a consequence, ground-based radio-astronomical systems remained state-of- the-art, valuefor money instruments with an approximate 5-10% yearly operational costing envelope of thenationally funded investments of order 100 MUS$ or less.


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With SKA, new roads need to be explored as required by its order of magnitude performanceimprovement. Some of the requirements as set out as a result of the workshop in [6] are shown inTable 1. To achieve these, different system concepts or combinations of these exist [7], each havingtheir own merits. With various levels of national support and organisation, these are topics of active R& D in a number of different countries. NFRA among others is for example pursuing the concept ofthe electronically controlled array antennas [8] for a number of reasons e.g. the multi-beam capabilityand adaptive interference rejection and undoubtedly a slight bias toward this concept, could be notedat this conference.

Sensitivity Aeff/Tsys: 2 x 104m

2/K, Surface Br. 1K@ 0.1(Continuum)

Frequency Range: 0.03 20 GHz, Instant. BW: 0.5+ f/5 GHz,10

4spectr. Channels, 2 widely separated simultaneous Freq. bands

Spatial Coverage: 2Sterad, FOV: 1square deg.@ 1.4 GHz, max. prim.beam separation 100 degree(low freq.) or 1 degree @ 1.4 GHz (highfreq.), Ang. resol. 0.1 @ 1.4 GHz, number of instantaneousBeams: 100

Imaging 108pixels, 10

6dynamic range clean beam @ 1.4 GHz

Polarisation - 40 dB (purity in map)

Table 1:Overview of prominent SKA straw-man requirements (adapted from [6]).

Other issues relate to the location and precise configuration/distribution of the telescope collectingarea the latter being closely related to imaging quality (more stations) and confusion requiring moreandlarger stations (smaller beams) versus resolution. Most of the (combined) requirements of Table 1are difficult to achieve. However, a realistic view combined with optimism will simply point to someof these as subsystem characteristics for which a relatively straightforward road ahead can be definedand specifications be defined. Others, are system level issues that, irrespective of the variousconcepts, are more difficult to translate into a set of engineering specifications.

For example how to achieve sub-microJansky sensitivity levels in images with a large (routinely, saybeyond 1 in a million or better) dynamic range vs. confusion, calibration, polarisation and interferenceissues over a large frequency range of a decade or more. With regard to RFI-mitigation, presentactivities led by the astronomical community are directed toward getting across that frequency spaceis not only a political, regulatory, astronomical or technical issue, but reaches beyond the regulatory-only needs of active and passive users toward an ecological issue of prime cultural importance. Itremains nevertheless essential that SKA R&D efforts are concentrated toward exploiting newtechniques to reduce adverse effects for astronomy. No doubt, implications and results are importantto other communities as well as the use of radio is so much increasing in todays wireless society.

While the different concepts can be elements of programs with national centres of gravity, executed inan atmosphere of friendly competition (cooptition), synergy must be maximised. A lot remains to be

done to co-ordinate all these activities in the international framework and to inform the communityabout our progress. This progress should be directed toward an agreed first level set of requirements,and a concept design and architecture. It is clear that the SKA proposition is an extremely interestingR & D vehicle for educational purposes and holds promise to involve a new generation of specialists.Fortunately, as a non-technical result of this conference, various countries have agreed that anappropriate level of R & D is mandatory to make possible a well thought choice between conceptsaround 2005. Also it formed the basis to a recently erected body i.e. the International SKA Co-ordinating Committee to steer, monitor and facilitate progress.


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2 The concepts as they evolve

