Overview of SATELLITE System & V-SAT
Long distance communication using conventional techniques like coaxial cable or microwave radio relay links involves a large number of repeaters. For radio relay links of repeater spacing is limited by line of sight and is of the order of tens of kms. As the number of repeaters increase system performance and reliability are degraded. Tropo scatter propagation can cover several hundred kms. but the channel capacity is limited and costs are high due to necessity of large antennas and high transmit power. HF communication is subject to fading due to ionospheric disturbances and channel capacity is severely restricted due to limited bandwidth available. Large areas could be covered if the height of microwave repeater could be increased by putting it on board an artificial earth satellite (Fig.1). Science Fiction writer Arthur C. Clarke in an article in Wireless World in 1945 proposed that worldwide coverage could be obtained by using three microwave repeaters placed in a geostationary orbit at the height of about 36000 kms. with a period of 24 hours (Fig.2).
Satellite communication provide a practical and economical means of long haul communication traffic in a country with a large geographical area.
It also enables communication service to those areas which are virtually INACCESSIBLE by other conventional forms of communication system due to natural physical barriers.
Principles and Features of Satellite Communications
Figure 2 shows the principles of satellite communications. Here, a geostationary satellite with microwave radio repeater equipment receives and amplifies radio waves sent from earth stations and returns them to the earth.
A geostationary satellite is launched above the equator 36,000 km high above the earth. Its period round the earth coincides with that of the earth rotation. Therefore, the satellite looks as if it is stationary from the earth. If three (3) communication satellites are launched equidistantly above the equator (See Fig.2), it can serve almost all communication network round the world. Therefore, to facilitate public international telecommunications, INTELSATS IV and V have been launched above the Atlantic, Pacific, and Indian Oceans. These networks cover almost all countries around the world.
For international communication, a submarine cable along the Atlantic Ocean was installed in 1857. Also, short–wave radio communication (invented by Marconi in 1886) has been in use. However, short wave radio communication has disadvantages of :
- Small transmission capacity; only small telephone channels can be used to transmit.
- Fading in wave propagation; interferes with stability of transmission. Although over–the–horizon propagation is used for short distance international communications, it is impossible to apply it to transoceanic long distance communications.
Unlike other system, geostationary satellite communication systems summarize as follows :
- Stable and large capacity communication.
- Costs of establishment and maintenance do not depend on communication distance. The costs of submarine and over–the–horizon systems are proportional to the length, but those of the satellite system do not affect the communication distance. Therefore, the satellite system is ideal for long distance communications.
- Multiple access is possible. Signals sent from an earth station can be received at several earth stations simultaneously. Therefore, it can transmit signals to many stations simultaneously, such as TV. Actually, increasing of submarine cable’s capacity and distance between repeaters, can make submarine cables competitive to satellite communication specially when very large capacity is required but for small traffic size countries, satellite communication is unavailable for the independent communication services.
Advantages of Satellite Communications
- Large coverage : Almost one–third of the earth with exception of polar regions is visible from geostationary orbit. It is, thus, possible to cover about 10,000 kms. distance irrespective of intervening terrain with a single satellite.
- High quality : Satellite links can be designed for high quality performance. The link performance is highly stable since it is free from ionospheric disturbances, multipath effects or fading.
- High reliability : Reliability is high since there is only one repeater in the link.
- High capacity : With microwave frequencies, wide bandwidths are available and large communication capacity can be obtained.
- Flexibility : In a terrestrial system, communication is tied down to the links installed. On the other hand, satellite communication is well suited for changing traffic requirements, locations and channel capacities.
- Speed of installation : Installation of earth terminals can be achieved in a short time as compared to laying of cables or radio relay links.
- Mobile, short–term or emergency communications : With ariliftable or road transportable terminals, short–term or emergency communications can be quickly provided. Reliable long distance land mobile, maritime mobile and aeronautical mobile services are feasible only by means of satellite.
- Satellite communication is ideally suited for point to multipoint transmission on broadcasting over large areas. Application of satellites for TV broadcasting, audio and video distribution and teleconferencing, facsimile, data and news dissemination is, therefore, increasing rapidly.
- All types of common services are possible.
Satellite Communication Network
Satellite Communication Network could be defined as an ensemble of earth stations of pre–determined size spread over a pre–defined coverage area, interconnected through a suitably designed satellite, placed at a pre–determined location in properly chosen orbit around the earth. Thus, two important elements of a satellite communication network are :
- Space Segment
- Ground Segment
Uplinks and Down Link
Uplink is the radio path from Ground segment, i.e. earth station to the Space segment, i.e. satellite, whereas Downlink is the radio path from space segment, i.e. satellite to the ground segment, i.e. earth station.
