Pointing Accuracy
The Pointing Accuracy figure of merit considers a set of effects in determining the uncertainty of the instrument's pointing. These effects are the number of vector measurements, the relative geometry of the beacons, and the uncertainty of the vector measurements. You can specify the uncertainty in vector measurements for each measurement as a constant value.
The Pointing Accuracy is related to the Pointing DOP in the definition of pointing DOP through an assumption of a single value of the uncertainty in vector measurements. The uncertainties in vector measurements are assumed to be uncorrelated in both Pointing Accuracy and Pointing DOP computations.
The value reported by the Pointing Accuracy and the uncertainty of vector measurements have angular units.
When using Pointing Accuracy to measure the quality of coverage, you need to specify the following:
- the method for calculating Pointing Accuracy
- the number of assets used to produce solutions
- the method for computing a static Pointing Accuracy value over the entire coverage interval
- the time step to be used when computing the static value of Pointing Accuracy across the coverage interval
Items 1 and 2 specify the dynamic definition of Pointing Accuracy. STK uses this dynamic definition to compute the Pointing Accuracy value for each grid point at the current time. Specifying items 3 and 4 then enables the calculation of a static value of Pointing Accuracy via sampling of the dynamic values.
Method
STK provides several methods to calculate Pointing Accuracy, which are described in the following table.
Compute options
You also need to set the method for computing the static definition for pointing accuracy from the options in the Compute field, which are described in the following table. The available options depend on the selected pointing accuracy and the number of available assets.
Option | Description |
---|---|
Average | This option applies an average pointing uncertainty in each direction over the entire coverage interval. |
Maximum | This option applies the maximum pointing uncertainty in each direction over the entire coverage interval. |
Minimum | This option applies the minimum pointing uncertainty in each direction over the entire coverage interval. |
Percent Below | The value of the pointing uncertainty is less than the computed value X percentage of time, where X is a Percent Level that you specify. |
Number of assets for computations (Type)
Although two beacons are needed for the attitude solution, you can use additional beacons to improve the accuracy of the solution. Options in the Type field are described in the following table.
The pointing accuracy type that you choose affects the dynamic and static pointing accuracy definition of the figure of merit.
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.
Direction to asset uncertainties
Click Uncertainties... to assign a direction to asset uncertainty value to an asset. The Figure of Merit Uncertainties Model dialog box will appear. The Direction to Asset Uncertainty Value is a constant value that represents angular standard deviation of measurements away from the direction between the object on which the coverage is defined and a particular asset. Constant values are specified in units of degrees.
Graphics display of pointing accuracy coverage
STK uses graphics to represent the static and dynamic values of Pointing Accuracy. Graphics representing the dynamic values are displayed only during animation. Select Enable under Satisfaction if you want the graphics to follow the general behavior for satisfaction graphics. If Satisfaction is not selected and no contour levels have been defined, then no graphics will display on the Attitude Sphere. See the Satisfaction help topic for details on settings. There are no default graphics for the Pointing Accuracy figure of merit. The satisfaction threshold value is the pointing Accuracy value.