At an earlier stage, the Square Kilometre Array, now known as SKA was considered as a singleconcept instrument of a decade bandwidth or more. The collecting area was dispersed over about 30stations mostly distributed in a 30 50 km radius with a number of outstations at 150 km in a Y-

shaped fashion, each having an approximate diameter of 300m [7]. The precise number of thesestations and their geometric arrangement and location are dictated by arguments of imaging, spatialresolution, frequency and brightness sensitivity and hence are elements of parameter space and proneto change. As the number for the newly proposed (sub) mm array ALMA [9] is now set at 64 for thepurpose of good imaging, it is unlikely that the number of stations for SKA remains to be 30. See e.g.[10]. Also, other arrangements may be more optimal see e.g. [20]. The physical realisation of eachstation can be different leading to different concepts. In principle this is because an aperture can besynthesised in a non-unique [11] way in order to realise a desired effective receiving area. This offersthe potential for a near infinite number of solutions but for various reasons including todaysinstruments, the major concepts so far are limited to five or six[8]. Briefly stated, the station conceptsand the major supporters thus far are a large spherical reflector pursued by BAO in China [12,13].Efforts are now concentrating on a first station called FAST of 500 m diameter with adaptive surface,

a number of low cost large reflector of about 20 25 m paraboloids promoted by the Indian NCRA,many mass produced Television Receive Only (TVRO)- like antennas by the US SETI institute. As aresult of recent studies for near future SETI, efforts are now concentrated to the realisation of the 1hectare telescope (1hT) [14 ], a large adaptive reflector (LAR) antenna in which the receiver(s)are located in a teetered balloon above an adaptive ground-based reflector with large F/D by theCanadian DRAO [15], the concept of electrically steerable active array antennas by the Dutch NFRAand combinations of these e.g. hybrid arrays [16] and focal plane arrays by ATNF in Australia

Other solutions are part of active research [17] and more recently, the study of a Luneberg Lens arrayconcept. More advanced possibilities may be different e.g. a tile of the electronic array may berealised as a systems in silicon (system on silicon) or as a 3D frequency selective surface with

electronically or optically controlled reconfigurable dipole elements [28].

20 GHz

LOFAR

.02 0.20 2.0

1hT

M-SKA/ Electr.

Adapt.Array

Indian Reflector

LAR

Large Sp. Refl./FAST

CONCEPTS

Figure 1: Presently studied concepts versus expected optimum frequency range

(GHz).


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At the Sydney workshop, I presented the case [6a] that the three-decade instrument resulting from thescience requirements (see Table 1) could not be based on the same concept and integration oftechnologies. It is more likely that a dedicated instrument of approximately a decade wide bandwidthwould cover each of three possible frequency bands. This would have the advantage that instrumentscould be optimised from different points of view i.e. science, technology, and costs or otherwise whilebeneficial in the international framework. Consequently SKA would be the generic abbreviation ofthe total of the low-, mid-, and high-band instruments.

At the occasions of the ESTEC conference [16] and the SKA technical meeting following the URSI-GA in Lille[18], I pointed out that electrically short active antennas are worth considering and notlimit ourselves to 50 ohm systems for the antenna to amplifier match. This was because electricallyshort active dipoles offer extremely wide frequency coverage as proven by various military andindustrial [eg.19] applications outside astronomy and was investigated for a decade-wide radio-astronomical purpose in a then recent ESA study [20]. In this case the application was at extremelylow frequencies not considered for SKA, but the point was made that the sky noise is much higherthan the receiver noise and given the essentially reactive antenna, voltage rather than power matchingwould offer optimum noise performance. In the context of SKA, Bregman [21 and earlier references]

solidified the argument for a different technology telescope by pointing out that this solution wouldactually be useful up to about say, 200 MHz. The basic argument being that only beyond thatfrequency, the receiver noise starts to dominate the sky noise and below that the sky noise increaseswith a 2.6 power law going to lower frequencies. As the effective area using electrically short active

dipole arrays, scales with wavelength squared i.e. Aeff = c.2, the brightness sensitivity would

nevertheless still improve with a 0.6 power law with increasing frequency.

Interesting enough, recent scientific discussions by part of the astronomical community in Hollandand the US call for a dedicated wide band low frequency array (ALOFAR@) [21] ranging from 15 MHz over 150 MHz. In view of the above, the concept now studied is an electrically short active dipole ina sparse electronically steered array configuration. Indeed, new technical possibilities in the area of

signal processing and in active antenna developments, now allow to divert from classical lowfrequency array approaches as implemented nowadays, sub-GHz array instruments.

