ASKAP Antennas & Control Systems Project Brief Project ... · The antenna and its control system need to be designed for a lifetime of 20 years, given normal operating conditions

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ASKAP Antenna Project CSIRO Australia Telescope National Facility Version 2: 20/07/07

ASKAP Antennas & Control Systems

Project Brief

Project Overview

To achieve the cost-specification for the ASKAP (previously known as xNTD) antennas we require at least

one novel, realistic, inexpensive complete antenna design which can be assembled at a remote radio-quiet

site, with minimal on-site commissioning and on-going maintenance requirements. Most notably, the

ASKAP specification requires that the antenna support high-dynamic range imaging via pipeline

processing, and this has raised a number of specification decision points based on the as-yet un-optimised

FPA receiver performance. We discuss these decision points, resolution actions and options in this paper

(refer to page 5 onwards).

To address aspects of this design challenge, CSIRO has entered into a 3-way R&D collaborative design

project with Connell Wagner and Tenix. The results of this work are being made publicly available on an on-

going basis. Any party who is interested in joining this collaboration on a similar co-investment basis should

contact the project team listed on the final page of this document.

This paper is intended to give an overview of the current overall ASKAP antenna requirements and some

background on the constraints to the design. This document is not an invitation to tender a solution to the

team, nor will an apparently-compliant design solution imply any established relationship with CSIRO. As

part of an ongoing R&D project, many aspects of the antenna specification will be refined: therefore any

party interested in pursuing a design option is urged to consult with the project team.

All antenna designs which meet the required specification will be assessed at the ASKAP project Concept

Design Review scheduled for late 2007: we expect to include antenna designs derived by other astronomy

projects (SKA pathfinders) as well as those offered by external vendors.

Due to the highly innovative nature of several aspects of the ASKAP design, we will initially build an

array of 6 antennas at the WA site to gain experience with this type of instrument. The addition of 24

(or more) antennas will be as soon as possible afterwards - minor technical modifications might be

necessary in the light of the experience gained.

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ASKAP Antenna specification

Operating conditions at the WA site The limits given in this section are provisional and are subject to refinement as data from the site is analysed on an ongoing basis - The antenna and its control system need to be designed for a lifetime of 20 years, given normal operating

conditions at the site are

(i) operating wind speed less than 45 km/hour; and safe drive to stow position up to 60 km/hour;

(ii) ambient T in the range -10 to +55 degrees C; and

(iii) operating to an elevation limit of 15 degrees above the horizon. The minimum instantaneous

accessible sky coverage that the antenna must achieve is discussed later.

The antenna must be capable of surviving wind gusts up to 160 km/hour when in safe stow mode.

Lightning protection must be built into all components of the control and drive system.

Timescale

We expect that the choice of ASKAP antenna design made in late 2007 will apply to all antennas which will

comprise the radio telescope - i.e. we do not anticipate radical design changes between the initial 6

antennas and the full ASKAP array.

Target costs

The target cost for the ASKAP antenna is for fabrication, installation and commissioning for a unit cost of

AUD 300k, including foundations, but not including any control systems. This is clarified later in the

document.

Target specification

The target specification for the antenna components are -

An antenna mount, pedestal, encoders and drive to support a reflector suitable for operation up to 10 GHz

We have adopted a 12-m unshaped paraboloid reflector, with a prime focus F/D of 0.4 as the base

specification. However given the FPA performance a secondary focus solution may be preferable –

consideration of these options will be finalised by the end of 2007.

For all antenna components the following applies -

(a) Minimisation of build, operations and maintenance costs and power consumption,

(b) Ease of erection and commissioning at a remote site,

(c) Materials and construction method are open for all components of the antennas

(d) A design lifetime of 20 years with minimal in-field servicing.

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ASKAP Antenna Mount

The mount should provide acceptable sky coverage determined to be at least 75% of the half-sphere above

the observatory over any 24-hour period.

The mount components must support a ‘typical’ day’s radio astronomy tracking: Taking the ATCA as an

example, each antenna cycles over a total of ~2400 degrees in azimuth and ~1000 degrees in elevation in

a 24-hour period. These total distances comprise reasonably short drives in that the average distance is 6

degrees, spread over 150 – 250 separate drives in a day.

