5 Syllabus Tentatively Week 1 Overview Week 2 Orbits and constellations: GEO, MEO and LEOWeek 3Satellite space segment, Propagation and satellite links , channel modellingWeek 4Satellite Communications TechniquesWeek 5Satellite error correction TechniquesWeek 6Multiple Access IWeek 7Multiple access IIWeek 8Satellite in networks IWeek 9INTELSAT systems , VSAT networks, GPSWeek 10GEO, MEO and LEO mobile communicationsINMARSAT systems, Iridium , Globalstar, OdysseyWeek 11PresentationsWeek 12Week 13Week 14Week 15Tentatively
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 SystemCommunications SystemAntennasMore usually TTC&M - Telemetry, Tracking, Command, and MonitoringTelemetry:Automatic transmission and measurement of data from remote sources by wire or radio or other meansWe will look at each in turn
10 Typical tracking, telemetry, command and monitoring system.
12 AOCSAOCS is needed to get the satellite into the correct orbit and keep it thereOrbit insertionOrbit maintenanceFine pointingMajor partsAttitude Control SystemOrbit Control SystemLook at these next
13 ORBIT MAINTENANCE - 1MUST CONTROL LOCATION IN GEO & POSITION WITHIN CONSTELLATIONSATELLITES NEED IN-PLANE (E-W) & OUT-OF-PLANE (N-S) MANEUVERS TO MAINTAIN THE CORRECT ORBITLEO SYSTEMS LESS AFFECTED BY SUN AND MOON BUT MAY NEED MORE ORBIT-PHASING CONTROL
14 FINE POINTINGSATELLITE MUST BE STABILIZED TO PREVENT NUTATION (WOBBLE) Move unsteadilyTHERE ARE TWO PRINCIPAL FORMS OF ATTITUDE STABILIZATIONBODY 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 AXISMoves around the axis perpendicular to the orbital plane (E-W on the earth)YAW AXISMoves around the axis of the subsatellite point
16 DEFINITION OF AXES - 2EarthoEquatorsYaw AxisRoll AxisPitch Axis
17 TTC&M MAJOR FUNCTIONS Reporting spacecraft health Monitoring command actionsDetermining orbital elementsLaunch sequence deploymentControl of thrustersControl 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 TEMPERATUREVOLTAGESCURRENTSSENSORSTRANSMIT DATA TO EARTHRECORD DATA AT TTC&M STATIONSNOTE: 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 phaseDuring “Safe Mode” operations when the spacecraft loses tracking dataEarth-pointingDuring parts of the launch phaseDuring routine operationsNOTE: for critical telemetry channels
20 TRACKING MEASURE RANGE REPEATEDLY CAN MEASURE BEACON DOPPLER OR THE COMMUNICATION CHANNELCOMPUTE ORBITAL ELEMENTSPLAN STATION-KEEPING MANEUVERSCOMMUNICATE WITH MAIN CONTROL STATION AND USERS
21 COMMAND DURING LAUNCH SEQUENCE IN ORBIT SWITCH ON POWER DEPLOY ANTENNAS AND SOLAR PANELSPOINT ANTENNAS TO DESIRED LOCATIONIN ORBITMAINTAIN SPACECRAFT THERMAL BALANCECONTROL PAYLOAD, THRUSTERS, ETC.
22 COMMUNICATIONS SUB-SYSTEMS Primary function of a communications satellite (all other subsystems are to support this one).Only source of revenueDesign to maximize traffic capacityDownlink usually most critical (limited output power, limited antenna sizes).Early satellites were power limitedMost satellites are now bandwidth limited.
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.
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 PolarisationCircular PolarisationElliptical 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 receiverRefractive 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 ofthe signal in %Example: satellite systems at 4-6 GHz5040rain absorption30fog absorptione2010atmospheric absorption5°10°20°30°40°50°elevation of the satellite
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 dBdBW = 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 antennaAntenna gain compared to isotropic radiatorEIRPFree space path lossSystem noise temperatureFigure of merit for receiving systemCarrier to thermal noise ratioCarrier to noise density ratioCarrier 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 antennaSource: 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 dBThe 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 atmosphereLta = Losses associated with transmitting antennaLra = Losses associates with receiving antennaLpol = Losses due to polarization mismatchLother = (any other known loss - as much detail as available)Lr = additional Losses at receiver (after receiving antenna)
47 Transmission FormulaSome intermediate variables were also defined before:Pt =Pout /Lt EIRP = Pt GtWhere:Pt = Power into antennaLt = Loss between power source and antennaEIRP = effective isotropic radiated powerTherefore, 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 GainAntenna Pointing LossFree Space LossAtmospheric Loss (gaseous, clouds, rain)Rx Antenna Pointing LossReception:Antenna gainReception Losses(cables & connectors)Noise Temperature ContributionRxPr
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 subtractionsTxEIRPTransmission:+ HPA PowerTransmission Losses(cables & connectors)+ Antenna GainAntenna Pointing LossFree Space LossAtmospheric Loss (gaseous, clouds, rain)- Rx Antenna Pointing LossReception:+ Antenna gainReception Losses(cables & connectors)+ Noise Temperature ContributionRxPr
51 Easy Steps to a Good Link Power Budget First, draw a sketch of the link pathDoesn’t have to be artistic qualityHelps you find the stuff you might forgetNext, think carefully about the system of interestInclude all significant effects in the link power budgetNote and justify which common effects are insignificant hereRoll-up large sections of the link power budgetIe.: TXd power, TX ant. gain, Path loss, RX ant. gain, RX lossesShow all components for these calculations in the detailed budgetUse the rolled-up results in build a link overviewComment the link budgetAlways, always, always use units on parameters (dBi, W, Hz ...)Describe any unusual elements (eg. loss caused by H20 on radome)
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 onthese items in thenext classes.
54 Closing the LinkWe need to calculate the Link Budget in order to verify if we are “closing the link”.Pr >= Cmin Link ClosedPr < Cmin Link not closedUsually, we obtain the “Link Margin”, which tells how tight we are in closing the link:Margin = Pr – CminEquivalently:Margin > 0 Link ClosedMargin < 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 knownRequired C/N is known for desired signal typeC/No: carrier/noise p.s.d. (dbHz)Allows simple calculation of allowable RX bandwidth if required C/N is known for desired signal typeCritical 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/TsEasily describes the sensitivity of a receive systemMust be used with caution:Some (most) vendors measure G/Ts under ideal conditions onlyG/Ts degrades for most systems when rain loss increasesThis is caused by the increase in the sky noise componentThis is in addition to the loss of received power flux density
57 System Noise Power - 1Performance 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 dBWe 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 systemTransmitted noise on linkScene noise observed by antennaInternal to RX systemThe 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