As it now turns out, the advanced techniques for LOFAR are partially overlapping with technicalapproaches developed in the context of the mid- range (say from 0.2 - 2 GHz) electronic array R & Dprogram at NFRA. This includes other aspects important to be addressed for the concept e.g. themulti-beaming, calibration and interference issues

From the point of view of the electronic (power matched, possibly not to 50 ohm) array concept, themaximum frequency of the mid-range is about 2 GHz. As before, in this type of antenna the apertureis coherently sampled and hence requires two receiving elements per wavelength. Based onstraightforward arguments of quantities, required power and cost, the concept will necessitate a

prohibitively large number of active antenna elements of over 108beyond this frequency.Furthermore, the effective area associated with a single basic element antenna in this concept shows anegative power law [-0, -2] with increasing frequency. In order to maintain a large effective area overa decade bandwidth, it is therefore desirable to synthesise the aperture with a minimum number ofelements say 2 or 3 [15] each operating at a different frequency range within the total decade. Clearly,a technological challenge then is to find a suitable element arrangement for dual polarisation with apower law vs. frequency close to 0. See e.g. work in [22, 23]. Subsequent studies should model themulti-element arrangement just mentioned (see [24] showing some preliminary work) and to find alow cost architecture that includes the optimum active low noise receiving part.

The Indian paraboloidal reflector antenna, when made cheap as a further development of the GMRT

dishes, will due to the large gaps in the wire mesh primarily be suitable for the low- and mid-bandSKA say from about 50 MHz 1.5 GHz. The large prime focus spherical reflector dish concept is


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suitable for lower frequency operation from 100 MHz but will not be suitable much beyond about 2GHz. The prime focus LAR concept aim to operate from a few hundred up to about 10 GHz andpossibly higher frequencies. On the opposite part of the spectrum, the 1hT dishes due to their smallsize of about 4 5 m probably inhibits useful observations below about 0.5 GHz. Owing to their solidsurface, they are designed to be best suitable to cover the mid- and high band SKA.

All in all, the mainstream effort today can therefore best be summarised as in Figure 1, showing theconcepts and the expected nominal frequency range.

All these efforts aim at a (combined) concept for SKA as a versatile multipurpose astronomicalinstrument roughly characterized by its multi-band (3 decades), multi-beam (>10) capability withimproved dynamic range and imaging capability (100-1000x) and an Aeff/Tsys> 10

4m

2/K. In terms of

keeping track on its operational performance, blind on-line quality assessment will be desirable.

As an example, Figure 2 shows the Aeff/Tsysof hypothetical telescopes implemented in threetechnologies i.e. a low band active dipole array, a mid band electronic adaptive array with threedifferent elements to cover a decade bandwidth and for frequencies beyond 2 GHz, a low noise array

of reflective paraboloids. The figure [also shown in 6a] is based on reasonable assumptions regardingsystem noise versus frequency i.e. Tsys= 800K at 100MHz and decreasing toward higher frequenciesdue to the reduced sky noise to about 100K at 200MHz where the receiver noise and the sky noise areassumed equal. For the midband array, Tsys= 100 at 200MHz and decreasing to 40K at 2 GHz and forthe high band is between 25-30 K beyond 2 GHz up to 10GHz. With regard to the effective areas ofthese instruments, the low band array constitute a square kilometre at 200 MHz and increasinglylarger (power 2 law) toward lower frequencies. For the mid band array, the nominal area is again asquare kilometre but as said, it decreases at increasing frequency ( -1 and 2 power laws are shown).For the high band array, the effective area is assumed constant and only 100.000 m

2which is at

least one order of magnitude larger than any other existing or planned telescope. For example, thenewly planned ALMA (sub)mm array has an effective area of less than 10.000 m

2. When combined

with an assumed 20 K low noise receiving system, the Aeff/Tsysof this hypothetical array is still arespectable 0.4 104m

2/K.

10 GHz.10 1.0

Aeff/Tsys(m2/K)

104

103

Act.dipole

array

Many Small

paraboloids

Electr. Adapt.Array

105

Figure 2:An example of the effective area versus frequency of hypothetical

telescopes Implemented in three technologies [6a]. See the text.


