Satellite Communication - LaunchingSatellite Communication - Launching

Satellites stay in space for most of their life time. We know that the environment of weightlessness isSatellites stay in space for most of their life time. We know that the environment of weightlessness ispresent in the space. That’s why satellites don’t require additional strong frames in space. But, thosepresent in the space. That’s why satellites don’t require additional strong frames in space. But, thoseare required during launching process. Because in that process satellite shakes violently, till theare required during launching process. Because in that process satellite shakes violently, till thesatellite has been placed in a proper orbit.satellite has been placed in a proper orbit.

The The design of satellitesdesign of satellites should be compatible with one or more launch vehicles in order to place the should be compatible with one or more launch vehicles in order to place thesatellite in an orbit.satellite in an orbit.

We know that the period of revolution will be more for higher We know that the period of revolution will be more for higher apogeeapogee altitude according to Kepler’s altitude according to Kepler’ssecond law. The period of geostationary transfer orbit is nearly equal to 16 hours. If second law. The period of geostationary transfer orbit is nearly equal to 16 hours. If perigeeperigee is isincreased to GEO altitude (around 36,000 km), then the period of revolution will increase to 24 hours.increased to GEO altitude (around 36,000 km), then the period of revolution will increase to 24 hours.

Launching of SatellitesLaunching of Satellites

The process of placing the satellite in a proper orbit is known as The process of placing the satellite in a proper orbit is known as launching processlaunching process. During this. During thisprocess, from earth stations we can control the operation of satellite. Mainly, there are four stages inprocess, from earth stations we can control the operation of satellite. Mainly, there are four stages inlaunching a satellite.launching a satellite.

First StageFirst Stage − The first stage of launch vehicle contains rockets and fuel for lifting the satellite − The first stage of launch vehicle contains rockets and fuel for lifting the satellitealong with launch vehicle from ground.along with launch vehicle from ground.

Second StageSecond Stage − The second stage of launch vehicle contains smaller rockets. These are − The second stage of launch vehicle contains smaller rockets. These areignited after completion of first stage. They have their own fuel tanks in order to send theignited after completion of first stage. They have their own fuel tanks in order to send thesatellite into space.satellite into space.

Third StageThird Stage − The third (upper) stage of the launch vehicle is connected to the satellite − The third (upper) stage of the launch vehicle is connected to the satellitefairing. This fairing is a metal shield, which contains the satellite and it protects the satellite.fairing. This fairing is a metal shield, which contains the satellite and it protects the satellite.

Fourth StageFourth Stage − Satellite gets separated from the upper stage of launch vehicle, when it has − Satellite gets separated from the upper stage of launch vehicle, when it hasbeen reached to out of Earth's atmosphere. Then, the satellite will go to a “transfer orbit”. Thisbeen reached to out of Earth's atmosphere. Then, the satellite will go to a “transfer orbit”. Thisorbit sends the satellite higher into space.orbit sends the satellite higher into space.

When the satellite reached to the desired height of the orbit, its subsystems like solar panels andWhen the satellite reached to the desired height of the orbit, its subsystems like solar panels andcommunication antennas gets unfurled. Then the satellite takes its position in the orbit with othercommunication antennas gets unfurled. Then the satellite takes its position in the orbit with othersatellites. Now, the satellite is ready to provide satellites. Now, the satellite is ready to provide servicesservices to the public. to the public.

Satellite Launch VehiclesSatellite Launch Vehicles

Satellite launch vehicles launch the satellites into a particular orbit based on the requirement. SatelliteSatellite launch vehicles launch the satellites into a particular orbit based on the requirement. Satellitelaunch vehicles are nothing but multi stage rockets. Following are the launch vehicles are nothing but multi stage rockets. Following are the two typestwo types of satellite launch of satellite launchvehicles.vehicles.

Expendable Launch Vehicles (ELV)Expendable Launch Vehicles (ELV)

Reusable Launch Vehicles (RLV)Reusable Launch Vehicles (RLV)

Expendable Launch VehiclesExpendable Launch Vehicles

Expendable launch vehicles (ELV) get destroyed after leaving the satellites in space. The followingExpendable launch vehicles (ELV) get destroyed after leaving the satellites in space. The followingimage shows how an ELV looks.image shows how an ELV looks.

