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Earth Station Communication System-Sagarmatha Earth Station

Abstract: This study is the outcome of the 'In-Plant Training in Satellite Communication' held at Sagarmatha Earth Station, Balambu, Kathmandu, Nepal on May-June 2006. This study has investigated the essential field of communication engineering i.e. Satellite Communication along with basic of communication between ground station (Earth Station) and Satellite. In this research program we study essential equipments needed for earth to satellite communication. Study also reveals the latest technology in antenna propagation used by Nepal Telecom for wireless communication. Advantages due to utilization of latest equipments and it cost effectiveness is also studied in this research program.

Benefit: This study reveals the importance of Earth Station in the field of Satellite Communication. Sagarmatha Earth Station being only Earth Station of Nepal, it has served to all the people of Nepal from the date of its establishment. Communication between remote district like Humla, Jumla, Mugu, Kalikot e.t.c where wired communication can never be thought of, is made possible only via this earth station. As we have entered into the modern era of high speed communication i.e. Optical Fibre communication still we are not able to utilize it efficiently being a landlocked country. So, satellite communication is only alternative for country like Nepal. Thus, functions of this station is very promising and appreciating in context of Nepal. As a whole, we can say this station is fully dedicated to Nepalese welfare and development.

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History of Satellite Communication [1] [2]

First satellite was Russian satellite Sputnik launched in 1957.

Early Bird, launched in 1965 was first geostationary satellite. It was first satellite launched by Intelsat later named as Intelsat I weighing 36kgs, 6/4 GHz transponder each 25 MHz bandwidth.

Canada was the first country to build a national telecommunication system using GEO satellites. Anik 1A was launched in May 1974, just 2 months before the first U.S domestic satellite WESTER 1.

The ability of satellite systems to provide communication with mobile users had long been recognized, and the International Maritime Satellite Organization (Inmrasat) has provided service to ships and aircraft for several decades.

LEO satellites were seen as one way to create a satellite telephone system with worldwide coverage; numerous proposals were floated in the 1990s, with three LEO systems eventually reaching completion by 2000(Iridium, Globstarm and Orbcomm).

DBS-TV and GPS had revolutionized the communication system in this 21st century.

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Introduction

Earth Station provides the communication pathway between the Geo-stationary Satellite and local networks. Signals from the all the local networks are send to the Satellite for long distant communication and signal received from the Satellite are send to desired location within a network is done by the Earth Station. International Calls, International Direct Dialing, Occasional TV Broadcasting, Internet Access, Weather Forecasting, VSAT (Very Small Aperture Terminal) Communication are prominent benefits of an Earth Station. Intelsat-1 to Intelsat-4, ThaiCom, Arabsat, TelStar, Early Bird, KoreaSat are some satellites send by many countries to guide wireless long distant communication. A signal also termed as baseband signal or message signal is send via antenna to the Satellite by the unique uplink frequency and signal are received from the satellite by the downlink frequency. Figure below shows the block diagram of the systems needed for efficient and accurate communication inside the earth station.

 

Figure: Schematic diagram of earth station communication.

 

Ku-Band Uplink Frequencies: 14 GHz – 14.5 GHz

Ku-Band Downlink Frequencies: 11.7 GHz – 12.2 GHz

C-band Uplink Frequencies: 5.925 GHz – 6.425 GHz

C-Band Downlink Frequencies: 3.7 GHz – 4.2 GHz

Up-link frequency is much higher than the down-link frequency because while sending information from the earth to the space, signal need to travel against the gravitational force of earth and need to penetrate the thick layer of atmosphere. Baseband band or Message signal is first send to the Multiplexer through a communication channel. Multiplexing is the process of combining a number of signals into a single signal, so that it can be processed by a single amplifier or transmitted over a single radio channel. Multiplexing can be done at baseband or at a radio frequency. The corresponding technique that recovers the individual signals is called Demultiplexing. Multiplexing is a key feature of all commercial long-distance communication systems, and is par of the multiple access capability of all satellite communication systems.

Multiplexed signal is send to the DACCS (Direct Access Cross Connect System) and DCME (Digital Circuit Multiplication Equipment). In communications systems DACCS performs following function, a digital system in which (a) access is performed by T-1 hardware architecture in private and public networks with centralized switching and (b) cross-connection is performed by D3/D4 framing for switching digital-signal-0 (DS-0) channels to other DS-0 channels. Modern digital access and cross-connect systems are not limited to the T-carrier system, and may accommodate high data rates such as those of SONET. In the EIA/TIA 568-A standard, a horizontal cross-connect is used to describe the distribution station when such a station is separate from the main backbone, and accessed via horizontal cabling. Digital Circuit Multiplication Equipment (DCME) was a type of voice compression equipment that is installed at either end of a long-distance link (typically communications satellite or submarine communications cable). The main characteristics of DCME are defined in ITU-T recommendation G.763. DCME consists of a voice interpolation stage, which is a form of statistical multiplexer applied to voice-band signals, and a low rate encoding stage which exploits correlation between successive voice-band samples on an individual input channel to reduce the transmitted bit-rate from that required by PCM of equivalent quality.

