Earth station

An Earth station is a terrestrial terminal station mainly located on the Earth’s surface. Three essential elements of any satellite communication network or system include the Earth segment, the space segment and the up/down link between the space segment and the Earth segment. Earth station design is mainly governed by the type of service to be provided such as fixed satellite service (FSS), broadcast satellite service (BSS), mobile satellite service (MSS) etc. ; quality of service to be provided mainly dictated by Earth station G/T ; type of communication requirements such as telephony, data, television etc.; international regulations; cost considerations and site constraints. The Earth station is characterized by frequency band (6/4 GHz, 14/12 GHz etc.), polarization (linear, circular etc.), antenna diameter, effective isotropic radiated power (EIRP), G/T , receive antenna gain, modulation type, access method (FDMA, TDMA etc.) and so on.

Earth station communicating with satellites

Types of Earth Station Based on the type of service provided by the Earth station 1. Fixed Satellite Service (FSS) Earth Stations 2. Broadcast Satellite Service (BSS) Earth Stations 3. Mobile Satellite Service (MSS) Earth Stations Depending upon usage of Earth Stations 1. Single function stations 2. Gateway stations 3. Teleports

1 )Fixed Satellite Service (FSS) Earth Station The large Earth stations (G/T∼=40 dB/K) medium Earth stations (G/T∼=30 dB/K), small Earth stations (G/T∼=25 dB/K), very small terminals with transmit/receive functions (G/T∼=20 dB/K) and Very small terminals with receive only functions (G/T∼=12 dB/K) The large Earth stations Very small terminal (Receive only) Very small terminals

Technically, broadcast satellite service or BSS as it is known by the 2 ) Broadcast Satellite Service (BSS) Earth Stations Large Earth stations (G/T∼=15 dB/K) used for community reception and Small Earth stations (G/T∼=8 dB/K) used for individual reception. Technically, broadcast satellite service or BSS as it is known by the International Telecommunications Union (ITU) refers only to the services offered by satellites in specific frequency bands. These frequency bands for different ITU regions include 10.7 GHz to 12.75 GHz in ITU region-1 (Europe, Russia, Africa), 12.2 GHz to 12.7 GHz in ITU region-2 (North and South America) and 11.7 GHz to 12.2 GHz in ITU region-3 (Asia, Australia).

3) Mobile Satellite Service (MSS) Earth Stations large Earth stations (G/T∼=−4 dB/K), medium Earth stations (G/T∼=−12 dB/K) and small Earth stations (G/T∼=−24 dB/K). Satellite phone is the most commonly used mobile satellite service Iridium and Globalstar are the two major LEO satellite systems offering mobile satellite services.

Single Function Stations Single function stations are characterized by a single type of link to a satellite or a satellite constellation. These stations may be transmit-only, receive-only or both. Some common examples of single function stations : Television receive-only (TVRO) terminals used for TV reception by an individual satellite radio terminals, receive-only terminalsused at a television broadcast station to pick up contribution feeds, two-way VSAT terminalsused at a retail stores for point-of-sale communications with the corporate hub, handheldsatellite telephone terminals designed to work with a single satellite constellation andmany more. TVRO terminal

Gateway Stations Gateway stations serve as an interface between the satellites and the terrestrial networks and also serve as transit points between satellites. A gateway station receives a large variety of terrestrial signals at any given time. These include telephone signals, television signals, and data streams and so on. These stations are connected to terrestrial networks by various transmission technologies, both wired such as coaxial cables, optical fibres etc. and wireless such as microwave towers. Unlike single function Earth stations where it is just up-linking and down-linking operations that comprise the core activity; in the case of gateway stations, signal processing is the major activity.

Earth Station Architecture Block schematic arrangement of a generalized Earth station

The major components of an Earth station : 1)The RF section, 2)The baseband equipment and 3)The terrestrial interface. In addition, every Earth station has certain support facilities such as Power supply unit with adequate back-up, monitoring and control equipment and thermal and environment conditioning unit (heating, air-conditioning etc.). 2)The baseband section: It performs the modulation/demodulation function with the specific equipment required depending upon the modulation technique and the multiple access method employed. 1)RF section: Antenna subsystem, the up-converter and the high power amplifier (HPA) in the up-link channel and the antenna subsystem, low noise amplifier (LNA) and the down-converter in the down-link channel.

