SMAD

34 Mission Characterization
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 OptIon Most Common FIreSat
DIscussed] Area OptIons Options
MIssion Concept Data delivery Direct downnnk to user, automated ground Direct downlink or
[Sec. 2.11
processing, man-In-the-Joop processing and through mission
transmission
control
Tasking Ground commanding, autonomous tasking, Simple operation or
simple operations (no tasking required) ground commands
ControDable Selection Standard ground stations, private TV NlA
Subject [Sees. receivers, ship or aircraft transceivers, [See Sec. 9.31
2.3, 13.4, 22.31 special purpose equipment
Performance
Steering
EIRP, G/T (See 13.3 for definitions)
fixed or tracking
Passive Subject What is to be Subject itseH, thermal environment, Heat or visible nght;
[Sec. 2.31 sensed em/Ued radiation, contrast with chemical composisurroundings
tion;particles
Payload Frequency Communlcalions: normal bands IR, visible
[Chaps. 9, 131
Observations: IR, visible, microwave
(some llems may
Radar: L, C, S bands, UHF
not apply,
depending on Complexity Single or multiPle frequency bands, Single or
mission type) single or multIple Instruments multiple bands
Payload Sizevs. SrneD aperture with high power Aperture
sensitivity (or sensitivity) or vice versa
Spacecraft Bus Propulsion Whether needed; cold gas, monopropellant, Determined
[Chap. 10] blpropenant by definition of
payload and orbit
Orbit control Whether needed, onboard vs. ground
Navigation
Onboard (GPS or other) vs. ground-based
Attitude deter- None, splnntng, 3-axis; articulated
mlnatlon and payload vs. spacecraft pointing;
control actuators and sensors
Power
Soler vs. nuclear or other; body-mounted vs.
1- or 2-axis pointed arrays
Launch System Launch SSLV, Atlas, Della, STS, Titan, Datarmined by
[Chap. 181 vehicle Pegasus, Ariane, other foreign definition of spacecraft
and orbit
Upper stage Pam-D, IUS, TOS, Centaur, integral
propulsion, other foreign
Launch site
Kennedy, Vandenberg, Kourou, other foreign
Orbit Special orbits None, geosynchronous, Sun-synchronous, SlngleGEO
[Chaps. 6, 7] frozen, Molnlye, repeating ground track satellite, lOw-Earth
consteDation
Altitude Low-Earth orbit, mlcl-altitude,
geosynchronous
Inclination 0°,28.5°,57",63.4°,90°,98"
Constellation Number of sateDites; Walker pattem, Min. inclination deconfiguration
other patterns; number of orbit planes pendent on altitude
2.2 Step 4: Identifying Alternative Mission Architectures 35
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 OptIon Most Common FlreSat
Discussed] Area OptIons Options
Ground System ExIsting or AFSCN, NASA control center, other shared Shared NOAA
[Chap. 15) dedicated system, dedicated systam system, dedicated
system
Communications T1mefiness Store and dump, real-time fink EIther option
ArchItecture
[Chap. 13) Control Single or multiPle ground stations, direct to 1 ground station;
anddeta user, user commanding, commercial finks commercial or
dissemination
direct date transfer
Relay TDRSS, satellite-to-sateOite crossnnks, TORS or
.
mechanism commercial communications relay commercial
Mission Automation Fully automated ground stations, part-time Any of the
Operetions level operations, fuD-tIme (24-11r) operations options itsted
[Chap. 14)
Autonomy
level
FuR ground command and control,
parllal autonomy, fuR autonomy
(not yet readily avaDable)
Steps C and D. Construct and prune a trade !Tee. of. av~ilable optio~. ~~g
identified options we next construct a trade tree which, m Its snnplest form, 18 a listing
of all possible combinations of mission options. Mechanically, i~ is easy to create a ~
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 ~ d~termined 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 the lowest cost. Again, although
bus options may not be in the trade tree, they may playa 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 imd 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 F"rreSat in Fig. 2-4. Our ~oaI
is to retain a small number of the most promising options to proceed to more detailed