Bedford Astronomy Club

Step 4 Identifying Alternative Mission Architectures

A mission architecture consists of a mission concept plus a specific set of options for the eight mission elements defined in Sec. 1.2. Although we need all of the elements to define and evaluate a mission architecture, some are more critical than others in determining how the space mission will meet its objectives. Typically, we define a mission architecture by specifying the mission concept plus the subject, oibit, communications architecture, and ground system. These provide a framework for defining the other elements. Alternatively, we may define the architecture by specifying a unique approach to mission operations or a unique payload which then drives the definition of the remaining elements

Our goal is to arrive at a set of candidate architectures for further evaluation large enough to encompass all approaches offering significant advantages, but small enough to make the more detailed definition and evaluation manageable. Table 2-4 summarizes the mechanism for doing this, which we describe below.

Step A. Identify the mission elements subject to trade. We begin by examining our basic mission concept and each of the eight mission elements in light of the requirements and constraints from Sec. 1.4 to determine which have more than one option.

TABLE 2-4. Process Summary for Identifying Alternative Mission Architectures. This highly creative endeavor can have a significant Impact on mission cost and complexity.

Step	Where Discussed
A. Identify the mission elements subject to trade.	Table 2-5
B. Identify the main options for each tradeable element	Table 2-6
C. Construct a trade tree of available options	IFig. 2-4, •»Table 2-7
D. Prune the trade tree by eliminating unrealistic combinations.
E. Look for other alternatives which could substantially influence how we do the mission.	Chap. 22

Usually this step greatly reduces the number of tradeable elements. Table 2-5 summarizes this process for FireSat. The FireSat mission has multiple options that will affect not only cost but also performance, flexibility, and long-term mission utility. Thus, for this mission we should cany through several different options so the decision-making audience can understand the main alternatives.

TABLE 2-5. Selecting FireSat Elements Which can be Traded. Many options exist for FireSat, not all of which are compatible with each other.

Element of Mission Architecture	Can be Traded	Reason
Mission Concept	Yes	Want to remain open to alternative approaches
Subject	No	Passive subject Is well defined
Payload	Yes	Can select complexity and frequencies
Spacecraft Bus	Yes	Multiple options based on scan mechanism and power
Launch System	Cost only	Choose minimum cost for selected orbit
Orbit	Yes	Options are low, medium, or high altitude with varying number of satellites
Ground System	Yes	Could share NOAA control facility, use dedicated FireSat facility, or direct downlink to users
Communications Architecture	No	Fixed by mission operations and ground system
Mission Operations	Yes	Can adjust level of automation

Table 2-5 lists one of the options as "Cost only," meaning that the trade depends mainly on cost and only secondarily on how or how well the mission is accomplished. An example would be the launch system, for which the main concern normally is what launch vehicle will get the spacecraft into orbit at the lowest cost Still, these trades may be important in selecting the mission concept For example, a major increase in the launch cost may outweigh being able to use a smaller number of identical satellites in a higher orbit

Step B. Identify the main options for each tradeable element. Although in theory we have almost an unlimited number of options, we normally draw them from a limited set such as those in Table 2-6. Thus, we first choose options that apply to our mission and then look for special circumstances which may lead us to consider alternatives not listed in the table.

TABLE 2-6. Common Alternatives for Mission Elements. This table serves as a broad checklist for identifying the main alternatives for mission architectures.

