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The New Loran C Time Scale & Possibilities for Future Enhancements S. R. Stein Timing Solutions Corporation. The Role of the Time Scale in TFE. GPS provides the link to UTC(USNO) 3 Cs clocks are used to average the data from the GPS receivers in order to Flywheeel in case of GPS outage

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slide1

The New Loran C Time Scale

&

Possibilities for Future Enhancements

S. R. Stein

Timing Solutions Corporation

the role of the time scale in tfe
The Role of the Time Scale in TFE
  • GPS provides the link to UTC(USNO)
  • 3 Cs clocks are used to average the data from the GPS receivers in order to
    • Flywheeel in case of GPS outage
      • 2 day time constant calibrates clocks to < 110-13 (10 ns/day)
    • Provide enhanced performance
      • scaley() = clocky() /3
    • Provide fault detection
      • Isolate frequency and time steps to individual Cs or GPS
    • Enable diagnostics
      • Provide long-term individual clock performance assessment
  • The mechanism to accomplish these results is called a time scale
gps time recovery
GPS Time Recovery
  • < 15 ns (RMS) time recovery expected based on performance of existing TSC UTC recovery systems
    • Uses C/A only GPS receiver
gps time recovery1
GPS Time Recovery
  • Kalman filter models clocks and predicts clock performance when measurements aren’t available
    • Provides optimal dynamic performance such as for start-up
    • System can flywheel through GPS system outages
tfe system level
TFE System Level
  • TFE consists of two redundant signal paths to generate transmitter drive signals with a known relationship to UTC(USNO)
    • Each redundant half of the system operates independently to control primary frequency standards, generate transmitter drive signals and measure time differences
    • Single (non-redundant) unit for distributing frequency signals from cesiums for diagnostic use
local lorsta time scale configuration

GPS

GPS

Receiver

Receiver

Local LORSTA Time-Scale Configuration

Measurements

  • Two independent time scales are computed from time difference measurements between three Cs clocks
  • Each time scale is independently steered to UTC(USNO) using GPS
  • Two clocks are steered to one time scale and one clock is steered to the second time scale

Time

Scale

Steer

Commercial

Cesium

Commercial

Cesium

Time

Scale

Steer

Steer

Commercial

Cesium

the answer is time scale what is the problem
The Answer is “Time Scale” -What is the Problem?
  • Einstein once said “Time is what a clock reads.”
  • But, what is the time when 2 or more clocks are needed for reliability?
  • Issue:
    • It is impossible to measure the error of each clock.
    • It is only possible to measure the difference between 2 clocks
    • We want to solve for two variables (the time of each clock) but only have one measurement (the time difference)
the solution time scale definition
The Solution: Time-Scale Definition
  • Traditional Solution
    • Add a relationship between the clocks that determines the solution and makes the time continuous on addition or deletion of a clock
  • KAS-2 Enhancements
    • Add two relationships between the clocks that control the performance at medium and long times
kas 2 time scale
KAS-2 Time Scale

Compute

Kalman Gain

Update

Covariance

Forecast

Update

State & Covariance

New

Measurements

  • FEATURES:
  • Operates in real time
    • Kalman filter provides
      • Optimum dynamic performance
      • Minimum squared-error estimates
    • Accepts data whenever acquired
    • Three weights per clock allow performance to be optimized for all noise types
    • Outstanding robustness due to
      • Outlier detection
      • Time step detection
      • Frequency step detection
simple example of a closed form solution
Simple Example of a Closed Form Solution
  • The Simplest Real Time Scale (Assumptions)
    • The clocks have white frequency noise only
    • The measurement noise is negligible compared to the clock noise
    • The clocks have equal performance
    • There are two time difference measurements
actual startup behavior and rms error
Actual Startup Behavior and RMS Error
  • 2 Cs clocks steered to UTC(USNO) using GPS
evaluating the performance of new algorithms
Evaluating the Performance of New Algorithms

Simulation

Real Clock Data

  • Monte-Carlo Simulations
    • Substitute a clock simulator for the real clocks
  • Comparison with NIST operational time scale using TAI as reference
slide16

Distributed Loran C Time Scale

A Possible Future Enhancement

rationale
Rationale
  • Improved survivability
    • Loran has 100 clocks distributed throughout CONUS
  • Enhanced performance
    • Time scale frequency stability would be 10 times better than an individual clock
  • Provide a GPS independent source of UTC(USNO)
    • Common view allows nearly GPS independent measurement of Loran clock time differences
      • No new hardware needed
    • Satcom allows total GPS independence
    • The two approaches may be combined
  • Coast Guard could contribute to the formation of UTC
gps common view
GPS Common View
  • Many errors are reduced or cancel completely
    • GPS clock errors and satellite cross-track ephemeris errors cancel
    • Propagation delay errors are reduced
  • Loran C clocks can be synchronized to each other and to UTC(USNO)
  • User A and User B each measure the local time vs GPS via individual satellites
distributed loran c time scale reduced gps dependence
Distributed Loran C Time ScaleReduced GPS dependence

Local Clocks - GPS SVs

LORSTA

GPS

NETWORK

Time Scale

Receiver

LORSTA

GPS

Time Scale

Receiver

NAVCEN

Time Scale

GPS

Receiver

LORSTA

GPS

Time Scale

Receiver

UTC(USCG)

LORSTA

GPS

Time Scale

Receiver

Local Clocks - UTC(USCG)

distributed loran c time scale minimum gps dependence
Distributed Loran C Time ScaleMinimum GPS dependence

DSCS Satellite

LORSTA

Time Scale

Local Clocks - GPS SVs

LORSTA

GPS

NETWORK

Time Scale

Receiver

LORSTA

GPS

Time Scale

Receiver

NAVCEN

Time Scale

LORSTA

GPS

Time Scale

Receiver

UTC(USCG)

LORSTA

GPS

Time Scale

Receiver

Local Clocks - UTC(USCG)

summary and conclusions
Summary and Conclusions
  • TFE time-scale implementation provides state-of-the-art timing, local to the LORSTA, for time-of-transmission control
    • sub 15 ns UTC recovery
    • redundancy
    • automatic operation
    • Quasi independence of GPS based on use of Cs standards
  • The TFE architecture can be enhanced to improve GPS independence
    • Add common view (differential) GPS processing for improved independence
    • Add two-way satellite time transfer for true independence
  • Loran C could become an extremely robust national time resource to preserve the national timing capability
    • Contribute to the formation of UTC