Step 3 Identifying Alternative Mission Concepts

The broad mission concept is the most fundamental statement of how the mission will work—that is, how it gets its data or carries out the mission to satisfy the end user's needs. The mission concept as we are using it here consists of the four principal elements in Table 2-1. Notice that most of these elements are somehow associated with data or information. Except for manufacturing in space and a small number of other space payloads, most space missions are concerned fundamentally with the generation or flow of information. Thus, FireSat's mission is to generate and communicate to an end user information about forest fires. Communications satellites move data and information from one place to another. Weather, surveillance, and navigation satellites are all concerned with generating and communicating information. Thus, data flow is centra] to most space missions. How will FireSat determine where a fire is and how big it is? How will the system communicate that information to the fire fighter in a truck or plane? Once we answer these broad questions, we begin to understand FireSat's abilities and limits.

TABLE 2-1. Elements of the Mission Concept of Operations. See Table 2-2 for a list of key trades and where discussed. Note that we discuss communications architecture in Sec. 13.1.

Element

Definition

FireSat Example

Data Delivery

How mission and housekeeping data are generated or collected, distributed, and used

How is imagery collected? How are forest fires identified? How are the results transmitted to the fire fighter In the field?

Communications Architecture

How the various components of the system talk to each other

What communications network is used to transmit forest fire data to the users in the field?

Tasking, Scheduling, and Control

How the system decides what to do in the long term and short term

What sensors are active and when is data being transmitted and processed? Which forested areas are receiving attention this month?

Mission Timeline

The overall schedule for planning, building, deployment, operations, replacement, and end-of-life

When will the first FireSat become operational? What Is the schedule for satellite replenishment?

As Table 2-2 shows, defining the mission concept consists of defining the various options that are available and then selecting the most appropriate. Section 2.2 describes how we define options and take a first cut at the broad choices available to us. The process of selecting among them described in Sec. 3.2 is called system trades. Here we are interested in what these trades are and what some of the broader alterna tives are to generate and transmit data. The process of defining how to transmit the data between the spacecraft and various users and controllers on the ground is called the communications architecture and is discussed in Chap. 13.

TABLE 2-2. Process for Defining the Mission Concept of Operations. See Table 2-1 for definitions and FireSat example.

Step

Key Trades

Where Discussed

1. Define data delivery process for - Mission and housekeeping data

Space vs. ground processing

Level of autonomy

Central vs. distributed processing

Sec. 2.1.1 Chap. 13

2. Define tasking, scheduling, and control for

- Mission and housekeeping data

- Long term and short term

Level of autonomy Central vs. distributed control

Sec. 2.1.2

3. Define communications architecture for - Mission and housekeeping data

Data rates bandwidth Timeliness of communications

Sec. 13.1

4. Define preliminary mission timeline for

- Concept development

- Production and deployment

- Operations and end-of-life

Replenishment and end-of-life options Deployment strategy for multiple satellites Level of timeline flexibility

Sec. 2.1.3

5. Iterate and document

N/A

N/A

The mission timeline differs from other elements of the mission concept in Table 2-1. It represents the overall schedule for developing, planning, and carrying out the mission. This defines whether it is a one-time only scientific experiment or long-term operational activity which will require us to replace and update satellites. In either case, we must decide whether the need for the mission is immediate or long term. Should we give high priority to near-term schedules or allow more extensive planning for the mission? Of course, much of this has to do with the funding for the mission: whether money is available immediately or will be available over time as we begin to demonstrate the mission's usefulness.

2.1.1 Data Delivery

Space missions involve two distinct types of data—mission data and housekeeping data. Mission data is generated, transmitted, or received by the mission payload. This is the basic information that is central to what the mission is all about For FireSat, this data starts out as infrared images on a focal plane and ends up as the latitude, longitude, and basic characteristics of a forest fire transmitted to a fire fighter on the ground. The mission data has potentially very high data rates associated with it. However, the need for this data may be sporadic. TTius, FireSat may generate huge quantities of raw data during periods of time that it is passing over the forests, but there is little need for this same level of data when it is over the poles or the oceans.

Ultimately, the processed mission data may go directly to the end user or through ground stations and communication networks associated with mission operations. This will, of course, have a fundamental effect on how the mission works. In the first case, FireSat would process its imagery and send the forest fire information as it is being observed to the fire fighters in the field. In the second case, data would go instead to an operations center, where a computer system or human operators would evaluate it, compare it with previous data, and determine the location and characteristics of a forest fire. Then, the operations center would transmit this information to the fire fighters in the field. The result is about the same in both cases, but the system's abilities, limits, characteristics, and costs may be dramatically different

In contrast to the mission data, housekeeping data is the information used to support the mission itself—the spacecraft's orbit and attitude, the batteries' temperature and state of charge, and the status and condition of the spacecraft's parts. Unlike the mission data, which is typically sporadic and may have huge data rates, the housekeeping data is usually continuous and at a low data rate. Continuously monitoring system performance does not require much information transfer by modem standards. In addition, rather than going to the end user, housekeeping data goes to the system monitoring and control activity within mission operations. Although housekeeping and mission data are distinct, we often need housekeeping data to make the mission data useful. For example, we must know the spacecraft's orbit and attitude to determine the ground lookpoint of the payload sensors and thereby locate the fire.

For both mission and housekeeping data, the data delivery system should be an information management-oriented process. We want to take a large amount of raw data, frequently from a variety of sensors, and efficiently transform it into the information the users need and provide it to them in a timely manner. We do not know at first whether sending FireSat data directly to the field or sending it first to a mission operations center for interpretation and analysis is the best approach. But we do know our choice will dramatically affect how well FireSat works and whether or not it is an efficient and effective system.

The principal trades associated with data delivery are:

• Space vs. ground—how much of the data processing occurs on board the spacecraft vs. how much is done at mission operations or by the end user?

• Centred vs. distributed processing—is one computer talking to another computer, or does one large central computer on the spacecraft or on the ground process everything?

• Level of autonomy*—how much do people need to intervene in order to provide intelligent analysis and minimize costs?

These trades are strongly interrelated. Thus, autonomy is important by itself, but is also a key element of the space vs. ground trade. If human intervention is required (i.e., it can't be done autonomously), then the process must be done on the ground—or it must be a very large spacecraft We will discuss each of these trades below after we have looked at the data delivery process as a whole. Autonomy is discussed in Sec. 2.1.2, because it is also critical to tasking and control.

The best way to start looking at the data-delivery problem is a data-flow analysis. This defines where the data originates and what has to happen to it before it gets to where it needs to go. To examine the data flow we can use a data-flow diagram as shown in Fig. 2-2 for the FireSat mission. A data-flow diagram lets us outline the tasks which we need to do, even though we don't understand yet how we will do most of them. For FireSat we know that we need some type of information collection, probably

* The language here can be confusing. An autonomous operation runs without human intervention. An autonomous spacecraft nms without intervention from outside the spacecraft a camera or imager or some other mechanism for detecting fires. As shown across the top row of Kg. 2-2, this imaging information must be digitized, probably filtered in some fashion, and transferred to a map of forest regions. We must then interpret the image to identify whether a fire exists, incorporate the results on a map, and distribute the map to the end user.

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