Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Satellite Communications Satellite subsystems Global Positioning System (GPS), NASA Lect 03 • • • • • • • • • • Overview Position control system Attitude control system Power control system Environmental control system Telemetry Transponders Antennas Beam shaping Reliability design © 2012 Raymond P. Jefferis III 1 Satellite Subsystems Overview • The communications mission of the satellite is supported by subsystems to maintain its position, orientation, electric power, and internal environment. • Fulfilling the mission may require producing a shaped communications beam to communicate. Lect 03 © 2012 Raymond P. Jefferis III 2 Orbital Position Control • A geosynchronous satellite must remain located within a 3-dimensional box despite the effects of gravitational anomalies and solar wind. (This is necessary for accurate ground station location of the satellite.) • Station-keeping is effected by thruster “burns” (For example: xenon ion engine) • Fuel/energy is expended to do this, which limits the effective life of the satellite. Lect 03 © 2012 Raymond P. Jefferis III 3 Schematic of Orbital Position Control • Orbital position measured by ground stations • Thruster burns calculated • Burn times sent to satellite • Orbital corrections made by thruster “burns” Lect 03 © 2012 Raymond P. Jefferis III 4 Simple Orbital Dynamics v d dr r 2 2 r 3 2 where μ = 3.986004418E5 r = orbital radius v = orbital velocity Note: As orbital radius decreases, velocity increases. Lect 03 © 2012 Raymond P. Jefferis III 5 Orbital Maneuvers In: Orbital and Celestial Mechanics Website http://www.cdeagle.com http://www.cdeagle.com/html/ommatlab.html Recommended download: Orbital Mechanics with MATLAB, Orbital Maneuvers http://www.cdeagle.com/ommatlab/maneuvers.pdf Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 6 Attitude Control • Earth station coverage requires that satellite remain in a fixed orientation with respect to the earth • On-board sensors measure orientation with respect to the earth, sun, and stars. • Attitude corrections are made by control thrusters in three axes. Little energy required for attitude corrections. Lect 03 © 2012 Raymond P. Jefferis III 7 Attitude Measurements Orientation measurements • Sun orientation • Star(s) orientation(s) • Earth orientation A-A´ angle and orientation B-B´ angle and orientation Lect 03 © 2012 Raymond P. Jefferis III 8 Attitude Control The governing differential equation is: d 2 J 2 T dt Where J is the moment of inertia of the satellite around the axis of interest, θ is the attitude angle with respect to a fixed reference direction, and T is the applied torque, supplied by thrusters. Lect 03 © 2012 Raymond P. Jefferis III 9 Simple Attitude Algorithm • • • • Estimate the required correction angles Burn thrusters to provide torques Let angles drift to calculated value Cancel angular velocity with opposing the thrusters • Repeat until the alignment is correct Lect 03 © 2012 Raymond P. Jefferis III 10 Power Control • Orients solar panels normal to solar radiation, for maximum output • Regulates system voltages and distributes current loads • Maintains battery conditioning by maintaining charge and discharge cycles for expected outages of 70 minutes (orbital darkness cycle) • Limits discharge to 70%, for battery protection Lect 03 © 2012 Raymond P. Jefferis III 11 Solar Power • Silicon solar cells produce electric power from “incident radiation” – Direct sunlight – Sunlight reflected from earth (albedo) • Power is proportional to incident energy • Temperature affects conversion efficiency • As solar cells age, power output is reduced Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 12 Solar Panel Characterization • • • • Short circuit currrent, Isc Open circuit voltage, Voc Maximum power point voltage, Vmpp Maximum power point current, Impp Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 13 Simple Circuit Model Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 14 Simple Model Explanation • Iph -> current delivered by photocell • D -> Diode characteristic of photocell (Some loss current, ID flows in diode) • Rs -> Equivalent internal series resistence of photocell • Ip -> effective current delivered (Iph – ID) • Vp-> effective photocell output voltage Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 15 Mathematical Model Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 16 Mathematica® Photocell Model