Or

• TT&C data is transmitted between the satellite and ground station, either directly or by a relay satellite.

A relay satellite can provide broadcast TT&C service to multiple satellites.

Satellite sensors coilect.data and transmit it to single ground station directly or by relay station.

Satellite sensors collect data and broadcast it to multiple ground stations.

Data-relay satellites relay data originating on ground or in smother satellite to single ground station.

Data-relay satellite broadcasts data originating on ground or in another satellite to multiple ground stations.

Fig. 13-3. Communications Architectures may be Defined by the Function Performed.

Open circles represent a relay satellite.

Fig. 13-3. Communications Architectures may be Defined by the Function Performed.

Open circles represent a relay satellite.

Operators at ground (air, ship) stations usually control the mission in (near) real time by transmitting commands to the satellite. When the satellite is not in view of the ground-control station during part of its orbit, commands previously received and stored in the satellite are executed by an onboard timer. The advantages of this approach are flexibility to changing requirements, greater reliability, and a less complex, lower-cost satellite. The disadvantages are vulnerability to human error or failure of the ground control facility. Costs for the ground control segment, especially operations, can be high.

On the other hand, the satellite itself can control a mission by using onboard data-sensing and programmed decision-making processes. This arrangement replaces ground control, is highly survivable, has fast response time (communication link delays eliminated), excludes errors introduced by human operators, and reduces ground equipment and operations cost But it is less responsive to changing or unan ticipated requirements, and the satellite itself is more complex, more costly, and potentially less reliable. Even when using an autonomous control architecture, a ground station is generally required to collect data from the spacecraft and to serve as a backup to the onboard control system.

Usually, ground stations control unmanned satellites to simplify the satellite design. In the future we expect more functions, such as stationkeeping, to be performed in the satellite to reduce the dependence on the ground station control (see Sec. 16.1).

During the operation of a satellite system, the communication links may need to be reconfigured, or its parameters, such as power or bandwidth, adjusted to accommodate a change in requirements. The process for doing this is called network control. Communications architectures may require a number of control functions (Table 13-3). Early satellites, such as Sputnik, did not need these functions because their systems used only one satellite, a single satellite-to-ground link, and a broadbeam antenna. On the other hand, a communications satellite system such as the NASA ACTS [Naderi and Kelly, 1988] contains many narrowbeam satellite antennas with demodulators and switching circuits. This architecture requires a sophisticated system for network control. Network control can be centralized using a single ground station or satellite, or distributed with multiple ground stations or satellites. Distributed configurations use a control hierarchy, or set of priorities, to avoid conflicts. Distributed control makes the network less vulnerable to failure of a single control element (see Chap. 14 for further details).

TABLE 13-3. Network Control Functions.

Function

Example

Resource Allocation

Frequency channel, bandwidth assignment Time slot assignment Date rate assignment Modulation/coding assignment Antenna beam assignment Transmitter power control Crosslink assignment

Link Acquisition

Antenna pointing Frequency acquisition Time acquisition Acknowledgment protocols Crypto synchronization

Performance Monitoring and Redundancy Switching

Spectrum analysis

Signal-to-noise ratio reduction (due to rain, etc.)

Interference identification

Fault identification

Redundancy switching

Reallocation of resources

Time/Frequency Standard

Provide universal time

Tracking, Telemetry, and Command (TT&C)

Range, range-rate measurement

Command signal formatting, verification, execution

Telemetry signal demultiplexing, processing, display

Stationkeeping

Ephemeris prediction Thrust control

13.1-3 Criteria for Selecting Communications Architecture

Individual users will assign different priorities to the criteria for selecting a communications architecture. For example, a commercial company will try to reduce cost and risk, but the military may make survivability the top priority. The factors which affect the criteria are explained below:

Orbit: The satellite orbit determines how much time the satellite is in view by the ground station and the potential need for intersatellite links. The satellite altitude determines the Earth coverage, and the satellite orbit determines the delay between passes over a specified ground station. Together, orbit and altitude set the number of satellites needed for a specified continuity of coverage (see Sec. 7.2). Transmitter power and antenna size depend on the distance between the satellites and the ground stations (see Sec. 133). Satellite view time determines the signal-acquisition and mission-control complexity (see Chap. 14).

In the satellite-cellular systems described above, intersatellite links are not necessarily used. Instead, the constellation is designed so that at least one satellite is in view by the gateway and every user at all times, so that there are no "outages." Coverage is determined by the number of satellites, the inclination of their orbits, the latitude of the gateway and user, and the number of gateways located around the world, if intersatellite links are not used.

If intersatellite links are used, then the number of gateways and their location becomes much less critical, as many satellites can connect to a single gateway through intersatellite links. Various systems proceeding now have used different philosophies with respect to intersatellite links, which can have great effect on the capital cost of the system. Intersatellite links make the satellites more expensive, but eliminate the need for many fairly expensive ground stations (gateways), for example.

There are many systems proposed in various frequency bands which use not only the geostationary orbit, the low-Earth orbit discussed above, and also what is called a medium-Earth orbit (MEO), which ranges in altitude from about 10,000 to 20,000 km. These are typically inclined with respect to the equator as the LEOs are, and can address users with small, hand-held UTs, but can see a much larger portion of the Earth at one time, so that only 10 or 12 of them are required to give nearly complete Earth coverage.

