Satellite earth stations are ground sites that are designed for extraterrestrial telecommunication with orbiting satellites. They often have large antennas for this purpose and we further dive in to what they are and how they are used.
Introduction
Satellite earth stations form a section of satellite communications systems. A satellite communications system consists of an earth station and a satellite in space. The satellite in space acts as a relay station to communicate to another earth station. Such a system may be subdivided into two segments:
- Space segment: This segment consists of orbiting satellites and the earth-based satellite control subsystem on the ground, such as the TT&C station, that is used to operate and maintain satellites in orbit. This infrastructure is owned by the satellite operators who may be private companies or government parastatals.
- Ground segment: This section consists of service provider infrastructure such as gateways and hubs as well as user terminals owned by companies and individuals.
The TT&C station provides functions of tracking, telemetry and command and often is equipped to receive various types of sensor information. This station determines a satellite’s orbital position and provides commands to adjust its altitude, orientation and trajectory. The station also relays information regarding the health and operational status of the satellite devices and equipment.
In the ground segment, gateways and hubs communicate with the user terminals using the satellite as a relay for the signals. Gateways connect to other terrestrial networks such as the Public Switched Telephone Network (PSTN), cellular networks and the Internet. Hubs connect different elements of the network by exchange of messages with the user terminals and can also relay messages between the terminals.
Figure 1 shows the Longonot Earth Station, a typical earth station found in Rift Valley, Kenya. The large antenna equipment offers a scenic view of the area.
Communication configurations
A satellite communication system communicates using microwave frequencies forming a microwave relay station. A satellite in space is used to link two or more microwave transmitters/receivers (transceivers) found in the earth stations.
Basically, one earth station sends a transmission to the satellite via an uplink that is in a certain frequency band. The satellite transponder receives this transmission, coverts and amplifies the signal and sends it to another earth station via a downlink. Transponder may be transparent or regenerative. A single satellite will operate in a number of frequency bands called transponder channels. Most satellites providing point-to-point service today use a frequency bandwidth in the range 5.925 to 6.425 GHz for transmission from earth to satellite (uplink) and a bandwidth in the range 3.7 to 4.2 GHz for transmission from satellite to earth (downlink). This combination is referred to as the 4/6-GHz band.
The two common communication configurations are:
- Point-to-point: This is communication between two earth stations using one satellite as a relay between them.
- Point-to-multipoint: This is communication between one earth station transmitter and a number of earth station receivers. This configuration is also called broadcast link. A great application of point-to-multipoint is the Direct-to-Home (DTH) television systems.
Earth Station Elements and Design
Major earth stations differ in size and complexity according to their functions which may be one of the following:
- Gateways, for telephone, cellular or Internet networks. They allow user terminals access a larger public or private network.
- Broadcasting uplink to originate various non-interactive forms of content such as video, audio (radio) programming and data.
- Hubs connecting remote user terminals back to a central location to access a host computer, servers, telephone switching equipment and private video transmission.
- Network control, a center to process requests from remote terminals for service and satellite bandwidth and to manage the overall satellite communication network.
A generic block diagram of a typical earth station is shown in the Figure 3.
From figure 3, we see that a typical earth station has a RF terminal which comprises of at least one transmitter channel and one receiver channel. The transmitter receives multiple data streams from the users and a multiplexer routes this signals to the appropriate modulator. The modulator may produce a modulated IF operating at 70 MHz with a channel bandwidth of 40 MHz or an IF of 140 MHz with a channel bandwidth of 80 MHz. The multiple channels are added together and upconverted to the RF carrier which includes frequencies in the L, C, X, Ku, K and Ka bands. The up-converters convert the signal to the desired carrier frequency. The signal is then amplified and transmitted out the earth station via a large antenna pointed to the satellite. The antenna may have a single, dual or circular polarization. A single antenna may be used for transmitting the uplink and receiving the downlink. A frequency duplexer is used to route the signals between the transmitter and receiver to the shared antenna (think switch). The duplexer is designed with a high isolation between transmit and receive channels.
The receiving block is similar to that of the transmit section but in reverse. The downlink received is filtered and amplified. A pre-select filter rejects out of band RF interference. The signal is then block down-converted to the lower IF making it easier for the demodulation process. Most downlinks use an L band IF (950 MHz to 1450 or 2150 MHz), so most validation and troubleshooting measurements are made in this range. If multiple data channels are required by the system, a signal divider and output multiplexer will route the signals to the appropriate output port.
This is only an introductory overview of satellite earth stations. Further understanding can be facilitated using a site visit or from engagement with an RF systems engineer.
REFERENCES
- Keysight Technologies, “Precision Validation, Maintenance and Repair of Satellite Earth Stations FieldFox Handheld Analyzers”, August 24, 2015.
- Bruce R. Elbert, “The Satellite Communication Ground Segment and Earth Station Handbook”, Artech House, 2001.
- William Stallings, “Data and Computer Communications”, 8th Edition, Pearson Education, Inc, 2007
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