Signal is then send to Modulator where signal is converted to high frequency called IF (Intermediate Frequency) equals 170 MHz. Modulation is converting low frequency signal to high frequency where property of modulating signal changes according to either Phase or Frequency or Amplitude called as AM, PM or FM. Modulation helps weaker signal to travel longer distance and also help to decrease the length of antenna needed for propagation. Frequency is inversely proportional to wavelength of wave and antenna length directly depends on the wave length of the baseband signal. Conversion of any signal to IF helps us in designing amplifiers and filters easily for small range or frequencies.

IF signal is then converted to large frequency called as uplink frequency order of GHz (C-Band uplink frequency is 6 GHz) by the Up-Converters. Larger the Signal frequency greater the signal has capacity to penetrate the thick atmospheric layer and greater chance to travels to longer distance with minimum attenuation. But frequencies greater than 12 GHz result in maximum rain attenuation hence suitable frequency is chosen during satellite communication.

Up-Converted signal is then send to SSPA (Solid State Power Amplifier) to increase the power of signal to prevent any kind of attenuation and errors (Bit Error Rate). Final signal is send to the antenna via a wave guide pipe. Wave Guide Pipe sends any signal with minimum attenuation and maximum efficiency and it is a wired communication channel laid from SSPA or any equipmemt directly to the antenna for distant communication. Antenna provides directivity and Impedance Matching to the signal for easy propagation over the air. Number of environmental factors need to be considered like Rain, Solar Flares, Snow Fall, Clouds, Auroras e.t.c for Satellite communication.

Satellite needs numbers of sophisticated equipment for receiving and transmitting the signal by FM technique to long distances. The payload of a satellite is all the specialized equipment needed to perform its designed function. A communications payload act like a communications repeater. RF signals to the satellite are received, converted, amplified, and transmitted back to Earth. The payload includes the antenna, wide-band receivers, input and output multiplexers, programmable attenuation devices, and amplifiers. Satellites designed with a single payload are only able to operate with a single band of frequencies, either C or Ku. Satellites with dual payloads, also known as Hybrid Satellites, are able to operate with both C and Ku bands (1 band per payload). Each payload has a set of components that operates with a specific band of frequencies.

While receiving the signal passes via the same equipments but under lower frequency than uplink frequency called download frequency. Signal received at the antenna is send to the LNA (Low Noise Amplifier) to remove all the noise and amplifies only the required signal. LNA is present at the antenna itself and present receiving end of the antenna. Received signal at the downlink frequency (4 GHz for the C-Band Communication) is send to the Down-converter from the wave guide pipe to convert the signal to IF at the frequency 40 MHz. Intermediate Frequency is send from the Demodulator to the Demultiplexer. Finally, baseband band signal is received with least attenuation and least error from the demultiplexer.

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Orbital Effects on the Satellite Communication

Following are the factors affecting satellite communication:

  • Doppler Shift: For a stationary observer, the frequency of a moving radio transmitter varies with the transmitter’s velocity, relative to observer. If the true transmitter frequency ( the frequency that the transmitter would send when at rest), the received frequency is higher when the transmitter is moving toward the receiver and lower that the transmitter is moving away form the receiver. The shift of frequency is called Doppler Shift.

  • Rage Variation: The position of satellite with respect to earth exhibit a cyclic variation. For TDMA careful attention must be done to the timing of the frames within the TDMA burst.

  • Solar Eclipse: During full solar eclipse satellite must run on its own battery power as it receives no power from the solar cell.

  • Sun Transit Outage: During equinox period (March 21 and September 23) earth station receive not only signal from satellite but also the noise temperature transmitted by the sun. This added noise temperature will cause the fade margin of the receiver to be exceeded and outage will occur.

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Equipments at Sagarmatha Earth Station

1. ARMA C400 SSPA

2. NEC UP/DOWN CONVERTER

3. STANDARD A OR B ANTENNAS
In Sagarmatha Earth Station we have two Standard ‘B’ antennas (11m and 9m) and one Standard ‘A’ antenna and other demos and two microwave antennas (not in operation). Both standard antennas are linked with four IntelSat Satellites out of 24. These all are either

Cassegrain Antenna or Gregorian antenna.

  • Standard ‘A’: Main purpose is International Calls, Internet Connection for NTC, and Occasional TV for NTV. Its diameter is 16.4m and largest in Nepal.

  • Standard ‘B’: There are two standard antennas. Used mainly for VSAT network. Their diameters are 11m and 9m. Placed for 66º E satellite. Consists of two circuits.

Multiplexer: In Sagarmatha Earth Station, multiplexer are used for Occasional TV, VSAT communication and Telecommunication.

STM Multiplexer: This multiplexer is used in VSAT network communication.

PCM Multiplexer: Used for telecommunication.

REP: Multiplexer used for telephone system. (Now-a-days not in operation or replaced).