Alt-Az mount (horizontal elevation axis mounted above a vertical azimuth axis)

Under normal operating conditions:

• The drive system must be capable of precision astronomical tracking, and provide slew rates of at least

1.0 deg/sec az and 0.5 deg/sec elevation which result in acceptable sky coverage, given the ‘keyhole’ i.e.

loss of sky coverage near the zenith.

• The limit for the azimuth cable wrap must be +/- 270 degrees about the stow position at az = 90 degrees.

Equatorial mount (declination axis installed normal to a polar axis, parallel to the earth's axis)

Under normal operating conditions:

• The drive system must be capable of precision astronomical tracking, and provide an effective slew rate of

at least 20 degrees/minute.

• The limit for the HA wrap must be +/- 180 degrees about the neutral position at HA = 0 degrees; and +/-

90 degrees about Dec =0 degrees.

For either mount type, the full drive speed should be reached within 1 sec of operation. Note, however that

this isn’t a primary requirement and if this is costly then a slower acceleration would be acceptable.

Consideration of power consumption may also limit acceleration.

Pointing accuracy must be 30 arcsec (rms) in all operational modes. (the pointing error here is defined as

the radial distance between the actual position of the antenna's optical axis and the requested position; it is

a true angle on the sky)

Due to the likelihood of operating one or more antennas in close proximity, the antenna must be designed

to operate within the smallest reasonable maximum operating footprint. The entire antenna should be

constrained within a vertical cylinder of diameter 17 metres, independent of the position it is pointing to.

If applicable, the additional cost and design issues to enable access a larger fraction of the sky should be

quantified for each design.

ASKAP antenna reflector surface

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The reflector surface must be suitable for radio astronomical observations up to 10 GHz. The rms fit of the

actual surface to a best-fit paraboloid must be maintained at less than 0.6mm under normal operating

conditions (i.e. over all elevation angles). The transmission loss must be less than 1%.

The dish surface need not be designed as load-bearing – i.e. no one will walk on it.

It is anticipated that a solid panel solution will be adopted.

Focus & receiver frame

The antenna design should include a quadripod focal plane receiver support structure.

1. attached to either the rim of the reflector or at a radial rib (refer to the discussion of sidelobes in

later sections)

2. to include a feed rotator to operate at 20 deg/min and to include a cable wrap mechanism

3. the rotator and support frame will house a FPA receiver (CSIRO built) of max dimension of 1.4m

diameter and a weight of no more than 200 kg..

- The front face of the FPA will be positioned at the prime focus; +/- 10 cm axial adjustment will

be required.

- the centre of the rotator to be fixed relative to the reflector surface within +/- 3mm at all

- operating elevations.

- the quadripod should resist torsion about the optical axis to within +/- 0.35 deg (3 GHz

- operation) at all operating elevations.

- the plane of the FPA should be normal to the antenna optical axis at all elevations, to within an

angle of +/- 0.2 degrees.

Shadowing of the dish surface by the quadripod support legs must be less than 10% of the reflector

aperture. Note that the calculation of the spherical shadowing is to be referenced to the on-axis primary

reflector focal point.

Other

A number of cable wrap mechanisms are required to guide the set of cables from the receiver frame to the

base of the pedestal. At present the specification includes a set of 128 receiver cables (of thin coax or fibre

type), plus power and communications. The size of the bundle of cables is to be determined (contact the

project for more information).

The pedestal should be designed to house ASKAP control systems (i.e. leave as much free space as

possible). At a minimum this is likely to be a commercial 19 inch wide rack, 2 metres high. The pedestal

and turning head may be built from any material but they must be water-tight and include natural

(vented/grill) ventilation. The CSIRO equipment housed in the pedestal will present a thermal load of

several kW (contact the project team for more information).

The antenna must be designed to include suitable safety limit switches and emergency stops to ensure

safe operation.