Mission Element [Where Discussed]	Option Area	Most Common Options	FireSat Options
Mission Concept [Sec. 2.1]	Data delivery Tasking	Direct downlink to user, automated ground processing, man-ln-the-loop processing and transmission Ground commanding, autonomous tasking, simple operations (no tasking required)	Direct downlink or through mission control Simple operation or ground commands
Controllable Subject [Sees. 2.3,13.4,22.3]	Selection Performance Steering	Standard ground stations, private TV receivers, ship or aircraft transceivers, special purpose equipment EIRP, G/T (See 13.3 for definitions) Fixed or tracking	[See Sec. 9.3]
Passive Subject [Sec. 2.3]	What Is to be sensed	Subject Itself, thermal environment, emitted radiation, contrast with surroundings	Heat or visible light; chemical composition; particles
Payload [Chaps. 9,13] (some items may not apply, depending on mission type)	Frequency Complexity	Communications: normal bands Observations: IR, visible, microwave Radar L, C, S bands. UHF Single or multiple frequency bands, single or multiple instrument	IR, visible Single or multiple bands
Payload	Size vs. sensitivity	Small aperture with high power (or sensitivity) or vice versa	Aperture
Spacecraft Bus [Chap. 10]	Propulsion Orbit control Navigation Attitude determination and control Power	Whether needed; cold gas, monopropellant, blpropellant Whether needed, onboard vs. ground Onboard (GPS or other) vs. ground-based None, spinning, 3-axis; articulated payload vs. spacecraft pointing; actuators and sensors Solar vs. nuclear or other; body-mounted vs. 1- or 2-axls pointed arrays	Determined by definition of payload and orbit
Launch System [Chap. 18]	Launch vehicle Upper stage Launch site	SSLV, Atlas, Delta, STS, Titan, Pegasus, Ariane, other foreign Pam-D, IUS, TOS, Centaur, integral propulsion, other foreign Kennedy, Vandenberg, Kourou, other foreign	Determined by definition ol spacecraft and orbH
Orbit Altitude Inclination Constellation configuration	None, geosynchronous, Sun-synchronous, frozen, Molniya, repeating ground track Low-Earth orbit, mid-altitude, geosynchronous Number of satellites; Walker pattern, other patterns; number of orbit planes	Single GEO satellite, low-Earth constellation Min. Inclination dependent on altitude

TABLE 2-6. Common Alternatives for Mission Elements. (Continued) This table serves as a broad checklist for identifying the main alternatives for mission architectures.

Mission Element [Where Discussed]	Option Area	Most Common Options	FIreSat Options
Ground System [Chap. 15]	Existing or dedicated	AFSCN, NASA control center, other shared system, dedicated system	Shared NOAA system, dedicated system
Communications Architecture [Chap. 13]	Timeliness Control and data dissemination Relay mechanism	Store and dump, real-time link Single or multiple ground stations, direct to user, user commanding, commercial links TDRSS, satellite-to-satellite crosslinks, commercial communications relay	Either option 1 ground station; commercial or direct data transfer TDRSor commercial
Mission Operations [Chap. 14]	Automation level Autonomy level	Fully automated ground stations, part-time operations, full-time (24-hr) operations Full ground command and control, partial autonomy, full autonomy (not yet readily available)	Any of the options listed

Steps C and D. Construct and prune a trade tree of available options. Having identified options we next construct a trade tree which, in its simplest form, is a listing of all possible combinations of mission options. Mechanically, it is easy to create a list of all combinations of options identified in Step B. As a practical matter, such a list would get unworkably long for most missions. As we construct the trade tree we need to find ways to reduce the number of combinations without eliminating options that may be important

The first step in reducing the number of options is to identify the system drivers (as discussed in Sec. 2.3) and put them at the top of the trade tree. The system drivers are parameters or characteristics that largely determine the system's cost and performance. They are at the top of the trade tree because they normally dominate the design process and mandate our choices for other elements, thus greatly reducing our options.

The second step in reducing options is to look for trades that are at least somewhat independent of the overall concept definition or which will be determined by the selection of other elements. For example, the spacecraft bus ordinarily has many key options. However, once we have defined the orbit and payload, we can select the spacecraft bus that meets the mission requirements at die lowest cost. Again, although bus options may not be in the trade tree, they may play a key role in selecting workable mission concepts because of cost, risk, or schedule.

The third tree-pruning technique is to examine the tree as we build it and retain only sensible combinations. For example, nearly any launch vehicle above a minimum size will work for a given orbit and spacecraft Because cost is the main launch-vehicle trade, we should retain in the trade tree only the lowest-cost launch vehicle that will fulfill the mission. This does not mean that we will ultimately launch the spacecraft on the vehicle listed in the trade tree. Instead, we should design the spacecraft to be compatible with as many launch vehicles as possible and then select the vehicle based on cost (which may well have changed) when we are deciding about initial deployment

Steps C and D produce a trade tree such as the one for FircSfll in Fig. 2-4. Our goal is to retain a small number of the most promising options to proceed to more detailed definition. For each option we will have selected most, though not necessarily all, of the elements shown in Table 2-7 for options 1 and 6. Of course we should reevaluate the trade tree from time to time as the system becomes more completely defined.

Orbft

Paylo&d

Comm.

Arch.

(DcmMk)

launch

Option No.

Low Earth orbft

0 0