The ELV contains three stages. First and second stages of ELV raise the satellite to an about 50 milesThe ELV contains three stages. First and second stages of ELV raise the satellite to an about 50 milesand 100 miles. Third stage of ELV places the satellite in transfer orbit. The task of and 100 miles. Third stage of ELV places the satellite in transfer orbit. The task of ELVELV will be will becompleted and its spare parts will be fallen to earth, when the satellite reached to transfer orbit.completed and its spare parts will be fallen to earth, when the satellite reached to transfer orbit.

Reusable Launch VehiclesReusable Launch Vehicles

Reusable launch vehicles (RLV) can be used Reusable launch vehicles (RLV) can be used multiple timesmultiple times for launching satellites. Generally, this for launching satellites. Generally, thistype of launch vehicles will return back to earth after leaving the satellite in space.type of launch vehicles will return back to earth after leaving the satellite in space.

The following image shows a reusable launch vehicle. It is also known as The following image shows a reusable launch vehicle. It is also known as space shuttlespace shuttle..

The functions of The functions of space shuttlespace shuttle are similar to the functions of first and second stages of ELV. Satellite are similar to the functions of first and second stages of ELV. Satellitealong with the third stage of space shuttle are mounted in the cargo bay. It is ejected from the cargoalong with the third stage of space shuttle are mounted in the cargo bay. It is ejected from the cargobay when the space shuttle reaches to an elevation of 150 to 200 miles.bay when the space shuttle reaches to an elevation of 150 to 200 miles.

Then, the third stage of space shuttle gets fired and places the satellite into a transfer orbit. After this,Then, the third stage of space shuttle gets fired and places the satellite into a transfer orbit. After this,the space shuttle will return back to earth for the space shuttle will return back to earth for reusereuse..

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VSAT index

How to make inclinometer

Finding dish offset angle

Guide about the orientation of elliptical dishes

Using a compass

Inclined orbit operation

Pictures and explanation of a complexC band feed

Multi-LNB dish configurations

Circular polarisation set-up

How to point an offset dish

Use of rear side struts

Satellite station-keeping

Geostationary satellites are in orbit above the equator and go around the earth in the same directionas the earth rotates and at the same angular speed. They appear stationary in the sky, so yousatellite dish can see the satellite all of the time.

The satellites are not fixed to anything and float in space and their orbit is affected by small forcesfrom the distant sun and also the earth's gravitational pull which is not perfectly spherical. Satelliteshave a supply of fuel allocated for station keeping and this fuel is used gradually over a plannedlifetime period of 7 to 15 years to keep the satellite within typically +/- 0.15 deg east west and +/-0.15deg north/south of its nominal orbit position.

North - South station keeping

The most significant effect is the pull of the sun which causes the inclination of the satellites orbit toincrease at the rate of about 0.8 deg per year. If a satellite was left alone, after one year it would begoing up and down daily in a north south direction with an inclination amplitude of +/- 0.8 deg. Thiswould cause the satellite to move well outside the beam width pointing of virtually all, except thesmallest, fixed pointing satellites dishes.

To keep a satellite's inclination down to a maximum of about 0.15 deg it is necessary to use fuel tofire its north/south thrusters every few weeks. About 90% of the station keeping fuel is allocated tocorrect this.

When initially positioned in orbit a satellite might have an inclination perhaps -0.5 degrees and thiswill be allowed to decrease to zero and increase to +0.15 deg before station-keeping fuel is used. Towards end of life north - soputh station keeping is be abandoned and inclination is allowed toincrease to say 4 degrees over a period 5 years while keeping the east west position within +/- 0.15deg. This allows the satellite to be retained in use for tracking antennas.

East - West station keeping

The strength of the earth's gravitational field had three slight bumps around the equator with dips inbetween. This "triaxialty" causes satellites to drift slowly sideways towards where gravity pulls themost. This needs regular sideways thruster firings, just before the satellite is about to leave itsstation keeping box. Basically you give it a good kick back the other way and if you judge it just rightit gradually goes across, stops just before it leaves the far side and then falls back sideways over thenext few weeks, ready for the next thruster firing.

