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... to set constraints on the time period of a Propagate segment. See the Advanced Propagator topic for more information.

Stopping 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 to select another object in the scenario to use as the base for the access computation.
Target	For an Access stopping condition, click to select an object in the scenario to use as the target for the access computation.
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 Options to open the Light Time Delay Options dialog box.
Eclipsing Bodies List Source	For a Lighting stopping condition, choose the eclipsing bodies for STK to consider: Propagator Central Body List - This is the list of eclipsing bodies for the propagator's central body. A central body's eclipsing bodies list consists of itself and its moons if it is a planet, or itself, its parent, and its siblings if it is a moon. User Defined List - Click beside the Eclipsing Bodies field to select a customized list of eclipsing bodies to consider. Vehicle Central Body List - This is the list of eclipsing bodies for the vehicle's central body. Vehicle User Defined List - This is the customized list of eclipsing bodies defined on the satellite's Lighting properties page. If a customized list is not defined for the satellite, then this selection point to the vehicle's central body list.
Sequence	Click to the right of this field to select an action or sequence of actions to trigger if the highlighted stopping condition is satisfied. You can add sequence options using the Automatic Sequence Browser. The Stop sequence will stop propagation in this segment and allow the next segment to run. If you select an alternate sequence, Astrogator will run it and, when finished, the current Propagate segment will continue to run until a Stop is triggered. If an automatic sequence ends with a Stop segment, then the current segment will stop at the end of the run of that sequence.
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 to open the Select Central Body dialog box; click the central body to highlight it, and then click OK.
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 to open the Select Reference Axes window, click the coordinate system to highlight it, and then click OK.
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 Edit to open the User Calculation Object Selection dialog box to configure the calculation object. In the User Calculation Object Selection dialog box, click Change... to select a calculation object from the Component Browser. You can use the button to copy the currently selected calculation object to the clipboard and the button to replace the current calculation object with a copy of the one that is on the clipboard. Click to save a copy of the current calculation object to the Component Browser.
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: Cross Increasing means the stopping condition is satisfied when the parameter reaches a value equal to the trip value while increasing. Cross Decreasing means the stopping condition is satisfied when the parameter reaches a value equal to the trip value while decreasing. Cross Either means the stopping condition is satisfied when either of the above situations occurs. For Access stopping conditions: Either means the stopping condition is satisfied when gaining or losing access. Gain Access means the stopping condition is satisfied only if gaining access. Lose Access means the stopping condition is satisfied only if losing access.
Condition	For a Lighting stopping condition, define the lighting event at which the condition is satisfied: Enter Direct Sun means the stopping condition is satisfied when the spacecraft enters direct sunlight. Enter Umbra means the stopping condition is satisfied when the spacecraft enters an umbra region. Exit Direct Sun means the stopping condition is satisfied when the spacecraft exits direct sunlight. Exit Umbra means the stopping condition is satisfied when the spacecraft exits an umbra region.
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 and, in the Select Constraints dialog box that appears, select a constraint for the highlighted stopping condition. This is a further condition that must be met in order for the stopping condition to be deemed satisfied.
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 and define them as you would a normal stopping condition. When running the MCS, Astrogator ignores the normal stopping condition until meeting the defined "before" stopping conditions. Astrogator then interpolates backward to the normal stopping condition just prior to the "before" stopping condition, and this is defined as the point at which the normal stopping condition is satisfied. The backward interpolation is subject to the Minimum Propagation Time, as defined by clicking Advanced..., meaning that the backward interpolation is performed for at least that specified amount of time before Astrogator checks for satisfaction of the condition.
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.