BASIC CONCEPTS OF SATELLITE COMMUNICATIONS
- A communication satellite is an orbiting artificial earth satellite that receives a communications signal from a transmitting ground station, amplifies and possibly processes it, then transmits it back to the earth for reception by one or more receiving ground stations.
- Communications information neither originates nor terminates at the satellite itself.The satellite is an active transmission relay, similar in function to relay towers used in terrestrial microwave communications.
- The commercial satellite communications industry has its beginnings in the mid-1960s, and in less than 50 years has progressed from an alternative exotic technology to a mainstream transmission technology, which is pervasive in all elements of the global telecommunications infrastructure. Today’s communications satellites offer extensive capabilities in applications involving data, voice, and video, with services provided to fixed, broadcast, mobile, personal communications, and private networks users.
Evolution of Satellite Communication:
- During early 1950s, both passive and active satellites were considered for the purpose of communications over a large distance.
- Passive satellites though successfully used communications, with the advancement in completely replaced the passive satellites.
Passive Satellites:
- A satellite that only reflects signals from one Earth station to another or from several Earth stations to several others.
- It reflects the incident electromagnetic radiation without any modification or amplification.
- It can't generate power, they simply reflect the incident power.
- The first artificial passive satellite Echo-I of NASA was launched in August 1960.
Disadvantages of passive satellite:
- Earth Stations required high power to transmit signals.
- Large Earth Stations with tracking facilities were expensive.
- A global system would have required a large number of passive satellites accessed randomly by different users.
- Control of satellites not possible from ground.
- The large attenuation of the signal while traveling the large distance between the transmitter and the receiver via the satellite was one of the most serious problems.
Active Satellites:
- In active satellites, it amplifies or modifies and re-transmits the signal received from the earth.
- Satellites which can transmit power are called active satellite.
- Have several advantages over the passive satellites.
- Require lower power earth station.
- Not open to random use.
- Directly controlled by operators from ground.
Disadvantages of active satellite:
- Requirement of larger and powerful rockets to launch heavier satellites in orbit.
- Requirement of on-board power supply.
- Interruption of service due to failure of electronics components
Two major elements of Satellite Communications Systems are:
The satellite communications portion is broken down into two areas or segments: the space segment and the ground (or earth) segment.
Space Segment:
The space segment includes the satellite (or satellites) in orbit in the system, and the ground station that provides the operational control of the satellite(s) in orbit. The ground station is variously referred to as the Tracking, Telemetry, Command (TT&C) or the Tracking, Telemetry, Command and Monitoring (TTC&M) station. The TTC&M station provides essential spacecraft management and control functions to keep the satellite operating safely in orbit. The TTC&M links between the spacecraft and the ground are usually separate from the user communications links. TTC&M links may operate in the same frequency bands or in other bands. TTC&M is most often accomplished through a separate earth terminal facility specifically designed for the complex operations required to maintain a spacecraft in orbit.
Ground segment:
The ground segment of the communications satellite system consists of the earth surface area based terminals that utilize the communications capabilities of the Space Segment. TTC&M ground stations are not included in the ground segment. The ground segment terminals consist of three basic types:
- fixed (in-place) terminals;
- transportable terminals;
- mobile terminals.
Fixed terminals are designed to access the satellite while fixed in-place on the ground. They may be providing different types of services, but they are defined by the fact that they are not moving while communicating with the satellite. Examples of fixed terminals are small terminals used in private networks (VSATs), or terminals mounted on residence buildings used to receive broadcast satellite signals. Transportable terminals are designed to be movable, but once on location remain fixed during transmissions to the satellite. Examples of the transportable terminal are satellite news gathering (SGN) trucks, which move to locations, stop in place, and then deploy an antenna to establish links to the satellite.
Mobile terminals are designed to communicate with the satellite while in motion. They are further defined as land mobile, aeronautical mobile, or maritime mobile, depending on their locations on or near the earth surface.
Satellite Control Center function:
- Tracking of the satellite
- Receiving data
- Eclipse management of satellite
- Commanding the Satellite for station keeping.
- Determining Orbital parameters from Tracking and Ranging data
- Switching ON/OFF of different subsystems as per the operational requirements
SATELLITE ORBITS
Satellite Orbits are classified in two broad categories:
- Non-Geostationary Orbit (NGSO)
- Geo Stationary Orbit (GSO)
Disadvantages of NGSO
- Complex problem of transferring signal from one satellite to another.
- Less expected life of satellites at NGSO.
- Requires frequent replacement of satellites compared to satellite in GSO.
Geo Stationary Orbit (GSO)
- There is only one geostationary orbit possible around the earth.
- Lying on the earth’s equatorial plane.
- The satellite orbiting at the same speed as the rotational speed of the earth on its axis.
Advantages of GSO
- Simple ground station tracking.
- Nearly constant range.
