Mission Evaluation

James R. Wertz, Microcosm, Inc.

3.1 Step 7: Identification of Critical Requirements

3.2 Mission Analysis

The Mission Analysis Hierarchy; Studies with Limited Scope; Trade Studies', Performance Assessments

3.3 Step 8: Mission Utility

Performance Parameters and Measures of Effectiveness: Mission Utility Simulation; Commercial Mission Analysis and Mission Utility Tools

3.4 Step 9: Mission Concept Selection

Chapter 2 defined and characterized alternative concepts and architectures for space missions. This chapter shows how we evaluate the ability of these options to meet fundamental mission objectives. We address how to identify the key requirements which drive the system design, how to quantify mission performance, and how to select one or more concepts for further development or to decide that we cannot achieve the mission within current constraints or technology.

Although essentially all missions go through mission evaluation and analysis stages many times, there are relatively few discussions in the literature of the general process for doing this. Fortescue and Stark [1995] discuss the process for generic missions; Przemieniecki [1993, 1994] does so for defense missions; and Shishko [1995] provides an excellent overview for NASA missions. Kay [1995] discusses the difficulty of doing this within the framework of a political democracy and Wertz and Larson [1996] provide specific techniques applicable to reducing mission cost

Hie key mission evaluation questions for FireSat are:

• Which FireSat requirement dominates the system design or is the most difficult or expensive to meet?

• How well can FireSat detect and monitor forest fires, and at what cost?

• Should the FireSat mission evaluation proceed, and if so, which alternatives should we pursue?

We must readdress these questions as we analyze and design the space mission. By addressing them when we first explore concepts, we cannot obtain definitive answers. But we can form the right questions and identify ideas, parameters, and requirements we should be monitoring throughout the design. More extensive discussions of this systems engineering process are provided by Rechtin [1991] and the System Engineer ing Management [Defense Systems Management College, 1990]. The NASA Systems Engineering Handbook [Shishko, 1995] provides an excellent and detailed account of the process used by NASA. Przemieniecki [1990a, b] provides a good introduction to mathematical methods associated with military programs and has an associated software package. Other software packages intended specifically to support mission evaluation include Satellite Tool Kit (STK) from Analytical Graphics (1998), the Mission Utility/Systems Engineering module (MUSE) from Microcosm (1998), and the Edge product family from Autometric (1998).

3.1 Step 7: Identification of Critical Requirements

Critical requirements are those which dominate the space mission's overall design and, therefore, most strongly affect performance and cost*. For a manned mission to Mars, the critical requirements will be clean get to Mars all of the required mass to explore the planet and return, and maintain crew safety for a long mission in widely varying environments. For less ambitious space missions, we cannot establish the critical requirements so easily. Because we want to achieve the best performance at minimum cost, we need to identify these key requirements as early as possible so they can be a part of the trade process.

Table 3-1 lists the most common critical requirements, the areas they typically affect, and where they are discussed. There is no single mechanism to find the critical requirements for any particular mission. Like the system drivers discussed in Sec. 2.3, they may be a function of the mission concept selected. Consequently, once we establish the alternative mission concepts, we usually can determine the critical requirements by inspection. Often, concept exploration itself exposes the requirements which dominate the system's design, performance, and cost. One approach to identification of critical requirements is as follows:

1. Look at the principal performance requirements. In most cases, the principal performance requirement will be one of the key critical requirements. Thus, for FireSat, the requirements on how well it must detect and monitor forest fires would normally be principal drivers of the system design.

2. Examine Table 3-1. The next step is to look at the requirements list in Table 3-1 and determine which of these entries drive the system design, performance, or cost.

3. Look at top-level requirements. Examine each of the top-level requirements established when we defined the mission objectives (Sec. 1.3) and determine how we will meet them. For each, ask whether or not meeting that requirement fundamentally limits the system's design, cost, or performance.

4. Look for hidden requirements. In some cases, hidden requirements such as the need to use particular technologies or systems may dominate the mission design, and cost

* Critical requirements should be distinguished from system drivers (as discussed in Sec. 23), which are the defining mission parameters most strongly affecting performance, cost, and risk. The goal of mission engineering is to adjust both the critical requirements (e.g., coverage and resolution) and the system drivers (e.g., altitude and aperture) to satisfy the mission objectives at minimum cost and risk.

TABLE 3-1. Most Common Critical Requirements. See text for discussion.

Requirement

What It Affects

Where Discussed

Coverage or Response Time

Number of satellites, altitude, inclination, communications architecture, payload field of view, scheduling, staffing requirements

Sees. 7.2,1 3.2

Resolution

Instrument size, altitude, attitude control

Sec. 9.3

Sensitivity

Payload size, complexity; processing, and thermal control; altitude

Sees. 9.5,13.5

Mapping Accuracy

Attitude control, orbit and attitude knowledge, mechanical alignments, payload precision, processing

Sec. 5.4

Transmit Power

Payload size and power, altitude

Sees. 11.2,13.5

On-orblt Lifetime

Redundancy, weight power and propulsion budgets, component selection

Sees. 6.2.3, 8.1.3,10.4,19.2

Survivability

Altitude, weight power, component selection, design of space and ground system, number of satellites, number of ground stations, communications architecture

Sec. 8.2

For most FireSat approaches, resolution and coverage are the principal critical requirements, and we could find them by any of the first three options listed above. The critical requirements depend on a specific mission concept For the low-cost FireSat of Chap. 22, they are coverage and sensitivity. Resolution no longer concerns us because the sensing is being done by ground instruments whose positions are known well enough for accurate location.

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