The diagram above shows the central horizontal equator line of the geostationary orbit with thenominal satellite [position and a 0.3 deg square size station keeping box. The daily movement of thesatellite is shown by the red line. Each loop cycle represents one day. The satellite crosses theequator twice a day. The shape of the curve is two sine waves plus sideways drifts and gradualincreases or decreases in the amplitudes. Thruster firings have dramatic sudden effects and thesmooth pattern restarts and needs to be relearned. Elliptical, near circular or figure of eight patternsare possible.

Superimposed on the above diagram is a dot which represents the pointing of an earth station dishantenna, with its beam width represented by the circle. As pointed it will receive a good signal whilethe satellite is in the lower right corner of the square buit service will fail for about 50% of the timewhen the satellite is inthe upper left. This dish needs re-pointing about 0.1 deg up and 0.1 deg to theleft so its beam width includes all of the satellites daily movement range. The best time to do thisadjustment is when the satellite goes through the exact centre of the station keeping box. This mayhappen only once ever few weeks.

Important: Many small dish antenna will have a beam width larger than the station keeping box andall these dishes may be fixed.

Higher gain larger dishes with beam widths comparable to the size of the station keeping box requireaccurate alignment with the centre of the box.

Even higher gain antennas require active tracking systems that will typically make multiplemeasurements with small movements up/down/left/right every 15 minutes or so and work out what isbest. Tracking systems are quite clever and can learn the satellite orbit to minimise motor wear andto keep the antenna on track during rain fades etc. Tracking does increase unreliability however, sofixed pointing with accurate alignment to the centre of the box should be considered for intermediatesized antennas. Poor tracking is often due to lack of skill and understanding in setting up the verymany parameters for a tracking control system. Lubrication, back lash, step size, motor run time,stopping distances, angle calibration, mechanical and electrical non-linearity, volts per dB, areexamples.

Narrow beam widths are the result or large diameter antennas, or high frequencies, or both. Use thisantenna gain and beamwidth calculator to determine beamwidth for your dish size and frequency.

► Page created 2006, amended 20 Jan 2019 HTML5. 25 Jan 2020.

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Time Division Multiple Access

Time-division multiple access (TDMA) is a channel access method for shared-medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity.

TDMA was first used in satellite communication systems by Western Union in its Westar 3 communications satellite in 1979. It is now used extensively in satellite communications.

Explanation:

TDMA is characterized by access to the channel during a time slot. Figure 1 shows the operation of a network according to the principle of TDMA. As an illustration, the uplink earth stations, ESi , transmit bursts Ii

occupying all of the channel bandwidth during a specified time Tbi. A burst corresponds to the transfer of traffic from each uplink station. A burst transmission is inserted within a longer time structure of duration Ts called a frame period—a periodic structure within which all uplink stations must transmit. A frame is formed at satellite level. A typical structure of a frame is shown in Figure 2.

Figure 1: Operational concept of the TDMA system. Each station ESi transmits data Ii at assigned time Tbi within frame time Ts at the same frequency, where i = 1, 2, …, n.

Figure 2: TDMA frame format

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What is TDMA ? An explanation of TDMA

Time Division Multiple Access or TDMA is a method used to enable multiple earth stations or VSAT terminals to transmit intermittently on thesame frequency, but with the timing of their transmissions so arranged that the bursts do not overlay when they arrive at the satellite butarrive in sequence and thus are all successfully received by the teleport hub modem burst demodulator.

The operation of TDMA requires an outlink control to all the remote sites which contains some control information. This outlink carrier alsohad a frame structure that provides accurate timing information for all the remote sites. The teleport hub equipment computer tells eachVSAT site what particular time slot to use in the TDMA frame and this time plan information is broadcast to all sites periodically. The bursttime plan may be fixed, so as to allocate each site a particular proportion of the total TDMA frame time or is may be dynamic, whereby thetime slot allocated is adjusted in response to the traffic needs of each site.

TDMA Frame

This image below shows a sequence of two successive TDMA frames passing through the satellite. The carrier bit rate is 250 kbit/s

Explanation: Site 1 transmits a burst, starting at the beginning of each TDMA frame. The burst lasts 180 mS, so at a rate of 250kbit/s site 1sends 45,000 bits per burst, or 45,000 bits per second. Site 2 transmits a burst, timed to arrive at the satellite just after the end of burst 1. The red, second, burst lasts 80 mS, so at a rate of 250kbit/s, site 2 sends 20,000 bits per burst, or 20,000 bits per second. The diagramshows a fixed time plan, where each VSAT has been allocated a predetermined portion of the total time.