 

5. MODEM

DTX 240E System:

The function of the respective units:

PSK MODEM: Used for satellite communication using PSK technique. Few years ago MODEM used BPSK (Binary Phase Shift Keying) technique but due to newer invention of modulation technique Sagarmatha Station uses QPSK (Quadrature Phase shift keying) and 8PSK(Eight Phase Shift Keying) technique. This leaded to minimize the bandwidth to be used in transponder and cost for communication is gradually decreasing.

  • SDM-309 Satellite MODEM: MODEM used for VSAT communication. Used TDMA technique in the earth station.

  • DVB Digital Video Modem: MODEM used for occasional TV used by NTV (Nepal Television).

  • N × 64Kbps Modem: Extensively used for Internet Access by NTC.

6. OCCASSIONALV TV EQUIPMENTS OF NTV

  • Digital Video MODEM (DVB 3030)

  • RCS-11 Redundancy switch

  • Audio Video Distribution Amplifier

  • Encoder

  • Digital Baseband patch for occasional TV

  • Video patch and TV Test Signal generation

  • TV sound monitor and webform monitor.

  • Solid State Power Amplifier

  • Extended Range DC Power Supply (0-60V and 20A).

7. DTX-360 Ethernet HUB for VSAT
Can be called as the heart of the VSAT network. Signal coming from any VSAT station from remote place is received by this HUB station and transmitted to desired place for efficient communication. Internet access, Local and International telephone calls are sent from remote village via VSAT antenna to main earth station and here HUB receives the signal and transmits to the satellite for communication. Efficient communication is possible in Sagarmatha Earth Station by utilizing DAMA technology and dedicated technology in this HUB. E.g. Due to use VSAT technology internet access in Base camp of Mount Everest has been made possible.

8. Antenna Steptrack Controller
Used to operate in different modes of antenna. It is fully automated system and software oriented control system. Controller works according to values of Look Angles i.e. Azimuth and Elevation. Azimuth is measured eastward (clockwise) from geographic north to the projection of satellite path on a (locally) horizontal plane at the earth station. Elevation is the angle measured upward from the local horizontal plane at the earth station to the satellite path. Operated by 7200 Antenna Control Unit, which is fully software oriented control system.

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Terminologies

8PSK: Eight Phase Shift Keying
ADPCM: Adaptive Differential Pulse Code Modulation
AM: Amplitude Modulation
BPSK: Binary Phase Shift keying
CDMA: Code Division Multiple Access
CELP: Code Excited Linear Prediction
DACCS: Digital Access Cross Connect System
DAMA: Demand Assigned Multiple Access
DBS-TV: Direct Broadcast Satellite Television
FDMA: Frequency Division Multiple Access
FM: Frequency Modulation
GEO: Geo-Stationary Earth Orbit
GPS: Global Positioning System
GPRS: Global Positioning Radio System
IF: Intermediate Frequency
IntelSat: International Telecommunications Satellite Organization
ITU-T: International Telecommunications Union
LHCP: Left Hand Circular Polarization

LEO: Lower Earth Orbit
LNA: Low Noise Amplifier
MEO: Medium Earth Orbit
NTC: Nepal Telecom
PCM: Pulse Code Modulation
PSK: Phase Shift Keying
QPSK: Quadrature Phase Shift Keying
RHCP: Right Hand Circular Polarization
SCPC: Single Channel Per Carrier
SSPA: Solid State Power Amplifier
TDMA: Time Division Multiple Access
TTCM: Telemetry, Tracking, Command and Monitoring
VSAT: Very Small Aperture Terminal

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Acknowledgement

I heartly thank Nepal Telecom, Kathmandu, Nepal for providing me a week to complete my 'In-Plant Research in Satellite Communication' on 23rd -30th May 2007 at the Sagarmatha Earth Station, Balambu, Kathmandu, Nepal. I am grateful to Mr. Mithilesh Chandra Jha (Senior Engineer) for giving me his precious time to complete research on Sagarmatha Satellite Earth Station as a technical advisor. Without his technical guidance this research program wouldn’t have been completed in time. I’m equally thankful to Mr. Birendra Gongol (Station Manager) for giving me opportunity to study each and every instruments involved in ground to satellite communication. I would like to thank Mr. Lok Raj Paneru (Senior Engineer) for his technical guidance. I’m equally thankful to all the staffs of Station for giving me chance to study sensitive parts of station. I would like to thank Nepal Telecom and Nepal Telecom Training Centre for giving me oppurtinity to study each and every sections of a Sagarmatha Earth Station. I sincerely thank to Mr. Dipeen Dhakal, Pulchowk Engineering College, Kathmandu, Nepal for his guidance in analysis of different Mirowave Signals.

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List of Refrences

[1] Timothy Pratt, Charles Bostian, Jeremy Allnutt. Satellite Communication, Wiley Student Edition (2006).

[2] Handbook of communication, Tata Mc-Graw Hill Edition (2005)

 

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Last Updated on July 14th, 2012 at 19:00 pm