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

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