Choosing a Type of Access Constraint | Choosing the Method for Selecting the Constraint Value | Choosing the Statistic to Be Used in the Static Definition | Specifying a Time Step | Displaying Access Constraints in a Graphics Window
Measuring Access Constraints
- Type of constraint to be measured
- Value of the constraint to be selected based on all currently available coverage assets
- Statistic to be computed as the static value
- Time Step to be used during the computation of the static value
The statistical value of the constraint across the set of available assets is needed because the Figure Of Merit can only compute one number for a grid point at a specific time.
Minimum, maximum or average value of the specified access constraint as applied to all visible coverage definition assets for each grid point at the current time.
The dynamic definition of the Access Constraint Figure Of Merit is the minimum, maximum or average of the specified access constraint type as applied to all coverage definition assets for each grid point at the current time.
Not all constraints available through the Constraint Properties of an object are available for use in the Figure Of Merit. Available selections for the Figure Of Merit are limited to those that provide meaningful coverage information.
The static definition of the Access Constraint Figure Of Merit is computed based on sampling the dynamic definition at the specified time step.
The computation of the static value of the Access Constraint Figure Of Merit is done via sampling over the coverage time period at each grid point. This is a computationally very intense process for which the required time scales linearly with the duration of the coverage interval and the number of grid points. Static computations are needed to support static satisfaction graphics, static contours and any reports and graphs displaying static definition results. In some cases, such as the evaluation of coverage from geostationary spacecraft, the values of the Access Constraint parameter may have very little variation in time. In these cases, analyses may often be performed over shorter time periods or at larger step sizes without significantly affecting the results. The dynamic contour graphics in these cases may also provide a very good representation of the static values in a much shorter time.
In order for access constraint values to be computed for assets providing coverage to a grid point location, the identity of the asset must be discernible. As this is not possible for grouped assets (constellations or chains with a constellation at the end where the Grouping option on the asset is set to Grouped), the computation of accurate figure of merit values is not possible.
Choosing a Type of Access Constraint
The types of constraints available to you through the Constraint field depend on the type of object associated with the grid points. For descriptions, see Figure of Merit Access Constraint Types.
By default, grid points are associated with the target object class. You can change this default object by using the Grid Constraint Option... button on the Grid page. If you have not associated an object type with grid points, the constraints applied are those defined for a target. If you have associated an object type with grid points using the Grid Constraint Option... feature, the constraints depend on the type of object chosen.
Choosing the Method for Selecting the Constraint Value
Once you have chosen the type of constraint to be computed for the Figure Of Merit, you also need to specify which value of that constraint is to be selected based on all currently available assets. Choices are discussed in the following table.
Options for applying constraints across assets
| Option | Description |
|---|---|
| Average | Constraint values are computed for all currently available assets and the average is selected |
| Maximum | Constraint values are computed for all currently available assets and the maximum is selected |
| Minimum | Constraint values are computed for all currently available assets and the minimum is selected |
| Sum | Constraint values are computed for all currently available assets and the sum is selected |
Choosing the Statistic to Be Used in the Static Definition
You also need to select the statistic used to define the static value of the access constraint using the options in the Compute field. Options are discussed in the following table.
Compute options for Access Constraints
If you select Minimum or Maximum, additional samples of the dynamic definition are added when the trend of values indicate that an extreme has occurred between samples and at the Start and End times for all access intervals.
Example of sampling access intervals to determine the Maximum statistic
In all cases, sample values are only computed at sample times when at least one asset has visibility to the grid point.
Specifying a Time Step
In the Time Step field, enter the value to be used during the sampling of the dynamic definition for use in the static definition.
Displaying Access Constraints in a Graphics Window
Graphics are used to represent the static and dynamic value of Access Constraints. Graphics representing the dynamic values are only displayed during animation. If Satisfaction is enabled, the graphics follow the general behavior for satisfaction graphics. If Satisfaction is disabled and no contour levels have been defined, no graphics display in the window. There are no default graphics for the Access Constraint Figure Of Merit.
The Satisfaction Threshold value depends on the access constraint imposed on the Figure Of Merit.
The use of Accumulation graphics options for Up to Current and Not Up to Current are not recommended for this Figure Of Merit since both options result in decreased animation.
During periods when no assets have access to a grid point, the dynamic (animation) value returned by the Figure Of Merit will be the value specified by the invalid data indicator. If this values lies within the range of values being used to display contours, unexpected graphical depictions may result.