28 Mobile communications Satellite systems (Part-1)

Suchit Purohit

Learning Objectives

• Communication satellite Segments
• History Satellite communication
• Classical Satellite Systems
• Applications
• Indian Communication Satellites and Critical Services
• Basics of Geo-stationary Satellites
• satellite Inclination angle
• Satellite Elevation Angle
• Propagation Loss
• Satellite Orbits
• GEO
• LEO
• MEO
• Frequency Bands, Spectrum

Introduction

Satellite communication supports the mobile communications. Satellites offer global coverage without wiring costs for base stations and are almost independent of varying population densities.

Figure 1

Satellite Communication can be broadly divided in three segments

Figure 2 : Space Communication Segments

1. Space Segments: consists of mainly Communication satellites in various orbits GEO, LEO, MEO and HEO depending upon the application needs to the users
2. Ground Segments: consists of mainly Hub or Gateway Earth Station user terminal.
3. Communication Channel Communication is established by Up link and Down link frequency in UHF, L band, S-band, Normal C-band, Extended C-band, Ku-band, X-band, Ka-band, Q-band, V-band, W-band, Optical-bands etc. depending on the user application, approval by ITU, band width, Geographical location, data rate, data volume and traffic density requirements.

The high speed of satellites with a low altitude raises new problems for routing, localization of mobile users, and handover of communication links.

History Satellite communication

Satellite communication began after the Second World War. In 1945, Arthur C. Clarke published his essay on ‘Extra Terrestrial Relays’. But it was not until 1957, in the middle of the cold war, that the  sudden launching of the first satellite SPUTNIK by the Soviet Union shocked the Western world. SPUTNIK is not at all comparable to a satellite today; it was basically a small sender transmitting a periodic ‘beep’.

Today, geostationary satellites are the backbone of news broadcasting in the sky. Their great advantage is their fixed position in the sky. Their rotation is synchronous to the rotation of the earth, so they appear to be pinned to a certain location.

The first commercial geostationary communication satellite INTELSAT 1 (also known as ‘Early Bird’) went into operation in 1965. It was in service for one-and-a-half years, weighed 68 kg and offered 240 duplex telephone channels or, alternatively, a single TV channel. INTELSAT 2 followed in 1967, INTELSAT- 3 in 1969 already offered 1,200 telephone channels. While communication on land always provides the alternative of using wires, this is not the case for ships at sea. Three MARISAT satellites went into operation in 1976 which offered worldwide maritime communication. However, Sender and receiver still had to be installed on the ships with large antennas (1.2 m antenna, 40 W transmit power). The first mobile satellite telephone system, INMARSAT-A, was introduced in 1982. Six years later, INMARSAT-C became the first satellite system to offer mobile phone and data services. (Data rates of about 600 bit/s, interfaces to the X.25 packet data network exist.) In 1993, satellite telephone systems finally became fully digital with INMARSAT-M.

Nineteen ninety-eight marked the beginning of a new age of satellite data communication with the introduction of global satellite systems for small mobile phones, such as, e.g., Iridium and Global star. There are more than 2000 geostationary satellites in commercial use which shows the impressive growth of satellite communication over the last 50 years.

The Indian Space Program was started by 1969, by Great Visionary Dr.Vikram Sarabhai. The first Indian Satellite INSAT-1A was launched in 1981. Currently INSAT and GSAT series 42 satellites are operational in orbit.

Classical Satellite Systems

Earlier satellites were simple transponders; today’s satellites rather resemble flying routers. Transponders basically receive a signal on one frequency, amplify the signal and transmit it on another frequency. While in the beginning only analog amplification was possible, the use of digital signals also allows for signal regeneration. The satellite decodes the signal into a bit stream, and codes it again into a signal. The advantage of digital regeneration compared to pure analog amplification is the higher quality of the received signal on the earth. Today’s communication satellites provide many functions of higher communication layers, e.g., inter- satellite routing, error correction etc.

Figure 3 : Classical Satellite Systems

Figure 3 shows a classical scenario for satellite systems supporting global mobile communication. Depending on its type, each satellite can cover a certain area on the earth with its beam (the so-called ‘footprint’. Within the footprint, communication with the satellite is possible for mobile users via a mobile user link (MUL) and for the base station controlling the satellite and acting as gateway to other networks via the gateway link (GWL). Satellites may be able to communicate directly with each other via intersatellite links (ISL).

Figure 4 : Communication Satellites – Intersatellite Link (ISL).

