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EPSRC Centre for Doctoral Training in Communications
‘A 4-year PhD programme for the next generation of entrepreneurial communications engineers’
@BristolCDTComms
www.bristol.ac.uk/cdt-communications [email protected]
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Time from the stars?
Presented by John Haine on behalf of
Moonas Ahmad, Faya Algahtani, Obada Alia, Sebastian Kudera, Sarman Ozan,
Ioannis Papoutsidakis, Simon Wilson
(2018 CDT Cohort)
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Motivation
• Time distribution is a critical global infrastructure
• Atomic time distributed by GNSS satellites
• Vulnerable to jamming, spoofing, solar storms etc
• Back-up alternatives desirable
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Time synchronization• Computer clocks are susceptible to clock drift and
require synchronisation for accurate operation
• Network Time Protocol (NTP) and Precision Time Protocol (PTP) are designed to coordinate computer times in a network
• Implement a hierarchical architecture where the very top level is a hardware reference clock obtained from GNSS.
• High precision and accuracy of PTP allows it to be used to synchronise and time stamp financial transactions and mobile phone transmissions
• NTP and PTP can both be fooled by DDOS or man-in-middle attacks in order to wrongly synchronise computer clocks
Is there an alternative to GNSS to synchronise clocks between sites?
GPS NTP server
Hierarchical NTP structure
https://timetoolsltd.com/products/ network-time-server-appliances/t300-gps-ntp-server/
https://timetoolsltd.com/time-sync/ the-fundamentals-of-time-synchronization/
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Concept
• Pulsars provide a globally* observable set of “lighthouses” that broadcast extremely regular time ticks
• These can be calibrated by reference to UTC (when available)
• Observing same pulse at two different locations allows local times to be compared
• …provided communication links remain available
• Hence provide a backup to UTC(*universally!)
➢Is this feasible and where could it be applied?
➢Are there “existence proofs”?
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
What is a Pulsar?
• Stars die in one of three ways depending on size• Small stars, with mass less than 1𝑀⊙ become white dwarfs.
• Large and heavy stars with mass greater than 3𝑀⊙ explode in aSupernova and form Black Holes
• Stars with a mass in between 1𝑀⊙ and 3𝑀⊙ also explode in aSupernova but form Neutron stars.
• Pulsars are small, dense, highly magnetized rapidly rotatingNeutron stars.
https://www.schoolsobservatory.org/learn/astro/stars/cycle
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Pulsar radiation
• Due to its rapid rotation, pulsar generates strong magnetic field lines
• During the rotation, charged particles are ripped off the star
• Particles rotate with the star, traveling along the closed field lines
• Their rotational speed increases with the distance from the star to the point where they reach the speed of light
• At this point called the “Light Cylinder”, those particles escape along the open field lines creating a pulsar beam
• Pulsar also “nutates” (wobbles) which sweeps the beam
• Whenever the beam is targeted towards Earth we can detect a pulse with a suitable radio receiver
• Rotational rate, hence PRF, is very stable – some comparable to atomic clocks
https://science.sciencemag.org/content/312/5773/539
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
https://gfycat.com/leadingflatbonobo-pulsar
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Some observed pulsars
Source:Grzegorz Szychlinski
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Don’t you need a huge telescope?
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Anthony Hewish &
Jocelyn Bell, MRAO 1967
~80 MHz4 Ha wire
array
Amateur Radio Telescopes
436 MHz
420 MHz
424 & 1294 MHz
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Antennas• Observations possible from 10MHz to 40GHz, most easily
received from 30MHz - 1.5GHz
• Flux density is strongest in the range 400MHz to 1400MHz
• ITU RAS frequency bands for high-precision timing observations: 1.4-1.427 (hydrogen line), 2.69-2.7, 4.99-5.0 GHz
• Optimum is 1.4 GHz – low interference
Issues
• Size / frequency
• Steerability (to minimize observation time)
• Phased array preferred
3D Corner 4m2
Double Bi-Quad 1.3m2
Dish >13m2Yagi >1.8m2
Array 16m2
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Antenna requirements
• Equivalent aperture needed > 2 sq m
• Low sidelobes to reject interference
• Minimum SNR 5 dB – needs integration over quite long time period
• LNA required to achieve necessary noise figure• E.g. Mitsubishi MGF491 – NF 0.15 dB gain 20 dB at ~1.4 GHz
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Antenna array design
• The smallest parabolic dish antenna currently used for pulsar detection at ~1.4GHz is 3.1m, with an efficiency of ~60-65%
• Effective aperture ~2m2
• Gain ~31dBi
• An equivalent antenna array would need to comprise ~2000 isotropic elements
• Spaced at λ/2, elements are ~10.6cm apart
• Resulting antenna array ~4.7m x 4.7m
• A 100% efficient array of ~1.5m x 1.5m would provide an effective aperture of 2m2 over ~±20°
• Tracking ±20° enables an integration time of ~150 minutes
• Array size 16 x 16 elements
• Isotropic element gain ~10dBi
➢Additional ~20dB needed from antenna or LNA chain.