- Very small frequency shift.
Disadvantages of GSO
- Transmission delay of the order of 250 millisecond.
- Large free space loss.
- No polar coverage.
Note: A geostationary orbit is a type of geosynchronous orbit. A geosynchronous orbit can be any orbit, like with an elliptical path, that has a period equal to the Earth’s rotational period, whereas a geostationary orbit has to be a circular orbit and that too placed above the equator.
Satellite orbits in terms of the orbital height:
According to distance from earth:- Geosynchronous Earth Orbit (GEO).
- Medium Earth Orbit (MEO).
- Low Earth Orbit (LEO).
Geostationary or geosynchronous earth orbit (GEO)
GEO satellites are synchronous with respect to earth. Looking from a fixed point from Earth, these satellites appear to be stationary. These satellites are placed in the space in such a way that only three satellites are sufficient to provide connection throughout the surface of the Earth (that is; their footprint is covering almost 1/3rd of the Earth). The orbit of these satellites is circular.
There are three conditions which lead to geostationary satellites. Lifetime expectancy of these satellites is 15 years.
- The satellite should be placed 35,786 kms (approximated to 36,000 kms) above the surface of the earth.
- These satellites must travel in the rotational speed of earth, and in the direction of motion of earth, that is eastward.
- The inclination of satellite with respect to earth must be 0°.
Geostationary satellite in practical is termed as geosynchronous as there are multiple factors which make these satellites shift from the ideal geostationary condition.
- Gravitational pull of sun and moon makes these satellites deviate from their orbit. Over the period of time, they go through a drag. (Earth’s gravitational force has no effect on these satellites due to their distance from the surface of the Earth.)
- These satellites experience the centrifugal force due to the rotation of Earth, making them deviate from their orbit.
- The non-circular shape of the earth leads to continuous adjustment of speed of satellite from the earth station.
These satellites are used for TV and radio broadcast, weather forecast and also, these satellites are operating as backbones for the telephone networks.
Advantages Of GEO
- Minimal Doppler shift.
- These factors make it ideal for satellite broadcast and other multipoint applications.
- GEO satellites have a 24 hour view of a particular area.
- A GEO satellite’s distance from earth gives it a large coverage area, almost a fourth of the earth’s surface.
Medium Earth Orbit (MEO) satellites
MEOs can be positioned somewhere between LEOs and GEOs, both in terms of their orbit and due to their advantages and disadvantages. Using orbits around 20,000 km, the system only requires a dozen satellites which is more than a GEO system, but much less than a LEO system. These satellites move more slowly relative to the earth’s rotation allowing a simpler system design (satellite periods are about six hours). Depending on the inclination, a MEO can cover larger populations, so requiring fewer handovers.
Disadvantages Of MEO: A MEO satellite’s distance gives it a longer time delay (70-78 ms) and weaker signal than a LEO satellite, though not as bad as a GEO satellite. The satellites need higher transmit power and special antennas for smaller footprints.
Low Earth Orbit (LEO) satellites
These satellites are placed 500-1500 kms above the surface of the earth. As LEOs circulate on a lower orbit, hence they exhibit a much shorter period that is 95 to 120 minutes. LEO systems try to ensure a high elevation for every spot on earth to provide a high quality communication link. Each LEO satellite will only be visible from the earth for around ten minutes.
Using advanced compression schemes, transmission rates of about 2,400 bit/s can be enough for voice communication. LEOs even provide this bandwidth for mobile terminals with Omni-directional antennas using low transmit power in the range of 1W. The delay for packets delivered via a LEO is relatively low (approx 10 ms). The delay is comparable to long-distance wired connections (about 5–10 ms). Smaller footprints of LEOs allow for better frequency reuse, similar to the concepts used for cellular networks. LEOs can provide a much higher elevation in Polar Regions and so better global coverage.
These satellites are mainly used in remote sensing an providing mobile communication services (due to lower latency).
- A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication.
- A LEO satellite’s smaller area of coverage is less and waste of bandwidth.
- A network of LEO satellites is needed, which can be costly.
- LEO satellites have to compensate for Doppler shifts cause by their relative movement.
- Atmospheric drag effects LEO satellites, causing gradual orbital deterioration.
Advantages Of Satellite Communication
- Universal: Satellite communications are available virtually everywhere.
- Versatile: Satellites can support all of today's communications needs.
- Reliable: Satellite is a proven medium for supporting a company's communications needs.
- Seamless: Satellite's inherent strength as a broadcast medium makes it perfect.
- Fast: Since satellite networks can be set up quickly, companies can be fast-to-market with new services.
- Flexible.
- Expandable.
- High Quality.
- Quick Provision of Services.
- Mobile and Emergency Communication.
- Suitable for both Digital and Analog Transmission.