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Antenna Control Systems

Given the specialised (astronomical) operations required, the ASKAP project will develop the antenna

control systems which interface to the drive system. Prospective designers will need to confirm that their

proposed drive and encoder systems are acceptable to the ASKAP group. The encoders, for example,

must be able to accommodate the required pointing precision.

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ASKAP Specification – Major Decision Points

As highlighted at the start of this document, there are a number of major decision points which the ASKAP

project cannot yet resolve: These decision points (for example, F/D, focus options etc) depend on the

performance of the FPA and finding the best combination of antenna and FPA mount/rotator to meet the

high dynamic range imaging specification.

It is anticipated that these issues will be resolved by the end of 2007 and in time for the project CDR.

ASKAP Antennas – focus options

The choice of focus type (primary or dual reflector) depends most critically on the FPA performance

(including the overall efficiency of the reflector/receiver systems).

The project started with the philosophy of a prime focus configuration antenna: However it is now clear

that a secondary focus system may have significant advantages, particularly if the FPA has more efficient

illumination of the reflector for an F/D > 0.4. CSIRO ICT experts are currently modelling Cassegrain and

Schwarzschild secondary optics and there are indications that a primary reflector with F/D = 0.4 and a

modest secondary (to yield F/D(eff) of about 0.6), results in a better illumination of the FPA. There may also

be some major mechanical and operational advantages in realising a dual reflector system (easier to

mount/control FPA, ease of cabling, especially for cooling systems).

ASKAP Antennas - Mount and FPA Feed rotator options The definition of mount type and FPA feed rotator are coupled, and these choices are also related to the

antenna optics. If a dual reflector system is chosen, then a simple monopod feed support is all that would

be required and the mount can be any type i.e. alt-az or equatorial – in practice this would most likely be

alt-az. The table below summarises the options. Note that a dual-reflector alt-az telescope with a monopod

feed support avoids the major sidelobe response issues which are discussed later in this document.

Reflector Type

Antenna Mount

Feed Support

Operation

Equatorial

Quadripod; but rigidity of FPA support is an issue

Antenna structures and FPA are stationary on sky due to equatorial mount

Prime focus

Alt-Az

Quadripod; but rigidity of FPA support is an issue and now needs precision rotator

FPA stationary on sky due to FPA rotator. Need to minimise quadripod sidelobes (use composite material?)

Dual-reflector

Equatorial or Alt-Az

Monopod

Rotate entire feed structure – FPA stationary on sky

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Figure – NASA JPL 34-m DSN antenna with monopod feed support.

Most radio telescopes have adopted an "alt-az" mount. However, this type of mount is not necessarily the

best choice from the perspective of achieving the highest dynamic range imaging. This becomes even truer

with the increasing complexity of imaging technology and the requirement to model (time-variable) sidelobe

responses.

The Westerbork Array antennas, built in the late 1960's, are recognised as demonstrating the benefits of

equatorial mounts (i.e. excellent dynamic range imaging).

Figure – Equatorial mount Westerbork array antennas (www.astron.nl)

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Conventional wisdom has it that the alt-az mount is significantly cheaper to build, and combined with the

capability of delivering all sky coverage the alt-az has become the mount of choice for radio telescopes

(e.g. the Australia Telescope Compact Array at Narrabri (ATCA) ).

Figure – alt-az mount ATCA antenna (www.atnf.csiro.au)

The choice of mount for ASKAP needs to be carefully considered – we are investigating whether critical

technologies have evolved to the point that the potential costs and benefits for the mount decision need to

be recast.

Impact of the sidelobe response on imaging

An antenna's response to a distant transmitter (source) depends on the orientation of the source relative to

the antenna's main axis. The response function is called the beam pattern; it consists of a main beam - the

response when the source is near boresight, and the sidelobes, the low level responses when the antenna

is pointing away from the source. Figure 1 shows the beam pattern for the ATCA 22m antenna. The plot

was obtained by executing a set of raster scans centred on a geostationary satellite. We see a strong

response on boresight; there is also a circular sidelobe around the main beam, whose peak is about 3% of

boresight. And there are a number of weaker circular sidelobes at larger radii. These all relate to the

general symmetry of the reflector. In addition, there are two linear sidelobe structures, one vertical, one

horizontal, which are the sidelobes due to the quadripod structure.

