Phil Diamond
e-MERLIN
Jodrell Bank Observatory
• Motivation and Design
Goals
• Key Science Applications
• Technical implementation
www.merlin.ac.uk/e-merlin
Science case and technical info
Jodrell Bank Observatory
Development of MERLIN: The quest for
angular resolution
• MERLIN grew from early
Jodrell Bank tests of radiolinked interferometry.
• Now consists of 7
telescopes distributed
throughout England.
Baselines range from 11217 km.
• Signals transmitted via 32
MHz (128 Mbps) analogue
radio-link at 7.8 GHz.
Jodrell Bank Observatory
Design Goals for e-MERLIN
• MERLIN has demonstrated
the power of matching the
resolution of Hubble at radio
wavelengths
• All new telescopes aiming for
0.1”
• MERLIN is the only 200-km
network in the world
• Need sensitivity to compete
• Fibre connection from
telescopes can increase
sensitivity by x10
• Together with upgraded
Lovell Telescope, will provide
total sensitivity increase of
factor 30 => 1.4µJy/bm in
12 hours
HST
Jodrell Bank Observatory
MERLIN
e-MERLIN Science Goals
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http://www.merlin.ac.uk/e-merlin
Cosmology
Galaxy Formation and Evolution
Star-formation
Main sequence and evolved stars
Extreme environment astrophysics
Jodrell Bank Observatory
Implementation
• Receivers: wider bandwidth: 2 GHz per pol. digital
output 30 Gb/s/antenna => 210 Gb/s => 10Pb/day
• IF/LO system : rebuild to handle wide bandwidth
• Data transmission : rest of this talk
• Correlator : scaled-down version of EVLA WIDAR
correlator, to be built by DRAO (Penticton, Canada)
• Software :
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Monitor/control – updated, rewritten
Correlator control – rewritten
Post-correlation – data correction new
Post-processing software deal with 0.5 TB/day
Archive will contain full FOV for each observation
• Timescales: 5 yrs to build, 10 – 15 yrs operation
• Funding: £8.6M ($13M) capital in place. Project kickoff April 1, 2002
Jodrell Bank Observatory
Data Transmission Requirements
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For each antenna
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Minimise development effort – ALMA/EVLA parts
Fit txs, with astro receivers, in small cabin on antenna
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Dual 2 GHz wide bands
Sampled at 4 Gsps to 3 bits precision
i.e. astronomical data rate is 24 Gbps per antenna
Notes: data are random noise, data to JB, timing.
Dispersion in coaxial cable means fibre-optics needed to
bring 30 Gbps signals down from antenna even over short
(100 m) distances.
Fit optical receivers inside correlator, BER < 10-6
Screen well to minimise EMI. Tx box approximately
4x8x16 inches
Jodrell Bank Observatory
Strawman Fibre System- Physical
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Dark fibre solution, loop Cam-Deff-Kno-JBO, total of
~400km fibre
3 x 10 Gbps streams per antenna
3 wavelength per antenna (around 1550nm), WDM
system, optical multiplexors onto single fibre.
Industry-standard LD-EAM transmitters
Industry-standard PIN-TIA receivers
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Optical demultiplexors, PIN diode photo-detectors, clock
recovery electronics – all inside correlator
EDFA plus, possibly, Raman optical
amplification
Majority of amplifiers at University sites
Jodrell Bank Observatory
Loop network dark fibre solution:
preferred option
Using e.g. Corning Cable
Systems micro-ducting technique,
or other available ducts.
Minimum 400 km total link
length, 115 km longest single
link, will require:
• amplifiers
• dispersion compensation
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Repeaters can be based at
telescope sites, except station X, a
hut somewhere.
Jodrell Bank Observatory
Loop network: opto-electronics system
Jodrell Bank Observatory
Strawman Transmission Protocol
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Non-industry standard. SDH not optimal for our data
ALMA/EVLA/e-MERLIN protocol
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Small data frames with time tags, data modulation
Frame error detection
Fast framing across colours
Built-in test patterns
FPGA plus industry-standard 16:1/1:16 mux/demux but
at exactly 10 Gbps
ALMA prototype systems
under construction at JBO.
Will be used for e-MERLIN
and EVLA
Jodrell Bank Observatory
Permissible Variations
• Data rates need not be 10 Gbps. 9.9xxx would be
possible.
• Some other protocol might be acceptable provided there
is some form of time-tagging.
• Extra layers of optical-electrical, e.g. black/white nonWDM transmissions to and from local POPs, would be
manageable.
• Loop/star topography would also be possible
Jodrell Bank Observatory
A Flexible Approach to Fibre
Procurement
• Most installed commercial networks are at 2.4 Gbps.
Some 10 Gbps links gradually being introduced. Most
use SDH encoding.
• Only ~ 2% of available capacity used at the moment.
• We are exploring:
– Lease of dark fibre
– Lease of wavelengths
– lease the full 30 Gbps capacity from a network
• Exact form of link dictates electronics requirements for
JBO, e.g. if SDH format conversion needed.
• Discussions with numerous fibre providers:
– 7 companies expressing serious interest
– ‘Creative’ solution required
– Competitive tender this summer
Jodrell Bank Observatory
Limits of 10Gbps transmission over fibre:
• Attenuation limit at ~60km
– Can be overcome using Erbium Doped Fibre Amplifiers
(EDFAs) to reach spans of hundreds kms
• Dispersion limit at ~80km
– Can be overcome using NZ-DSF and dispersion
compensation methods to reach spans of hundreds of kms
• Polarisation Mode Dispersion limit at ~400km
– Fibre is birefringent and paths vary continuously giving
rise to timing jitter. Regeneration required at this limit.
Jodrell Bank Observatory
Service provider solution:
(Managed bandwidth)
• Depends on points of
presence (POP)
• Requires ~150 km of
own fibre (the last mile
problem)
• Dispersion etc. taken
care of by provider
• Use of SDH/Sonet
could be a problem
• Negotiations with
commercial providers
are underway.
Jodrell Bank Observatory
Fibre Summary
• ‘Lean’ network relative to a telecoms provider.
– 10-6 BER, no redundancy, relatively low reliability
• Minimise
– network build/installation cost
– co-location sites, can be expensive
– the amount of fibre required using DWDM &
consolidation where practical.
– equipment development by re-using ALMA
transmitter/receiver designs
• G.655 fibre would minimise dispersion compensation
• Very encouraging discussions with fibre companies,
initial budgetary estimates within 50% of our data
transmission budget.
Jodrell Bank Observatory
Milestones
• Funding available now
• Oct 2003 – new C-band system, compatible
with upgraded Lovell Telescope – 3x
sensitivity
• Oct 2005 – new L-band system, frequency
flexibility
• 2005 – fibre test system. Test observations
with reconstituted analogue band.
• 2006/7 – e-MERLIN first fringes
Jodrell Bank Observatory