Time scale principles Time scale principles


НазваTime scale principles Time scale principles
Дата конвертації20.04.2013
Розмір463 b.
ТипПрезентации



Time scale principles

  • Time scale principles

    • Examples of special cases
      • AT1 and EAL
      • Large Drift or Long averaging
      • Large measurement noise or near real-time
    • The general problem
      • Kalman Solution
  • Adding a steered clock

  • Steering the time scale



A time scale is a procedure for combining the data from several clocks

  • A time scale is a procedure for combining the data from several clocks

  • Inputs:

    • (Initial estimates of the statistical characteristics of each member)
    • Measurements of times or frequencies of all members with respect to a reference device
      • Reference device need not be special


A time scale is a procedure for combining the data from several clocks

  • A time scale is a procedure for combining the data from several clocks

  • Outputs:

    • ensemble time and frequency
    • Statistical performance of each member
    • Update to model for each clock
    • (Physical realization of ensemble time)


Advantages:

  • Advantages:

    • Minimize single points of failure
    • Output does not depend on a single device
    • Ensemble provides error detection
    • Get the best of each contributor
      • Nominally identical clocks may not be equal
      • Combine clocks with different properties


Noise of the measurement process

  • Noise of the measurement process

    • Time noise with no frequency aspect
  • Deterministic model of each clock

  • Stochastic contribution of each clock

  • Non-statistical glitches for each clock









Measured time differences represent differences of time states of clocks

  • Measured time differences represent differences of time states of clocks

  • Frequency estimate has deterministic and white noise contributions

    • Averaging statistically appropriate
      • Time constant determined by flicker frequency floor
    • Frequency estimate (x/t)  freq. state y(tk)
  • Drift parameter determined outside of algorithm

    • Treated as a constant by AT1


Computed as weighted average of each clock

  • Computed as weighted average of each clock

    • Weight derived from prediction error on previous cycles
    • Sum of weights is 1


AT1 algorithm does not explicitly calculate these parameters

  • AT1 algorithm does not explicitly calculate these parameters

    • Ensemble frequency is time evolution of ensemble time
    • Ensemble frequency drift is time evolution of ensemble frequency


Every clock is a member of ensemble used to evaluate its performance

  • Every clock is a member of ensemble used to evaluate its performance

  • Prediction error is always too small

    • Weight is biased too large
    • Error detection is degraded
    • Positive Feedback loop








Short measurement interval

  • Short measurement interval

    • Frequency variance approximately wfm
  • Mixed ensemble computed iteratively

    • Separate computation for clocks with negligible drift
  • Full Kalman algorithm

    • Complex and difficult to handle errors




Problem important when time differences are noisy or as t 0

  • Problem important when time differences are noisy or as t 0

    • AT1 algorithm cannot be used for near real-time systems
  • Measured time differences must be partitioned into measurement noise and clock noise

    • Measurement noise must not degrade clock parameter estimates


Partition variance of measurements based on initial estimates of noise parameters and covariance matrix

  • Partition variance of measurements based on initial estimates of noise parameters and covariance matrix

    • Jones and Tryon, TA(NBS)
    • GPS Composite clock (Brown)
    • KAS2 (Sam Stein, Symmetricom)


AT1-type algorithms assign variance to frequency noise

  • AT1-type algorithms assign variance to frequency noise

    • Measurement noise very small
    • Frequency drift constant (or 0)
  • Errors are modeled as simple time steps with no change in parameters



AT1-type algorithms are appropriate only over a range of averaging times determined from the clock statistics

  • AT1-type algorithms are appropriate only over a range of averaging times determined from the clock statistics

    • Lower limit from measurement noise
    • Upper limit from frequency variance
  • Kalman-type algorithms can handle more complex noise types

    • More sophisticated partition of measured variance
    • Reset/Error detection more difficult to handle
      • Reset machinery is outside of statistical considerations


Time scale algorithms assume variance of clocks is not cross-correlated

  • Time scale algorithms assume variance of clocks is not cross-correlated

  • Common-mode effects are a serious problem

    • Common time step in high-weight clocks
      • Wrong clocks are reset








Steered clock usually steered based on time:

  • Steered clock usually steered based on time:

    • Simple steering drives xs 0
      • Steered clock realizes ensemble time
    • More complex steering
      • Steered clock is UTC(lab) steered to UTC
        • Error signal is UTC(lab)-UTC from Circular T
      • xsx0+y(t-t0)+0.5*d*(t-t0)2










Advantages of time scale algorithms:

  • Advantages of time scale algorithms:

    • More robust – no single point of failure
    • Provides statistical evaluation of members
    • Provides a natural platform for steering
  • Problems with time scale algorithms

    • Real clock behavior may not conform to model
    • High-weight clocks are difficult to handle
    • Error handling/Reset algorithm is arbitrary and not always statistically robust
    • Variable frequency drift hard to model
    • Real-time time scales have special problems


Realizing UTC(NIST) at a Remote Location

  • Realizing UTC(NIST) at a Remote Location

    • Metrologia, Vol. 45, page S23, 2008
  • Other papers in this volume of Metrologia

  • The Statistical Model of Atomic Clocks and the Design of Time Scales

    • Review of Scientific Instruments, Feb. 2012


Схожі:

Time scale principles Time scale principles iconA kind, forgiving, charitable, pleasant time; the only time I know of, in the long calendar of the year, when men and women seem by one consent to open their shut-up hearts freely

Time scale principles Time scale principles iconPlan teoretical principles of ir- spectroscopy

Time scale principles Time scale principles iconInternational Bureau of Credit Histories Principles of work

Time scale principles Time scale principles iconNasa-jpl/ucla june, 2011 Outline Data assimilation and a multi-scale scheme

Time scale principles Time scale principles iconMaking up sentences: Listen and draw the time

Time scale principles Time scale principles iconEvolution of Severe Convection in the New York City Region What are large-scale conditions favoring severe (tornadic) convective weather?

Time scale principles Time scale principles iconGame time listen to the words and clap your hands when you hear the words connected with our topic

Time scale principles Time scale principles iconРішення для керування територіально розподіленими іт ресурсами. Наші вендори твердження №1
Високопродуктивні обчислювальні системи (hpc-кластери, суперкомп’ютери, Real-Time database & Database in memory)
Time scale principles Time scale principles iconAdverbs of time: today, yesterday, now, soon, early, then, before…
За значенням прислівники в англійській мові поділяються на 5 основних груп: прислівники часу прислівники місця прислівники повторюваності...
Time scale principles Time scale principles iconCтворення єдиного інформаційного простору дистанційного навчання Семенов М. А
На обкладинці журналу "Time" зображена клавіатура, монітор комп'ютеру та написано "Людина року це ти" користувач мережі Інтернет,...

Додайте кнопку на своєму сайті:
dok.znaimo.com.ua


База даних захищена авторським правом ©dok.znaimo.com.ua 2013
звернутися до адміністрації
dok.znaimo.com.ua
Головна сторінка