Choice of Frequency band for space communication depends upon
- Band–width required.
- Noise consideration
- Propagation factors
- Technological developments with regard to component and device.
As the signal levels from the satellite are expected to be very low, any natural phenomenon to aid the reception of the incoming signals must be exploited. Note in Figure 3 that between the frequencies of 2 GHz to 10 GHz, the level of the sky–noise reduces and this band of frequencies is known as the ‘microwave window’.
The most of the communication satellites as on today are using a frequency of 6 GHz for “Up link” and 4 GHz for “Down link” transmission.
These frequencies are preferred because of
- Less atmospheric absorption than higher frequency.
- Less noise both galactic and manmade.
- Less space loss compared to higher frequency.
- A well developed technology available at these frequencies.
- 6 GHz/4 GHz bands are shared with terrestrial services, creating interference problem.
- As equatorial orbit is filling with geostationary satellites, RF interference is increasing from one satellite system to another is increasing.
- 14/11 and 30/20 GHz systems for telecommunication and broadcasting satellite services are slowly coming being.
Frequency bands in use for satellite communication are :
|“L” BAND||1830–2700 MHz|
|“S” BAND||2500–2700 MHz||INSAT IS USING|
|“C” BAND||5925–6425 MHz UP
3700–4200 MHz DOWN
|INSAT IS USING|
|“X” BAND||7900–8400 UP
|“KU” BAND||14.000–14.500 Hz. UP
|“K” BAND||27.5–30 GHz UP
17.7–21.2 GHz DOWN
|EXTENDED C BAND||6725–7025 UP
|INSAT IS USING|
|V BAND||40–51 GHz UP
40–41 GHz DOWN
|V Band Inter-satellite||59–64 GHz
The total earth–satellite–earth path length may be as much as 74,000 km thus giving a one–way propagation delay of 250 ms. The effect of this delay on telephone conversations, where a 500 ms gap can arise between one person asking a question and hearing the other person reply, has been widely investigated, and was found to be less of a problem than had been anticipated. With geostationary satellites, two–hop operation sometimes unavoidable and gives rise to a delay of over one second.
Fig. 4. A station which is located closer to the sub–satellite point, as demonstrated in Fig.4 will have an advantage in received signal level with respect to one at the edge of the service area of the satellite. For a global coverage satellite, this can be as much as 4.3 dB.
Satellite communication systems classify that :
- Communication system
(1) Multiplexed telephone channels with one
- carrier frequency system, and
- Single channel per carrier system.
(2) Modulation system
- Analog modulation (Frequency Modulation system), and
- Digital modulation system
(3) System configuration
- Pre–assignment system
- Demand assignment system, and
- Various other systems combined with those above.
Kinds and Systems of Communication Satellite
- Kinds of Communication Satellites – depends on type of orbit and freq. band used.
During the early experimental stage of communication satellites, a passive satellite was used without any amplifiers and it only reflected radio waves sent from the earth station. But, later on active satellite with amplifiers was developed and put into practical use. Communication Satellite can be classified by the orbit used and also by frequency band used. Before discussing satellite orbits in a more generalized manner, however, it is necessary to be aware of the natural laws that control the movement of satellites. These are based on Kepler’s laws and basically stated are :
- The orbit plane of any earth satellite must bisect the Earth centrally.
- The Earth must be at the centre of any orbit.
The choice of orbit is restricted to three basic types, namely : polar, equatorial and inclined as illustrated in Fig.5. The actual shape of the orbit is limited to circular and elliptical. Any combination of type and shape is possible but observations are made only of the circular polar, elliptically inclined and the circular equatorial.
Circular polar orbit
This is the only orbit that can provide full global coverage by one satellite, but requires a number of orbits to do so. In a communications sense where instantaneous transfer of information is required, full global coverage could be achieved with a series of satellites, where each satellite is separated in time and angle of its orbit. However, this produces economic, technical and operational disadvantages and is thus not used for telecommunications though it is favoured for some navigation, meteorological and land resource satellite system.
Elliptically inclined orbit
An orbit of this type has unique properties that have been successfully used for some communications satellite system, notably the Russian domestic system. For this system, the elliptical orbit has an angle of inclination of 63 degrees and a 12–hour orbit period. By design, the satellite is made to be visible for eight of its 12–hour orbit period to minimize the handover problem while providing substantial coverage of the temperate and polar regions. By using three satellites, suitably phased, continuous coverage of particular temperate region can be provided that would not be covered by other orbits.
The elliptically inclined orbit is used exclusively by the Russians for their Orbital and Molniya systems, but since coverage is limited to particular areas (higher latitudes), it is, therefore, not suitable for a global network.
