Step 2 Preliminary Estimate of Mission Needs Requirements and Constraints

Having defined the broad objectives that the space mission is to achieve, we wish to transform them into preliminary sets of numerical requirements and constraints on the space mission's performance and operation. These requirements and constraints wiU largely establish the operational concepts that will meet our objectives. Thus, we must develop requirements which truly reflect the mission objectives and be willing to trade them as we more clearly define the space system.

To transform mission objectives into requirements, we must look at three broad areas:

• Functional Requirements, which define how well the system must perform to meet its objectives.

• Operational Requirements, which determine how the system operates and how users interact with it to achieve its broad objectives.

• Constraints, which limit cost, schedule, and implementation techniques available to the system designer.

The needs, requirements, and constraints for any specific mission will depend upon the mission itself and how we implement it. For example, the mission may be a commercial venture, a government scientific program, or a crash emergency program responding to dire need. Still, most space missions develop their requirements according to the basic characteristics in Table 1-5.

Establishing top-level mission requirements is extremely difficult, depending on mission needs and on the perceived complexity or cost of meeting them. Therefore, contrary to frequent practice, we should iterate the numerical requirements many times in the design process. The first estimate of mission requirements must come from the goals and objectives combined with some view of what is feasible. Often, we can reiterate or slightly modify requirements and specifications from previous missions, thus carrying over information known from those missions. Of course, we must be prepared to trade these requirements as we develop the mission concept, thereby avoiding the problem of keeping old and inappropriate requirements.

The next step in setting up preliminary mission requirements is to look for the "hidden agenda" discussed in Sec. 13 and Chap. 2. This agenda contains the developer's implicit goals and constraints. For example, the FireSat mission may need to be perceived as responding quickly to public demand. Thus, an extended R&D program to develop the most appropriate FireSat satellite may not be acceptable.

As discussal further in Chap. 21, we must recognize that developing a space mission depends on political, legal, and economic elements, as well as technology. Thus, the most appropriate solution must meet mission technical requirements and the developer's political and economic goals. For example, satellite systems for a small nation may use components built in that nation or develop some new components locally, even though they would cost less if bought in other countries. In this case, we would spend more money to meet a political constraint: using the space mission to

TABLE 1-5. Examples of Top-Level Mission Requirements. We typically subdivide these top-level requirements into more specific requirements applicable to specific space missions.

Requirement

Where Discussed

Factors which Typically Impact the Requirement

FlreSat Example

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