3.6 Timeliness of data distribution

The communications architecture transfers the required mission data (payioad and housekeeping data) from the spacecraft down to the mission operations control center. In addition, we must send commands back to the spacecraft, and meet other requirements such as encryption. Thus, we select the communications relay elements along with the mission control system after most payioad and orbit trades are complete. Typical options are SGLS for Air Force missions or TDRSS/GSTDN with the NASA mission control centers. Custom systems are required for some applications and aie commonly used for commercial missions in geosynchronous orbit Chapter 13 describes communications architectures, and Chap. 14 treats operations.

F. Design the Spacecraft to Meet Payload, Orbit, and Communications Requirements (Chapter 10)

The spacecraft and its subsystems support the payload in the mission orbit—point it and supply power, command and data handling, and thermal control. They must be compatible with the communications architecture and mission-operations concept. These elements, along with the launch system, drive the spacecraft design. We usually choose the launch system that costs the least to place the minimum required weight in the mission or transfer oibit Once we make this selection, the spacecraft's stowed configuration is constrained by the shroud volume of the selected vehicle or vehicles. Table 2-15 summarizes the items we need to specify while defining the spacecraft Chapter 10 covers how we synthesize spacecraft concepts and their definition and sizing.

TABLE 2-15. Summary of Spacecraft Characteristics. See text for discussion.

1. General arrangement including payload fields of view (deployed and stowed)

2. Functional block diagram

3. Mass properties, by mission phase (mass and moments of inertia)

4. Summary of subsystem characteristics

4.1 Electrical power (conversion approach; array and battery size; payload power available, average/peak overall spacecraft power, orbit average, peak)

4.2 Attitude control (attitude determination and control components; operating modes; ranges and pointing accuracy)

4.3 Navigation and orbit control (accessing requirement, use of GPS; onboard vs. ground)

4.4 Telemetry and command (command/telemetry format; command and time resolution; telemetry storage capacity; number of channels by type)

4.5 Computer (speed and memory; data architecture)

4.6 Propulsion (amount and type of propellant; thruster or motor sizes)

4.7 Communications (link margins for all links; command uplink data rate; telemetry downlink data rates)

4.8 Primary structure and deployables

4.9 Unique thermal requirements

4.10 Timing (resolution and accuracy)

5. System parameters

5.1 Lifetime and reliability 52 Level of autonomy

A key spacecraft-versus-launch-system trade is the use of integral propulsion. Many commercial spacecraft ride the launch system to transfer orbit and then insert themselves into the mission orbit using an internal propulsion or an internal stage. Some DoD spacecraft, such as DSCS III and DSP, depend on a launch system with an upper stage for insertion directly into the mission orbit They do not carry large integral propulsion subsystems. We should consider this trade whenever the spacecraft and payload cost enough to justify the reliability offered by an expensive upper stage.

Another trade between the spacecraft and launch system involves guidance of the upper stage. Often, the spacecraft control system can guide the upper stage, which may allow deletion of equipment from that stage, thereby increasing performance and lowering cost. This trade is particularly important when using three-axis-stabilized stages.

G. Select a Launch and. Orbit Transfer System (Chapter 18)

The launch system and its upper stage need to deliver the spacecraft and payload to the mission orbit or to a transfer orbit from which the spacecraft can reach the mission orbit on its own. The chosen launch system usually determines the launch site. The launch site organization provides pre-launch processing, checkout, and installation to the launch system, usually on the launch pad.

Launch vehicles and upper stages may be combined in many ways to match almost any reasonable combination of payload and mission orbit Chapter 18 details the characteristics and selection of launch systems. Selecting a launch system typically involves the trades with the spacecraft discussed above. In addition, we must decide between a single spacecraft launch and manifesting two or more spacecraft in a shared launch. In general, multiple manifesting costs less, but constrains the schedule. Finally, we should bring certain launch-system parameters to the system level design process: type of vehicle, cost per launch, and flow times for processing and prelaunch activities at the launch site.

H. Determine Logistics, Deployment, Replenishment, and Spacecraft Disposal

Strategies (Sections 7.6,19.1, and 21.2)

Logistics is the process of planning to supply and maintain the space mission over time. Whereas only military missions typically demand formal plans, the process described in Sec. 19.1 can strongly affect costs for any multi-year mission requiring extended support Historically, most life-cycle costs have been locked in by the end of concept exploration, so we must evaluate operations, support, replenishment, and mechanisms during this phase.

Planners often overlook the sequence for building up and maintaining satellite constellations. To deploy a constellation effectively, we must create performance plateaus which allow us to deploy in stages and to degrade the system gracefully if individual satellites fail. These performance plateaus develop from the constellation design, as described in Sec. 7.6.

Section 212 describes the ever-increasing problem associated with orbital debris, consisting of defunct satellites and associated parts. Because of this problem, all new satellite designs should plan for deorbiting or otherwise disposing of satellites at the end of their useful life. In particular, satellites must be removed from areas such as the geostationary ring, where they would seriously threaten other spacecraft or any low-Earth orbit constellation.

I. Provide Costing Support for the Concept-Definition Activity (Chapter 20)

Developing costs for system elements is vital to two objectives: finding the best individual mission architecture and comparing mission architectures at the system level. Chapter 20 describes parametric, analogous, and bottoms-up methods for costing. Typically, for concept exploration, we use only the first two because we lack a detailed definition of the design. At this level, we simply want relative comparisons rather than absolute estimates, so we can accept the greater uncertainty in these methods.

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