Evaluation of Loran Timing Receivers at NPL

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Evaluation of Loran Timing Receivers at NPL

• R. Hlavac and P. W. Stacey
• Time and Frequency, Division of Enabling
  Metrology, NPL
• Location Timing Applications
• 9-11 November 2004

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Contents

• Coordinated Universal Time (UTC)
• Timing via eLoran and Loran-C/Eurofix
• Experiments with Locus CsSync 1000 Timing
  Receivers
• Discussion of Obtained Results
• Future Work

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Coordinated Universal Time (UTC)

• UTC - Coordinated Universal Time
• International timescale
• Computed by BIPM, Paris
• Post processed monthly, feedback via Circular T
• UTC(NPL)
• Base for UK legal time
• Realized at National Physical Laboratory
• Derived from a single hydrogen maser
• Kept within 30ns from UTC

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Timing via eLoran and Loran-C/Eurofix

• eLoran
• Extensive modernization since late 1990s
• New Timing and Frequency Equipment (TFE)
• 3x Cs clock- 5071A
• Steered via GPS to stay within 15ns (rms) from
  UTC(USNO)
• Kalman filter to flywheel GPS outages

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Timing via eLoran and Loran-C/Eurofix

• Loran-C/Eurofix
• Expanding on contemporary Loran-C functionality
• Transmission of short messages
• Extra time modulation of Loran-C signal
• Timing controlled by Control Centre Brest (CCB,
  France)

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Experiments with Locus CsSync 1000Timing
Receivers

• CsSync 1000 (DSP-based, all-in-view)
• Navigation mode (over 40 stations)
• Improved signal to noise ration
• Timing mode (locked to one station)
• E-Field antenna

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Experiments with Locus CsSync 1000Timing
Receivers

• Experiment between 9thJune 6th September 2004
  at NPL
• Receivers locked to Loran-C station in Lessay,
  France
• Rcvrs 10MHz output (derived from Lessay)
• Rcvrs 1pps output not aligned to UTC
• Sampling rate
• 30s for 1pps data
• 10s for phase data

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Experiments with Locus CsSync 1000Timing
Receivers
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Discussion of Obtained Results

• Offset removed
• Daily cycling
• Temperature
• Sky-wave
• Variations
• 80ns p-p
• Scatter of 50-60ns

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Discussion of Obtained Results

• Continuation
• 1 pps derived from int. 10MHz
• Loran-C clock drift (underlying process)

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Discussion of Obtained Results

• Fractional frequency
• fr (f-f0)/f0 -
• Frequency stability (Allan deviation)
• 2x10 -13 (t 1 day)
• Comparable to Cs clock
• Better than MSF

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Common-View Method
CV ARefA-Loran BRefB-Loran A-BRefA-RefB
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Discussion of Obtained Results

• Offset removed
• Variations down to 40ns p-p
• Scatter of 30ns
• Standard deviation 9ns (1s)

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Discussion of Obtained Results

• GPS-CV perform better than Loran-C CV
• Chip rate
• Coding
• L-band
• Loran-C CV
• Short baseline
• Instabilities get cancelled

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Discussion of Obtained Results

• Two processes
• At t60s rcvrs locking constant
• At t43000s (noon) atmospheric and/or
  environmental effects

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Conclusions

• eLoran and Loran-C/Eurofix can be used as source
  of precise frequency( signal controlled by Cs
  clock)
• Neither eLoran nor Loran-C/Eurofix has
  implemented UTC service as yet (100ns rms at user
  site, meets stratum 1)
• Loran-C performs well above its current timing
  and frequency requirements (frq. stability
  1x10-12 t1 day)
• Can be used as a GPS backup

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Conclusions

• Frequency stability of 2x10-13 or 1.8x10-13 (t
  1day) for standalone receiver or common view
  mode, respectively
• Loran system instabilities contribution is very
  small, compared to receiver instabilities, short
  baseline CV
• Standard deviation of Loran-C CV data 9ns (1s),
  variations down to 40ns, scatter down to 30ns
  (24hours)

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Future work

• Continue in evaluation of Loran-C as a source of
  accurate time and stable frequency
• Perform Loran-C common-view experiment on longer
  base lines
• Extend our measurement capabilities to implement
  Loran-C monitoring, should Loran-C usage in the
  UK expand

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Acknowledgements

• Authors would like to express their thanks to
• Locus Inc. for lending us receivers and antennas
• John Davis and Peter Whibberley at NPL and Linn
  Roth at Locus Inc. for their valuable advice

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Contact Details

• Roman Hlavac
• Time and Frequency
• Division of Enabling Metrology
• National Physical Laboratory
• Teddington, Middlesex, TW11 0LW, United Kingdom
• Email roman.hlavac_at_npl.co.uk
• Tel  [phone]/6351
• Fax [phone]