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Together with considerations like expected performance, technology, cost and maximum sciencereturn, maintainability and location/site among others, a mechanism to conclude on the final (choiceof) concept(s) must be set up around 2005. Before that time, these and other as of today newerconcepts, must engage in active R & D programs addressing at least some if not all of the majorchallenges.

3 Challenges

Not only in leading the path toward the realisation of SKA, but also when in use the challenges for theastronomer are many. To mention a few:

1. Achieving (sub)microJansky sensitivity in the calibrated map in spite of image plane andionospehric effects, the effect of beam smoothness and stability versus scanning andadaptivity and the application of new observing modes e.g. short exposures and/or multi-beaming.

2. The presence of much stronger (say order 107) sources in the field of view in spite of

confusion- and self noise requiring multipatch- multi source selfcalibration techniques.3. Achieve an adequate minimum level of radio interference through frequency filtering, spatio-

temporal filtering eg. deterministic nulling.4. Deal with new methods of data handling (non deterministic) eg. automatic feature searches.5. To configure a three decade instrument.

Qualitatively, the astronomical requirements will impact the technical realisation (e.g. architecturaldesign and implementation) for the different system concepts in their own specific and a priori neitheralways obvious nor clearly defined way. A system-level approach is required opening the way toachieving consensus about an optimised system.

For the SKA system designers, the question is therefore how to translate the requirements from Table1 into engineering specifications and subsequent design to cost and specifications. This is to be done

together with additional ones e.g. high dynamic range in fully polarimetric maps given the harsherRFI environment and the increased confusion problem due to the brighter sky. From these follows theneed for generic system descriptions and modelling which includes polarisation. Also the importanceof calibrating the sky and the effects of the ionosphere mostly at lower frequencies and the need to setup a program for RFI - mitigation strategies will effect design parameters throughout the system. Thesimulations should lead to an optimised architectural and functionally integrated design withpredictable, robust and reliable behaviour. This conference will at least touch upon some of theseaspects. See for example the contributions on Calibration and Simulation- and Data processingtechniques. Worth mentioning, is the international AIPS++ activity that among other essentialfunctionalitys, aims at suitably modelling essential parameters including ionosphere through theMeasurement Equation also essential to calibrating the instrument[33]. This activity when suitablydirected for the purpose of SKA perhaps at a later stage may well prove a prime example of directedinternational co-operation.

There is also an issue here about the availability of simulation tools of such a completed system,being dependent on the telescopes conceptual approach. Over time, the approach should reflect therapid pace of technology change and consider the modularity vs. upgradability issue. We mayconclude that depending on the concept, SKA will be build as a non-upgradable instrument for a finitelifetime of say 10-15 years. Although contrasting todays realities, this may politically prove thecorrect approach while at the same time probably easing the technical project aspects.

The issue of dual band polarised receiving systems over a wide decade bandwidth taking the systemnoise into account is not trivial. For paraboloidal or spherical reflectors, this comes closest to

extrapolating performance of todays wide band systems and at least intuitively, is likely to be doablefrom the technology maturity point of view. These systems have the added advantages that with an


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appropriate feed design, the collecting area is effectively constant and even although more than onefeed may be required for optimal performance, the problems are embedded in a single advancedreceiver per telescope within a traditional mechanical platform. This is contrasting the concept ofelectronically scanned aperture arrays. Although fully in he spotlight for the purpose of military andtelecom e.g. base-station applications in general, the emphasis remains largely on technology and theissues are about identifying enabling technologies, their trends and the integration of technologies andfunctions.

Furthermore, assuming that the project investment as a whole should not exceed todays 600MUS$limit, the cost per square meter is required to be of order a few hundred US$ or less. This is to over anorder of magnitude lower than paraboloidal reflectors commonly in use now and a few times less thanthe GMRT. Hence, the costing issue should be intrinsically part of the design and development effort.With regard to the electronic array concept new to radio astronomy, a priori experience while learningas we go, required to be addressed as soon as possible. Hence, NFRA embarked on an extensiveinitial technical R & D program [25] which is about to enter its fifths year and which now starts toactually produce new and valuable insights. Others working on other concepts, have put into placeprograms which are also underway.

