EPSRC Centre for Doctoral Training in …...2019/05/01 · EPSRC Centre for Doctoral Training in Communications ‘A 4-year PhD programme for the next generation of entrepreneurial

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)


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


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/


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”?


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


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


https://gfycat.com/leadingflatbonobo-pulsar

https://gfycat.com/leadingflatbonobo-pulsar


Some observed pulsars

Source:Grzegorz Szychlinski


Don’t you need a huge telescope?



Anthony Hewish &

Jocelyn Bell, MRAO 1967

~80 MHz4 Ha wire

array

Amateur Radio Telescopes

436 MHz

420 MHz

424 & 1294 MHz



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


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


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.


Receiver can be based on low-cost SDR & LNA devices

Mitsubishi MGF491 – NF 0.15 dB gain 20 dB at ~1.4 GHz


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.


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.


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


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


PULSAR CLOCK

Historical Museum of Gdańsk

Tower Clock Department

Time Keeping Laboratory







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


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?


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]

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EPSRC Centre for Doctoral Training in …...2019/05/01 · EPSRC Centre for Doctoral Training in Communications ‘A 4-year PhD programme for the next generation of entrepreneurial