Communication satellites

Introduction

• The essential elements of any communication system is shown in the figure below.
• For long distance communication system, we use electromagnetic waves upon which the information is coded and sent. The electromagnetic waves by virtue of its ability to travel long distances are used for any long-distance communication system including communication satellites.
• Suitability of electromagnetic waves for communication system depends on 2 factors, a. it should travel unimpeded and b. it should be harmless.
• Electromagnetic waves that satisfy these conditions include those with the frequency range 3Khz to 300 Ghz. (Note this is the range suitable for any communication system). Communication satellites, on the other hand, operate in the frequency band 3 GHz to 300GHz.
• Now we are also exploring the possibility of terahertz radiation for novel applications requiring high-speed and high-volume data transfer.
• Terahertz radiation use microwaves frequencies that penetrates many materials except metals. This is used for biomedical imagery, security, remote sensing and spectroscopy. However, there are reports that terahertz radiation damage skin cells.
• Note: Waves beyond UV are harmful due to their ability to penetrate biomolecules, even DNA.
• Note: Visible light and infrared are subject to attenuation (weakening) due to their ability to interact with matter.

Principle Of Satellite Communication

Satellite communication is based on a line-of-sight 1-way or 2-way transmission of radio waves in the frequency range 3GHz to 300GHz.

The essential components of satellite communication system are

• Satellite in GSO
• Ground station to send the data, say news or cricket match (uplink channel: earth-to-space)
• One or more receiving stations (downlink channel: satellite-to-earth)
• Transponders: Within the satellites are devices called transponders which uptake the signal (which is attenuated) and amplify is and send it back.

Frequency And Bandwidth

• The amount of data that you can send and how fast you can send the data are the main performance metrics of any communication system.
• These are decided by frequency and bandwidth which in-turn define spectral efficiency of a communication system.
• Frequency refers to the number of times that an electromagnetic wave is waving. More the number of waving more data you can represent. (every crest of the wave can represent zero and every trough can represent 1).
• Satellite communication operate in the range of 3GHz to 300GHz.
• Bandwidth refers to how much data can be transmitted over a given frequency range. Think of it like a water hose. Wider the hose, more the water that flows through it at once. Thus, wider the bandwidth, more the data that can be transmitted at once.

Common Frequency Bands For Satellite Communication

C band

• C-band operates within 4-8 Ghz.
• The C band is primarily used for voice and data communications.
• Because of its weaker power it requires a larger ground antenna.
• It provides lower transmission power over wide geographic areas.

Ku band

• Ku-band operates within 12-18 Ghz.
• Ku band is used typically for consumer direct-to-home, tele-education applications.
• The antenna sizes are much smaller than C band because the higher frequency.

Ka band

• Ka-band operates within 26-40 Ghz.
• The Ka band is primarily used for two-way consumer broadband and military networks.
• Ka-band frequency bands facilitate high transmissions speed and significant information transfer with the use of small ground equipment and thus used in broadband applications.
• Due to the higher frequencies of this band, it can be more vulnerable to signal quality problems caused by rain fade.

Evolution of communication satellites: from broadcasting to satellite-internet (applications)

One way to look at the evolution of communication satellites is from the lens of application and their corresponding design.

• The satellites have moved from being capable of sending low-frequency waves thus less data to being more and more powerful capable of sending high-frequency waves thus more data.
• Following from 1st statement the antennas on earth have moved from being very very large to very very small today.
• Following from above the nature of communication links has changed from being that between a satellite and one base station to that which has one satellite and multiple user terminal communicating at the same time.
• At first, they were used to communicate with one base station on earth which would send and receive signals to and from satellite respectively. The base station would in-turn distribute it to end uses using say a wire or radio. This was what the TV stations, Radio stations, telephone exchanges did (at least trunk calls). This is because satellites used were not powerful.
• As the satellites became more powerful (meaning they could beam high-energy waves, thus high frequency) they could send more data to more receivers at the same time. The VSAT services (Very Small Aperture Terminals) which had smaller size of antennas were mainly used for professional services, like banks or MNC networks etc.
• With the arrival of DTH (direct-to-home) in 1980s, satellites became very powerful (Ku-band) that could beam TV content to smaller dish antennas in every house.
• Now the communication satellites are expected to beam high-speed internet broadband for which they must be very powerful (Ka-band) These satellites are called high throughput satellites.