The following outlines one approach to calculating the full beam pattern -

1. Invoke the Reciprocity Theorem which shows that the radiation patterns for receive and transmit are

identical.

2. Launch a wave from the feed horn and follow its progress to the reflector and then out into space. In

this way we can account for all the obstacles in the path - diffraction around, and blockage by the quadripod

legs and any structure in the focal plane.

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It is useful to take a snapshot of the progress at the aperture plane - the plane parallel to the plane defined

by the rim of the main reflector, positioned just above the focus. The far-field pattern is obtained from a

Fourier transform of the aperture plane distribution.

Figure 2 shows the aperture plane distribution for the 22m ATCA antenna with a single pixel receiver. The

main features are:

1. A symmetrical "pill-box" distribution. This is the dominant feature which provides the main beam. The

transform of a "pill box" is a Bessel function J1(r)/r, with a strong central peak, followed by sidelobes which

decrease gradually in strength as a function of the radial distance

2. A central blockage due to the 2.8m sub-reflector. This provides an additional set of circular sidelobes.

3. Blockage shadows of the quadripod legs. These shadows come in two flavours:

a) The inner section, where rays from the feed hit the underside of the quadripod legs after

reflection from the main reflector. These shadows are rectangular in appearance. This section

extends from the edge of the subreflector blockage out to the point where the quadripod leg

attaches to the main reflector surface.

b) The outer section, where the rays hit the underside of the quadripod and are prevented from

reaching the reflector. This section extends from the end of the inner section out to the rim of the

main reflector. This shadow is far from rectangular: it flares out at the main reflector rim.

The Fourier transform of the aperture plane is the sum of the transform of each component - items 1, 2 and

3. The magnitude of the quadripod sidelobes is set by the area of the blockage shadows scaled to the 22m

reflector area. Fat, opaque legs produce significant sidelobes; transparent, or very narrow legs have faint

sidelobes. The requirement for high-dynamic range images with the ATCA has led us to implement

complex corrections in the imaging software.

It is important to note that the sidelobes are related to the location of the receiving element with respect to

the antenna structures – in the case of ASKAP with FPA receivers, each made up of N x N elements, each

element will see the quadripod legs from a different angle, leading to different shadowing.

Basic theory of radio astronomical imaging (interferometry)

We observe a celestial target (field) for a period of time, T, with an array of N antennas. The data collected

will consist of all the cross-correlations between all the antennas in the array.

∫+

−=

τ

τ

t

tdttjVtiVtjiC ),(),(),,(

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is the correlation between antennas (i) and (j) at time t, averaged over a small interval (2.

The imaging process, which is achieved with a 2-dimensional Fourier transform, in effect looks for the

signature of all the possible point sources in the field. This will work well as long as the system is stable

and calibrated. A source offset from the field centre will appear with an intensity related to its position in the

full beam pattern. In an ideal world the beam pattern's sidelobes are all concentric circles centred on the

boresight, and the offset sources will appear with constant amplitude, and will image correctly. In the real

world there will be linear shadows and non-circular sidelobes. As a result the imaging process will be

imperfect, with artefacts generated by the apparent gain fluctuations. The magnitude of the artefacts will be

related to the magnitude of these non-linear sidelobes, and temporal variations due to field rotation in these

exacerbates the difficulty of calibration.

A number of strategies have been suggested to address this problem and the two simplest options are

either to use

- RF transparent quadripod legs, to eliminate the sidelobes, or

- equatorial antenna mount to lock the sidelobes to the sky, to eliminate the apparent variation in amplitude,

or

- rotate monopod feed support.

These, and other options, are discussed in more detail in the following sections.

The case for ‘Sky Mount’ antennas

The simplest (and traditional) implementation of a ‘sky mount’ antenna is one using an equatorial mount.

This fixes the relationship between the pattern of the receiver “pixels” and the sky, so that there is no

rotation of the field as it is tracked across the sky. To the receiver, the sky and the feed mount (i.e.

quadripod) will be stable on the sky.

