category (C). Here we might have a gravity gradient-stabilized system accurate to a few degrees with no attitude determination cost at all, an horizon sensor system accurate to 0.05-0.10 deg, or a much more expensive star sensor system accurate to better than 0.01 deg (see Sec. 11.1).

This process allows us to balance cost between the appropriate components and to go back to the mission definition and adjust the real requirements. For example, achieving a mapping accuracy of 100 m on the ground might triple the cost of the space mission by requiring highly accurate attitude determination, a new system for determining the orbit, and a detailed list of target altitudes. Reducing the accuracy requirement to 500 m might lower the cost enough to make the mission possible within the established budget constraints. This is an example of trading on mission requirements, described in Chaps. 2 to 4. Requirements trading is extremely important to a cost-effective mission, but we often omit this in the normal process of defining mission requirements.

To carry out this trade process, we need to know how an error in each of the components described in Table 5-5 relates to the overall mapping and pointing errors.

Table 5-6 gives formulas relating the errors in each of the seven basic components to the overall error. Here the notation used is the same as in Fig. 5-20. For any given mission conditions, these formulas relate the errors in the fundamental components to the resulting pointing and mapping accuracies. Table 5-6 provides basic algebraic information which we must transform into specific mapping and pointing requirements for a given mission. The general process of deriving these requirements is given below. Representative mapping and pointing budgets based on these formulas are given in Table 5-7

TABLE 5-7. Representative Mapping and Pointing Error Budgets. See Rgs. 5-21 and 5-22 for corresponding plots.


Error hi Source

Error Budgets

Mapping Error (km)

Pointing Error (deg)

£=10 deg

£=30 deg


6=30 deg

Attitude Errors:

0 0

Post a comment