Choice Of Orbit For Communication Satellites

• Most communication satellites are launched in Geo-synchronous orbit (GSO) at an altitude of about 36,000 km.
• This is because in broadcast services or DTH we need continuous communication, and the ground antenna is fixed and should always face the satellite. These conditions are met only by putting the satellite in GSO above the equator.
• Note: A geosynchronous (GEO) satellite circles the earth at the earth’s rotational speed and in the same direction as the rotation.  In addition, when the satellite is above the equator it effectively appears to be permanently stationary when observed at the earth’s surface, so that an antenna pointed to it will not require periodic adjustments.
• Note: However, some communication satellites are also put in LEO and MEO, particularly the High-throughput satellites used for satellite-based internet. (see below of HTS)

High-Throughput Satellites (HTS) And Satellite-Based Internet

• HTS are next-gen communication satellites that will drive satellite-based internet services.
• Can be placed in LEO, MEO, and GEO. Lower orbits have substantially lower latency than the higher orbits but they require a higher number of satellites to cover the same area.
• A constellation of about 150 satellites will be sufficient to cover most of the developing countries.

Spot-beam technology and Frequency-reuse: How are high through put satellites different? (optional)

• 2 important principles make high throughput satellites powerful apart from the usage of Ka-band. These are spot-beam technology and frequency re-use technique. 
• Traditionally a communication satellite’s coverage is wide. This is because it flashes one broad beam or a few beams, from 36000 km above, which cover large areas (entire continent sometimes). Thus, the amount of information that can be sent through one beam is small.
• Under spot beam technology, on the other hand, satellites have the ability to flash multiple beams each targeting a different geographic area at the same time. This is like cellular networks where a geographical area is divided into multiple cells and a mobile tower covers a number of cells in that area.
• This design helps the satellites to use a concept called frequency-band reuse which is very useful to increase the capacity of communication system.
• Frequency-band reuse is a fundamental principle of cellular networks, which are designed to reuse the same set of frequencies across multiple cells in order to increase the capacity of the network. Only check is neighbouring cells do not use same frequency band to avoid interference. (represented by same coloured cells in the figure)
• Thus, frequency reuse allows the use of same frequency-band for multiple bandwidth.

Scope And Application Of HTS

• Enables high-bandwidth, high-speed, low-latency communication.
• More bits per second per unit of spectrum
• HTS is lower orbits have low-latency (time for data to make one full round from ground station to satellite back to receivers
• Capable of supporting over 100 Gbps of capacity
• Capable of providing broadband connectivity to people on the move where there is no connectivity. (Ocean, airplanes etc.)
• Ultra High Definition Television (UHDTV) (8K videos) provides video quality that is equivalent to 8-to-16 HDTV screens. This requires a lot more bandwidth per channel than currently used.
• Will deliver Internet service to the home using a small terminal (about 70 cm)
• Being the backbone of non-terrestrial networks HTS will enable connectivity to remote areas.
• Provides more bandwidth at much lower cost than terrestrial infrastructure at least in the remote areas.
• HTS will help create Wi-Fi hotspots for community internet access, backhaul to mobile network in remote locations.

Non-Terrestrial Networks

• It is a modem technology to enable communication between 2 unconnected regions separated by a barrier like mountain, ocean etc.
• Conventional mobile telephony like 5G technology focus on providing coverage to a specific geographical area. However terrestrial network infrastructure may not be viable for say rural or remote areas or difficult areas.
• Non-Terrestrial Networks address these areas by providing connectivity through satellites.