Function Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 17 Photocell Output Voltage Calculation Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 18 Calculated Photocell Output Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 19 Current vs Voltage Output of Solar Cell Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 20 Maximum Power Point of Solar Cell Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 21 Class Work • Plot graphs for photocell panels with the following power outputs: – 500 W/m2 – 250 W/m2 • See notes Lect 03 © 2012 Raymond P. Jefferis III Lect 00 - 22 Temperature Control • Temperature regulation (0 - 75 degC) • Temperature and its cycling stresses all components, shortening operational life • Satellites have multiple heat sources – Solar cell power dissipated on board – Direct absorption of solar radiation • Waste heat can be dumped into space by: – Radiation – Evaporation (if fluid available) Lect 03 © 2012 Raymond P. Jefferis III 23 Heat Balance Lect 03 © 2012 Raymond P. Jefferis III 24 Telemetry • Transmitted data about conditions in a satellite – Operational status information • Subsystems data • Utility data – Environmental data • Temperatures • Pressures (propellant tanks) Lect 03 © 2012 Raymond P. Jefferis III 25 Telemetry Block Diagram Lect 03 © 2012 Raymond P. Jefferis III 26 Ground Station Telemetry • Telemetry data received by earth station(s) • Satellite tracking data is generated by the earth station • Orbital control processing is calculated on the earth • External data & commands are sent by earth station(s) to the satellite using telemetry channels Lect 03 © 2012 Raymond P. Jefferis III 27 Layered Telemetry Model Waqas Afzal and Adnan Mahmood, Proceedings of the International MultiConference of Engineers and Computer Scientists 2008 ,Vol II IMECS 2008, 19-21 March, 2008, Hong Kong Lect 03 © 2012 Raymond P. Jefferis III 28 Transponders • Receive weak communication signals on one frequency • Amplifly these weak signals • Simultaneously retransmit the communication on another frequency at much higher power. • Electromagnetic isolation is required to prevent transmitted signals from interfering with the reception of weak signals from a ground station. Lect 03 © 2012 Raymond P. Jefferis III 29 Channel Isolation Methods • Frequency separation between Reception and Transmission channel frequencies • Electronic filtering (bandpass amplifiers) • Transmitting antennas oppositely polarized (electromagnetically decoupled) from the receiving antennas in each channel • Circulators with high degree of isolation Lect 03 © 2012 Raymond P. Jefferis III 30 Double Conversion Transponder H and V indicate Horizontal and Vertical polarization, respectively. Lect 03 © 2012 Raymond P. Jefferis III 31 Data Processing Transponder H and V indicate Horizontal and Vertical polarization, respectively. Lect 03 © 2012 Raymond P. Jefferis III 32 In-Band Frequency Allocations • Transmission and reception frequencies are channelized into discrete bands • Band allocation (1 of n) 1 - 36 MHz Channel (useful capacity) 2 - 2 MHz guard bands • Bands on 40 MHz centers Lect 03 © 2012 Raymond P. Jefferis III 33 Transponder Frequency Plan • The Intelsat GALAXY-11 communications satellite uses the plan that follows. • Each channel has 36 MHz bandwidth, with a 2 MHz guard band on each end • Transmit (XMT) and Receive (RCV) pairs of frequencies are about 2500 MHz apart, to provide enough isolation that simultaneous reception and transmission can take place. Lect 03 © 2012 Raymond P. Jefferis III 34 G11 C-Band Transponders Uplink (RCV) Downlink (XMT) Note 2225 MHz RCV/XMT separation on each channel. Lect 03 © 2012 Raymond P. Jefferis III 35 G11 Ku-Band Transponders Uplink (RCV) Downlink (XMT) Note 2300 MHz RCV/XMT separation on each channel. Lect 03 © 2012 Raymond P. Jefferis III 36 Antennas • Receive weak signals and couple them to a low noise amplifier • Transmit power signals and shape the beam for planned reception patterns on the ground Note: Satellite is stabilized in all axes • Can be horizontally or vertically polarized. Polarized signals are received best by similarly polarized receiving antennas. • Have “gain” due to focusing of energy Lect 03 © 2012 Raymond P. Jefferis III 37 Antenna Types • Wire (monopoles and dipoles) Low gain (4 - 8 dB) not focused • Horn (tapered waveguide) Intermediate gain (23 dB), 10˚ beam focus Often used to feed dish antenna • Reflecting (dish, many wavelengths in diameter) High gain (45 