Propagate Segment

The Propagate segment can be used to model the movement of the spacecraft along its current trajectory until meeting specified stopping conditions. The segment uses the defined propagator 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.

The parameters of the Propagate segment are described below.

Propagator

To define the propagator to be used for the segment, click the ellipsis button and select one from the list that appears.

Advanced

To specify constraints on the time period during which Astrogator will attempt to satisfy the Stopping Conditions, click Advanced... to open the Propagator Advanced window; the constraint parameters are described in the following table:

Field	Description
Minimum Propagation Time	Specify the minimum time that must elapse from the beginning of the segment until Astrogator will begin checking stopping conditions for satisfaction.
Maximum Propagation Time	Select to specify a maximum propagation time, after which the segment will end regardless of whether the stopping conditions have been satisfied.
Warning Message on Maximum Propagation Time	Select to have Astrogator issue a warning message if propagation is stopped by the Maximum Propagation Time parameter.
Duration and Epoch Stopping Conditions Override Max Prop Time	Select to have Astrogator ignore the Maximum Propagation Time if the segment contains an enabled Duration or Epoch Stopping Condition.

Stopping Conditions

Stopping conditions are Astrogator components that are used by the Propagate segment 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 component selection window, which allows 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

Each stopping condition assigned to a segment is listed in the table as a separate item and the properties of the currently selected stopping condition are displayed beneath it. Each stopping condition's table entry is comprised of the following fields that describe it:

On. Select to use the stopping condition when running the MCS.
Name. Displays the name of the stopping condition. This field can be edited for easier reference.
Sequence. Displays the name of the automatic sequence that the stopping condition calls, or STOP if no automatic sequence has been selected.
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 can be used to control how stopping conditions will be applied:

Field	Description
Trip	Specify the desired value - the value at which the condition will be satisfied - for the stopping condition.
Tolerance	Specify the desired tolerance for achieving the highlighted stopping condition. If the stopping condition algorithm completes computation without reaching the tolerance value, an error message will display in the Message Viewer window and the MCS will stop running.
Base	For an Access stopping condition, select Current Satellite or click the ellipsis button to select another object in the scenario to use as the base for the access computation.
Target	For an Access stopping condition, click the ellipsis button 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 that is 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 8 digits or the time interval in which the event lies is within the time tolerance.
Use Light Time Delay	For an Access stopping condition, choose whether to consider light time delay in access computations. Click Options to open the Light Time Delay Options window.
Eclipsing Bodies List Source	For a Lighting stopping condition, choose the selection of eclipsing bodies to be considered: Propagator Central Body List -- 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 siblings if it is a moon. User Defined List -- Click the ellipsis button beside the Eclipsing Bodies field to select a customized list of eclipsing bodies to consider. Vehicle Central Body List -- the list of eclipsing bodies for the vehicle's central body. Vehicle User Defined List -- the customized list of eclipsing bodies defined on the satellite's Eclipse Bodies properties page; if a customized list is not defined for the satellite, then this selection will use the vehicle's central body list.
Sequence	Click the ellipsis to the right of this field to select an action or sequence of actions to trigger if the highlighted stopping condition is satisfied. Further sequence options can be added using the Automatic Sequence Browser. The Stop sequence will stop propagation in this segment and allow the next segment to run. If an alternate sequence is selected, it will be run 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 apparent only for stopping conditions that take the central body into account (i.e., Periapsis). The default central body of a stopping condition is the Earth. To select a different central body for the stopping condition, click the ellipsis button to open the Select Central Body window; click the desired 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 apparent only for stopping conditions that take the coordinate system into account (i.e., 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 the ellipsis button to open the Select Reference Axes window; click the desired 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 apparent 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 user-defined stopping conditions, use this field to specify what kind of value on which you want to stop. Click Edit to open the User Calculation Object Selection window and configure the calculation object. In the User Calculation Object Selection window, 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 additional condition that must be met 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 -- the stopping condition is satisfied when the parameter reaches a value equal to the trip value while increasing. Cross Decreasing -- the stopping condition is satisfied when the parameter reaches a value equal to the trip value while decreasing. Cross Either -- the stopping condition is satisfied when either of the above situations occurs. For Access stopping conditions: Either -- the stopping condition is satisfied when access is gained or lost. Gain Access -- the stopping condition is satisfied only if access is gained. Lose Access -- the stopping condition is satisfied only if access is lost.
Condition	For a Lighting stopping condition, define the lighting event at which the condition will be satisfied: Enter Direct Sun -- the stopping condition is satisfied when the spacecraft enters direct sunlight. Enter Umbra -- the stopping condition is satisfied when the spacecraft enters an umbra region. Exit Direct Sun -- the stopping condition is satisfied when the spacecraft exits direct sunlight. Exit Umbra -- 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 in the course of an orbit propagation). Specify the number of times the condition must be satisfied before the propagation ends or moves on to the designated automatic sequence.
Max Trip Times	Enter the maximum number of times that the stopping condition will be applied - and any resulting automatic sequences executed - during the propagate segment. Once the stopping condition has been met for a number of times equal to the value of this field, the propagate segment will ignore the stopping condition.
Constraints	Click the ellipsis button and, in the selection window 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	This field can be used to set "before" stopping conditions. A "before" stopping condition is used to define a stopping condition that depends on two events. To add "before" stopping conditions, click the ellipsis button and define them as you would a normal stopping condition. When the MCS is run, Astrogator will ignore the normal stopping condition until the "before" stopping conditions defined for it are met. Astrogator will then interpolate backwards to the normal stopping condition just prior to the "before" stopping condition, and this will be defined as the point at which the normal stopping condition is satisfied. The backwards interpolation is subject to the Minimum Propagation Time, as defined by clicking Advanced... - meaning that the backwards interpolation will be performed for at least that specified amount of time before Astrogator will check for satisfaction of the condition.
Condition Inherited by Automatic Sequences	Select 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 will be ignored. Segments that do not normally evaluate stopping conditions (e.g., 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 will be ignored.

Light Time Delay Options

If you have selected an Access stopping condition and are using Light Time Delay in the computation, you can set Light Time Delay Options by clicking Options. The following table describes the light time delay options available:

Option	Description
Signal Sense (Base Object)	Allows you to set the direction of the signal. Transmit-- The signal is being sent from the object you selected as the Clock Host. Receive -- The signal is being received by the object you selected as the Clock Host.
Clock Host	Time values are reported with a clock collocated with the clock host object. Base-- Refers to the object that brought up the Access Tool. Target-- Refers to an object from the Associated Objects list.
Aberration Type	Sets the model of aberration to be used in access computations. The values are: None-- Ignores aberration. Annual-- Considers annual aberration only. Thus, aberration is applied only in cases where the light time delay is computed using the solar system barycenter frame. Total-- Applies the full aberration correction, when light time delay is computed.
Light Time Delay Convergence Tolerance	Sets the tolerance used when iterating to determine the light time delay. The iteration stops when the improvement in the value is less than this tolerance.