Application

• Air-to-ground networks can provide inflight connectivity for airplanes with the help of a base station placed in a specific geographical area.
• Application: providing communication services to disaster areas, future urban air mobility, air-to-ground networks can provide

Comparing terrestrial and non-terrestrial networks

Conventional optical fibre (Terrestrial Network)	Satellite-based internet (Non-terrestrial Network)
Higher capacity in a small region concentrated fashion (short-range)	Distributes capacity over a large area(long-range)
High cost in remote regions	Low cost in remote regions
Viable in the densely populated urban areas,	Better for sparsely populated regions

GMPCS: Global Mobile Personal Communication by Satellite  

• A regulatory framework established by the International Telecommunication Union to ensure the effective use of frequency spectrum for satellite-based mobile communication services.
• Many satellite operators around the world provide satellite based mobile communication services. The difference between terrestrial and satellite-based mobile operators is that while different spectrum (frequency bands) are allotted to different operators in terrestrial network, same spectrum is allotted to multiple operators under satellite-based mobile communication system.
• Currently Jio and Oneweb have been granted GMPCS licence by TRAI to operate in India.

Direct-to-Mobile Technology (DTM)

• Imagine an FM radio for video content you get on TV. This is what DTM is.
• It is a way to consume video content without having to connect to internet.
• It is the convergence of broadcast and broadband
• Recently, IIT Kanpur and Prasar Bharati have come out with a proof-of-concept.
• Advantage:  will allows telecom providers to divert video traffic from their mobile network thereby decongesting mobile spectrum minimising call dropouts and increased internet speeds.

Indian Regional Navigation Satellite System NaviC

• Also called NavIC is similar to the GPS.
• While GPS has a constellation of 24 satellites, IRNSS has a 7-satellites constellation.
• NavIC has a position accuracy of 20 metres in its primary coverage area.
• It can service regions extending up to 1500 km around India’s boundary.
• There are currently seven IRNSS satellites (1A to 1G) in orbit.
• A, B, F, G are placed in a geosynchronous orbit. (1A is replaced by 1I recently)
• C, D, E, are located in geostationary orbit.
• NavIC provides two types of services
  

• Standard positioning service- meant for all users.
• Restricted service-Encrypted service provided only to authorised users like military and security agencies.

Applications

• Terrestrial, Aerial and Marine Navigation
• Disaster Management
• Vehicle tracking and fleet management
• Integration with mobile phones
• Precise Timing
• Mapping and Geodetic data capture
• Terrestrial navigation aid for hikers and travelers
• Visual and voice navigation for drivers

Indian Data Relay Satellite System

• ISRO will launch a new satellite series called Indian Data Relay Satellite System.
• It is primarily meant for providing continuous/real time communication of Low-Earth-Orbit satellites including human space mission to the ground station.
• Under IDRSS, 2 satellites will be launched in geostationary orbit spaced 180 degrees apart to provide continuous contact for any spacecraft in LEO.

Use

• Faster data transfer
• Currently the satellites in the low earth orbits communicate with ground stations directly.
• However the limitation with direct communication with ground station is that the satellites in LEO orbit the earth once in 90 minutes.
• Thus these satellites are in the line of sight (above the horizon) only for 45 minutes giving very little time for data transfers.
• Besides the new satellites like HySiS (Hyper-spectral Imaging satellite) which provides high-precision images in multiple spectra, requires high data transfer at faster speeds. IRDSS satellite will communicate with all these satellites in LEO and downlink the data directly to one single ground station.
• Continuous monitoring of Gaganyaan
• Real-time communication with IRNSS system
• The Indian Remote Sensing Satellite System which monitors agriculture, water resources, urban development, mineral prospecting, environment, forestry, drought and flood forecasting etc requires real-time continuous communication.
• This requires a number of ground-based stations in order to keep communication intact at all times.
• A data relay satellite instead in the geo stationary orbit can overcome the need for large number of ground based stations.