This facilitates direct communication between users within different footprints without using base stations or other networks on earth. Saving extra links from satellite to earth can reduce latency for data packets and voice data. Some satellites have special antennas to create smaller cells using spot beams (e.g., 163 spot beams per satellite in the ICO system (ICO, 2002)). The required terrestrial service infrastructure for satellite control and the control links between. Satellite systems will continue to be, a valuable addition to the many networks already in existence on earth. Users might communicate using ISDN or other PSTN, even cellular networks such as  GSM and UMTS. Many  gateways provide seamless communication between these different networks. A real challenge, for example, is the smooth, seamless handover between a cellular network and a satellite system (vertical handover) as it is already well known from within cellular networks (horizontal handover). Users should not notice the switching from, e.g., GSM, to a satellite network during conversation.

• Applications  Traditionally, satellites have been used in the following areas:
• Weather forecasting: Satellites are used to capture picture of the earth and relay on ground station to predict the weather conditions. It is very useful to predict anomalies like Hurricane beforehand so that preventive measures can be taken,
• Radio and TV broadcast satellites: Satellites and easy and cheap alternative to cable network for radio and television broadcasting.
• Military satellites: Communication links managed through satellite because they are safer from attack by enemies Most of the Military satellites operate on X-band.
• Satellites for navigation: The global positioning system (GPS) provides precise localization information with high precision of few meters. All ships and aircraft use GPS for their navigation systems. Information obtained from GPS receivers installed in vehicles is used for fleet management localization of vehicles etc. The Indian Space Research Organization (ISRO) has developed indigenous Indian Regional Navigation Satellite System called “NavIC” as deshi GPS with 7 GEO/GSO satellites. It has become operational. It can replace the most popular GPS of US, Glonass of Russia, Beidou of China or Galileo of European. NavIC System can develop potential application for Road Navigation, Rail Navigation and safety, Generate warning for unmanned Level Crossing, Vessel Monitoring and Tracking for costal Surveillance, Surveying –Land, Marine, Differential correction with Geodetic Receivers, Disaster management and warning, Emergency Calling, Location based services, Ionospheric monitoring and scintillation studies, Time and Frequency synchronization for Internet and Intranet application and several applications for Scientific and Research communities and Services to society.
• Global telephone backbones: Communication satellites are emerging as an alternative to cables for international telephone backbone .The satellites are increasingly being replaced by fiber optical cables crossing the oceans. The main reason for this is the tremendous capacity of fiber optical links (commercially some 10 Gbit/s using wavelength division multiplexing, several Tbit/s in labs) and, in particular, the much lower delay compared to satellites. While the signal to a geostationary satellite has to travel about 72,000 km from a sender via the satellite to the receiver, the distance is typically less than 10,000 km if a fiber-optical link crossing the Pacific or Atlantic Ocean is used. Unfortunately, the speed of light is limited, resulting in a one-way, single-hop time delay of 0.25 s for geostationary satellites. Using satellites for telephone conversation is sometimes annoying.
• Connections for remote or developing areas: Satellites offer a simple and quick connection to global networks to the places which are inaccessible due to their geographical location.
• Global mobile communication: Most recent advantage of satellites is mobile data communication. Geostationary satellites are not ideal for this because of high latency rate therefore; there is need of satellites using using lower orbits. Use of satellites for mobile communication is not as a replacer but as an extension to existing systems. They have an edge over existing cellular systems such as AMPS and GSM because of world wide coverage. For the UMTS system frequency bands directly adjacent to the terrestrial bands have been allocated for the satellite segment (S-Band: 1980–2010 MHz uplink, 2170–2200 MHz downlink).

INDIAN Critical Services through Indigenous Communication Satellites

Figure 5 : Indian Critical Services via Communication Satellites.

India’s INSAT and GSAT series of communication satellites are offering UHF, S-band Normal C band, Cext Cband, Ku-Band and Ka band multi band multipurpose Transponders for Telecommunication, Broadcasting, DTH, VSAT/Business Communication Societal Development, Dedicated satellite for Tele-education, e-learning, Telemedicine, METSAT and Disaster services, mobile services, advance communication services for internet, high throughput satellite for more cannels and higher data rate and many more services for country’s development program.

Indian Communication Satellites

Table 1 :