Circular Equatorial Orbit
Circular orbits in the equatorial plane permit fewer satellites and ground stations to be used, and satellites with long orbital periods (at high altitudes) have greater mutual visibility. A satellite in a circular orbit at 35,800 km has a period of 24 hours and consequently appears stationary over a fixed point on the earth’s surface. The satellite is visible from one third of the earth’s surface, up to the Arctic circle, and this orbit is almost universally preferred for satellite communications system.
Stabilization of the satellite is necessary since the earth is not truly spherical, and the moon, sun and the earth’s tidal motion have gravitational effects on the satellite, tending to make it drift from its correct position. Inclination to the equatorial plane produces a sinusoidal variation in longitude, seen from earth as motion around an ellipse once every 24 hours, with peak deviation equal to the inclination angle. Incorrect velocity results incorrect altitude, and a drift to the east or to the west. When a non re–usable launcher is utilized, injection of the satellite into geostationary orbit requires two rocket burns : the first to get the vehicle into a parking orbit, and the second via an elliptical transfer orbit to geostationary altitude. The spacecraft’s own apogee motor then increases its velocity to about 10,000 fps to maintain the geostationary orbit. When launched from the Space Transportation System (Shuttle), a booster rocket is attached to the satellite to boost it to the geostationary orbit.
The satellite must then be correctly positioned, and held in position for its required lifetime (typically 7 to 10 years). This is done by using hydrazine (liquid nitrogen plus ammonia) and cold gas jets. About 40 lbs. of hydrazine are required for corrections to maintain geostationary position within q 0.1x for five years, but since hydrazine is also used for initial positioning, the quantity available depends on the accuracy of the launch. To extend the life of the satellites, less frequent corrections may be made allowing the satellite to drift.
F = Noise Figure of Receiver. The antenna noise is expressed in degrees Kelvin and is called noise temperature of antenna. It can be converted to familiar units of power, watts by multiplying it with Boltzmann’s constant K = 1.38 x 10–29 joule/kelvin and the bandwidth. Noise temperature of an antenna is of the order of 20–50oK.
This satellite revolves above the equator round the earth at a height of 35,790 km. Its period of revolving round the earth is same as that of the earth rotation on its own axis. Therefore, it looks as if it is stationary. This system was contributed to the “WIRELESS WORLD” by Mr. A.C.Clark, Dr. Rosen (an American) and others. It launched a Syncom communication satellite in 1963. Syncom No. 1 failed to launch in February, 1963. But, Syncon No. 2 finally succeeded in July 1963. This satellite centered the equator and moved like a figure eight (8). This was not a complete geostationary satellite, but it came into practical use (24 hours) as synchronous satellite. This satellite is advantageous because :
- Its large antenna at an earth station is easy to track.
- Twenty–four (24) hours communication can be made with even only one satellite.
- The satellite looks at the earth as if it were stationary, and it radiates highly effective wave power.
- Visibility from one (1) satellite is very wide, and global communication can be made using only three (3) satellites.
Its drawback, however, is its delay caused in long distance transmission. But, the system is economical and accordingly, it is widely used for both international and regional domestic communications.
Figure of Merit (G/T)
The earth station are classified on the basis of figure of merit of earth station which is defined by the parameter ‘G’ by ‘T’ (G/T). G is the receive gain of the earth station antenna and T is the equivalent noise at LNA input. This noise includes :
- Antenna noise
- Equivalent noise of receive chain (LNA, down converter) referred to LNA input.
The total noise is expressed in terms of noise temperature (Kelvin). Thus, G/T of an earth station in dB/K is given by.
G/T = GR – 10 log T in dB
where G is the receive gain of earth station antenna and T is noise temperature of the receive chain. A high G/T implies that an earth station can receive very weak signal because antenna gain is high and noise is low. Note that an LNA is specified by its noise temperature, i.e. by noise its generates.
The amount of receiver noise present is defined as receiver noise temperature To eq. The parameter To eq is an effective equivalent temperature that an external noise source would have to produce the same amount of receiver noise. The equivalent temperature is written as
To eq = Tb + (F–1) 290o
where, Tb = back ground noise temperature accounting for contribution collected by antenna.
HVNET – BSNL V-SAT NETWORK
This is the first High Speed Satellite based VSAT network of Department of Telecom., Govt. of India. It provides for high speed data transfers and voice communication covering the entire country.