Figure 3 shows a generic functional lay-out of a telescope receiving system. Depending on the(combination of) concept(s), the details will differ in terms of implementation, technology etc.To optimise the system and interplay of technologies to cost and function, a systematic and stepwisedevelopment process up to and including industrialisation is required, starting as early as possible butin any case after the selection of concepts.

For the Antenna- and Receiver designers, there are challenges to be found on the aspects of electro-magnetic modelling of wide band antennas, the matching to the low noise amplifier and integration(MMIC, RFIC, RF-beamforming, packaging etc.) versus semiconductor technology like (InP,GaAs,Si/SiGe, bipolar vs. CMOS) and function. In achieving a large dynamic range of say order 10

7,

up to the A/D converters (see Fig. 3) for the purpose of RFI mitigation after which digital signalprocessing, takes over, active and passive filtering techniques versus gain distribution should beconsidered. This in itself opens a range of technological opportunities of relevant approaches also to

ObjectSignal

conditioningA/DSensor

Digital

SignalProcessing

Beamforming/

Correlation

Control/Configuration/data acq.

Offline/imaging SW

Telescope as receiving system basic functional lay-out

Figure 3:Generic functional layout of a smart receiving system.Depending on the specific implementation, the sensor may for examplebe seen as an individual antenna array element or as the paraboloidalreflector. Also, the architectural and functional complexity of the digitalsignal processing is largely depending of the specific implementation.


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fields outside radio astronomy. For single optimised systems (e.g. paraboloids), it seems obvious toconsider the issue of cryogenically cooled systems which may or may not include high Tcsuperconducting filters combined with the low noise pre-amp. For these, the more or lessconventional closed cycle approaches eg. Stirling cycle machines or Pulse Tube refrigerators whichlack moving parts, can be considered. It remains to be seen if emerging low cost cryogenictechnologies for mass market applications now considered for high speed computers, will becomeattractive e.g. considering cost, power and reliability. In as far as the frontend (package) as a whole isconcerned, it is likely the area where cost, power, signal distribution and packaging are mostimportant considerations. Again, this is primarily so for the electronic adaptive array approach due tothe large quantity of receiving elements. As an example, Table 2 depicts the expected technologyversus cost of an all-electronic frontend, based on present work [26].

Time scale 1995-2003 2000-2008 2007-2013RF front-end

Technology Multilayer RF board Singlelayer RF board Kapton/foam+FR4# components 50 SMD 15 SMD Single MCM-C/DCost (US$) 35 55 15 35 7 - 15

RF-IC

Technology GaAs/PHEMT SiGe/HBT


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simulation (again) is essential. Together with model- and component (e.g. reusable designs and code)based development in the context of hard-software co-design and verification, state-of-the-artapproaches offer challenging prospects. Non-deterministic approaches based on neural networksand/or fuzzy logic e.g. for the purpose of on-line quality assessment need to be considered. As statedearlier, the large amount of astro-data, may also require such approaches so far uncommon forexample operating on (calibrated) data bases according to associative search rules (e.g. featuresearch).

All electronic technologies are not necessarily the ultimate answer. For the electronic array, photonictechnologies may reach up into the frontend for the purpose of optical beamforming. In this and theother concepts, the role of (in)coherent photonics will also be important in the LO- and timegeneration and dissemination, the interconnects, data networks and, more speculative, even for thepurpose of optical processing [30]. Some of these are areas are actively pursued as developmentprojects for ALMA [31,32] while others are elements of research in NFRAs SKA R&D program[25].

4 In Closing

An enterprise like SKA will only succeed with the largest persistence the community can offer. Notonly, will a new generation of technologists and astronomers alike, find a vehicle for many years ofchallenging R&D vehicle up to the first real observations, but with some vision it is also essential forkeeping a field of utmost importance vividly alive amidst other cultural developments.New technologies and their integration induce new and innovative approaches to old problems, andinherently also lead to new functional capabilities beneficial to astronomy. It is for this reason thatactive R&D programs for SKA also involving the wider community outside traditional astronomyinstitutes to tackle the numerous challenges, is essential to its ultimate success. Fortunately, thetimeliness of SKA as a complement to other major astronomical endeavours and the progress made so

far, leave no doubt as to its ultimate success as a major science instrument for the next decades.

