Step 5 Identifying System Drivers

System drivers are the principal mission parameters or characteristics which influence performance, cost, risk, or schedule and which the user or designer can control. For example, the temperature at which a forest fire bums may heavily influence how easily it can be detected; however, this is beyond the system designer's control and, therefore, is not a system driver. Correctly identifying the key system drivers is a critical step in mission analysis and design. Misidentifying system drivers is one of the most common causes of mission analysis error. For example, we may focus a lot of time and effort on getting the most coverage for an orbit when the system's ultimate performance depends mainly on data rates or cloud cover.

Table 2-8 lists the most common system drivers for space missions, along with what limits them, what they limit, and where they are discussed. The table helps us ensure that we do not overlook system drivers. In identifying these drivers we must clearly determine whether we are looking for drivers of performance, cost, risk, or schedule. These may be the same or different To identify system drivers, we:

1. Identify the Area of Interest

Explicitly identify the area of interest, typically performance, cost, risk, or schedule.

2. Identify Parameters Which Measure the Area of Interest

Define numerical parameters which measure the identified area of interest (See Sec. 3.4 on measures of effectiveness and performance parameters for more details on how to do this.) The important point is to find parameters which genuinely measure the goal rather than ones which simply are easy to compute.

3. Develop First-Order Algorithms

Develop a formula or algorithm to express the first-order estimate for the value of the parameter identified above. This could include either system algorithms as defined in Sec. 3.1, or unique algorithms for the identified parameter. (See Table 2-9 for the FireSat example.)

4. Examine the Factors

Examine each of the factors in the expression identified above. Those which can be adjusted and which have the strongest effect on results are the system drivers.

5. Look for Possible "Hidden Drivers"

Examine each of the first-order algorithms for implicit variables or for factors affecting more than one characteristic. For example, altitude will influence the ground resolution of a given instrument, the area covered by the field of view, and the spacecraft's velocity relative to the Earth. Therefore, it will more strongly influence effective area search rates than a single formula may show.

TABLE 2-8. Common System Drivers. System drivers can frequently be identified by examining the parameters in this list

Driver

What Limits Driver

What Driver Limits

Where Discussed

Size

Shroud or bay size, available weight, aerodynamic drag

Payload size (frequently antenna diameter or aperture)

Chaps. 9,10

On-orbit Weight

Altitude, inclination, launch vehicle

Payload weight, survivability; largely determines design and manufacturing cost

Sec. 10.4.1

Power

Size, weight (control Is secondary problem)

Payload & bus design, system sensitivity, on-orbit life

Sees. 10.2,11.4

Data rate

Storage, processing, antenna sizes, limits of existing systems

Information sent to user; can push demand for onboard processing

Sec. 13.3

Communications

Coverage, availability of ground stations or relay satellites

Coverage, timeliness, ability to command

Sec. 7.2, Chap. 13

Pointing

Cost, weight

Resolution, geolocation, overall system accuracy; pushes spacecraft cost

Sec, 5.4

Number of Spacecraft

Cost

Coverage frequency, and overlap

Sees. 72,7.6

Altitude

Launch vehicle, performance demands, weight

Performance, survivability, coverage (instantaneous and rate), communications

Sees. 3.3,7.1, 7.4,7.6

Coverage (geometry and timing)

Orbit, scheduling, payload field of view & observation time

Date frequency and continuity, maneuver requirements

Sees. 5.2,72.

Scheduling

Timeline & operations, decision making, communications

Coverage, responsiveness, mission utility

Sec. 3.2.4, Chap. 14

Operations

Cost, crew size, communications

Frequently principal cost driver, principal error source, pushes demand for autonomy (can also save "lost" missions)

Chap. 14

The way we have defined our particular problem, or which parameters are available to us, may affect our list of system drivers. Thus, defining system drivers depends in part on the physical and technical nature of the problem and in part on the constraints imposed on the mission analyst Usually, we want to make these constraints explicit, so we will know which variables are available for adjustment and which are assumed to be given. Table 2-9 shows the major performance drivers for FireSat

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