GPS Time Synchronization Overview

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GPS Time Synchronization Overview

• Prasanth Jeevan

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GPS Introduction

• 24 active satellites, 2 rotations per day
• At least 4 visible to a terrestrial receiver
• Transmit pseudorandom noise (PRN) codes
• Two that are really accurate for the military
• One for civilians (shorter period, lower chip
  rate)
• Receiver can get 3D location and GPS time

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GPS - Trilateration

• 3D works in a similar way with 3 satellites
• Accuracy on the order of tens of meters
• With 4 satellites you get GPS time too
• If you just want GPS time

http//Illumin.usc.edu
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GPS for Time Sync

• you can do it with one satellite!
• Why do you want the time from GPS?
• Each satellite has an atomic clock (1 part in
  1012)
• Can synchronize multiple clocks via GPS
• Also known as time transfer
• GPS time-transfer unit (TTU), a receiver for this
  purpose
• Wireless communications would benefit
• GPS timescale is CDMA telecommunications time
  scale (accuracy on the order of 10s of
  microseconds)

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Common View (CV) Method

• 2 stations with GPS receivers look at the same
  satellite
• They simultaneously measure their offset from GPS
  time
• Repeat many times
• Few parts in 1014 depending on user/site
  specifics
• A B (A GPS) (B GPS)
• A is Station 1s time
• B is Station 2s time
• GPS is the GPS satellite time

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CV C/A-code single-channel measurements

• The original way to do CV
• Based on single-channel receivers that could only
  look at one satellite at a time
• If the two receivers are at the same location you
  can ignore coordinate differences, ephemerides,
  and ionosphere/troposphere errors

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All-View method

• Uses new multichannel receivers
• Usually can observe 5 of 12 satellites
  simultaneously
• International GPS Service (IGS) provides clock
  drift and clock offset data
• Dont need to just compare common satellites

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Aside World Time

• Universal Time Coordinated (UTC) is the weighted
  mean of 250 atomic clocks throughout the world
• CV is used to compare the clocks

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Limitations

• Hardware temperature sensitivity
• Ionosphere/troposphere delays
• Ephemeris
• Precise antenna coordinates
• Expense

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Other Enhancements

• GPS carrier phase measurements
• L1 and L2 carriers
• Temperature stabilized hardware
• Standardization of receiver software
• Satellite enhancements

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Future of GPS

• GPS Modernization Program
• More Satellites
• IIR-M and IIF
• New Bands (with newer generation satellites)
• L2c, L5, more?
• European Galileo, 2010
• Open Service, Commercial Service, Public
  Regulated Service and Safety of Life Service
• GLONASS, not reliable for now
• Joint development with India started recently
  (2005)
• Wide Area Augmentation System (WAAS)
• GPS satellite orbit, clock drift, and signal
  delays caused by the atmosphere and ionosphere

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Other GPS Work

• Indoor GPS
• Massively parallel correlators
• Longer duration coherent processing
• Assisted-GPS
• Differential GPS
• A receiver at a known location that tracks the
  satellites constantly
• similar to/extension of A-GPS concept?
• Centimeter accuracy with WAAS

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References

• M.A. Weiss et al. A Comparison of GPS
  Common-View Time Transfer to All-in-View, IEEE,
  2005.
• N. Agarwal et. al. Algorithms for GPS operation
  indoors and downtown, GPS Solutions, 2002.
• W. Lewandowski et al. GPS Primary Tool for Time
  Transfer, Proceedings of the IEEE, vol. 8, No. 1,
  January 1999.
• T. Gotoh. Improvement GPS Time Link in Asia with
  All in View, IEEE, 2005.
• http//www.wikipedia.com
• http//tf.nist.gov/time/twoway.htm

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Questions?
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• Differential gps and timing
• A-gps and timing
• Wide Area Augmentation System (WAAS)
• GPS timing and cellular
• Cellular to relay gps time better?