Propagate Segment
Use the Propagate segment to model the movement of the spacecraft along its current trajectory until meeting specified stopping conditions. The segment uses the defined propagator and integrator to propagate the state, adding each point to the ephemeris as it goes.
After each step, the segment checks to see if any stopping conditions were met during the step. If so, it then finds the exact point, within tolerance, where the stopping condition is satisfied. From that point, the segment either executes an automatic sequence or stops the propagation and passes the state at that point to the next segment. Astrogator also adds an ephemeris point at the time that the stopping condition is triggered.
The following sections provide descriptions of the parameters of the Propagate segment.
Propagator
Astrogator will apply the propagator named in the Propagator text field. To change this selection, click next to the text field and choose a different propagator from the list, which reflects the current propagator list in the Component Browser.
Integrator
Astrogator will apply the integrator named in the Integrator text field. To change this selection, click next to the text field and choose a different integrator from the list. This list reflects the current integrators listed for the selected propagator, as designated in the Component Browser. If the ellipsis is unavailable, that means there is only one integrator choice for the selected propagator.
You must have at least one integrator entry for the propagator you select. The STK-provided propagators have this. You must check this for imported or your custom-configured propagator components you use from the Component Browser.
Propagator time period constraints
Click Advanced Propagator topic for more information.
to set constraints on the time period of a Propagate segment. See theStopping conditions
Stopping conditions are Astrogator components that the Propagate segment uses to define the point at which it ceases propagation. When more than one stopping condition is active for the segment, propagation stops as soon as one of them is satisfied. You can add, copy, delete and perform other actions on the stopping conditions by right-clicking in the table that lists them or by using the following toolbar buttons:
Button | Name | Function |
---|---|---|
![]() | New... | Opens the New Stopping Condition dialog box, which enables you to select a stopping condition component to assign to the segment. |
![]() | Cut | Removes the currently selected stopping condition from the list and copies it to the clipboard. |
![]() | Copy | Makes a copy of the currently selected stopping condition to the clipboard. |
![]() | Paste | Inserts a copy of the stopping condition that is currently on the clipboard. |
![]() | Delete | Deletes the currently selected stopping condition. |
![]() | Add To Component Browser | Copies the currently selected stopping condition to the Component Browser as a stopping condition. |
Each stopping condition assigned to a segment appears in the table as a separate item, and the properties of the currently selected stopping condition appear beneath it. Each stopping condition's table entry is comprised of the following fields that describe it:
- On. Select this check box to have the segment use the stopping condition when running the MCS.
- Name. Displays the name of the stopping condition. You can edit this field for easier reference.
- User Comment. Click in this field to enter your own comment on the stopping condition.
- Description. Displays a brief definition of the nature of the stopping condition.
The following table describes the settings that you can use to control how the segment applies the stopping conditions:
Field | Description |
---|---|
Trip | Specify the value at which the condition will be satisfied. |
Tolerance | Specify the tolerance for achieving the highlighted stopping condition. If the stopping condition algorithm completes computation without reaching the tolerance value, an error message appears in the Message Viewer window and the MCS stops running. |
Base | For an Access stopping condition, select Current Satellite or click ![]() |
Target | For an Access stopping condition, click ![]() |
Time Convergence | For an Access stopping condition, this option sets the time tolerance used by the access algorithms to determine the start times and stop times of access intervals. The event detection routine declares that an event is found when either the value of the access constraint converges to the desired value within eight digits or the time interval in which the event lies is within the time tolerance. |
Use Light Time Delay | For an Access stopping condition, select this check box to have STK consider light time delay in access computations. Click Light Time Delay Options dialog box. | to open the
Eclipsing Bodies List Source | For a Lighting stopping condition, choose the eclipsing bodies for STK to consider:
|
Sequence | Click ![]() |
Central Body | This field is interchangeable with the Coordinate System and Reference Point fields and is used only for stopping conditions that take the central body into account (e.g., Periapsis). The default central body of a stopping condition is Earth. To select a different central body for the stopping condition, click ![]() |
Coordinate System | This field is interchangeable with the Central Body and Reference Point fields and is used only for stopping conditions that take the coordinate system into account (e.g., Argument of Latitude). The default coordinate system of a stopping condition is Earth Inertial. To select a different coordinate system for the stopping condition, click ![]() |
Reference Point | This field is interchangeable with the Central Body and Coordinate System fields and is used only for the R Magnitude stopping condition. |
User Calc. Object | Use this field, if appropriate, to select a user Calculation Object for the highlighted stopping condition. For stopping conditions you defined, use this field to specify what kind of value on which you want to stop. Click to open the User Calculation Object Selection dialog box to configure the calculation object. |
Criterion | A criterion is an extra condition to meet for the stopping condition to be satisfied. This field is only available for certain stopping conditions and the options are dependent upon the type of stopping conditions: For all position stopping conditions:
For Access stopping conditions:
|
Condition | For a Lighting stopping condition, define the lighting event at which the condition is satisfied:
|
Repeat Count | This field is active for most stopping conditions, those capable of being satisfied more than once. Specify the number of times the condition must be satisfied before the condition is considered to be met. |
Max Trip Times | Enter the maximum number of times to apply the stopping condition — and any resulting automatic sequences executed — during the propagate segment. After the stopping condition has been met for a number of times equal to the value of this field, the propagate segment ignores the stopping condition. |
Constraints | Click ![]() |
Before | Use this field to set "before" stopping conditions. Use a "before" stopping condition to define a stopping condition that depends on two events. To add "before" stopping conditions, click ![]() |
Condition Inherited by Automatic Sequences | Select this check box to apply the stopping condition to any automatic sequences activated within the same segment. If the stopping condition is met while an automatic sequence is executing, and this option is not selected, then the stopping condition is ignored. Segments that do not normally evaluate stopping conditions, such as impulsive maneuvers, cannot inherit stopping conditions. If the stopping condition that you are passing to an automatic sequence is met in a segment that does not evaluate for them, then the stopping condition is ignored. |