SATELLITE	Launch Date	Launch Mass	Launch Vehicle	Orbit	Application
INSAT-3B	Mar 22, 2000	2,070 Kg	Ariane-5G	GSO	Communication
INSAT-2E	Apr 03, 1999	2,550 Kg	Ariane-42P H10-3	GSO	Communication
INSAT-2D	Jun 04, 1997	2079 Kg	Ariane-44L H10-3	GSO	Communication
INSAT-2C	Dec 07, 1995	2106 Kg	Ariane-44L H10-3	GSO	Communication
INSAT-2B	Jul 23, 1993	1906 kg	Ariane-44L H10+	GSO	Communication
INSAT-2A	Jul 10, 1992	1906 kg	Ariane-44L H10	GSO	Communication
INSAT-1D	Jun 12, 1990		Delta 4925	GSO	Communication
INSAT-1C	Jul 22, 1988		Ariane-3	GSO	Communication
INSAT-1B	Aug 30, 1983		Shuttle [PAM-D]	GSO	Communication
INSAT-1A	Apr 10, 1982		Delta	GSO	Communication
HAMSAT	May 05, 2005		PSLV-C6	SSPO	Communication
EDUSAT	Sep 20, 2004	1950.5 kg	GSLV-F01	GSO	Communication
INSAT-3E	Sep 28, 2003	2,775 Kg	Ariane5-V162	GSO	Communication
GSAT-2	May 08, 2003	1800 Kg	GSLV-D2	GSO	Communication
INSAT-3A	Apr 10, 2003	2,950 Kg	Ariane5-V160	GSO	Climate & Environm., Communication
KALPANA-1	Sep 12, 2002	1060 Kg	PSLV-C4	GSO	Climate & Environm. Communication
INSAT-3C	Jan 24, 2002	2,650 Kg	Ariane5-V147	GSO	Climate & Environ., Commun
GSAT-1	Apr 18, 2001	1530 Kg	GSLV-D1	GSO	Communication
HAMSAT	May 05, 2005		PSLV-C6	SSPO	Communication
GSAT-12	Jul 15, 2011	1410 kg	PSLV-C17	GSO	Communication
GSAT-8	May 21, 2011	3093 kg	Ariane-5 VA-202	GSO	Communication, Navigation
GSAT-5P	Dec 25, 2010	2310 kg	GSLV-F06	GSO	Communication
GSAT-4	Apr 15, 2010	2220 Kg	GSLV-D3	GSO	Communication
INSAT-4CR	Sep 02, 2007	2,130 kg	GSLV-F04	GSO	Communication
INSAT-4B	Mar 12, 2007	3025 Kg	Ariane5	GSO	Communication
INSAT-4C	Jul 10, 2006		GSLV-F02	GSO	Communication
INSAT-4A	Dec 22, 2005	3081 Kg	Ariane5-V169	GSO	Communication
GSAT-17	Jun 29, 2017	3477 kg	Ariane-5 VA-238	GTO	Communication
GSAT-19	Jun 05, 2017	3136 Kg	GSLV Mk III-D1	GSO	Communication
GSAT-9	May 05, 2017	2230 kg	GSLV-F09	GSO	Communication
GSAT-18	Oct 06, 2016	3404 kg	Ariane-5 VA-231	GSO	Communication
GSAT-15	Nov 11, 2015	3164 kg	Ariane-5 VA-227	GSO	Communication, Navigation
GSAT-6	Aug 27, 2015	2117 kg	GSLV-D6	GTO	Communication
GSAT-16	Dec 07, 2014	3181.6 kg	Ariane-5 VA-221	GSO	Communication
GSAT-14	Jan 05, 2014	1982 kg	GSLV-D5	GSO	Communication
GSAT-7	Aug 30, 2013	2650 kg	Ariane-5 VA-215	GSO	Communication

The INSAT-1 series satellites were multipurpose satellites made by Ford Aerospace US based on Indian design and services requirements. These satellite were launched by foreign launcher Ariane and Delta. INSAT-2 series and onwards all satellites were indigenous satellite made by ISRO. After the INSAT series, the GSAT series started. Several.Satellite of GSAT and INSAT-2,-4,-3 series whose weight are less than 3000 Kg have been launched from Indian Launch Pad at SHAR by PSLV or GSLV launch vehicles. The earlier satellites were designed for 10 year’s operational life.  Nowadays satellites in Geostationary orbit have more than 15 year’s designed life.

Operational Indian Satellites in Space

• Currently operational 42 satellites in orbit
• 18 Earth observation (including here are meteorological)
• 15 communication
• 7 navigational and
• 2 Space Science satellites
• 26 satellites are currently in various stages of realization

Major Application Areas

• Resources monitoring, infrastructure planning, enabling weather forecasting, disaster management support, location based services, host of societal applications, including the demands of satellite communication.
• Agriculture, forestry & environment, water resources, urban & rural planning, asset mapping, mineral prospecting, ocean resources, meteorology, satellite communication, location based services, tele-education, tele-medicine and disaster management support.

Basics of Geo-stationary Satellites

Satellites orbit around the earth in circular or elliptical orbits. They are at same distance to the earth’s surface as per the given law:

• The gravitational force of the earth given as Fg = m.g.(R/r)2
• The centrifugal force trying to pull the satellite away given as Fc = m·r·ω2.

