SMAD

42 Mission Characterization 2.4
2.4 Step 6: Characterizing the Mission Arcbitecture 43
TABLE 2-12. Summary of Orbit and Constellation Characteristics. See text for discussion.
TABLE 2-13.
1 AltItude
2 Incnnatlon
3 Eccentricity
4 Argument of perigee for nonclrcular orbits
5 l1V budget for orbit transfer
6 l1V budget for orbit maintenance
7 Whether orbit will be controlled or uncontrolled
8 Number and relative orientation of orbit planes (constellations)
9 Number and spacing of spacecraft per orbit plana (consteHations)
Summary of MissIon-Payload Characteristics. For multiple payloads, we must
determine parameters for each payload.
1. Physical Parameters
1.1 Envelope dimensions
1.2 Mass properties
2. Viewing and Pointing
2.1 Aperture size and shape
2.2 Size and orientation of clear field of view required
2.3 PrimaJy pointing dlrectlon*
2.4 PoIntIng direction range and accuracy required
2.5 Tracking or scanning rate
2.6 Pointing or tracking duration and duty cycle
3. Electrical Power
3.1 VoHage
3.2 Average and peak power
3.3 Peak power duty cycle
4. Telemetry and Commands
4.1 Number of commend and telemetry channels
4.2 Commend memory size and time resolution
4.3 Data rates or quantity of data
5. Thermal Control
5.1 Temperature Omits (operatlnglnon-operatlng)
5.2 Heat rejection to spacecraft (average/peak wattage/duty cycle)
*e.g., Sun, star, nadir, ground target, another spacecraft
Payload vs. orbit trades typically try to balance the resolution advantages of low
altitudes against the fewer spacecraft needed for the same coverage at higher altitudes.
The counterbalancing factor is that we need a sensor with a larger aperture and better
sensitivity to obtain the same resolution at higher altitudes; the more capable sensor
costs more and needs a larger spacecraft and launch system.
Payload vs. spacecraft trades usually try to meet pointing and tracking requirements
at the lowest cost. At one extreme, the payload does all the pointing independently of
the spacecraft attitude; an example is the use of gimballed scan platforms on the JPL
Mariner MK-ll spacecraft. At the opposite extreme, Space Telescope and Chandra
X-Ray Observatory point the entire spacecraft with the required level of accuracy. An
intermediate approach used on RME points the entire spacecraft to a lower level of accuracy,
allowing the payload to do fine pointing over a limited field of regard.·
E. Select the Mission Operations Approach (Chapters 13-15)
We next select and size the elements needed to support communications and c0ntrol
of the spacecraft and payload. Table 2-14 gives the key parameters. Typically a
mission operations control center commands and controls the spacecraft and delivers
data to the user. With rare exceptions, we would choose an existing control renter,
based on the user's needs, downlink data rates, and, in some cases, security considerations.
Both NASA and the Air Force have existing systems. Particular institutions,
such as Intelsat or Comsat, use custom systems. Most commercial operators employ
system-peculiar control centers. If needed, we can interconnect most systems withdifferent
options for relaying communications. Chapter 15 details the specification,
selection, and design of this element.
TABLE 2-14.
Summary of Mission Operations CharacteristIcs.
1. Communications Architecture
1.1 Number and distribution of ground stations
1.2 Downnnk and upUnk path design
1.3 Crossnnk characteristics, H used
1.4 Relay satenltes used
1.5 Communications link budget
1.6 Space-to-ground data rates
2. Ground System
2.1 Use of existing or dedicated faciDties
2.2 Required transmit and receive characJerlstics
2.3 Required data handUng
3. Operations
3.1 Level of automation
3.2 Software lines of code to be created
3.3 Fun-time or part-time staffing
3.4 Number of personnel
3.5 Amount of commanding required
3.6 Timeliness of data distribution
The communications architecture transfers the required mission data (payload 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 payload and orbit trades are complete.
Typical options are SGLS for Air Force missions or IDRSS/GSIDN with the NASA
mission control centers. Custom systems are required for some applications and are
commonly used for· commercial missions in geosynchronous orbit. Chapter 13
describes communications architectures, and Chap. 14 treats operations.