Communications Architecture

Fred J. Dietrich, Globalstar LP: Richard S. Davies, Stanford Telecommunications, Inc.

13.1 Communications Architecture Communications Architecture Defined by Satellite-Ground Station Geometry; Communications Architecture Defined by Function; Criteria for Selecting Communications Architecture

13.2 Data Rates

TT&C; Data Collection; Data Relay

13.3 Link Design

Derivation of link Equation; link Design Equations; Modulation and Coding; Atmospheric and Rain Attenuation; Frequency Selection; link Budgets

13.4 Sizing the Communications Payload

13.5 Special Topics

Multiple Access: Sharing Communication Links; Payloads with Onboard Processing; Antijam Techniques; Security; Diversity Techniques; Optical Links

A communications architecture is the arrangement, or configuration, of satellites and ground stations in a space system, and the network of communication links that transfers information between them. This chapter discusses this arrangement of links, their operation, and their effect on system design. More detailed information on satellite communications is available in Morgan and Gordon [1989] and Sklar [1988].

Table 13-1 lists the steps required to specify the communications architecture. The first step is to define the mission objectives and requirements in enough detail to evaluate and compare alternative architectures. Section 13.1 describes alternative configurations and the criteria used in their selection.

The second step is to determine the data rates for each of the links identified in step 1. To do this it is necessary that the required data throughput accuracy of data transmission be specified, and whether or not there will be data processing on board the satellite. This process is described in Sec. 13.2.

The third step is to design each link in the network, as explained in Sec. 133. Principal factors are the availability of a radio frequency spectrum, coverage area of the satellite antenna beam, and path length between satellite and ground station. These

TABLE 13-1. Specifying a Communications Architecture. Evaluating alternative architectures may require designing the links and sizing the communication payload as described in Table 13-9.

Step

Process (= Detailed Step)

Reference

1. Identify Communication Requirements

• Develop mission data flow diagram based on mission requirement

- Date sources, data end users, & locations

- Quantity of data per unit time -Access time

- Transmission delay -Availability, reliability

Sec. 2.1 Chap. 4

2. Specify Alternate Communications Architectures

• Identify links and ground station locations

• Consider use of relay satellites and relay ground stations

• Determine data processing location

Sec. 2.1

3. Determine Data Rates for Each Link

• Determine sampling rates, quantization levels

• Specify bits per sample

Sea 9.5.1 Sec. 13.2 Table 13-4

4. Design & Size Each Link

• Evaluate alternatives and compare

Tables 13-2,13-9

5. Document Reasons for Selection

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