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EC 723 Satellite Communication Systems

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Presentation on theme: "EC 723 Satellite Communication Systems"— Presentation transcript:

1 EC 723 Satellite Communication Systems
Mohamed Khedr

2

3 Grades Load Percentage Date Midterm Exam 30% Week of 3 December 2007
Final Exam Participation 10% Report and presentation Starting week 11th

4 Textbook and website Textbook: non specific

5 Syllabus Tentatively Week 1 Overview Week 2
Orbits and constellations: GEO, MEO and LEO Week 3 Satellite space segment, Propagation and satellite links , channel modelling Week 4 Satellite Communications Techniques Week 5 Satellite error correction Techniques Week 6 Multiple Access I Week 7 Multiple access II Week 8 Satellite in networks I Week 9 INTELSAT systems , VSAT networks, GPS Week 10 GEO, MEO and LEO mobile communications INMARSAT systems, Iridium , Globalstar, Odyssey Week 11 Presentations Week 12 Week 13 Week 14 Week 15 Tentatively

6 Exploded view of a spinner satellite based on the Boeing (Hughes) HS 376 design. INTELSAT IVA (courtesy of Intelsat).

7 a) A spinner satellite, INTELSAT IV A (courtesy of Intelsat).

8 (b) A three-axis stabilized satellite, INTELSAT V (courtesy of Intelsat).

9 SPACECRAFT SUBSYSTEMS
Attitude and Orbital Control System (AOCS) Telemetry Tracking and Command (TT&C) Power System Communications System Antennas More usually TTC&M - Telemetry, Tracking, Command, and Monitoring Telemetry:Automatic transmission and measurement of data from remote sources by wire or radio or other means We will look at each in turn

10 Typical tracking, telemetry, command and monitoring system.

11 Bathtub curve for probability of failure.

12 AOCS AOCS is needed to get the satellite into the correct orbit and keep it there Orbit insertion Orbit maintenance Fine pointing Major parts Attitude Control System Orbit Control System Look at these next

13 ORBIT MAINTENANCE - 1 MUST CONTROL LOCATION IN GEO & POSITION WITHIN CONSTELLATION SATELLITES NEED IN-PLANE (E-W) & OUT-OF-PLANE (N-S) MANEUVERS TO MAINTAIN THE CORRECT ORBIT LEO SYSTEMS LESS AFFECTED BY SUN AND MOON BUT MAY NEED MORE ORBIT-PHASING CONTROL

14 FINE POINTING SATELLITE MUST BE STABILIZED TO PREVENT NUTATION (WOBBLE) Move unsteadily THERE ARE TWO PRINCIPAL FORMS OF ATTITUDE STABILIZATION BODY STABILIZED (SPINNERS, SUCH AS INTELSAT VI) THREE-AXIS STABILIZED (SUCH AS THE ACTS, GPS, ETC.)

15 DEFINITION OF AXES - 1 YAW AXIS ROLL AXIS PITCH AXIS
Rotates around the axis tangent to the orbital plane (N-S on the earth) PITCH AXIS Moves around the axis perpendicular to the orbital plane (E-W on the earth) YAW AXIS Moves around the axis of the subsatellite point

16 DEFINITION OF AXES - 2 Earth o Equator s Yaw Axis Roll Axis Pitch Axis

17 TTC&M MAJOR FUNCTIONS Reporting spacecraft health
Monitoring command actions Determining orbital elements Launch sequence deployment Control of thrusters Control of payload (communications, etc.) TTC&M is often a battle between Operations (who want every little thing monitored and Engineering who want to hold data channels to a minimum

18 TELEMETRY - 1 MONITOR ALL IMPORTANT TRANSMIT DATA TO EARTH
TEMPERATURE VOLTAGES CURRENTS SENSORS TRANSMIT DATA TO EARTH RECORD DATA AT TTC&M STATIONS NOTE: Data are usually multiplexed with a priority rating. There are usually two telemetry modes.

