Displaying the Average Length of the Coverage Gap

Measuring the Average Length of the Coverage Gap

Time Average Gap measures the average length of the coverage gap found if the time line for grid points is sampled randomly. Because Time Average Gap does not possess dynamic behavior, no time-dependent information is computed. STK computes the static definition of Time Average Gap as:

STK Coverage uses an "optimistic" algorithm when coverage gaps that share a boundary with the coverage interval exist. The span of the coverage gap is measured using the coverage interval bound. This is important for the correct interpretation of the Age Of Data, Revisit Time, Time Average Gap and Response Time figures of merit.

The Time Average Gap represents the size of the coverage gap that you would expect to fall into if you selected a random moment in time. It is constructed as a weighted average of the existing gap durations where the weight for each gap is defined as the likelihood that you will select a time within that gap.

For example, say there is an overall coverage analysis duration of 100 time units, with three coverage gaps of duration 5 units, 10 units and 15 units. In this example, the likelihood that the sample would fall into each gap would be (5/100), (10/100) and (15/100), respectively. Additionally, there would be a (70/100) chance of not falling into a gap at all. If you select a time that is not in a gap, you are in a gap of zero duration. The time average gap is computed as the weighted average of the duration of the existing gaps:

Time Average Gap = 5 * (5/100) + 10 * (10/100) + 15 * (15/100) + 0 * (70/100) = 3.5

Note that you do not need the final term since it is identically zero and that all terms involve division by 100. For computational efficiency, the computation is reordered as:

Time Average Gap = (5*5 + 10*10 + 15*15) / 100

You can immediately see that this is the summation of the squared gap times divided by the overall interval length as given above.

Displaying the Average Length of the Coverage Gap in a Graphics Window

Graphics are used to represent the static and dynamic value of Time Average Gap. If Satisfaction is enabled, the graphics follow the general behavior for satisfaction graphics. If Satisfaction is disabled, the graphics appear as described in the following table:

Static or Animation?	Grid points highlighted if they...
Static	are in a coverage gap at some time during the coverage interval, regardless of the average length of the gap

The Satisfaction Threshold value is the average time between access intervals for a point. The default behavior for this Figure Of Merit corresponds to a Satisfaction setting of Greater Than 0.