There is designed in 20mS guard period between each burst. This allows for slight mistiming in the transmission of the bursts. Severemistiming would cause bursts to arrive overlapping or on top of each other, causing loss of service to both sites involved in the mutualinterference. The long 20mS guard period is illustrative only, so you can see the white space in the figure above. In actual TDMA systemsthe guard band may be very much less and there may be very many more bursts per frame.

The above is just an example. TDMA frame length may be as short as 2000 microseconds or as long as 1 second, as in the exampleabove. The shortest TDMA frame periods are associated with the highest speed TDMA systems, operating at say 120.832 Mbit/s. On lowspeed 250kbit/s VSAT return links, with perhaps 2 to 50 sites sharing, and used for internet browsing and emails, the TDMA frame period istypically 500mS.

TDMA burst time plan.

The allocation of bursts to time slots within the TDMA frame is the burst time plan. As shown above the plan is:

VSAT site 1 start 0 mS, time allocated 180 mSVSAT site 2 start 200 mS, time allocated 80 mSVSAT site 3 start 300 mS, time allocated 180 mSVSAT site 4 start 500 mS, time allocated 280 mSVSAT site 5 start 800 mS, time allocated 180 mS

This information is broadcast to all sites, which then follow the timing instructions. This burst time plan might be applied unchanged forseveral days or weeks or it might be changed every few seconds or minutes according to the traffic demand.

Burst timing

The time of arrival of each burst at the satellite is critical and to get this right each siteis told when to transmit.

This instruction is based on the burst time plan start time value plus also a time delaybased on the range of VSAT site to the satellite. The range is initially calculated basedon the latitude and longitude of the earth station and the orbital position of the satellite. The amount of time to allow for, due to range, is calculated using the speed of light,which is 300m per microsecond.

For a new site to be introduced into the burst time plan, a slot is prepared for it by thehub staff. Using the site lat and long, an approximate timing value is estimated. Whenthe site is activated the burst should appear in the correct place. Fine burst timingadjustment may then be possible to fit the burst exactly without wasting too much

space in the guard times either side. Time wasted in the guard times is effectivelycapacity lost out of the total 250kbit/s available.

VSAT terminals further away from the satellite, like site 1 on the left, need to transmitearlier than site 2 which is already nearer the satellite.

This is an explanation why you need to tell the VSAT hub what is your latitude andlongitude before starting first transmissions to the teleport hub.

TDMA burst format.

This image represents a burst, whose time length may be 180 mS. At the start of the burst the bit sequence follows a predetermined burstpreamble or header designed to help the hub receiver demodulator lock onto the carrier. This is described as the "carrier and bit timingrecovery sequence and unique word". A brief period of un-modulated CW carrier is a good start. This helps the demodulator automaticfrequency control (AFC) adjust its centre frequency and also get the automatic gain control (AGC) so that the level of the carrier is madenominal. Next a period of alternate ones and zeros helps the demodulator to set the exact bit rate e.g. 249,999 bit/sec. Following this is theunique word. This is a pre-selected complex sequence of ones and zeros which the receiver attempts to match against two oppositematching patterns. When a hit is obtained the demodulator knows two things. It can now distinguish a one from a zero and it know its exactposition in the burst. At the end of the unique word the User data message starts. Right at the end may be a cyclic redundancy (CRC)checksum to check if there have been any errors during the burst. Additional bits for Forward Error Correction (FEC) are normally addedalso. These may be distributed along the packet or added as a extra group of bits at the end. The CRC checksum simply detects if therehave been any errors. FEC will put right and repair errors, provided there are not too many.

TDMA frame efficiency

Allowing for all the guard time slots between the bursts and the burst preambles the overall efficiency of use is typically 90%. The situationimproves for fewer, longer bursts and deteriorates for large numbers of short bursts.

