Fig. 1-2. Organizations and People that Play Key Roles In Space Missions. During concept exploration users and operators provide requirements, developers create the design of the mission and systems, while sponsors provide funding.

continuing process which culminates in a new program start. Operators and end users develop potential mission requirements based on the considerations shown in the left-hand column of Table 1-3. The process is different for each organization, but at some point a new program begins with a set of mission objectives, concept of operations, and desired schedule. In DoD, the Mission Needs Statement documents this information and becomes part of the planning, programming, and budgeting system [Defense Systems Management College, 1990]. If approved, the program receives funding and proceeds to concept development

At the Program Initiation milestone, the funding organization commits to proceeding through concept development. The program will receive different levels of scrutiny depending on its scope, political interest, and funding requirements. In DoD, programs classed as major programs receive the utmost attention at the highest levels of the Defense Department. Various components of the military use distinct criteria to identify major programs [Defense Systems Management College, 1990]. A DoD critical requirements program is "major" if it requires more than $200 million in research, development, test, and evaluation funds or more than $1 billion in production costs. Programs that require participation by more than one component of the armed forces or have congressional interest may also be classified as major programs.

During Concept Development the developer must generate alternative methods of meeting the operator's and end user's needs. This procedure includes developing and assessing different concepts and components for mission operations, as well as

TABLE 1-3. Further Breakdown of Concept Exploration Phase. During concept exploration the operator and end users define their needs and requirements and pass them to the developing organization for concept development A basic premise of this book is that the operator, user, and developer should work together to create realistic and affordable mission objectives and requirements that meet user needs.

TABLE 1-3. Further Breakdown of Concept Exploration Phase. During concept exploration the operator and end users define their needs and requirements and pass them to the developing organization for concept development A basic premise of this book is that the operator, user, and developer should work together to create realistic and affordable mission objectives and requirements that meet user needs.

Concept Exploration and Definition

Needs Analysis

Concept Development

Generate potential requirements based on Mission objectives Concept of operations Schedule

Life-cycle cost and aftordability Changing marketplace Research needs National space policy Long-range plan for space Changing threats to national defense Military doctrine New technology developments Commercial objectives

Reassess potential requirements generated during needs analysis

Develop and assess alternative mission operations concepts

Develop and assess alternative space mission architectures

Estimate performance supportablllty life-cycle cost produceability schedule funding profiles risk return on Investment

estimating the factors shown in the right-hand column of Table 1-3. The information becomes part of an overall system concept High-level managers in the user, operator, and development communities evaluate whether the concepts, initial mission objectives, and potential requirements meet the mission's intentions. If the program satisfies the need at a reasonable cost, it passes the Requirements Validation milestone and proceeds into the Detailed Development Phase.

This book provides the technical processes and information necessary to explore concepts for many space missions. Table Í-3 identifies a major concern that can undermine the entire process: in many cases, users and operators analyze the needs and formulate mission requirements apart from the development community. Then they pass these requirements "over the wall" without negotiating. The developer often generates alternatives without the operators and users. These autonomous actions produce minimum performance at maximum cost

To explore a concept successfully, we most remove the walls between the sponsor, space operators, users or customers, and developers and become a team.

A good team considers the mission's operations, objectives, and requirements as well as the available technology to develop the best possible mission concept at the lowest possible life-cycle cost

All space missions consist of a set of elements or components as shown in Fig. 1-3. The arrangement of these elements form a space mission architecture. Various organizations and programs define their mission elements differently, although all of the elements are normally present in any space mission.

The subject of the mission is the thing which interacts with or is sensed by the space payload: moisture content, atmospheric temperature, or pressure for weather missions; types of vegetation, water, or geological formations for Earth-sensing missions; or a rocket or intercontinental ballistic missile for space defense missions. We must decide

Fig. 1-3. Space Mission Architecture. All space missions include these basic elements to some degree. See text for definitions. Requirements for the system flow from the operator, end user, and developer and are allocated to the various mission elements.

what part of the electromagnetic spectrum to use in order to sense the subject, thus determining the type of sensor as well as payload weight, size, and power. In many missions, we may trade off the subject For example, if we are trying to track a missile during powered flight, the subject could be the rocket body or exhaust plume, or both.

For communications and navigation missions the subject is a set of equipment on the Earth or on another spacecraft, including communication terminals, televisions, receiving equipment for GPS navigation, or other user-furnished equipment The key parameters of this equipment characterize the subject for these types of missions.

The payload consists of the hardware and software that sense or interact with the subject Typically, we trade off and combine several sensors and experiments to form the payload, which largely determines the mission's cost, complexity, and effectiveness. The subsystems of the spacecraft bus support the payload by providing orbit and attitude maintenance, power, command, telemetry and data handling, structure and rigidity, and temperature control. The payload and spacecraft bus together are called the spacecraft, space segment, or launch vehicle payload.

The launch system includes the launch facility, launch vehicle and any upper stage required to place the spacecraft in orbit, as well as interfaces, payload fairing, and associated ground-support equipment and facilities. The selected launch system constrains the size, shape, and mass of the spacecraft

The orbit is the spacecraft's path or trajectory. Typically, there is a separate initial parking orbit, transfer orbit, and final mission orbit There may also be an end-of-life or disposal orbit The mission oibit significantly influences every element of the mission and provides many options for trades in mission architecture.

The communications architecture is the arrangement of components which satisfy the mission's communication, command, and control requirements. It depends strongly on the amount and timing requirements of data to be transferred, as well as the number, location, availability, and communicating ability of the space and ground assets.

The ground system consists of fixed and mobile ground stations around the globe connected by various data links. They allow us to command and track the spacecraft, receive and process telemetry and mission data, and distribute the information to the operators and users.

Mission operations consist of the people, hardware, and software that execute the mission, the mission operations concept, and attendant policies, procedures, and data flows. Finally, the command, control, and communications (C3) architecture contains the spacecraft, communications architecture, ground segment, and mission operations elements.

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