Selected NASA Requirements Formats

A very large system that is being procured by the National Aeronautics and Space Administration (NASA) is called EOSDIS, the Earth Observing System Data and Information System. This system is likely, when considering all related costs, to be in the multibillion dollar range. The preliminary requirements for this system were documented in a rather voluminous NASA report [8.5]. EOSDIS was divided into three major segments, each of which was further delineated in terms of functions, as shown in Exhibit 8.2. We note the functional description as a key aspect of defining the system.

Exhibit 8.2: Functional Description of EOSDIS [8.6]

1. Flight Operations Segment Functions: 1.1 Mission Control

1.2 Mission Planning and Scheduling

1.3 Instrument Command Support

1.4 Mission Operations

2. Science Data Processing Segment Functions: 2.1 Data Processing

2.2 Data Archive

2.3 Data Distribution

2.4 Data Information Management

2.5 User Support for Data Information

2.6 User Support for Data Requests

2.7 User Support for Data Acquisition and Processing Requests

3. Communications and System Management Segment

Functions: 3.1 Distribution of EOS Data and Information to EOSDIS Nodes

3.2 Distribution of Data Among Active Archives

3.3 Interface with External Networks

3.4 Network/Communications Management and Services

3.5 System Configuration Management

3.6 System/Site/Elements Processing Assignment and Scheduling

3.7 System Performance, Fault, and Security Management

3.8 Accounting and Billing

This functional description sets the stage for all further systems engineering activities and is central to the systems engineering process. NASA, in turn, establishes a set of requirements for the system for each and every function. At the top (system) level, however, NASA elected to organize the requirements as shown in Exhibit 8.3:

Exhibit 8.3: Organization of EOSDIS Systemwide Requirements [8.5]

5.0 EOSDIS SYSTEMWIDE REQUIREMENTS

5.1 OPERATIONAL REQUIREMENTS

5.2 FUNCTIONAL REQUIREMENTS

5.3 PERFORMANCE REQUIREMENTS

5.4 EXTERNAL INTERFACES

5.4.1 EOS Project

5.4.2 Space Station Information System (SSIS)

5.4.3 Networks and NASA Institutional Elements

5.4.4 Cooperating Institutions

5.4.5 The EOSDIS User Community

5.4.6 International Partners

5.5 SECURITY

5.5.1 Technical Security

5.5.1.1 Computer Systems Selection Criteria

5.5.1.2 EOSDIS Security Procedures

5.5.2 Physical Security

5.5.3 Contingency

5.5.3.1 Restart/Recovery

5.5.3.2 Software/Data

5.5.3.3 People

5.5.3.4 Equipment

5.6 RELIABILITY, MAINTAINABILITY, AVAILABILITY (RMA)

5.6.1 Reliability

5.6.2 Maintainability

5.6.3 Operational Availability

5.6.4 EOSDIS System-Level RMA

5.6.5 Fault Detection Requirements

From Exhibit 8.3, two important points are

1. The system is described by function, and specific requirements are defined for each such function.

2. A set of top-level requirements is also defined, which applies at the system level.

An example of the system-level requirement for reliability is as follows:

The reliability for EOSDIS shall be measured by the Mean Time Between Failures (MTBF) for a system or component over the entire life cycle of the equipment. Failures are those equipment and software malfunctions which result in interruptions in service. Interruptions in service resulting from external factors beyond the control of EOSDIS . . . shall not be considered failures unless EOS-DIS equipment or software contribute to the interruption.

Beyond this systemwide reliability requirement, NASA invokes two military standards and one handbook:

• Mil-Std-785B for Reliability

• Mil-Hdbk-472 for Maintainability Prediction

• Mil-Std- 470A for Maintainability Status Reporting

The reader interested in NASA approaches to requirements definition may refer to the EOSDIS requirements specification [8.5] and similar requirements documents for other NASA programs [8.6, 8.7].

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