3)Terrestrial interface. It may be connected directly to the user in some applications. The terrestrial network could be a fibre optic cable link or a microwave link or even a combination of the two. In addition to the three above mentioned components of an Earth station, every Earth station has support facilities such as tracking, control and monitoring equipment, power supply with back-up and environmental conditioning unit

Earth station design consideration Design of an Earth station is generally a two-step process: 1)The first step involves identification of Earth station requirement specifications, which in turn govern the choice of system parameters. 2)The second step is about identifying the most cost effective architecture that achieves the desired specifications. Requirement specifications includes type of service offered (Fixed satellite service, Broadcast satellite service or Mobile satellite service), communication requirements (telephony, data, television etc.), required base band quality at the destination, system capacity and reliability. Major system parameters relevant to Earth station design include transmitter EIRP (Effective Isotropic Radiated Power), receiver figure-of-merit (G/T ), system noise and interference and allowable tracking error.

Performance parameters 1)EIRP (Effective or Equivalent Isotropic Radiated Power) 2)The figure-of-merit (G/T ). 1)EIRP (Effective or Equivalent Isotropic Radiated Power) - high power amplifier (HPA) + transmitting antenna -It is given by the product of the power output of HPA at the antenna and the gain of the transmitting antenna(Pt * Gt) -Expressed in decibels -EIRP is defined for both Earth station transmitting antenna as well as satellite transmitting antenna.

-G/T is expressed in dB/K. 2) Receiver Figure-of-merit (G/T ) -It tells us about the sensitivity of the receiving antenna and the Low Noise Amplifier (LNA) combine to weak received signals. -It is the ratio of receiving antenna gain to system noise temperature. -G/T is expressed in dB/K. -G/T of the Earth station may be enhanced by increasing the receiving antenna gain or lowering the noise temperature or both.

Earth Station Design Optimization The generalized expression for G/T is given by equation : G/T = C/No − EIRP + Lp + Lm + k Where C/No, EIRP, Lp, Lm and k are carrier- to-total noise power spectral density, satellite’s effective isotropic radiated power, path loss, link margin and Boltzmann constant (in dBs) respectively. For a minimal cost Earth station, G/T should be minimized. This can be possible by either using relatively higher EIRP in the satellite or being able to afford a lower carrier-to noise ratio or both. Complexity and cost issues depend on: Earth station EIRP, antenna tracking requirements, traffic handling capacity and terrestrial interface requirements. In addition, there are international regulatory issues and technical constraints that drive the optimization process.

Earth station testing 1)Unit and Subsystem Level Testing -done at the manufacturer’s premises -test data is made available to the subsystem designer making use of the components -different subsystems are comprehensively tested for their electrical, mechanical and environmental specifications 2) System Level Testing -Carried out after subsystem testing and integration has been completed * Mandatory Tests : -Transmit Cross-polarization Isolation Measurement -Receiver Figure-of-merit Measurement. -EIRP Stability -Spectral Shape *Additional Tests: Antenna patterns measurement *Line-up test

R.F. equipment for earth station

Up-converters, High power amplifiers (HPA) The transmit antenna in the transmit channel, and The receive antenna, Low noise amplifiers (LNA) and Down-converters in the receive channel. 1)Antenna These mainly include the prime focus fed parabolic reflector antenna, offset fed sectioned parabolic reflector antenna and cassegrain fed reflector antenna. Prime focus fed parabolic reflector antenna Cassegrain antenna

High Power Amplifier Single amplifier HPA configuration Multiple amplifier HPA configuration

Up-converters/Down-converters (a) Up-converter (b) down-converter

Low Noise Amplifier (LNA) The low noise amplifier (LNA) is one of the key components deciding the system noise temperature and hence the figure-of-merit G/T of the Earth station Gallium Arsenide FET (GaAs FET) or High Electron Mobility Transistors (HEMT). Two types: In one of the variants, particularly where small size antennas are used such as those for TVRO or small business applications, the low noise amplifier section feeds a single stage down-converter in a single block called low noise block (LNB). DTH dish and co-located LNB

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