RF Spectrum: The RF carrier frequency affects the satellite and ground station transmitter power, antenna size and beamwidth, and requirements for satellite stabilization. In turn, these factors affect satellite size, mass, and complexity. The carrier frequency also determines the transmitter power needed to overcome rain attenuation (see Sec. 13.3). Finally, it is necessary to apply for and receive permission to use an assigned frequency from a regulatory agency such as the International Telecommunication Union, the Federal Communications Commission, or the Department of Defense's Interdepartmental Radio Advisory Committee, and every nation's regulatory agency. These agencies also allocate orbit slots for geostationary satellites (Chap. 21).

Data Rate: The data rate is proportional to the quantity of information per unit time transferred between the satellite and ground station (see Sec. 13.2). The higher the data rate, the larger the transmitter power and antenna size required (Sec. 13.3). Processing the spacecraft-generated data on board the satellite reduces the data rate without losing essentia] information, but makes the satellite more complex (see Sec. 13.2).

Duty Factor: The fraction of time needed for operation of a satellite link is the duty factor, which is a function of the mission and the satellite orbit A low duty factor enables a single ground station to support more than one satellite (usually the case for telemetry and command). Alternatively, several users may share a single satellite link (see Sec. 13.5).

In the case of LEOs used for cellular service, one gateway will typically have several antennas communicating simultaneously with several satellites^ each of which may be carrying 1,000 or more individual circuits. In this case, the ground station duty factor will be nearly 100% as antennas switch from satellite to satellite; the UT use factor will be quite small, however, as is the use of a telephone.

Link Availability'. Link availability is the time the link is available to the user divided by the total time that it theoretically could be available. It depends on equipment reliability, use of redundant equipment, time required to repair equipment, outages caused by rain, and use of alternate links. Typical goals for link availability range from 0.99 to 0.9999, the latter value applying to commercial telephone networks. (See Chap. 19 for a discussion of reliability.)

Link Access Time: The maximum allowable link access time, or time users have to wait before they get their link, depends on the mission. For example, we usually demand access to a voice circuit in seconds. Meteorological data is needed in less than an hour to be useful in weather forecasting. On the other hand, X-ray data from a scientific satellite can be stored and transmitted later. Tracking, telemetry, and command links are often required in near real-time (a few seconds), especially if a problem requires an immediate response from the satellite-control operator. l ink access time depends strongly on orbit selection, which determines when a satellite is in view of the ground station. Note that a real-time response is impossible for deep space missions, because the radio propagation time is minutes or hours long.

Threat: Various kinds of threats may influence system design. For military applications, choices of frequency, antenna, modulation, and link margin need to be evaluated for susceptibility to jamming. At the same time, a high-altitude nuclear detonation can disturb the propagation of radio signals. A physical threat to the satellite might dictate multiple satellites or a hardened design (see Chap. 8). A physical threat to a ground station might demand a data-relay satellite with crosslinks to allow the ground station to be relocated in safe territory.

The FireSat sample mission uses low-altitude satellites with limited coverage. If a ground station is near the forest area under surveillance, a store-and-forward or crosslink architecture is not required. The communications architecture is then simply a single satellite operating when in view of its ground station. A separate ground station is required for each major area under surveillance.

13.2 Data Rates

In designing a communications architecture for space missions, we must ask: what is the information to be transferred over our communication links? How fast must the transfer rate be? Keeping in mind that higher rates of data transmission mean higher system costs, we need to decide how we will transfer information to the user.

Satellite links originally used analog modulation techniques to apply the data onto the RF carrier for transmission over the link. Since 1980, however, most space-ground communication links use digital modulation. To implement a digital system, we must first sample the amplitude of the analog signal at a rate equal to at least twice the highest frequency in the signal spectrum,/m. In 1928 Nyquist* showed that if we meet this condition we can theoretically reconstruct the original analog signal from the samples (see Sklar [1988], Sec. 2.4). For example, the normal human voice has a frequency spectrum range of about 3.5 kHz. Thus, to reproduce it digitally, the sampling rate must be at least 7,000 samples/sec. However, practical considerations, such as realizable filter limitations, suggest that the sampling frequency should be at least 2.2 times the maximum input frequency [Sklar, 1988]:

Using Eq. (13-1), our 3.5-kHz voice signal must be sampled at a rate of 7.7 ksamples/s. In fact, the sampling rate of commercial digitized voice systems is 8 ksamples/s. Another example is the sampling rate of the audio compact disc player which is 44.1 ksamples/s—about 2.2 times 20 kHz, the maximum source frequency of interest for high-quality music.

The analog amplitude sample is next converted to a digitized word composed of a series of bits. Consider the analog-to-digital converter process illustrated in Fig. 13-4, where three bits designate one of eight amplitude levels. For example, a 6.3 V amplitude converts to a 3-bit word—110. At the receiver, a digital-to-analog converter converts this word according to the algorithm 22 + 21 + 0° + 0.5 = 6.5 V, leaving a quantization error of 0.2 V. This quantization error can be reduced by increasing the number of bits in the word.

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