Info

Minimum

Acceptable .

Desired

Subject Characteristics

Detect presence or absence of large fires

Identify, locate, and track progress of fires

Determine thermal conditions within fires and products of combustion

Quantity

Measure existence of 1 fire

Simultaneously measure and track 7 fires

Simultaneously measure and track 20 fires

Timeliness

Report detection of fire within 6 hours

Report detection of fire within 2 hours

Report detection of fire within 20 min

Revisit Interval

Update status of fire every 2 hours

Update status of fire every 90 mln

Update status of fire every 45 min

Geolocation Accuracy

Determine location of fire within ±100 km

Determine location and extent of fire within ±1 km

Determine location and extent of fire within ±100 m

Completeness

Map fires in continental U.S.

Map fires in North America and one other selectable region (e.g., Persian Gulf)

Map fires globally

We need to parameterize the mission, such as identifying and locating forest fires, in such a way that we can evaluate, size, and design candidate sensors. This parameterization involves a process of requirements analysis that focuses on matching the tasks involved in the mission with categories of discipline capabilities. If we match mission requirements with existing or probable capabilities the result is a set of potential information requirements. We then try to identify the characteristics of the subject (signatures) that correspond to the information requirements through a set of rules. For the FireSat example, these rules consist of the spectral wavelengths and thresholds needed to detect fires. The rules yield a set of mission observables, such as specific wavelength bands and spectral sensitivities that we need in our sensor. These observables provide the basis for the payload characteristics that comprise the baseline design to satisfy the mission. In the case of FireSat, the basic mission categories that might satisfy tins mission and the corresponding information type are shown in Table 94.

TABLE 9-4. Simplified Subject Trades for FlreSat Mission. The information type allows for subject trades to be made among the different signatures that can be exploited to satisfy FlreSat mission requirements.

Sensor Type

Information Type

~Bectro-optical Imager

Spectrometer

Radiometer

Visible return from light or smoke cloud produced by the fire Spectral signatures from products of combustion Thermal Intensity

A unique signature exhibited by fires is the flickering light in a fire. This flicker has a characteristic frequency of about 12 Hz and can be exploited by processing the data stream from an electro-optical sensor to search for this frequency [Miller and Friedman, 19%]. Light flickering at this frequency produces an irritating effect on the human vision system, possibly as a survival adaptation for the species against the threat of wildfires.

There are many choices and types of sensors, more than one of which might be a candidate to perform a given mission. In the case of FireSat it may be possible to satisfy basic mission requirements by observing a number of different phenomenologies: visible signatures associated with flame and smoke, thermal infrared signatures from the fire, spectral analysis of the products of combustion, or an algorithm combining all of these. The selection of a spacecraft payload represents the fundamental leap in determining how to satisfy mission requirements with a space sensor. In the previous section we introduced a top-down framework for considering the general problem of spacecraft design. Here we turn our attention to the payload; in particular, a methodology for determining the type of payload to employ and the physical quantities to measure.

Figure 9-3 illustrates the framework for the heart of the payload design process. The process begins with a task or mission requirement and ends with a spacecraft payload design. We have divided this process into intermediate steps to focus the effort along the way to a final design. In this section we focus on describing the process illustrated in Fig. 9-3; Sec. 9.4 provides some of the specific techniques that are employed in this process for visible and IR systems.

For the FireSat mission design, we need to identify specific signatures that would allow candidate sensors to provide viable solutions to the mission requirement We observe physical phenomena through signatures, and we must choose which signature will provide the desired information. The specific signatures that a payload senses must be evaluated in light of the particular focus of the mission. For example, a spectrometer that is sensitive enough to detect all fires, but which cannot be used to differentiate campfires from forest fires could generate a large false alarm rate and render it operationally useless. Defining the key signatures and observables that support the information content needed to satisfy the mission determine the performance limits for the payload design.

92.1 Subject Trades

The objective of a space mission is typically to detect communicate, or interact The subject, as an element of the space mission, is the specific thing that the spacecraft will detect communicate, or interact with. For GPS, the subject is the GPS receiver, For FireSat we would assume that the subject is the heat generated by the forest fire. But other subjects are possible: light smoke, or changes in atmospheric composition.

Mission Observables

Payload -Characierisiics

Fig. 9-3. Process for Unking Mission Requirements to Payload Design. The process moves from mission requirements to a payload design in three steps: requirements analysis, subject trades, and payload analysis.

What we choose as the subject will dramatically affect performance, cost, and the mission concept Thus, we must do this trade carefully and review it from time to time to ensure it is consistent with mission objectives and our goal of minimizing cost and risk.