Where m is the mass of the satellite; R is the radius of earth with R = 6,370 km;     r is the distance of the satellite to the centre of the earth; g is the acceleration of gravity  with g

= 9.81 m/s2; ω is the angular velocity with ω = 2·π·f, f is the frequency of the rotation. To keep the satellite in a stable circular orbit, the following equation must hold:

Fg = Fc

• Attractive force Fg = m g (R/r)²
• Centrifugal force Fc = m v²/r | since mgR²/r² = mv²/r For Stable orbit

Fg = Fc

Or r = g R²/v²

Solving the equation for the distance r of the satellite to the center of the earth results in the following equation:

The distance r = (g·R2/(2·π·f)2)1/3

From the above equation it can be concluded that the distance of a satellite to the earth’s surface depends on its rotation frequency. Figure 6.shows this dependency in addition to the relative velocity of a satellite. The interesting point in the diagram is when the satellite period equals 24 hours. This is exactly the case for a distance of 35,786 km. Having an orbiting time of 24 hours implies a geostationary satellite if it is additionally placed above the equator.

Fig. 6 Dependency of satellite period and distance to earth

Figure 6 :

Satellite Inclination angle

Inclination angle (δ): The angle between the equatorial plane and the plane described by the satellite orbit(Figure 7). An inclination angle of 0 degrees means that the satellite is exactly above the equator. If the satellite does not have a circular orbit, the closest point to the earth is called the perigee.

Figure 7 : Inclination angle of satellite

Satellite Elevation Angle

Elevation angle ε : The angle between the center of the satellite beam and the plane tangential to the earth’s surface.

Footprint: The area on earth where the signals of the satellite can be received.

Figure 8 : Satellite Elevation Angle

Satellite Orbits

Four different types of orbits can be identified as shown in Figure 10:

• Geostationary (or geosynchronous) earth orbit (GEO): GEO satellites have a distance of almost 36,000 km to the earth. TV and radio broadcast weather satellites and satellites operating as backbones for the telephone network are some of the examples.

Figure 9 : Different types of satellite orbits

• Medium earth orbit (MEO): MEOs operate at a distance of about 5,000–12,000 km some upcoming systems ICO are example of this type of orbit.
• Low earth orbit (LEO): Initially LEO satellites were mainly used for espionage, nowadays many satellites use this class with altitudes of 500–1,500 km. Globalstar, Iradium are examples of LEO satellites.

Figure 10 : One way satellite Delay

• Highly elliptical orbit (HEO): All the satellites with noncircular orbits belong to this class. Currently, only a few commercial communication systems using satellites with elliptical orbits are planned. These systems have their perigee over large cities to improve communication quality.

Figure 11 : Frequency Bands, Spectrum Characteristics

Frequency Bands

Figure 12 :

There are specific frequency ranges used by commercial satellites.

• L-band (Mobile Satellite Services)  1.0 – 2.0 GHz
• S-band (MSS, DARS – XM, Sirius)   1.55 – 3.9 GHz
• C-band (FSS, VSAT)             3.7 – 6.2 GHz
• X-Band (Military/Satellite Imagery)  8.0 – 12.0 GHz
• Ku-band (FSS, DBS, VSAT)     11.7–14.5 GHz
• Ka-band (FSS “broadband” & inter-satellite links) 17.7 – 21.2GHz & 27.5 – 31GHz.  Indian Communication satellites are being done are in all several frequency band like L, S, Lower  extended  C  (Cext-lower), Normal  C  (Cn),  Extended  Upper  C(  Cext-upper),  Ku,  Ka Frequency bands. The Up-Link frequency and Down-Link Frequency has been indicated in Table-2,

Table 2 : Frequency Bands – Indian Communication Satellites

Conclusions

The communication satellites are essential elements in Mobile Communications. All orbital configurations LEO,  MEO and GEO are required for  MSS purpose. India has several communication satellite in GEO with various frequency band. The space and Ground segments together are helpful in mobile services in global network. In past 50 years the tremendous growth took place in the satellites field and their service which has potentially increased the data rate as well as band width. Advance multi-beam satellites, along with free space RF and Optical link will govern the future applications in mobile services for seamless connectivity.

Suggested Reading:

1. Mobile Communication 2nd edition by Jochen Schiller, Pearson education
2. Mobile Computing by Asoke Talukder, Roopa Yavagal (Tata McGraw Hill)
3. “Wireless communication and networking” by William Stallings
4. Mobile Cellular Telecommunications — W.C.Y. Lee, Mc Graw Hill
5. Wireless Communications – Theodore. S. Rapport, Pearson Education
6. Reza B’Far (Ed), “Mobile Computing Principles”, Cambridge University Press.