Space Payload Design and Sizing

Bruce Chesley, U.S. Air Force Academy Reinhold Lutz, Daimler Chrysler Aerospace Robert F. Brodsky, Microcosm, Inc.

9.1 Payload Design and Sizing Process

9.2 Mission Requirements and Subject Trades Subject Trades

9.3 Background

The Electromagnetic Spectrum; Basic Telescope Optics; Diffraction Limited Resolution

9.4 Observation Payload Design

Candidate Sensors and Payioads; Payload Operations Concept; Required Payload Capability

9.5 Observation Payload Sizing

Signal Processing and Data Rates; Estimating Radiometric Performance; Estimating Size, Weight, and Power; Evaluate Candidate Payioads; Observation Payload Design Process; Assess Life-cycle Cost and Operability of the Payload and Mission

9.6 Examples

The FireSat Payload; MOD1S—A Real FireSat Payload

As illustrated in Fig. 1-3 in Chap. 1, the payload is the combination of hardware and software on the spacecraft that interacts with the subject (the portion of the outside world that the spacecraft is looking at or interacting with) to accomplish the mission objectives. Payioads are typically unique to each mission and are the fundamental reason that the spacecraft is flown. The purpose of the rest of the spacecraft is to keep the payload healthy, happy, and pointed in the right direction. From a mission perspective it is worth keeping in mind that fulfilling these demands is what largely drives the mission size, cost, and risk. Consequently, a critical part of mission analysis and design is to understand what drives a particular set of space payioads so that these elements can become part of the overall system trade process designed to meet mission objectives at minimum cost and risk.

This chapter summarizes the overall process of payload design and sizing, with an emphasis on the background and process for designing observation payioads such as FireSat (Communications payioads are discussed in Chap. 13.) We begin with the flow of mission requirements (from Chap. 1) to payload requirements and the mission operations concept (from Chap. 2) to a payload operations concept which defines how the specific set of space instruments (and possibly ground equipment or processing) will be used to meet the end goals. We then summarize key characteristics of electromagnetic radiation, particularly those which define the performance and limitations of space instruments. Finally, we provide additional details on the design of observation payloads and develop a preliminary payload design for FireSat, which we compare with the MODIS instrument, a real FireSat payload for the Terra spacecraft in NASA Earth Observing System.

Several authors have discussed space observation payload design in detail, such as Chen [1985], Elachi [1987], and Hovanessian [1988]. More recently Cruise, et al. [1998] provides a discussion of a full range of payload design issues including optics, electronics, thermal, structures and mechanisms, and program management In addition, a number of authors provide extended discussions of specific types of observations missions. Schnapf [1985], Buiten and Clevers [1993], and Kramer [1996] provide surveys of Earth observing missions and sensors. Huffman [1992] discusses UV sensing of the atmosphere. Meneghini and Kozu [1990] and Kidder and Vonder Haar [1995] discuss meteorology from space. Kondo [1990] and Davies [1997] discuss astronomical observatories in space. Finally, Chap. 13 provides numerous references on space communications payloads and systems.

Spacecraft missions have been flown to serve many purposes, and while virtually every mission has unique elements and fulfills some special requirement, it is nonetheless possible to classify most space missions and payloads into the following broad categories: communications, remote sensing, navigation, weapons, in situ science, and other. Table 9-1 provides a sample of missions that fall within these categories along with a primary payload and spacecraft that fits that particular mission. Many other types of space missions have been proposed or demonstrated. We include these in Table 9-1. We will introduce each of these spacecraft mission types, then focus on first-order system engineering analysis of remote sensing payloads.

Communications. The purpose of the majority of spacecraft is to simply transfer information. Communications missions range from wideband ftdl-duplex telecommunications connectivity to one-way broadcast of television signals or navigation messages. Communications has traditionally been dominated by large geosynchronous spacecraft, but constellations of smaller spacecraft in lower orbits are emerging with alternative architectures for global coverage. New technologies are developing rapidly, including research into using lasers for spacecraft communication. A detailed discussion of communications payloads and subsystems is included in Sec. 11.2, Chap. 13, and Morgan and Gordon [1989].

Remote Sensing. Spacecraft remote sensing represents a diverse range of missions and applications. Any observation that a spacecraft makes without directly contacting the object in question is considered remote sensing. Imaging the Earth's surface, sounding the Earth's atmosphere, providing early warning of a ballistic missile launch, or observing the characteristic chemical spectra of distant galaxies are all remote sensing missions. Fundamentally we focus on measurements in the electromagnetic spectrum to determine the nature, state, or features of some physical object or phenomenon.

Depending on the particular mission, we can evaluate different aspects of electromagnetic radiation to exploit different characteristics of the target with respect to spatial, spectral, and intensity information content We also evaluate this information in a temporal context that supports comparisons and cause-and-effect relationships. The types of information and sensors used to provide this information are illustrated in Fig. 9-1.

TABLE 9-1. "TVpes of Spacecraft Missions and Payloads.

Spacecraft Mission

Payload

Full-duplex broadband Message broadcast Personal comm

Transceiver Transmitter Transceiver

Milstar, Intelsat DirecTV, GPS Iridium

Remote Sensing

Imaging

Intensity measurement Topographic mapping

Imagers and cameras

Radiometers

Altimeters

LandSat, Space Telescope SBIRS early warning, Chandra X-Ray Observatory, TOPEX/Poseidon

Navigation

Ranging Nav signal

Transceiver Clock and transmitter

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