The data processing pipeline is predicated on the presence of some form of stabilisation of the focal plane

with respect to the sky, so that the processor is presented with N data streams, from N beams fixed on the

sky. The image quality is dependent on the quality of those data streams. Ideally, no source should exhibit

temporal variations, irrespective of where it is in the synthesised (by the beamformer) antenna primary

beam. The advantage of the sky mount is that it minimizes such temporal variations, which greatly

simplifies the data processing process. The equatorial mount does this by the nature of its geometry, and

minimizes the need for additional compensating mechanisms or software. It is possible that a

feed rotator may still be desirable on an equatorial mount telescope, but it would only be used for

calibration and thus have a lower drive specification.

With an alt-azimuth mount the sky rotates with respect to the antenna, so some form of image stabilisation

is required, either as a virtual rotator (via software in the beamformer) or a mechanical rotator. The software

rotator may be highly complex, costly and may have significant issues regarding calibration.

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In the discussion below the term rotator can mean either the mechanical or software solution. The software

solution has not yet been demonstrated, however we are confident that it can be realised.

.

Defining ‘Sky Mount’ solutions

Following an internal ASKAP antenna design workshop in December 2006, we define a limited set of

“acceptable” options for the ASKAP antenna mount/feed combination: Assuming a prime focus system

these all de-rotate the sky (in terms of the receiver) and minimise the sidelobe response function. In

increasing order of acceptability, there are four options -

Acceptability Option Mount/feed types

Marginal (I) Standard alt-az with metal quadripod and prime focus feed rotator (this has

significant sidelobe variation implicit);

Better (II) Standard alt-az with Kevlar (or other “rf transparent” material) quadripod aprime

focus feed rotator

Good (III) Alt-az with rotating reflector or rotating monopod (“sky mount” – feedleg sidelobes

fixed wrt sky) – feed leg structures can be metal or kevlar (better)

… or ….

(IV) Equatorial with modest feed rotator (only for calibration)

Figure 1: Extended beam pattern on the sky (ATCA) showing the sidelobe responses.

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Figure 2: Measured aperture plane response for a single 22-m ATCA dish (alt-az) – single on-axis feed.

Equatorial mounts - technical issues

Several design studies (eg, Freeman-Fox, 1957, Connell-Wagner, 2006) have suggested that an equatorial

antenna incurs a cost premium of around 20% compared to an alt-az antenna. The designers find that a

heavier, taller mount is needed if the sky coverage specifications are to be met, when compared to an alt-

az mount.

The central issues with equatorial mount design are the polar axis bearing and the declination axis drive.

Bearing technology has evolved, and this may no longer be a serious problem. The declination axis drive

may be more of a challenge.

Polar axis bearing

The polar axis is a problem because it is inclined to the vertical, so that the dead weight of the telescope is

likely to generate a bending moment at the bearing. An alt-az telescope with a counter-weighted reflector

will have its centre-of-mass on axis for all azimuth and elevation pointings. A wide variety of optical

telescope mounts have been developed, all avoiding a cantilevered bearing . These all had the declination

axis bearings outside the telescope - equivalent, in our case, to supporting the reflector with bearings at the

rim. The Mt. Palomar design (and many of the large telescopes built subsequently) has a polar axis

bearing structure which is several times larger than the telescope itself.

The Westerbork antennas have a clean equatorial design: They have a polar axis supported by two

bearings several metres apart. As a result, the declination bearing structure is big - a large offset from the

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polar axis is needed to ensure that the reflector does not foul the polar axis structure when pointing towards

sources significantly south of the celestial equator. The figure below illustrates the layout.

Figure: Typical equatorial mount design

Wind loading is an additional force that has to be supported by the main bearing; the torque at the bearing

cannot be cancelled with a counterweight. This may lessen the advantage of this mount, depending on the

relative magnitudes of dead-weight and wind loading.

The drive machinery on the polar axis is an additional problem: the loads are heavier than is the case for

an alt-az antenna. Unless there is a counterweight the weight of the reflector will lead to an hour angle

dependent moment that the drive system will need to support.

