The space segment is known to be another crucial aspect of satellite communication systems other than the control and user segment. The space segment undoubtedly includes satellites however it also comprises the ground-based units that help to maintain the operationality of satellites orbiting the earth.
The space segment is designed to receive, store and retransmit the radio signals forwarded by the control segment.
Content: Satellite Space Segment
Introduction
The satellite orbiting in space forms a major unit of space segment however, the equipment which carries the satellite into space is classified according to their respective function. Payload and bus are the two necessary equipment associated with satellite.
The payload offers those services to the system to attain which the satellite launching has been performed. More simply, it is equipment that provides all the services which the launching of the satellite requires. Another equipment is a satellite bus that is a vehicle that carries a satellite payload along with holding subsystems that offer facilities like power, attitude and orbital control, thermal control, etc. by which the payload gets serviced.
Elements of Space Segment
The various elements that combinedly constitute the space segment are as follows:
- Power Supply
- Attitude Control
- Station Keeping
- Thermal Control
- TT&C subsystem
- Transponder
Let us now discuss each one individually in brief.
Power supply: The primary source of power used by the electronically operating equipment in space is solar cells. However, batteries are also incorporated in such systems that offer standby power in case of unavailability of solar energy. It is to be noted here that power generated by a single solar cell is quite small than the one which is actually required by the equipment to operate thus, instead of using a single cell, arrays of cells, in series and parallel configuration, are used to achieve the desired power level.
The HS 376 satellite was discovered by Hughes Space and Communications Company with dimensions 660 cm and 216 cm for length and diameter respectively.
During the launching, the outer and inner panel of the spacecraft is exposed to sunlight. In the initial years, the power produced is nearly 940 W dc but with time nearly around 10 years or so, there can be power droppage of up to 760 W. However, when the solar power is unavailable i.e., during the conditions of the eclipse, two nickel-cadmium long-life batteries are used that deliver the power of around 830W. This battery takes a recharge time of less than 16 hours.
In some conditions when higher energization is required by the spacecraft then the rectangular arrangement of solar cells is done called solar sails. At the time of launch, the solar sail is folded and in the geostationary orbit, it gets extended, offering a stretchability of around 67 feet. The sails are rotatory in nature and this allows adjustment according to the sun so as to get maximum power output.
Attitude Control: Attitude control of satellite corresponds to controlling the orientation of satellite orbiting earth. This is a crucial part of satellite system designing and maintenance because attitude controlling enables the satellite’s antenna to accurately point over the specific region on the ground.
More simply, we can say that, in attitude control, the orientation of the satellite in space is to be sensed and then proper controlling action must be enabled in case of any error. Basically, the orientation of the satellite shows variation with time thus requires monitoring and correction.
The figure below shows a simple diagrammatic representation of a system performing attitude control:
In such systems, the position of the satellite in orbit is sensed then according to the respective control command adjustments, the actuators get the desired command which as an output adjusts the orientation of the satellite.
Though attitude controlling occurs abroad the satellite, the control signals are sent from the earth according to the received attitude data from the satellite.
Station Keeping: Similar to attitude control, maintaining proper orbital position is quite necessary. Station keeping is a technique that deals with the issue of orbital drift. A satellite in geostationary orbit shows drift in latitude, generally because of the gravitation pull of the sun and moon. However, within certain limits, these shifts are acceptable, but measures must be taken to limit the same from further exceeding and so jets are pulsed as and when required in order to maintain the inclination to 0.
TT&C subsystem: Here the T, T, and C stand for tracking, telemetry, and command, respectively. This is designed to perform various routines functions for the satellite in space. The TT&C system holds the authority of producing electrical signals in proportion to the quantity required to be measured, encoding it, and further transmitting it to the desired earth-based station.
The telemetry function is associated with the measurement at a distance. The telemetry signals comprise of the attitude information, environmental information, satellite information, along with power supply voltages and stored fuel pressure.
The command performs the opposite action of that of telemetry as in telemetry subsystem, information is transmitted from a satellite to an earth station, while command subsystem deals with getting the signal from earth station which is generally the response of telemetered information. It performs the necessary demodulation of the received signal and further retransmits the decoded signal to a unit where the required action is completed. From a security perspective, the encryption of the command signal is performed.
The tracking subsystems are responsible for tracking the satellite by monitoring the transmit beacon signals which get received at the TT&C earth stations. It is an important subsystem because tracking is a crucial aspect while dealing with transfer and drift orbital phases of the satellite. In case of correction when a position is tracked, the correcting signals are provided to the telemetry channels using special tracking antennas.
These functions are complex in nature hence require well-defined ground facilities along with TT&C systems within the satellite.
Thermal Control: The satellites orbiting earth suffer great temperature variations and there are various reasons for this. The temperature of the satellite varies because of radiations coming from the sun, thermal radiations from the earth. Other than these, the satellite bus also generates heat and all these factors increase the temperature level and for proper operation, the temperature must be maintained within limits. To achieve this, one of the important factors is that the satellite equipment must be operated within a stable temperature atmosphere.
For this, various methods are adopted such as the use of thermal blankets that provide insulation to the system, use of radiation mirrors that remove the heat from the payload. Sometimes mirror drums are employed that surround the equipment through which the radiation gets escaped into space.
Transponders: For a communication satellite, a transponder corresponds to a channel formed of serially connected units between transmit and receive antennas. Transponders are not single equipment rather it’s a combination of equipment forming a channel. The requirement of the channel is the basis of transmission operation because the signal needed to be propagated between the two ends is sent through the channel. Hence, the transponder forms a crucial unit of satellite communication.
The bandwidth of the transponder is dependent on the technology used for satellite as well as the nature of the signal which is getting transmitted. It is incorporated both in uplink as well as downlink.