Table 9-5 summarizes the subject-trade process. We begin by looking at the basic mission objectives and then ask what subjects could meet these objectives. To do this, we should look at what we are trying to achieve, the properties of space we intend to exploit, and the characteristics of what we are looking at or interacting with. Table 9-6 shows examples of subject trades for four representative missions. As the missions change, the nature of the subject trades will also change. For FireSat, we are looking for a well-defined subject (the forest fire), and we want to do this at minimum cost and risk. With the Space Telescope, we must ask, "What am I looking for? What am I trying to detect and how can I detect it?" For any of the science missions, we would ask, 'Is the subject some distant and unknown object, or is it part of the electromagnetic spectrum I am trying to explore?"

For a space system intended to detect airplanes, the main subject trades would concern mission goals. Are the targets cooperative or noncooperative? Do we need to track over the poles? Should we track in high-density areas around airports or over the open oceans? Hie answers to these questions will determine the nature of the subject

Perhaps the easiest subject trades are" those in which the system will be interacting with a ground element that is a part of the system, such as direct broadcast television or a truck communication system. In this case, the subject trade becomes simply an issue of how much capacity should go on the spacecraft vs. how much should go in the unit on the ground.

TABLE 9-5. Subject Trade Process. Note that the subject trades lead directly to the payioad trade process as discussed in Sec. 9.2

Step

Fire Sat Example

Where Discussed

1. Determine fundamental mission objectives

Detect and monitor forest fires

Sec. 1.3

2. Determine what possible subjects could be used to meet these objectives (Le-> what could the system detect or interact with to meet the objectives)

Heat, fire, smoke, atmospheric composition

Sec. 9.2

3. Determine broad class of ways that the spacecraft can detect or interact with the possible subjects

flame, smoke -> visual atmospheric composition -> lidar

Sec. 9.2

4. Determine if subject is passive or controllable

Initially assume passive fire detection

Sec. 92

5a. For controllable subjects, do trade of putting functionality at the subject, in the space system, or in the ground system

N/A

Sees. 2.1, 3.2.3

5b. For passive subjects, determine general characteristics that can be detected

Forest fire temperature range and total heat output

Sec. 9.2

6. Determine whether multiple subjects and payloads should be used

Not initially

Sec. 92

7. Define and document initial subject selection

IR detection of heat

N/A

8. Review selection frequently for alternative methods and possible use of ancillary subjects

See Sec. 22.3, alternative low cost for FireSat

N/A

The next step for subject trades is to determine whether the subject is controllable or passive. The system designer knows and can control characteristics of controllable or active subjects. This includes ground stations, antennas, receivers, and transmitters such as those used for ground communications, direct broadcast television, or data relay systems. Because we can control the subject, we can put more or less capability within it Thus we might choose to have a simple receiver on the ground with a high-power, accurately pointed, narrow-beam transmitter on the spacecraft Or we could place a sophisticated, sensitive receiver on the ground with a small, lower-cost system in space. Usually, the solution will depend on the number of ground stations we wish to interact with. If there are many ground stations, as in direct-broadcast television, we will put as much capability as possible into the satellite to drive down the cost and complexity of the ground stations. On the other hand, if there are only a few ground stations, we can save money by giving these stations substantial processing and pointing capability and using a simpler, lighter-weight, and lower-cost satellite.

Passive subjects are those in which the characteristics may be known but cannot be altered. This includes phenomena such as weather, quasars, or forest fires. Even though we cannot control the object under examination, we can choose the subject from various characteristics. We could detect forest fires by observing either the fire itself or the smoke in the visible or infrared spectrum. We could detect atmospheric composition changes or, in principle, reductions in vegetation. Thus, even for passive subjects, the subject is part of the system trades.

TABLE 9-8. Representative Subject Trades. Subject trades for the Space Telescope are particularly Interesting in that a significant goal of the system is to discover previously unknown phenomena or objects.

Mission

FireSat

Airplane Detection

Truck Communications System

Space Telescope

Property of space used

Global perspective

Global perspective

Global perspective

Above the atmosphere

General oblectofstudy or interaction

Forest fires

Airplanes

Portable telecommunication centers

Distant galaxies +

unknown phenomena

Alternative mission subjects

Fire

(visible or IR)

Smoke (visible or IR)

Increased C02

Decreased vegetation

Skin (radar, visible)

Plume (IR)

Radio emissions (RF)

Current radio Current CB Standard TV New telecommunication center

Cellular relay

Quasars Galaxies Planets

Visible spectrum Unknown objects

Key subject trades

None—IR detection probably best choice

Radar vs. IR vs. active RF

Complexity of truck element vs. complexity of space & ground station

Isthe subject known or unknown? Is It objects or spectral regions?

Comments

See low-cost alternative in Chap. 22

Need to examine goals; cooperative vs. noncooperative targets; high density vs. ocean tracking

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