Declination axis drive

The problem with the declination axis drive is the large range of angles that we need to accommodate -

roughly +/- 90 degrees. This is more than is comfortably provided by a jack-screw mechanism. For

comparison, the elevation drive needs less than 90 degrees. This has meant, in previous equatorial

designs, that a 180 degree bull-gear of large diameter is required.

Counterweights

Counterweighting an antenna has advantages and disadvantages. It allows the drive system in the relevant

axes to use less power (other than for acceleration, where the increased rotational inertia requires more

power), and be under less continuous strain when compared with unbalanced systems. The additional

power consumption to move unbalanced systems can be a significant cost over the operating life of the

telescope. In addition, unbalanced systems are potentially more dangerous if a critical part of the drive

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system fails, with more serious runaway consequences possible. The disadvantage of counterweighted

systems is the cost of the additional, “dead” material that increases weight, bearing loads, etc.

Equatorial mounts - new options?

Polar axis

A slewing ring looks now to be a viable option for the polar axis. Affordable strong slewing rings are now

available for antennas comparable to the ASKAP size and (light) weight. Both the ATA and Patriot

antennas, for example, are alt-az mounts with no counterweights; these provide a realistic demonstration

that suitable HA bearings could be obtained for an equatorial mount.

Figure – ATA dish – alt-az with offset mounting and no counterweight (ral.berkeley.edu)

Figure: Polar axis design as proposed within Sinclair Knight Merz’s Cost-feasibility study for the xNTD antenna, April 2006

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FPA Feed Rotators

As noted previously, some form of focal plane rotator is almost certainly required independent of whichever

antenna mount type is adopted. A mechanical feed rotator will need to meet the following specifications:

• To support a load of 200 kg

• Shape: circular

• Access from above is required

• Rotation required : +/- 190 degrees.

• Positioning precision: 0.1 degrees

• A cable wrap is required.

A possible implementations for a mechanical feed rotator is shown below.

Figure: Feed Rotator “Model 1” Assuming that the FPA is light weight, and symmetrical, no significant safety measures are required - a

simple brake on the motor will be adequate.

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Resolving the Mount & FPA rotator specifications

The ASKAP antenna design project is tackling the range of issues to understand the options available, this

includes

- EM modelling of composite feed legs (monopod and quadripod) – there are concerns that a

composite structure

- will induce a standing wave which will degrade the performance of the FPA receiver,

- EM testing (holography) of potential feed leg materials (e.g. composite, aluminium) at the

ATCA during April and May 2007,

- Exploring mechanical designs for equatorial mounts which may prove cost-effective compared

to alt-az

The results of these studies will be made available to interested parties.

Other relevant decision points

Additional considerations to the ASKAP antenna specifications outlined above which are not yet fixed:

• Will there be cooling of the receiver element? Are coolant lines to be incorporated into the cable

wrap?

• How substantial/heavy will the electronics infrastructure be; can it be accommodated away from the

FPA itself?

Given the inexorable escalation in these matters it is also possible that 200 kg is on the wrong side of

conservative - but we have to have a realistic estimate before we embark on the final antenna design.

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Further Project Information & Contacts

ASKAP – the large-Number small-diameter antenna Array is a new radio telescope to be built in WA from

2008 onwards.

Up-to-date information on ASKAP can be found at www.atnf.csiro.au

CSIRO Australia Telescope National Facility Cnr Vimiera & Pembroke Roads Marsfield NSW 2122 Postal: PO Box 76 EPPING NSW 1710 Tel: +61 (0) 2 9372 4100 ATNF Director, and SKA Director, Australia Dr Brian Boyle ATNF Assistant Director – ASKAP & SKA Phase I Project Leader Dr David Deboer ASKAP Antenna Project personnel

Carole Jackson ([email protected]) Mike Kesteven ([email protected]) Ross Forsyth ([email protected])

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ASKAP Antennas & Control Systems Project Brief Project ... · The antenna and its control system need to be designed for a lifetime of 20 years, given normal operating conditions