In a random demand TDMA system the actual fill efficiency will fall to 0% when nobody is active, such as night. During busy times, efficiencyof fill may still need to be kept very low, such as 35% to avoid of burst collisions and congestion. Much effort has gone into the design ofdemand assigned TDMA systems where earth stations are temporarily assigned to certain sequence time slots for the duration oftransmission of a large file for example. This improved the fill factor significantly.

Page started 20 Nov 2006, amended 9 June 2018 ECJ

Block Diagram of a Communication Satellite

Fig: Block diagram of a Communication satellite

The figure is a general block diagram of a satellite. All the major subsystems are illustrated. The solar panels supply the electric power for the spacecraft. They drive regulators that distribute dc power to all other subsystems. And they charge the batteries that operate the satellite during eclipse periods. And ac-to-dc converters and dc-to-ac inverters are used to supply special voltages to some subsystems. Total power capability runs from a few hundred watts in the smaller units to several kilowatts in the largest systems. The communication subsystem consists of multiple transponders. These receive the uplink signals, amplify them, translate them in frequency, and amplify them again for retransmission as downlink signals. The transponders share an antenna subsystem for both reception and transmission. The telemetry, tracking, and command (TT&C) subsystem monitors onboard conditions such as temperature and battery voltage and transmits this data back to a ground station for analysis. The ground station may then issue orders to the satellite by transmitting a signal to the command subsystem, which then is used to control many spacecraft functions such as firing the jet thrusters.

The jet thrusters and the apogee kick motor (AKM) are part of the propulsion subsystem. They are controlled by commands from the ground. The attitude control subsystem provides stabilization in orbit and senses changes in orientation. It fires the jet thrusters to perform attitude adjustment and station-keeping maneuvers that keep the satellite in its assigned orbital position.

Frequency Reuse

One system for effectively doubling the bandwidth and information carrying capacity of a satellite is known as frequency reuse.

In this system, a communication satellite is provided with two identical sets of 12 transponders. The first channel in one transponder operates on the same channel as the first transponder in the other set, and so on. With this arrangement, the two sets of transponders transmit in the same frequency spectrum and, therefore, appear to interfere with each other. However, this is not the case. The two systems, although operating on exactly the same frequencies, are isolated from each other by the use of special antenna techniques.

One technique for keeping transmissions separate is to use different antenna polarizations. For example, a vertically polarized antenna will not respond to a horizontally polarized signal and vice versa. Or a left-hand circularly polarized (LHCP) antenna will not respond to a right-hand circularly polarized (RHCP) signal and vice versa.

Another technique is to use spatial isolation. By using narrow beam or spot beam antennas, the area on the earth covered by the satellite can be divided up into smaller segments. Earth stations in each segment may actually use the same frequency, but because of the very narrow beam widths of the antennas, there is no interference between adjacent segments. This technique is referred to a spatial-division multiple access (SDMA) in that access to the satellite depends on location and not frequency.

BISHNUPUR PUBLIC INSTITUTE OF ENGINEERING

1st Mock Internal Exam Subject-ADVANCE COMMUNICATION ENGINEERING

Stream- ELECTRONICS AND TELECOMMUNICATION ENGINEERING

3rd Year (6th Semester) Date :06/04/2020 FM: 20

1. Answer all questions from the following: 1x4=4

i. What is the up linking frequency in C band? ii. Define station keeping. iii. What are the names of the satellite launch vehicles? iv. What are the functions of a transponder in a communication satellite?

2. Answer all the questions from the following: 8x2=16

i. Draw and explain the block diagram of a communication satellite. Explain Code Division Multiple Access technique with necessary diagram. (4+4)

ii. Write short notes on: a. Time Division Multiple Access Technique b. Frequency Reuse (4+4)

� Read the instructions carefully:

i. At first Go to the reply section of the given question paper and type “ok sir/got it sir/any other formal words” so that I can record your attendance within 1 hour.

ii. It’s a pen-paper based test. So, keep your pen and paper ready with you and start writing. iii. Please mention your Name, Year, Trade, Subject and other necessary details on the top of the

answer script. iv. After having your examinations scan your answer script with your mobile/laptop by using

some applications (e.g. Cam scanner) and make a pdf of your answer script. v. Finally you have to submit it in reply section of the given question within 1:00 pm. vi. Sufficient time is given to you. Wish you all the best for this exam.