Other Errors


Budget ) TisW Moderate Loose Position

Budget J Loose Moderate Tight

Design Margin

Other Errors


Budget ) TisW Moderate Loose Position

Budget J Loose Moderate Tight

Fig. 4-6. Typical Options In Error Budgets for Attitude and Position. Variations in attitude and ephemeris accuracy requirements have implications on allocation and attendant design risk. A balance of cost, performance, and implementation risk must enter the evaluation of options. Details of mapping budget development are given In Sec. 5.4.

risks degraded performance without a full constellation. Resorting to remote tracking stations or other sources of information can require excessive response times. A third option allows some risk for both attitude and position error budgets, but balances that risk against the cost of achieving the required geopositioning accuracy.

Table 4-2 lists the elements we would normally budget with the chapter and paragraph where we discuss each element. Budgeted items may come directly from requirements such as geolocation or timing, or they may be related to elements of the overall system design such as subsystem weight, power, or propellant.

Timeline budgets at the system level are also typical mission drivers. For FireSat, tight timelines for tip-off response and data distribution will require developing an initial budget We must define and decompose all functions necessary to meet this timeline, as well as define their allocation and control sequences (functions which cannot start without completion of others and potential data hand-offs). Simulation will help us estimate delays in processing and communication. Applying experience or data from related systems provides some calibration. But this initial budget is just that, since as the design process progresses, we will introduce changes from design iterations among different levels.

It is, however, extremely important to recognize the nature of initial design budgets. They are typically developed by system engineers with a broad understanding of the

TABLE 4-2. Elements Frequently Budgeted In Space Mission Design. Primary budgets are directly related to mission requirements or ability to achieve the mission (e.g., weight). These primary requirements then flow down Into secondary budgets.



Where Discussed



Sees. 10.3,10.4 Sees. 10.3,10.4,11.4 Sees. 10.3,10.4,17.4

GeolocaSon or System Pointing Errors

Pointing & Alignment Mapping Attitude Control Attitude Determination Position Determination

Sees. 4.2,10.4.2,11.1 Sees. 4.2,10.4.2,11.1 Sees. 4.2,6.1


Coverage Communications Operations Processing

Sees. 52,7.2 Sec. 13.1 Sec. 14.2 Sec. 16.2.1


Reliability Operations

Sees. 10.5.2,19.2 Sec. 14.2


Development cost Deployment cost

Operations and maintenance cost

Sec. 20.3 Sec. 20.3 Sec. 20.3

system and its elements. But the details of new technology and lower-level design studies can and should result in adjustments to these budgets as experts familiar with specific subsystem and component design review the initial allocations. A key aspect of the system design is a robust initial allocation (i.e., one which can tolerate changes at subsequently lower design levels) and adaptable to iterations as noted previously. Just as it is important to involve representatives of all affected levels of design in the development of the initial budgets, it is also important to recognize the iterative nature and that a system solution which minimizes total cost and risk may impose more stringent demands on certain aspects of lower-level designs than others. The process of reconciling the imposed costs and allocated risks involves a high degree of negotiation.

Table 4-3 shows how the response timeline may affect the space and ground segments of the system. While it may seem desirable to assign responsibility for a specified performance parameter to a single segment, we must evaluate and integrate critical system parameters across segments. For example, FireSat must respond quickly to tip-offs in order to provide the user timely data on suspected fires. This single response requirement alone may define the size and orbit envelope of the satellite constellation to ensure coverage when needed. Thus, time budgets for the following chain of events will be critical to the mission control segment's performance:

• Formulating the schedule for pass & time intervals

• Developing and scheduling commands to the spacecraft

• Developing, and checking constraints on the command load

• Establishing communications with the spacecraft

TABLE 4-3. Impact of Response-Time Requirement on FlreSafs Space and Ground Segments. The assumed requirement is for fire data to be registered to a map base and delivered to a user within 30 min of acquisition.

Impact on Space Segment

Spacecraft constellation accessibility to specified Earth coordinates Command load accept or Interrupt timelines Communication timelines to ground segments Satellite availability

Impact on Ground Segment

Time to determine and arbitrate satellite operations schedule

Manual Interrupt of scheduled operations

Command load generation and constraint checking time

Availability of mission ground segments and communications

Image processing timelines

Image sorting and distribution timelines

The space and ground segment budgets may involve interrupting current command loads, maneuvering the spacecraft, collecting die mission data, establishing communications links scheduled from the ground, and communicating the mission data. Mission data processing must receive, store, and process the mission data, sort it by user or by required media, and send it to the user. We must consider all of these activities in establishing budgets to meet the system requirement of delivering specified data and format within 30 min of acquiring iL

Requirements Budget Allocation Example

Pointing budget development, described in Sec. 5.4, is a problem on space missions using pointable sensors. Another common budget example is the timing delay associated with getting mission data to end users. It can be a critical requirement for system design, as is the case of detecting booster plume signatures associated with ballistic missile launches. In that case, coverage (i.e., the time from initiation of a launch to initial detection) as well as the subsequent transmission, processing, distribution, and interpretation of the detection, is time critical. Because of the severe coverage requirement, geosynchronous satellites with sensitive payloads and rapid processing are needed.

The FireS at mission does not require timing nearly as critical as missile detection, but clearly the detection of forest fires is a time-sensitive problem. Figure 4-7 shows both the timeline and the requirements budget associated with it For FireSat's Earth coverage (i.e., Time Segment 1), it would be ideal to provide continuous surveillance using a geosynchronous satellite. However, cost and ground resolution favor a low-Earth orbit implementation which results in Time Segment 1 being three to six hours, depending principally on the number of satellites in the constellation.

Once detection occurs, a series of shorter timeline events must occur to achieve the 30-minute requirement for Time Segment 2. The system may need to validate each detection to minimize the number of false alarms transmitted to the ground for processing. This may impose design specifications for onboard detection processing and additional payload "looks." The time spent downlinking the data after validating a detection could have a significant impact on the communications architecture that assures rapid acquisition of the required links. The availability of direct or relay links

Detectable Fire

Time Segment 1

Actual Detection

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