Document and Iterate

Chap 20

A. Define the Preliminary Mission Concept (Chapter 2)

As described in Sec. 2.1, the key elements are data delivery; tasking, scheduling, and control; communications architecture; and mission timeline. We begin with a broad concept and refine this concept as we define the various mission elements in the steps below. (See Tables 2-1 and 2-2 for a further definition of these elements and how to define them.)

B. Define the Subject Characteristics (Chapter 9)

We can divide space missions into two broad categories. One services other system elements, typically on the user premises, such as Comsat ground stations or GPS navigation receivers. The other category senses elements that are not a part of the mission system, such as the clouds observed by weather satellites. Our first step in defining the system elements (Chap. 9) is to determine the subject's key characteristics.

If a mission interacts with user equipment, we must define the subject characteristics either from known information for well-established services or by a trade study involving the rest of the system. The parameters for specifying passive subjects are laigely the same as those for specifying user elements, except that we don't have a

"receiver" to characterize, and the effective isotropic radiated power (EIRP) specification for the transmitter is replaced by definition of the object's emission intensity as a function of bandwidth. Table 2-11 summarizes the characteristics of both types of elements.

TABLE 2-11. Summary of Main Characteristics of Space Mission Subjects. See Chap. 13 for definitions of communications parameters.

Controllable Subjects

Passive Subjects

1. Quantity

2. Location or range

3. Transmitter EIRP 4- Receiver G/T

5. Frequency and bandwidth

2. Location or range

3. Emission intensity (W/sr) as a function of frequency or spectral band

4. Needed temporal coverage (duty cycle)

C. Determine the Orbit and Constellation Characteristics (Chapter 7)

The mission orbit profoundly influences every part of space mission development and operation. Combined with the number of spacecraft, it determines all aspects of space-to-ground and ground-to-space sensor and communication coverage. For the most part, the orbit determines sensor resolution, transmitter power, and data rate. The orbit determines the spacecraft environment and, for military spacecraft, strongly influences survivability. Finally, the orbit determines the size and cost of the launch and delivery system.

Chapter 7 gives detailed directions for orbit design. As Table 2-12 shows, the design should include parameters for the mission and transfer orbits, propellant requirements, and constellation characteristics.

D. Determine the Payload Size and Performance (Chapters 9 and 13)

We next use the subject characteristics from Step 2 and orbit characteristics from Step 3 to create a mission payload concept. We can divide most mission payloads into six broad categories: observation or sensing, communications, navigation, in situ sampling and observations, sample return, and crew life support and transportation. More than 90% of current space-system payloads observe, sense, or communicate. Even the navigation payloads are basically communications payloads with ancillary data processing and stable time-base equipment to provide the navigation signal. Detailed directions for sizing and definition appear in Chap. 9 for observation payloads and in Chap. 13 for communications payloads. Table 2-13 summarizes the key parameters we need to specify.

System-level payload trades typically involve the user element, selecting a mission orbit, and allocating pointing and tracking functions between the payload and spacecraft elements. User element trades involve balancing the performance of the payload and elements on the user's premises to get the lowest overall system cost for a given orbit and constellation design. As an example, if a single geosynchronous spacecraft must service thousands of ground stations, as for direct broadcast TV, we would minimize the system cost by selecting a large, powerful spacecraft that can broadcast to simple and inexpensive ground stations. A system designed for trunkline communication between half a dozen ground stations uses more complex and capable ground systems and saves cost with simpler spacecraft

TABLE 2-12. Summary of Orbit and Constellation Characteristics. See text for discussion.

1 Altitude

2 Inclination

3 Eccentricity

4 Argument of perigee for nonclrcular orbits

5 &V budget for orbit transfer

6 AV 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 plane (constellations)

TABLE 2-13. Summary of Mlsslon-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

22. Size and orientation of clear field of view required

2.3 Primary pointing direction*

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 Voltage

3.2 Average and peak power

3.3 Peak power duty cycle

4. Telemetry and Commands

4.1 Number of command and telemetry channels 42 Command memory size and time resolution 4.3 Data rates or quantity of data

5. Thermal Control

5.1 Temperature limits (operating/non-operating) 52 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.

Payioad vs. spacecraft trades usually try to meet pointing and tracking requirements at the lowest cost At one extreme, the payioad does all the pointing independently of the spacecraft attitude; an example is the use of gimballed scan platforms on the JPL Mariner MK-II spacecraft At die 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 payioad 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 control of the spacecraft and payioad. 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 center, 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 with different options for relaying communications. Chapter 15 details the specification, selection, and design of this element.

TABLE 2-14. Summary of Mission Operations Characteristics.
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