Stochastic Propagation in Astrogator
The STK/Astrogator® capability supports the propagation of time-varying corrections to drag, density, and SRP models according to a specified stochastic model. In the Astrogator capability, this should replicate the behavior of propagating a correction on a satellite model in the Ansys Orbit Determination Tool Kit (ODTK®) application without measurements, such as when running the ODTK Filter in predict mode.
The initial parameters for the stochastic propagation, listed in the table below, are specified on the Stochastic Parameters tab on the Initial State segment.
Stochastic parameters
The following tables define the parameters on the Stochastic Parameters tab.
Drag Parameters
| Field | Description |
|---|---|
| Coefficient (Cd) | This is a read-only copy of the drag coefficient value. |
| Area | This is a read-only copy of the drag area value. |
| Ballistic Coefficient | This is a read-only computation of the Ballistic Coefficient, computed as B = Cd A / M. |
| Initial Correction | Enter the initial correction applied to the drag model. This is only active for segments that have enabled stochastic propagation for a drag model. |
| Long Term Initial Correction | Enter the long-term correction applied to the drag model. This is only active for segments that have enabled stochastic propagation for a drag model and are using a Vasicek model. |
| N-Plate Corrections | Click this button to see a table of initial and long-term corrections applied to scale factor groups for an N-Plate drag model. You cannot create this natively in the Ansys Systems Tool Kit® (STK®) application. You must import it as part of a Flight Dynamics Record created by the ODTK application when it has estimated corrections to a N-Plate drag model. |
| Density Model Initial Correction | Enter the initial correction applied to the density model. This applies only to active for segments that have enabled stochastic propagation for a density model. |
Solar Radiation Pressure Parameters
| Field | Description |
|---|---|
| Coefficient (Cr) | This is a read-only copy of the solar radiation pressure coefficient value. |
| Area | This is a read-only copy of the solar radiation pressure area value. |
| Cr A / M | This is a read-only computation of the Cr A / M coefficient. |
| Initial Correction | Enter the initial correction applied to the solar radiation pressure model. This is only active for segments that have enabled stochastic propagation for a solar radiation pressure model. |
| Long Term Initial Correction | Enter the long-term correction applied to the solar radiation pressure model. This is only active for segments that have enabled stochastic propagation for a solar radiation pressure model and are using a Vasicek model. |
| N-Plate Corrections | Click this button to see a table of initial and long-term corrections applied to scale factor groups for an N-Plate solar radiation pressure model. You cannot create this natively in the STK application. You must import it as part of a Flight Dynamics Record created by the ODTK application when it has estimated corrections to a N-Plate solar radiation pressure model. |
The properties that determine the time-varying behavior of these corrections are specified on the propagator functions. These vary depending on which model type is used, but they typically enable the use of stochastic propagation for Drag, Density, and SRP as well as the specification of which stochastic model to use. See Propagator Functions for more information.
Multiple propagate segments with stochastic propagation
It's possible to have two or more MCS segments that have stochastic propagation enabled for a force model. In all cases, the reference epoch for the stochastic behavior of the corrections is set to the last Initial State segment that was run in the MCS. If there are two or more propagate segments that have stochastic propagation enabled but the second segment has different settings, then the stochastic propagation will be reinitialized with the settings on the new segment using the reference epoch from the last Initial State segment that was run. If there are two or more propagate segments with stochastic propagation enabled with the same settings, then the stochastic propagation should continue smoothly between segments.