Measuring Revisit Time

Revisit Time measures the intervals during which coverage is not provided (also known as “the gaps”). The dynamic definition of Revisit Time computes the duration of the current gap in coverage for each grid point. If a grid point is accessible at the current time, the gap duration is computed as zero. Options for the static definition of Revisit Time are described in the following table.

Compute options for Revisit Time

Option	Description
Average	The computed value is the average of the durations of all the gaps in coverage over the entire coverage interval. If there are N number of gaps, then the average is:
Maximum	The computed value is the duration of the longest gap in coverage over the entire coverage interval.
Minimum	The computed value is the duration of the smallest gap in coverage over the entire coverage interval.
Num Percent Below	The computed value indicates that x percent of the gaps are of a duration less than R, where R is the computed value and x is a Percent Level that you specify. For example, if you specified a value of 90 percent, and there were 100 coverage gaps, the value for Num Percent Below would be the duration of the 90th gap.
Percent Below	The computed value indicates that X percent of the time the point has a revisit time less than or equal to R where X is the percentage that you specified and R is the value that is calculated. See the example below. For instance, the 90th percentile Revisit Time would be computed by using the Percent Below option with a Percent Level of 90.
Percent Below Gaps Only	The computed value indicates that the point has a revisit time of at most R, X percent of the time when in a coverage gap, where R is the computed value and X is a Percent Level that you specify.
Std Deviation	The standard deviation of the duration of gaps in coverage calculated over the entire coverage analysis interval.

If no accesses exist, the revisit time is reported as the duration of the coverage interval.

Example

Consider a point that is analyzed over an interval of 100 minutes. It has the following coverage times (in minutes relative to the beginning of the analysis interval):

Start (min)	Stop (min)	Duration (min)
0.5	5	4.5
10	20	10
22	24	2
28	33	5
45	49	4
58	66	8
80	82.5	2.5
89	93	4
	Total Time	40

The resulting coverage gaps are then (in order of appearance):

Gap Duration (min)	Percentage of Coverage Interval
0	40 percent
0.5	.5 percent
5	5 percent
2	2 percent
4	4 percent
12	12 percent
9	9 percent
14	14 percent
6.5	6.5 percent
7	7 percent

In the first row, the gap duration is zero by definition because there is no gap during the coverage times. The first gap begins with the start of the coverage analysis interval and the last gap ends at the end of the coverage analysis interval.

Now suppose we want to know the 75th percentile revisit time. To calculate this, we sort the gaps by duration with the following results:

Gap Duration (min)	Percentage of Coverage Interval	Cumulative Percentage	Cumulative Percentage (gaps only)
0	40 percent	40 percent	N/A
0.5	0.5 percent	40.5 percent	0.5 percent
2	2 percent	42.5 percent	2.5 percent
4	4 percent	46.5 percent	6.5 percent
5	5 percent	51.5 percent	11.5 percent
6.5	6.5 percent	58 percent	18 percent
7	7 percent	65 percent	25 percent
9	9 percent	74 percent	34 percent
12	12 percent	86 percent	46 percent
14	14 percent	100 percent	60 percent

We then walk through the table, adding up the coverage interval percentages until we reach 75 percent and return the value of the gap duration that caused us to reach 75 percent. In this case, we see that 74 percent of the time the revisit time is 9 minutes or less and 86 percent of the time the revisit time is 12 minutes or less. Therefore, the 75th percentile revisit time is 9 minutes.

This analysis included the coverage times as part of the overall calculation. Sometimes the percentage below value is required to exclude the coverage times and only look at the gaps themselves. In this case, use the Percent Below Gaps Only option. The result is that the first row (zero duration gaps) of the gap duration table above is ignored. The 75th percentile then becomes 14 minutes.

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.

Min # Assets

Use this field to specify the minimum number of simultaneous Assets that are required for Coverage. For example, if you want to calculate the revisit time between stereo imaging opportunities, then you would set this value to 2. STK would calculate the time intervals when two or more Assets are available, then calculate the revisit time between those intervals.

End Gaps

Set this field to Include to have gaps at the ends of the analysis interval included in the revisit time computations. Set it to Ignore to eliminate gaps at the ends of the analysis interval from the revisit time computations. The Ignore option is strongly recommended for the case where Minimum revisit time is to be computed. In the example above, the gaps at the beginning and end of the coverage analysis interval (0.5 min and 7 min, respectively) would be ignored.

Displaying Revisit Time in a Graphics Window

Graphics are used to represent the static and dynamic value of Revisit Time. 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, the graphics appear as described in the following table:

Static or Animation?	Grid points highlighted if they...
Static	have at least one gap in coverage during the coverage interval
Animation	are currently in a gap between access intervals, regardless of the duration of the gap

The Satisfaction Threshold value is the duration between access intervals for a point; the time between accesses is called a coverage gap. The default behavior of this Figure Of Merit corresponds to a Satisfaction setting of Greater Than 0.

STK defines points that are currently in access to be in coverage gaps of zero duration.