Tdrs

Kinetic energy Directed energy

Warhead

High-energy weapon

BriDlant Pebbles concept Space-Based Laser concept

In Situ Science

Crowed Robotic

Physical and life sciences Sample collection/return

Space Shuttle, Mir Mars Sojourner, LDEF

Other

Mlcrogravlty Manufacturing Space power

Resource utilization Tourism Space burial

Physical plant and raw materials

Solar collector, converter, and transmitter

Lunar soil collector and processor Orbital hotel Remains container

Space Shuttle SPS

Lunar Base Various Pegasus XL

We make an additional distinction depending on the source of the electromagnetic radiation being sensed. If the instrument measures direct or reflected solar radiation in the environment, then we call it a passive sensor. Active sensors, on the other hand, emit radiation that generates a reflected return which the instrument measures. The principal active remote sensing instruments are radar and lidar.

Although our focus is on remote sensing of Earth, many scientific missions observe electromagnetic phenomena elsewhere in the universe. The physical principles of remote sensing and the categories of sensors are the same, regardless of whether the payload is looking at deep space or the planet it is circling.

Navigation. GPS, GLONASS, and other international navigation systems have demonstrated a wealth of applications for military, civilian, academic, and recreational users. As discussed in Sec. 11.7.2, GPS provides information for real-time position, velocity, and time deteimination. It is available worldwide on a broad range of platforms, including cars, ships, commercial and military aircraft, and spacecraft The heart of GPS is a spiead-spectrum broadcast communication message that can be exploited using relatively low-cost receivers.

Weapons. While remote sensing, communication, and navigation applications are quite mature and dominate the use of space, space-based weapons remain conceptual, occupying a small niche in the realm of space mission design. In particular, concepts

Spatial

Information

Spatial

Information

Fig. 9-1. Electromagnetic Information Content and Sensor Types. Sensor types inside the

Spectral Information

Intensity Information

Fig. 9-1. Electromagnetic Information Content and Sensor Types. Sensor types inside the triangle can observe the features shown outside the triangle. For example, each pixel collected by an imaging radiometer reflects both spatial and intensity information. Active instruments (such as radar) are printed in bold italic text (Modified from Elachi for weapons in space became a topic of intense study and debate as part of the Strategic Defense Initiative and space-based strategic missile defense. Development of certain operational space weapons has been prohibited under the Anti-Ballistic Missile Treaty of 1972. Although some experts view widespread weaponization of space as inevitable, it has not become a stated objective of U.S. national policy [DeBlois, 1997]. Of course, space has been used to support military objectives since the dawn of artificial spacecraft [Hall, 1995; McDougall, 1985], but the vast majority of military space applications fall into the categories of remote sensing and communications.

In Situ Science. Sample collection and evaluation serves an important role in planetary and space science. Perhaps the most elaborate instance of sample collection took place in the Apollo missions when approximately 300 kg of samples from the Moon were returned to Earth for analysis. Other examples of sample collection and analysis include planetary landers (such as Viking and Mais Sojourner) and collection of solar wind particles.

Other. Exploitation of physical resources in space—either from the Moon or asteroids—has sparked innovative and imaginative concepts for augmenting Earth's limited resources or enabling human exploration of the solar system. In the nearer term, however, space-based materials processing and manufacturing are more likely to mature and exploit the characteristics of the microgravity environment (Sec. 8.1.6). Glaser et al. [1993] has done extensive studies of satellite solar power, i.e., generating solar power in space for use on Earth. Many authors have created designs for lunar colonies and space tourism facilities, but all require a dramatic reduction in launch cost (See, for example, the CSTS Alliance's Commercial Space Transportation Study [1994].)

9.1 Payload Design and Sizing Process

Payload definition and sizing determines many of the capabilities and limitations of the mission. The payload determines what the mission can achieve, while the size of the payload, along with any special structural, thermal, control, communications, or pointing restrictions, will influence the design of the remainder of the spacecraft support systems.

We begin with the assumption that mission objectives are defined and the critical mission requirements are understood. This section concentrates on a top-down methodology for bounding the trade space of possible payloads and making an informed selection among them. This process is a useful guide for moving from a blank slate to a preliminary set of payloads. Iterating on the process produces a more detailed definition and more useful set of payloads that can meet the mission objectives at minimum cost and risk.

As shown in Table 9-2, the process begins with an understanding of mission requirements described in Chaps. 3 and 4. The mission requirements have a major effect on all aspects of space vehicle design, but it is frequently necessary to treat the components and subsystems separately for preliminary design and sizing. We begin with the payload because it is the critical mission element bounding spacecraft performance. Chapters 10 and 11 treat the remainder of the spacecraft systems and trade-offs involved in the overall spacecraft design.

