Future VLBI systems Tasso Tzioumis Facilities Program Director
Future VLBI systems Tasso Tzioumis| Facilities Program Director– Technologies for Radio Astronomy CSIRO ASTRONOMY AND SPACE SCIENCE IVTW November 2018
Why VLBI? Resolution Atmosphere gives 1" limit without corrections which are easiest in radio 1 arcmin 2 | IVTW November 14, 2018 Jupiter and Io as seen from Earth 1 arcsec 0. 05 arcsec 0. 001 arcsec Simulated with Galileo photo
Essential Features of VLBI systems – (issues) • Widespread antennas – Baselines 100 s 1000 s km • Precise and stable timing critical. • Synchronisation requires independent very stable reference clocks H-masers • Raw data samples required for Correlation of antenna signals • Transmit baseband / voltage data Extremely large data rates • Mitigation: Only need 2 -bit samples • Data transport – internet changed issue e-VLBI – real-time • Mass storage disk systems available cheaply. • Disk transport rare (remote antennas). Mainly on internet. Delayed or real time. • Data Format Compatibility mitigated with software correlators • Simple to re-format on-the-fly OR ingest multiple formats OR use standard VDIF format • Software correlators freely available and supported (e. g. Di. FX) 3 | IVTW November 14, 2018
Current VLBI systems • Custom VLBI systems are implemented (needed? ) • Feeds/receivers: mainly standard telescope systems (but see VGOS systems for IVS) • RF/IF chain: Standard till IF (e. g. IF of 0. 5 -1 GHz) • Digitisation: Custom. Special h/w required. • Digital processing: Custom h/w and firmware (DBE; DBBC; KVN/VSOP; LBA) – FPGA based e. g. ROACH • Data format: Custom. Many legacy formats. New standard VDIF. – Mitigated by disks and internet access. • Correlation: Custom. Software correlators. – Mitigated by freely available correlators on HPC clusters (e. g. Di. FX, STXC) • VLBI as special telescope sub-system • Special and time-consuming setups. Difficult and disruptive to schedule. • Only for “vlbi gurus”. ***Are custom VLBI systems always necessary? ? !! 4 | IVTW November 14, 2018
Radio telescope signal flow block diagram Universal backend RF Antenna systems (Drives, Feeds) . . . RF/IF System (LNAs, Cryo, RF, LO/IF) Data streams IF . . . ADC system (Sampling, Digitisation) . . . Digital system (FPGAs) Channels Packets RFSOC Timing systems (H-maser; GPS; Freq references; UTC; …. ) 5 | IVTW November 14, 2018 COTS systems Data packets . . . Digital switch Compute cluster Outputs (CPUs, GPUs)
VLBI with modern telescope Receivers-Backends-GPUs • Problem faced with Parkes Ultra. Wideband (UWB) system • Digitisation at Receiver in Focus cabin. • RF/IF not easily available for VLBI system. • Need different/new VLBI systems? ? ? • BUT • New telescope systems already produce data-packets in GPU cluster!! • Voltage data samples transmitted to GPUs – e. g. Pulsar timing • Samples tagged with precise timing (nsec) for pulsar timing! • H-maser stability already for all systems. Also needed for pulsars. • Data in GPUs already VLBI-ready!! (Baseband & Timing) !!! • Can use GPUs to channelise to VLBI channels • Reformat to 2 -bit and VDIF in software. • Already connected to Disks and Internet. • NO need for Special VLBI h/w or Firmware !! 6 | IVTW November 14, 2018
Huge advantages of new Systems • Can access any Receiver or Band the telescope supports • Potential for wide-band VLBI • No need for special setups taking hours…. • Just another telescope back-end mode. • Easier to schedule and run. • Cost reductions – no costly special VLBI systems • No special maintenance. No VLBI h/w experts. • Observations as per “normal” telescope operations. • Natural fit to modern telescope subsystems (e. g UWB; PAF) • Can adopt for current telescopes when modern receivers installed – E. g. Parkes adopts UWB back-end for ALL receivers! 7 | IVTW November 14, 2018
Characteristics of New systems • Front-end (Feed; RF/IF/LO) – Any of the receivers available on antenna • Can use new UWB systems to cover large band ranges (6: 1) – UWL 0. 7 -4. 2 GHz already at Parkes. Tsys ~20 K; – Planned: UWH 4 -24 GHz – Alternative: UWB-mid 4 -16 GHz & UWB-H 15 -30 GHz; share IF conversion. • Use Multibeam or PAF feeds • Universal back-end: ADCs and FPGAs • Already available: ADCs at 4 Gsps and 6 Gsps • Just available: RFSOC from Xilinx (16 ADCs+FPGAs) on 1 chip!! • Commercial of the self (COTS) systems: Switches and GPUs • Massive switches are getting cheap • GPU clusters already available • Astronomy software on GPUs (e. g Pulsars) available free in astro community (e. g Parkes package) – Very large community investment in software 8 | IVTW November 14, 2018
Summary and Conclusions • New telescope systems already produce data suitable for VLBI !! • Synergies with other fields (e. g. pulsar observations and timing) • Can leverage Receiver developments (eg UWB) to expand Freq and BW range of VLBI • Rapid developments in ADCs+FPGAs compact and simpler digital systems • NO need to always have “special” VLBI h/w and s/w at the telescopes • Simpler maintenance • Simpler setups and schedules. • Part of normal telescope operation – just another “mode” • Opportunity to move VLBI into the mainstream of telescope operations • More accessibility to VLBI technique 9 | IVTW November 14, 2018
CSIRO Astronomy and Space Science Tasso Tzioumis Facilities Program Director– Technologies for Radio Astronomy +61 2 9372 4350 Tasso. tzioumis@csiro. au www. csiro. au CSIRO ASTRONOMY AND SPACE SCIENCE
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