覆盖品质

如同评价人一样，是否诚实、正直，英勇、果敢、都是考察的要素，卫星覆盖品质也是一个多维度的评价，STK给出了下列的表格，这些都是考察的因子，因为实在太多，我们准备挑选几个因子，视图在这几个因子上，将我们自己开发的软件同STK做个比较。

Figures of Merit

STK allows you to specify the method by which the quality of coverage is measured using the options in the FigureOfMerit Definition page. The following Types are available:

Access Constraint	Access Duration
Access Separation	Age of Data
Coverage Time	Dilution of Precision
N Asset Coverage	Navigation Accuracy
Number of Accesses	Number of Gaps
Response Time	Revisit Time
Scalar Calculation	Simple Coverage
System Response Time	Time Average Gap

我们先看看 simple Coverage的描述

Measuring Simple Coverage

Simple Coverage measures whether or not a point is accessible by any of the assigned assets.An evaluation of the dynamic behavior of simple coverage computes a value of 1 for points that are currently in an access period and 0 for points that are not. The static behavior of simple coverage computes a value of 1 for grid points that have access to an asset at any point in time and 0 for points that are not accessible.The Satisfaction option is not available for this Figure Of Merit since satisfaction is implicit in the definition of simple coverage.

简单覆盖指的是某点是否被至少一个设备（Assets 指的是卫星、火箭、飞机等，不知如何翻译Asset, 我姑且翻译为设备）访问。简单覆盖又分为动态的简单覆盖和静态的简单覆盖。动态覆盖给每个在当前访问周期可以访问的点计数1，给不能访问的点，计数0。静态简单覆盖同样也是给点计数，不同的是，它考虑的时间周期不一样，只要在整个时间段可以访问到的点，都计数为1. 那么怎么定义 access period 呢？

Displaying Simple Coverage in a Graphics Window

Graphics are used to represent the static and dynamic value of Simple Coverage.

Static or Animation?	Grid Points highlighted if they...
Static	are covered by at least one asset at some point during the coverage interval
Animation	are currently covered by at least one asset

简单覆盖的显示方式，分静态显示和动画显示两种。静态方式：只要一个点被覆盖过，就高亮显示该点。

Figures of Merit & the Coverage Interval

When analyzing figures of merit that compute average values of a quantity over the coverage interval or on a per-day basis, it is important to use a coverage interval of sufficient duration for the result to "smooth out." Results based on intervals of short duration may be heavily biased by the conditions at the beginning or end of the interval or may not represent a sufficient variation of geometry between the assets and coverage area. This is especially true when the number of assets is small or there is a repeating ground track pattern.

FOM 和覆盖间隔

在计算质量平均值（基于时间区间或每天）的时候，为了得到光滑的结构，使用有足够持续长度的时间区间很重要。短持续时间的区间可能导致严重偏差。。。。

Defining a Figure of Merit's Dynamic & Static Behavior

Figures of merit can exhibit two types of behavior: dynamic and static. The dynamic behavior of a Figure Of Merit allows you to compute values corresponding to a specific time. Not all figures of merit can exhibit dynamic behavior. Those which do not exhibit dynamic behavior are not able to compute time-dependent information.

FOM动态行为让你针对特定时间计算数值，并非所有的merit能展现动态行为。

The static behavior for a Figure Of Merit specifies the value of quality over the entire coverage period. Depending on the size of the coverage problem being analyzed, the computation of the static values for each grid point can be time consuming. For this reason, the static values are cached to allow for rapid generation of reports and modifications to graphical representations once the static values have been computed. The computed static values will also be saved and restored with the scenario if the Save Mode of the parent Coverage Definition object is set to “Save accesses”.

静态行为描述了完整覆盖周期的质量值，计算静态行为的值，可能很耗时，静态值也被保存在场景中，假如......

The dynamic and static values for the grid points are computed to support graphical displays, reports and graphs for the Figure Of Merit.

Computing Per-day Values

Some figures of merit allow you to compute values on a per-day basis. Three quantities (minimum, maximum and average) are possible. The average per-day selection is computed as a value for the entire coverage interval divided by the duration of the coverage interval in days. For example, if a grid point is covered for nine hours of a 1.5-day coverage interval, the average per-day value would be 9 divided by 1.5 for an average of 6 hours a day.

有些FOM计算的是一天的值，最小、最大和平均值都有可能。平均覆盖的定义： n天内某点的覆盖时间/n天 = 某点每天平均覆盖时间。

The minimum and maximum per-day values are computed in a slightly different manner. First, calendar day intervals are determined using the coverage interval. The value of the Figure Of Merit across each interval is then determined and the minimum or maximum of these values is reported. Calendar days are determined based on Coordinated Universal Time (UTC).

最小和最大每天覆盖时间的计算方式略有不同，首先，。。。。。。这段话很难懂，不过结合下面这个图，就很容易理解，10月2日这一天，一共干了7+6+3=16小时，这就是最大值，最小覆盖发生在10月3日这一天，这个区间被截断了，10月3日只占了这个区间的2个小时，故此，min per day为2小时。

Figure of Merit Computation-Sampling versus Interval Method

Figure of merit values are functions of the visibility times between grid points and assets or, in the case of Attitude Coverage, from the associated object pointed via the grid points to the assets. The computation of values for figures of merit are usually computed in one of two ways: using a sampling method or using direct computations based on access intervals.

FOM 是可见时间的函数，通常有两种计算方式。基于采样的和基于访问间隔的直接计算。

In the sampling method, a sequence of times is determined or accumulated for each grid point to be used for sampling of the Figure Of Merit. An example of this method would be to evenly distribute 100 times across the coverage interval and compute the percentage of time covered for each point as the number of times when the point is able to access an asset (or, in the case of Attitude Coverage, when the associated object pointed via the point is able to access an asset). The accuracy of the answer produced by sampling is dependent on the frequency of the sampling and the duration of the accesses between grid points and assets.

在采样计算方法中，针对每个网格点，时间序列被决定或累积，用来计算FOM. 举例，采样点在时间区间内均匀分布100次，对每个点来说，他的时间覆盖百分比就是它能被Assets看到的次数。

STK Coverage uses algorithms based directly on the access intervals whenever possible to ensure accurate Figure Of Merit values. For example, Percent Time Covered is computed as the sum of the total time covered for each point divided by the duration of the coverage interval. This yields the same result as using a sampling method with an infinitely small sample time.

STK覆盖分析尽可能采用的直接基于access intervals的算法，来确保FOM的准确性。例如，时间覆盖百分比就是这样计算的，将每个点的总覆盖时间求和，除以时间长度。

The only figures of merit that use a sampling method are Dilution of Precision, Navigation Accuracy, Pointing Accuracy, Pointing DOP and Access Constraint figures of merit since direct solutions are not possible.

Dilution of Precision, Navigation Accuracy, Pointing Accuracy, Pointing DOP and Access Constraint 只能用采样的方法计算。

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