Once the mission requirements are understood, we must determine the level of detail required to satisfy different aspects of the mission. For FireS at, varying levels of detail are required if the task is to identify the existence of a fire, assess the damage caused by fires, or characterize the combustibles in a fire. Additionally, the temporal (timeliness) demands placed on the mission could be vastly different depending on whether the data is to support long-term scientific analysis or real-time ground activity.

We summarize the basic steps in this process below and discuss them in more detail in the remainder of this chapter for remote sensing payloads and in Chap. 13 for communications payloads.

1. Select Payload Objectives. These objectives will, of course, be strongly related to the mission objectives defined in Chap. 1 and will also depend on the overall mission concept, requirements, and constraints from Chaps. 2,3, and 4. However, unlike the mission objectives which are a broad statement of what the mission must do to be useful, the payload objectives are more specific statements of what the payload must do (i.e., what is its output or fundamental function). For FireS at, this is specific performance objectives in terms of identifying fires. For the space manufacturing example in the table, called WaferSat, the payload objective is a definition of the end product to be manufactured.

2. Conduct Subject Trades. The subject is what the payload interacts with or looks at As discussed in detail in Sec. 9.2, a key part of the subject trade is determining what the subject is or should be. For a mobile communications system, it is the user's handheld receiver. Here the subject trade is to determine how much capability to put in the user unit and how much to put on the satellite. For FireS at, we may get very different results if we define the subject as the IR radiation produced by the fire or as the smoke or visible flickering which the fire produces. In addition to defining the subject, we need to determine the performance thresholds to which the system must operate. For FireS at, what temperature differences must we detect? For WaferSat, how pure must the resulting material be? For mobile communications, how much rain attenuation

TABLE 9-2. Process for Defining Space Payloads. See text for discussion. See Chap. 13 for a discussion of communications payloads.

Process Step

Product

FireSat (Remote Sensing) Example

Space Manufacturing Example

Where Discussed

1. Use mission objectives, concept, requirements, and constraints to select payload objectives

Payload performance objectives

Identify smoldering and flaming fires

Manufacture ultra-pure silicon wafers

Chaps.1,2

2. Conduct subject trades

Subject definition and performance thresholds

Distinguish smoldering fires that are 3 K warmer than the background from flaming fires that are 10 K warmer than the background

Less than 1 ppb Impurities over 50 cm square wafers

Sec. 9.2

3. Develop the payload operations concept

Ertd-to-end concept for all mission phases and operating modes

Determine how end users will receive and act on fire detection data

Define user method to specify product needs, recover and use materials

Sees. 2.1,9.4, Chap. 14

4. Determine required payload capability to meet mission objectlvespdentify key characteristics of Interest]

Required payload capability

12-bit quantization of radiometric Intensity in the 3-5 |im wavelength

Throughput of 5,(MX) wafers/day on orbit

Sec. 9.4.3

5. Identify candidate payloads

Initial Dst of potential payloads

Specifications for Sensors #1 and 82

Specifications for Factories #1 end «2

Sec. 9.6

6. Estimate candidate paytoad capabilities and diaracterlstlcs [mission output, performance, size, mass, and power]

Assessment of each candidate payload

Sensor #1 meets the sensitivity requirement but requires a data rate of 10 Mbps.

Sensor #2 can only Identify flaming fires that are 10 K warmer than the background but requires a data rate of only 1.5 Mbps

Factory #1 produces 6,000 wafers/day, weighs 80 kg, and uses 2 kW

Factory #2 produces 4,000 wafers/day (some of which will have >1 ppb Impurities), weighs 100 kg, and uses 500 W

Sec. 9.5.3

7. Evaluate candidate payloads and select a baseline

Preliminary. paytoad definition

Spacecraft and ground architecture based on 1.5 Mbps data rate. Adjust mission requirement to Identify flaming fires only (not smoldering)

Select #1 with 1,000 wafers/day margin to be sold to reduce cost

Sees. 9.5.4,9.6.1

8. Assess de-cycle cost and operabllity ofthepaybadand mission

Revised payload performance requirements constrained by cost or architecture limitations

FireSat spacecraft with acceptable mission performance and cost

Payload repackaging to accommodate launch as an Ariane secondary payload on ASAP ring

Sec. 9.5.6, Chap. 20

9. Identify and negotiate payload-derlved requirements

Derived requirements for related subsystems

Data handing subsystem requirement to accommodate payload data rate of 1.5 Mbps

ACS system to provide 140 continuous mln of Jitter less than ±1 run

Sec. 9.5.4

10. Document and Iterate

Baseline paytoad design

Baseline FireSat payload

Baseline WaferSat payload

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