The BSNL VSAT network consists of a HUB Station located at Yeur Earth Station of BSNL near Thane (about 40 kms from Mumbai) and number of VSATs/Personal Earth Stations (PES) located throughout the country.The VSAT communicate to the Hub through the INSAT Satellite. All VSATs are connected in STAR topology and VSAT to VSAT communication is through HUB at Mumbai. The VSAT which is required to be installed at subscribers premises consists of three units, namely an Outdoor Unit, an Indoor Unit and Inter Facility Link (IFL). Cable interconnecting the two Units along with a 2.4 meter diameter Antenna assembly and can be installed easily in any open space and requires a floor area of about 4 mt x 4 mt. The IFL cable, which carries the telecom signals and power supply, the IFL cable can be up to 100 meters long.
HVNET Data Network Hub station is located at Yeur Earth Station, Yeur in Thane District.
HVNET caters to the 200 remote V–SAT Terminals.
This Hub consists of the following equipment :
- RF Equipment Rack : consisting of UP/DOWN Converters.
- Base Band Subsystem Rack.
- IF Subsystem Rack.
- IPN Switch : (Integrated Packet N/W Switch) which does the switching of V–SAT data calls to different N/W.
- EPABX : (Small Telephone Exchange) which switches the V–SAT to V–SAT voice calls and switches to PSTN N/W as per the type of call.
- PCM RACK : It is for HVNET Hub connecting to terrestrial network for DATA/VOICE/TLX and GPSS.
- System Control Centre :
- MicroVAX : It is a central Computer which records the call records and provides these call record details to the Billing System.
- Illuminate Operator Console : This is a workstation used for configuration and monitoring the system parameters in real time basis. We can control Powers/Frequencies/Identification numbers, etc.
- VT 520 event terminal : It is used to display events in the network.
- GNOC PC : This is connected to packet switch exchange for monitoring and configuring the IPN Ports.
- NARS PC : This is connected to NCP MicroVAX. This gives the network analysis and reports.
- Line printers : These are used for fault event printing and summary printing of the network.
- Biling PC : The billing software is installed in this PC. The required call record files are transferred to this PC from MicroVAX Computer and data billing is done on the basis of volume of traffic. The call records consists of calling address, called address, start time and end time with corresponding data and data volume transmitted and received. The back up of call records file are taken on magnetic media regularly.
Services Available on HVNET
- Data communication at speed up to 64 Kbps (Synchronous X.25) and speed up to 19.2 Kbps (Asynchronous X.28).
- Access to PSTN (National and International Calls) and vice versa in addition to VSAT to VSAT voice calls.
- Access to RABMN customers.
- Access to INET – I and II.
- Access to International Data Networks through GPSS of VSNL.
- Access to Telex customers of National and International Network.
- Access to Internet Shell Account.
HVNET VSAT Indoor Section
It has indoor unit which is also called PERSONAL EARTH STATION (PES). The PES comprises the following cards.
- MPC (Multiport Card)
- VDPC (Voice Data Port Card)
- IFM (Intermediate Frequency Module).
- IOD (Inroute/Outroute Controller CHIP) mounted on backplane.
Function of MPC
- Multiport Card provides up to 8 users interface ports.
- Each port can be configured to process a different protocol, e.g. x–28, x–25.
- Each port level converter (PLC) provides the electrical signal conversion for two ports.
- Each junction box attached to the back of card provides 4 nos. of DB–25 connections.
Function of VDPC
- VDPC is used by itself to provide a two wire (RJ–11) or four wire (RJ–45) telephone interface.
- FIM (Fax Interface Module) card is used to provide fax facility.
- COLC (Central Office Line Card) 600 ohms are used to provide 2–wire (RJ–11) telephone facility.
Function of IFM (Intermediate Frequency Modules)
- It is connected to RF head by IFL cable.
- It receive outroute down link signal from RF head, downconverts, demodulates, decodes and sends the signal to the IOC chip on the backplane.
- It receives inroute signal from IOC (Inroute/Outroute controller) chip, modulates to 170–190 MHz and sends signal to rf head.
- It performs overall control and monitoring of the PES.
- It contains EEPROM (Electrically Erasable Programmable Read Only Memory) which stores configuration information.
- It has a Config port (RJ–11) which will be connected to Remote Site Installation Computer and to read and write to the EEPROM.
Function of IOC Chip
– Outroute signal processing
- receives outroute data stream from IFM.
- descrambles the Outroute bit stream.
- detects CRC (Cyclic Redundancy Check) errors in the outroute.
- acquires and maintains received superframe header synchronization.
- generates system clocks and data packet timing.
- address filters serial; outroute data packets and converts to parallel for port cards (De–mux).
– In-route signal processing“““““`
- accepts parallel in-route data packets from port cards (Mux) and convert to serial.
- scrambles in-route data and provides Forward Error Correction coding (FEC).
- generates and inserts a preamble sequence for the in-route packet burst.
- sends data stream to IFM.