19 TELEMETRY - 2 TWO TELEMETRY PHASES OR MODES Non-earth pointing
During the launch phase During “Safe Mode” operations when the spacecraft loses tracking data Earth-pointing During parts of the launch phase During routine operations NOTE: for critical telemetry channels

20 TRACKING MEASURE RANGE REPEATEDLY
CAN MEASURE BEACON DOPPLER OR THE COMMUNICATION CHANNEL COMPUTE ORBITAL ELEMENTS PLAN STATION-KEEPING MANEUVERS COMMUNICATE WITH MAIN CONTROL STATION AND USERS

21 COMMAND DURING LAUNCH SEQUENCE IN ORBIT SWITCH ON POWER
DEPLOY ANTENNAS AND SOLAR PANELS POINT ANTENNAS TO DESIRED LOCATION IN ORBIT MAINTAIN SPACECRAFT THERMAL BALANCE CONTROL PAYLOAD, THRUSTERS, ETC.

22 COMMUNICATIONS SUB-SYSTEMS
Primary function of a communications satellite (all other subsystems are to support this one). Only source of revenue Design to maximize traffic capacity Downlink usually most critical (limited output power, limited antenna sizes). Early satellites were power limited Most satellites are now bandwidth limited.

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25 Typical satellite antenna patterns and coverage zones
Typical satellite antenna patterns and coverage zones. The antenna for the global beam is usually a waveguide horn. Scanning beams and shaped beams require phased array antennas or reflector antennas with phased array feeds.

26 Typical coverage patterns for Intelsat satellites over the Atlantic Ocean.

27 Contour plot of the spot beam of ESA’s OTS satellite projected onto the earth. The contours are in 1 dB steps, normalized to 0 dB at the center of the beam.

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29 Radio Propagation: Atmospheric Losses
Different types of atmospheric losses can perturb radio wave transmission in satellite systems: Atmospheric absorption; Atmospheric attenuation; Traveling ionospheric disturbances.

30 Radio Propagation: Atmospheric Absorption
Energy absorption by atmospheric gases, which varies with the frequency of the radio waves. Two absorption peaks are observed (for 90º elevation angle): 22.3 GHz from resonance absorption in water vapour (H2O) 60 GHz from resonance absorption in oxygen (O2) For other elevation angles: [AA] = [AA]90 cosec  Source: Satellite Communications, Dennis Roddy, McGraw-Hill

31 Radio Propagation: Atmospheric Attenuation
Rain is the main cause of atmospheric attenuation (hail, ice and snow have little effect on attenuation because of their low water content). Total attenuation from rain can be determined by: A = L [dB] where  [dB/km] is called the specific attenuation, and can be calculated from specific attenuation coefficients in tabular form that can be found in a number of publications; where L [km] is the effective path length of the signal through the rain; note that this differs from the geometric path length due to fluctuations in the rain density.

32 Radio Propagation: Traveling Ionospheric Disturbances
Traveling ionospheric disturbances are clouds of electrons in the ionosphere that provoke radio signal fluctuations which can only be determined on a statistical basis. The disturbances of major concern are: Scintillation; Polarisation rotation. Scintillations are variations in the amplitude, phase, polarisation, or angle of arrival of radio waves, caused by irregularities in the ionosphere which change over time. The main effect of scintillations is fading of the signal.

33 Signal Polarisation: What is Polarisation?
Polarisation is the property of electromagnetic waves that describes the direction of the transverse electric field. Since electromagnetic waves consist of an electric and a magnetic field vibrating at right angles to each other it is necessary to adopt a convention to determine the polarisation of the signal. Conventionally, the magnetic field is ignored and the plane of the electric field is used.

34 Signal Polarisation: Types of Polarisation
Linear Polarisation (horizontal or vertical): the two orthogonal components of the electric field are in phase; The direction of the line in the plane depends on the relative amplitudes of the two components. Circular Polarisation: The two components are exactly 90º out of phase and have exactly the same amplitude. Elliptical Polarisation: All other cases. Linear Polarisation Circular Polarisation Elliptical Polarisation

35 Signal Polarisation: Satellite Communications
Alternating vertical and horizontal polarisation is widely used on satellite communications to reduce interference between programs on the same frequency band transmitted from adjacent satellites (one uses vertical, the next horizontal, and so on), allowing for reduced angular separation between the satellites. Information Resources for Telecommunication Professionals [

36 Signal Polarisation: Depolarisation
Rain depolarisation: Since raindrops are not perfectly spherical, as a polarised wave crosses a raindrop, one component of the wave will encounter less water than the other component. There will be a difference in the attenuation and phase shift experienced by each of the electric field components, resulting in the depolarisation of the wave. Polarisation vector relative to the major and minor axes of a raindrop. Source: Satellite Communications, Dennis Roddy, McGraw-Hill