FREQUENCY ALLOCATIONS FOR SATELLITE SERVICES
Allocation of frequencies to satellite services s a complicated process which requires international coordination and planning. This is done as per the International Telecommunication Union (ITU). To implement this frequency planning, the world is divided into three regions:
Region 2: North and South America and Greenland.
Region 3: Asia (excluding region 1 areas), Australia and south-west Pacific.
Within these regions, he frequency bands are allocated to various satellite services. Some of them are listed below.
- Fixed satellite service: Provides Links for existing Telephone Networks Used for transmitting television signals to cable companies.
- Broadcasting satellite service: Provides Direct Broadcast to homes. E.g. Live Cricket matches etc.
- Mobile satellite services: This includes services for: Land Mobile, Maritime Mobile, Aeronautical mobile.
- Navigational satellite services: Include Global Positioning systems.
- Meteorological satellite services: They are often used to perform Search and Rescue service.
Below are the frequencies allocated to these satellites
| Band | Frequency Range | Total Bandwidth | General Application |
|---|---|---|---|
| L | 1 to 2 GHz | 1 GHz | Mobile satellite Service(MSS) |
| S | 2 to 4 GHz | 2 GHz | MSS, NASA, deep space research |
| C | 4 to 8 GHz | 4 GHz | Fixed Satellite Service(FSS) |
| X | 8 to 12.5 GHz | 4.5 GHz | FSS military, terrestrial earth exploration and meteorological satellites |
| Ku | 12.5 to 18 GHz | 5.5 GHz | FSS, Broadcast Satellite Service(BSS) |
| K | 18 to 26.5 GHz | 8.5 GHz | BSS, FSS |
| Ka | 26.5 to 40 GHz | 13.5 GHz | FSS |
APPLICATIONS OF SATELLITE COMMUNICATION
1) Weather Forecasting: Certain satellites are specifically designed to monitor the climatic conditions of earth. They continuously monitor the assigned areas of earth and predict the weather conditions of that region. This is done by taking images of earth from the satellite. These images are transferred using assigned radio frequency to the earth station. (Earth Station: it’s a radio station located on the earth and used for relaying signals from satellites.) These satellites are exceptionally useful in predicting disasters like hurricanes, and monitor the changes in the Earth's vegetation, sea state, ocean color, and ice fields.
2) Radio and TV Broadcast: These dedicated satellites are responsible for making 100s of channels across the globe available for everyone. They are also responsible for broadcasting live matches, news, world-wide radio services. These satellites require a 30-40 cm sized dish to make these channels available globally.
3) Military Satellites: These satellites are often used for gathering intelligence, as a communications satellite used for military purposes, or as a military weapon. A satellite by itself is neither military nor civil. It is the kind of payload it carries that enables one to arrive at a decision regarding its military or civilian character.
4) Navigation Satellites: The system allows for precise localization world-wide, and with some additional techniques, the precision is in the range of some meters. Ships and aircraft rely on GPS as an addition to traditional navigation systems. Many vehicles come with installed GPS receivers. This system is also used, e.g., for fleet management of trucks or for vehicle localization in case of theft.
5) Global Telephone: One of the first applications of satellites for communication was the establishment of international telephone backbones. Instead of using cables it was sometimes faster to launch a new satellite. But, fiber optic cables are still replacing satellite communication across long distance as in fiber optic cable, light is used instead of radio frequency, hence making the communication much faster (and of course, reducing the delay caused due to the amount of distance a signal needs to travel before reaching the destination.). Using satellites, to typically reach a distance approximately 10,000 kms away, the signal needs to travel almost 72,000 kms, that is, sending data from ground to satellite and (mostly) from satellite to another location on earth. This cause’s substantial amount of delay and this delay becomes more prominent for users during voice calls.
6) Connecting Remote Areas: Due to their geographical location many places all over the world do not have direct wired connection to the telephone network or the internet (e.g., researchers on Antarctica) or because of the current state of the infrastructure of a country. Here the satellite provides a complete coverage and (generally) there is one satellite always present across a horizon.
7) Global Mobile Communication: The basic purpose of satellites for mobile communication is to extend the area of coverage. Cellular phone systems, such as AMPS and GSM (and their successors) do not cover all parts of a country. Areas that are not covered usually have low population where it is too expensive to install a base station. With the integration of satellite communication, however, the mobile phone can switch to satellites offering world-wide connectivity to a customer. Satellites cover a certain area on the earth. This area is termed as a "footprint" of that satellite. Within the footprint, communication with that satellite is possible for mobile users. These users communicate using a Mobile-User-Link (MUL). The base-stations communicate with satellites using a Gateway-Link (GWL). Sometimes it becomes necessary for satellite to create a communication link between users belonging to two different footprints. Here the satellites send signals to each other and this is done using Inter-Satellite-Link (ISL).
Next lecture: Satellite Communication - Orbital Mechanics.