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Receiver can be based on low-cost SDR & LNA devices
Mitsubishi MGF491 – NF 0.15 dB gain 20 dB at ~1.4 GHz
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Signal processing
R. Grootjans, “Detection of dispersed pulsars in a time series by using a matched filtering approach,” Master’s thesis, University of Twente, 2016.
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
De-dispersion by “FDE”
Signal sorted into FFT “bins”
Pulses arrive with frequency-dependent delay because of ISM dispersion
Bin contents delayed by varying amounts according to centre frequency and added
R. Grootjans, “Detection of dispersed pulsars in a time series by using a matched filtering approach,” Master’s thesis, University of Twente, 2016.
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Epoch folding & matched filtering
• Epoch folding is used to search for a coherent signal in large amounts of data with low SNR• Add successive capture periods so that received data is
“folded” on to itself• Pulse profile increases in power as “N” while the noise
increases as N1/2
• Needs prior approximate knowledge of pulsar period
• Matched filter optimises SNR at the signal peak• Convolve received signals with time-reverse of expected
pulse• Equivalent to generating autocorrelation function
R. Heusdens, S. Engelen, P. Buist, A. Noroozi, P. Sundaramoorthy, C. Verhoeven, M. Bentum, and E. Gill, “Match filtering approach for signal acquisition in radiopulsarnavigation,” Proceedings of the International Astronautical Congress, IAC, vol. 5, pp. 3756–3760, Jan 2012
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Pulsar Signal Processing Tool (SIGPROC v4.3)
• Software package developed by Duncan Lorimer, professor of Physics and Astronomy at West Virginia University – open source under GPL
• Used by numerous pulsar astronomers since 2001
• Designed to standardize the initial analysis of the many types of fast-sampled pulsar data
• All the programs written in C and can be run from the UNIX command-line
• Used as a core module in almost all modern pulsar software
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
PULSAR CLOCK
Historical Museum of Gdańsk
Tower Clock Department
Time Keeping Laboratory
Source:Grzegorz Szychlinski
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Source:Grzegorz Szychlinski
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Source:Grzegorz Szychlinski
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Conclusions
• There are “existence proofs” for the key elements for a compact pulsar telescope• Requires ~4 m square 16 element phased array
• Compatible with roof mounting where a GNSS time backup is sufficiently important
• Modern SDR products and RF components make it reasonably economical
• Receiver architecture can learn from MIMO & massive-MIMO beam-forming techniques
• Signal processing with well-proven open-source software
• Stand-alone Pulsar clock is feasible
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Issues & further work
• Given multiple sites receiving pulsars and comparing time, what accuracy is attainable and what applications might be excluded?
• “Bootstrapping” – how dependent on GNSS would the system be for initialisation?
• Possibilities for distributing the telescope – reduce the array size and share observation data?
• Could a single fixed pointing antenna with long integration time give sufficient accuracy?
EPSRC Centre for Doctoral Training in Communicationswww.bristol.ac.uk/cdt-communications
Acknowledgements
• Grzegorz Szychlinski of the Pulsar Clock project at Historical Museum of Gdańsk, Poland
• CDT in Communications 2018 cohort, University of Bristol
EPSRC Centre for Doctoral Training in Communications
‘A 4-year PhD programme for the next generation of entrepreneurial communications engineers’
@BristolCDTComms
www.bristol.ac.uk/cdt-communications [email protected]