References

[1] Science with the Square Kilometre Array, Ed. A. R.Taylor and R. Braun, Calgory, 1999 resultingafter the Calgory workshop, Aug. 1998.

[2] Scientific Imperatives at cm and meter Wavelengths, Amsterdam April 1999, Kluwer, ISBN,1999.

[3] K. Rohlfs and Wilson, Tools of Radio Astronomy, Springer , ISBN 3-540-60981-4, 1996.[4] B. Burke and F. Graham-Smith, An introduction to Radio Astronomy, Cambridge Univ. Press,

ISBN 0521 55604 X/0521 5545 3, 1997.[5] A. G. de Bruyn,The Westerbork Northern Sky Survey (WENSS), IAU Symp. 175, Bologna 1995,

Eds. R. Ekers, C. Fanti and L. Padrielli, Publ. Kluwer , ISBN 0-7923-4121-X,1996.[6] URSI Large Telesc. Work.Gr.& 1kT Intern. Techn. Workshop, ATNF-CSIRO, Sydney, Australia,

Dec 1997.[6a] A. van Ardenne, System requirements for the Square Kilometre Array, ibid.[7] R. Braun, The concept of the Square Kilometre Array Interferometer, Proc. High Sensitivity

Radio Astronomy, Cambridge Univ. Press, 1996 .[8] A. van Ardenne, F. Smits, Technical Aspects of the Square Array Interferometer, Ibid.[9] ALMA Project, www.hg.eso.org/projects/ALMA.[10] C. Lonsdale, Concepts for a Large-N SKA, This Conference.[11] O. D. Bucci et al., Antena Pattern Synthesis: A New General Approach, Proc. IEEE- AP, 358-

371 May 1994.[12] Proc. of the 3

rdMeeting of The Large Telescope Working Group and Workshop on Spherical


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Radio Telescopes, Oct. 1995, Guizhou, China, Intern. Acad. Publ. ISBN 7-80003-363-5?TN.29,1996.

[13] B. Peng, The Technical Scheme for FAST, This Conference.[14] J. Dreher, The One Hectare Telescope, This Conference.[15] P. Dewdney, Recent progress in the development of the Large Adaptive Reflector, This

Conference.[16] A. van Ardenne, F. M. A. Smits, Technical Aspects for the Square Array Interferometer, Proc.

Large Antennas for Radio Astronomy, ESTEC WPP-110, 1996.[17] A. Parfitt, A low - cost reflector antenne element for SKA, This Conference.[18] A.van Ardenne , Informal SKA workshop, Proc. collected as overhead copies, Delft Technical

University, 1996.

[19] U. L. Rohde, J. Whitker, T. T. N. Bucher, Communication Receivers, sec ed., McGraw-Hill,1997.

[20] Very Low Frequency Array on the Lunar Far Side, Report by the Very Low FrequencyAstronomy Study Team, ESA SCI(97)2, October 1997.

[21] J. Bregman, LOFAR, This Conference.

[22] D. Schaubert, Wideband Vivaldi Arrays for large aperture antennas, This Conference.[23] B. Smolders, Phased-array system for the next generation of radio telescopes, This Conference.[24] Z. Popovic, Broadband Antennas for SKA, This Conference.[25] A. van Ardenne, The SKA technical R & D program, NFRA Newsletter, Sept. 1998.[26] J.G bij de Vaate, Personal communications, NFRA, 1999.[27] A. Leshem, Some comments on deconvolution and RFI removal, This Conference.[28] Z. Popovic, Photonic approaches and components, This Conference.[29] A. Kokkeler, D.Kant, A.Gunst, A/D converter research for SKA, This conference.[30] Multi Univ. Res. Initiative (MURI) on RF/Photonics, USA, 1997.

[31] MMA Project Book, www.tuc.nrao.edu/demerson/project_book.[32] A. van Ardenne, A. Bos,The ALMA future correlator; proposal for a prototype study.

[33] J.Noordam, Calibrating SKA, a Challenge, This Conference