Smart Antennas in Radio Astronomy

Transcription

1 Smart Antennas in Radio Astronomy Wim van Cappellen

2 Netherlands Institute for Radio Astronomy Our mission is to make radio-astronomical discoveries happen ASTRON is an institute for applied research Strategic collaborations with universities/institutes

3 Overview Context: Square Kilometre Array (SKA) Array concepts and system overview Technology challenges Demonstrator results

4 The Square Kilometre Array 4 Prime characteristics 1. Very large collecting area (km 2 ) sensitivity to detect and image hydrogen in the early universe sensitivity ~ 50 x EVLA, LOFAR 2. Very large field of view fast surveying capability over the whole sky survey speed, up to one million times faster than EVLA 3. Wide frequency range required for the key science projects: low : 70 MHz 450 MHz mid: 300 MHz 10 GHz high: 10 GHz 25+ GHz 4. Large physical extent (3000+ km) capability for detailed imaging of compact objects, and astrometry with thousandth arc second angular resolution from R. Schillizi

5 SKA Artist Impression Up to 1500 dishes (15m diameter) in the central 5 km plus another 1500 from 5 km to km + aperture arrays Radio camera All-sky monitor Connected to a data processor by an optical fibre network from R. Schillizi

6 SKA Structure GHz Wide FoV MHz Wide FoV Dense AA.... Sparse AA Digital Signal Processing Data Time Control To 250 AA Stations 16 Tb/s Central Processing Facility - CPF Correlator AA & Dish Mass Storage Post Processor GHz SPF DSP &/or GHz PAF 12-15m Dishes DSP 80 Gb/s &/or 640 Gb/s... Time Standard Control Processors & User interface Array Technology To 2400 Dishes User interface via Internet

7 SKA ASTRON Aperture arrays: LOFAR Science capable instrument, MHz EMBRACE Technology demonstrator, MHz 144 m WSRT + 80 m Nançay Focal Plane Arrays APERTIF FPA s on the WSRT, MHz

8 Principle of dense FPA s Aim is to provide adequate Field of View to enable large surveys with reflector telescopes Multiple feed horn systems have widely separated beams on the sky (depending on f/d) For a contiguous field-of-view, a dense array feed consisting of electrically small elements is required Multiple elements are combined into compound beams Elements are re-used for various beams FPA feed compound beam reflector compound beam

9 Station Architecture to correlator Optional RF beamformer between antenna and receiver (EMBRACE, LOFAR HBA)

10 Technical challenges Low noise T sys < 50 K (requires <30 K LNA (0.4 db)) LNA s operating at room temperature Mutual coupling variation of antenna impedance with scan angle Approach Design LNA to be insensitive to impedance variations (low r n ) Minimize antenna losses 0.1 db loss gives 7 K T sys contribution 14% of Tsys Avoid lossy materials (e.g. dielectrics)

11 Noise Matching Which optimum are we going to choose from? Selecting LNA device: low F min Low r n

12 Technical challenges Low-cost antenna Cheap manufacturing processes (=less accuracy) Cheap materials (=usually higher losses) Some examples: Laser-cut aluminum plates Antenna screen printed on foil and copper plated (RFID technology) Involve industrial partners at an early stage

13 Beamformer RF-ASIC Channel 2 Channel 1 Beam 1 Vcc B2 Channel 3 Vcc 2 Vcc 3 Vcc 1 Vcc 4 Channel mm OPAR V2 BFC: 1SIGe BiCMOS 0.25μm NXP foundry 4 diff inputs 2 diff outputs degr. 3 bits 0 5 db 3 bits Gain 18 db Z 0 50 Ohm Digital control Vdd Beam mm Vcc B1 Digital control

14 Technical challenges Calibration Requirement: beam should be temporal stable and smoothly varying with frequency Sidelobes and cross-pol can be accepted as long as they are stable (within limits) Important difference with traditional systems: the beam is not determined by mechanics only, but also by electronics. Online calibration is necessary Approach LOFAR: continuous station calibration on A-team sources in parallel to regular observations FPA: calibration source on dish surface

15 Technical challenges Massive amount of data Real-time processing required Costs of digital processing and data transport Power consumption Self-generated RFI Approach FPGA based processing boards (I/O limited!!)

16 From steel to software... Low Frequency Array

17 LOFAR 18 core stations, 18 remote stations 96 Low Band Antennas High Band Tiles per station Station diameter: m (LBA) m (HBA)

18 LOFAR Status First test stations operational Roll-out of central core and remote stations in progress

19 Station based processing ~ 460 Gbps in / ~ 2 Gbps out ~ 1.5 Tmul/s; 96 GByte storage

20 CEntral Processing Facility 10 Tbyte/day Tbyte/day 250 Tbyte/ day

21 LOFAR All-sky synthesis image Observing frequency: 58 MHz 508 s integration time

22 LOFAR as a passive radar All sky image at 55 MHz Duration: 60 sec Cas A Television transmitter DR1 in Fyn, Denmark Reflected on airplane Cygnus A

23 APERTIF prototype APERTIF prototype One dish fully dedicated to FPA Stand alone system (so far) 8 x 7 x 2 elements Vivaldi array Dual polarisation 60 Receiving chains Frequency range GHz 30 MHz bandwidth Element separation: 10 cm 1.5 GHz) Data recording backend (6.7 s) Output is full covariance matrix

24 Element patterns on the sky 6 deg High sidelobes Non-circular main beam Some symmetry in the array, but not perfect 6 deg

25 Beamformer weights Beamformer weights are determined on a strong point source (e.g. Cas A) Effects of antenna array and mutual coupling, blockage are included Airy ring structure is recognized in the weighting coefficients Magnitude of max SNR weights for on-axis 1421 MHz

26 Scanned compound beam

27 Image of M31 M31 with APERTIF prototype 1 telescope, 1 pointing, 121 beams, 6.7s integration time M31 with WSRT 14 telescopes, 163 pointings Single dish, single pointing image!!

28 Summarizing Smart Antennas are an enabling technology for the SKA Technology is demonstrated through SKA Pathfinders ASTRON is developing both Aperture Array and Focal Plane Array systems Wideband Low noise