37 Signal Polarisation: Cross-Polarisation Discrimination
Depolarisation can cause interference where orthogonal polarisation is used to provide isolation between signals, as in the case of frequency reuse. The most widely used measure to quantify the effects of polarisation interference is called Cross-Polarisation Discrimination (XPD): XPD = 20 log (E11/E12) To counter depolarising effects circular polarising is sometimes used. Alternatively, if linear polarisation is to be used, polarisation tracking equipment may be installed at the antenna. Source: Satellite Communications, Dennis Roddy, McGraw-Hill

38 Illustration of the various propagation loss mechanisms on a typical earth-space path
The ionosphere can cause the electric vector of signals passing through it to rotate away from their original polarization direction, hence causing signal depolarization. the sun (a very “hot” microwave and millimeter wave source of incoherent energy), an increased noise contribution results which may cause the C/N to drop below the demodulator threshold. The absorptive effects of the atmospheric constituents cause an increase in sky noise to be observed by the receiver Refractive effects (tropospheric scintillation) cause signal loss. The ionosphere has its principal impact on signals at frequencies well below 10 GHz while the other effects noted in the figure above become increasingly strong as the frequency of the signal goes above 10 GHz

39 Atmospheric attenuation
Attenuation of the signal in % Example: satellite systems at 4-6 GHz 50 40 rain absorption 30 fog absorption e 20 10 atmospheric absorption 10° 20° 30° 40° 50° elevation of the satellite

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41 Signal Transmission Link-Power Budget Formula
Link-power budget calculations take into account all the gains and losses from the transmitter, through the medium to the receiver in a telecommunication system. Also taken into the account are the attenuation of the transmitted signal due to propagation and the loss or gain due to the antenna. The decibel equation for the received power is: [PR] = [EIRP] + [GR] - [LOSSES] Where: [PR] = received power in dBW [EIRP] = equivalent isotropic radiated power in dBW [GR] = receiver antenna gain in dB [LOSSES] = total link loss in dB dBW = 10 log10(P/(1 W)), where P is an arbitrary power in watts, is a unit for the measurement of the strength of a signal relative to one watt.

42 Link Budget parameters
Transmitter power at the antenna Antenna gain compared to isotropic radiator EIRP Free space path loss System noise temperature Figure of merit for receiving system Carrier to thermal noise ratio Carrier to noise density ratio Carrier to noise ratio

43 Signal Transmission Equivalent Isotropic Radiated Power
An isotropic radiator is one that radiates equally in all directions. The power amplifier in the transmitter is shown as generating PT watts. A feeder connects this to the antenna, and the net power reaching the antenna will be PT minus the losses in the feeder cable, i.e. PS. The power will be further reduced by losses in the antenna such that the power radiated will be PRAD (< PT). (a) Transmitting antenna Source: Satellite Communications, Dennis Roddy, McGraw-Hill

44 Antenna Gain sphere = 4p solid radians
We need directive antennas to get power to go in wanted direction. Define Gain of antenna as increase in power in a given direction compared to isotropic antenna. P() is variation of power with angle. G() is gain at the direction . P0 is total power transmitted. sphere = 4p solid radians

45 Signal Transmission Link-Power Budget Formula Variables
Link-Power Budget Formula for the received power [PR]: [PR] = [EIRP] + [GR] - [LOSSES] The equivalent isotropic radiated power [EIRP] is: [EIRP] = [PS] + [G] dBW, where: [PS] is the transmit power in dBW and [G] is the transmitting antenna gain in dB. [GR] is the receiver antenna gain in dB. [LOSSES] = [FSL] + [RFL] + [AML] + [AA] + [PL], where: [FSL] = free-space spreading loss in dB = PT/PR (in watts) [RFL] = receiver feeder loss in dB [AML] = antenna misalignment loss in dB [AA] = atmospheric absorption loss in dB [PL] = polarisation mismatch loss in dB The major source of loss in any ground-satellite link is the free-space spreading loss.

