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.
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.
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.
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 |
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
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