Hold Segment

Use the Hold segment to model landing or rendezvous operations by setting the spacecraft to maintain a fixed position in reference to another object or body, until meeting specified conditions. The segment holds the state (the final state of the previous segment) constant in the Hold Frame. It adds points to the ephemeris, separated in time by the step size, that have a constant position and zero velocity in the Hold Frame. The hold segment continues until meeting the stopping conditions.

To define a Hold segment, you must set the general parameters to describe the nature of the holding operation, and then specify stopping conditions that determine when the hold is completed.

The following table describes the general parameters of the Hold segment:

Field	Description
Step Size	Specify the time interval between calculated ephemeris output points.
Hold Frame	Specify the reference frame of the Hold segment. The default coordinate system of a Hold segment is Earth Inertial. To select a different coordinate system for the segment, click to open the Select Reference Axes window; click the desired coordinate system to highlight it and then click OK.
Hold Attitude	Select to fix the spacecraft's attitude within the hold frame.

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 following table describes the constraint parameters:

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 ends regardless of whether the stopping conditions are satisfied.
Warning Message on Maximum Propagation Time	Select to have Astrogator issue a warning message if propagation stops because of 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.
Stop Immediately for Initially Surpassed Epoch Stopping Conditions	Select to immediately stop propagation on a particular segment if the propagation begins beyond an active Epoch Stopping Condition.

Stopping Conditions

Stopping conditions are Astrogator components used by the Hold segment to define the point at which it ceases the hold operation. When more than one stopping condition is active for the segment, the hold operation 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 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

Each stopping condition assigned to a segment displays in the table as a separate item and the properties of the currently selected stopping condition display 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.
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 stopping conditions are applied:

Field	Description
Trip	Specify the desired value - the value at which the condition is 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 displays 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 base 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 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.
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 (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 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 used 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 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 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 window and 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: 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 gaining or losing access. Gain Access - the stopping condition is satisfied only if gaining access. Lose Access - 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 - 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). Specify the number of times the condition must be satisfied before the condition is considered to be met.
Constraints	Click 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.

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)	Enables 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 use 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 computing light time delay.
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.