46 More complete formulation
Demonstrated formula assumes idealized case. Free Space Loss (Lp) represents spherical spreading only. Other effects need to be accounted for in the transmission equation: La = Losses due to attenuation in atmosphere Lta = Losses associated with transmitting antenna Lra = Losses associates with receiving antenna Lpol = Losses due to polarization mismatch Lother = (any other known loss - as much detail as available) Lr = additional Losses at receiver (after receiving antenna)

47 Transmission Formula Some intermediate variables were also defined before: Pt =Pout /Lt EIRP = Pt Gt Where: Pt = Power into antenna Lt = Loss between power source and antenna EIRP = effective isotropic radiated power Therefore, there are many ways the formula could be rewritten. The user has to pick the one most suitable to each need.

48 Link Power Budget Tx Rx EIRP Transmission: HPA Power
Transmission Losses (cables & connectors) Antenna Gain Antenna Pointing Loss Free Space Loss Atmospheric Loss (gaseous, clouds, rain) Rx Antenna Pointing Loss Reception: Antenna gain Reception Losses (cables & connectors) Noise Temperature Contribution Rx Pr

49 Translating to dBs The transmission formula can be written in dB as:
This form of the equation is easily handled as a spreadsheet (additions and subtractions!!) The calculation of received signal based on transmitted power and all losses and gains involved until the receiver is called “Link Power Budget”, or “Link Budget”. The received power Pr is commonly referred to as “Carrier Power”, C.

50 Link Power Budget Now all factors are accounted for Tx
as additions and subtractions Tx EIRP Transmission: + HPA Power Transmission Losses (cables & connectors) + Antenna Gain Antenna Pointing Loss Free Space Loss Atmospheric Loss (gaseous, clouds, rain) - Rx Antenna Pointing Loss Reception: + Antenna gain Reception Losses (cables & connectors) + Noise Temperature Contribution Rx Pr

51 Easy Steps to a Good Link Power Budget
First, draw a sketch of the link path Doesn’t have to be artistic quality Helps you find the stuff you might forget Next, think carefully about the system of interest Include all significant effects in the link power budget Note and justify which common effects are insignificant here Roll-up large sections of the link power budget Ie.: TXd power, TX ant. gain, Path loss, RX ant. gain, RX losses Show all components for these calculations in the detailed budget Use the rolled-up results in build a link overview Comment the link budget Always, always, always use units on parameters (dBi, W, Hz ...) Describe any unusual elements (eg. loss caused by H20 on radome)

52 Simple Link Power Budget

53 Why calculate Link Budgets?
System performance tied to operation thresholds. Operation thresholds Cmin tell the minimum power that should be received at the demodulator in order for communications to work properly. Operation thresholds depend on: Modulation scheme being used. Desired communication quality. Coding gain. Additional overheads. Channel Bandwidth. Thermal Noise power. We will see more on these items in the next classes.

54 Closing the Link We need to calculate the Link Budget in order to verify if we are “closing the link”. Pr >= Cmin  Link Closed Pr < Cmin  Link not closed Usually, we obtain the “Link Margin”, which tells how tight we are in closing the link: Margin = Pr – Cmin Equivalently: Margin > 0  Link Closed Margin < 0  Link not closed

55 Carrier to Noise Ratios
C/N: carrier/noise power in RX BW (dB) Allows simple calculation of margin if: Receiver bandwidth is known Required C/N is known for desired signal type C/No: carrier/noise p.s.d. (dbHz) Allows simple calculation of allowable RX bandwidth if required C/N is known for desired signal type Critical for calculations involving carrier recovery loop performance calculations

56 System Figure of Merit G/Ts: RX antenna gain/system temperature
Also called the System Figure of Merit, G/Ts Easily describes the sensitivity of a receive system Must be used with caution: Some (most) vendors measure G/Ts under ideal conditions only G/Ts degrades for most systems when rain loss increases This is caused by the increase in the sky noise component This is in addition to the loss of received power flux density

57 System Noise Power - 1 Performance of system is determined by C/N ratio. Most systems require C/N > 10 dB. (Remember, in dBs: C - N > 10 dB) Hence usually: C > N dB We need to know the noise temperature of our receiver so that we can calculate N, the noise power (N = Pn). Tn (noise temperature) is in Kelvins (symbol K):

58 System Noise Power - 2 System noise is caused by thermal noise sources
External to RX system Transmitted noise on link Scene noise observed by antenna Internal to RX system The power available from thermal noise is: where k = Boltzmann’s constant = 1.38x10-23 J/K( dBW/HzK), Ts is the effective system noise temperature, and B is the effective system bandwidth

59 Thank you


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