GNSS Receiver: Clock
The Clock attribute of a GNSSReceiver object lets you set spacecraft receiver clock parameters. Note that the clock is only used if CA, P1, P2 or DF Pseudo-range is selected. It is not used if CA SD or DF SD Pseudo-range is selected.
| Clock Parameters | |
|---|---|
| Parameter | Description |
| Epoch |
The epoch of the PhaseBias associated with the clock. When the GNSSReceiver is added to a facility, the Epoch, which is used to transition the phase to the state creation time, is set to the scenario start time. When the GNSS receiver is added to a satellite, the Epoch is set to the satellite epoch, and an initial phase value will be moved to the state creation time accounting for relativistic effects. The Epoch and PhaseBias are typically set when the user does a GNSS Navigation Solution IOD. The Navigation Solution IOD generates an estimate of the phase of the receiver clock in addition to a position and velocity estimate. |
| PhaseBias | Clock phase offset estimate at the specified epoch. |
| PhaseSigma | Uncertainty associated with the phase bias estimate at the initial filter time. Used to initialize the covariance. |
| FreqBias | Clock frequency offset estimate at the specified epoch. |
| FreqSigma | Uncertainty associated with the frequency bias estimate at the initial filter time. Used to initialize the covariance. |
| AgingBias | Clock aging offset estimate at the specified epoch. |
| AgingSigma | Uncertainty associated with the aging bias estimate at the initial filter time. Used to initialize the covariance. |
| A0 | Frequency-modulated (FM) clock white noise statistic. See below. |
| Aminus2 | FM clock random walk statistic. See below. |
| AgingWN | FM clock drift statistic. |
| SamplingInterval | Update interval for the stochastic clock model used during the simulation of GNSS measurements. |
| FreqAdjustment |
Options are None and Remove Secular Drift. If you select None (the default), then the clock experiences both the secular and periodic effects of general relativity. If you select Remove Secular Drift, then the clock only experiences the periodic effect due to the eccentricity of the orbit. This option is to model the case where the clock oscillator is intentionally offset to cancel out the drift due to the mean distance of the clock from the center of the Earth, as is done with the GNSS satellites themselves. |
| Estimate | Set to true to include the GNSSReceiver clock phase and frequency in the state. This controls the deviation of the clock during simulation in addition to the estimation of the clock during filtering and smoothing. |
| ClockResets | A list of clock phase, frequency and/or aging reset events. |
Setting A0 and Aminus2
One method of representing the accuracy and stability of a clock is with an Allan Variance diagram, which can usually be obtained from a clock vendor. This is a log10-log10 graph of the autocorrelation of clock phase errors, graphed against the autocorrelation time. A typical Allan Variance diagram for a crystal clock has the form of:
On the log10-log10 scale, frequency white noise produces a slope of approximately -1/2, while frequency random walk produces a slope of approximately +1/2. The user can control where the two processes intersect by setting A0 and Aminus2. A0 is set by choosing any point on the frequency white noise curve, say at (t1,σ( t1)), and setting A0 = σ2(t1) × t1. Similarly the value of Aminus2 is set by choosing any point on the frequency random walk curve (t2,σ(t2)), and setting Aminus2 = σ2(t2) / t2.
See the attached PDF file for further information on deriving clock coefficients from an Allan Variance Diagram.