dB), 3˚ beam focus • Array (multiple phased antennas in pattern) Adjustable gain and beam shape possible Lect 03 © 2012 Raymond P. Jefferis III 38 Dipole Antennas • Broad radiation pattern (78 degrees) • Low gain (2.15 dB over isotropic for halfwave antenna) • Longer versions have more gain (radiation pattern is altered) • Low gain limits missions to only those that can be accomplished with low orbit Lect 03 © 2012 Raymond P. Jefferis III 39 Reflector Antennas • Narrow radiation pattern (for wavelength λ and diameter D, and factor k), BW k( / D) (k = 60 for parabolic antenna) • High gain (for diameter D, wavelength λ, and area, A), G ( D / ) 4 A / BW k( / D) 2 Lect 03 2 © 2012 Raymond P. Jefferis III 40 Parabolic Dish Antenna • Symmetric • The feed faces the reflector at its focal point Wikipedia Lect 03 © 2012 Raymond P. Jefferis III 41 Center Fed Parabolic Dish Antenna Lect 03 © 2012 Raymond P. Jefferis III 42 Offset Parabolic Dish Antenna Wikipedia Lect 03 • Asymmetric • Feed is offset; faces the reflector • Reflector is shaped above feed horn line to compensate for offset © 2012 Raymond P. Jefferis III 43 Offset-Fed Parabolic Dish Lect 03 © 2012 Raymond P. Jefferis III 44 Cassegranian Antenna Wikipedia Lect 03 • Symmetric • Feed horn extends through center of reflector • Hyperboloid secondary reflector positioned at focus of primary reflector © 2012 Raymond P. Jefferis III 45 Cassegranian Antenna Cassegrain radar antenna at Sondrestrom, Greenland ( Diameter: 32 m Normal operating frequency: 1290 MHz ) Photo by L. Chang (wikipedia) Lect 03 © 2012 Raymond P. Jefferis III 46 Double Reflector Antennas • Cassegranian – Feed horn through center of reflector – Hyperboloid secondary reflector at the focus of the primary reflector • Gregorian – Feed horn through center of reflector – Ellipsoid secondary reflector at focus of the primary reflector • Offset Gregorian – Gregorian with feed horn at edge of the primary reflector Lect 03 © 2012 Raymond P. Jefferis III 47 Gregorian Antenna Feed Flickr Photo: http://flickr.com/photos/ekirsche/87736375/ Lect 03 © 2012 Raymond P. Jefferis III 48 Antenna Gain • The angular dependence of radiation from an antenna is its radiation pattern. It is measured as radiated power per solid angle. • The ratio of radiated power per solid angle to that of an isotropic dipole is the gain of the antenna. Lect 03 © 2012 Raymond P. Jefferis III 49 Antenna Power Flux Density • Isotropic • Radiated power per unit spherical area • Equivalent to the square of the RMS E-field voltage divided by the impedance of free space, 377 Ohms. • Ψ = P/4πr2 = E2/ZFS [Watts/m2] Lect 03 © 2012 Raymond P. Jefferis III 50 Antenna Aperture • The received power, Pr , is equal to the raditated flux density, Ψ, multiplied by the effective aperture, Aeff , of the receiving antenna • Pr = Aeff ψ = ηAψ [Watts] • Aeff is a fraction, η , of the actual antenna aperture area because of edge effects and other losses. Lect 03 © 2012 Raymond P. Jefferis III 51 Gain of Aperture Antenna G 4A A / 2 G = aperture antenna gain A = aperture efficiency A = aperture area [m2] = operating wavelength [m] For circular aperture, G A ( D / )2 Lect 03 G = aperture antenna gain A = aperture efficiency D = aperture diameter [m] = operating wavelength [m] © 2012 Raymond P. Jefferis III 52 Example Calculation • A circular antenna has D/ = 25 [wavelengths] A = 63% • Gain = 0.63*(*25)2 = 3886 = 36 dB Lect 03 © 2012 Raymond P. Jefferis III 53 Beamwidth of Aperture Antenna 3dB 58 / D (Large circular aperture) 3dB 75 / D (Parabola) θ = 3dB beamwidth in degrees λ = operating wavelength D = aperture diameter [m] Ref: J. D. Kraus and R. J. Marhefka, Antennas for All Applications, Third Edition, McGraw-Hill, 2002. Lect 03 © 2012 Raymond P. Jefferis III 54 Example Calculation • If a circular antenna has D/ = 25 [wavelengths], 3dB = 75/25 = 3 [degrees] • Note that the same reflector diameter will yield a gain of 6 dB at half the wavelength Lect 03 © 2012 Raymond P. Jefferis III 55 Antenna Beamwidth D/ 75 3dB where, θ = 3dB beamwidth [degrees] λ = operating wavelength [m] D = aperture diameter [m] Note: For = 3˚, D/ = 25 Lect 03 © 2012 Raymond P. Jefferis III 56 Approximate Gain vs Beamwidth Run Mathematica(R) program: mAntGain LECT 04 © 2012 Raymond P. Jefferis III Lect 00 - 57 GALAXY-11 Calculation Intelsat GALAXY-11 at 91W (NORAD 26038) • 39.1 dBW on C-Band (20W, 24 ch, Bw: 36 MHz) • 47.8 dBW on Ku-Band (75/140W, 40 ch, Bw: 36 MHz) Lect 03 © 2012 Raymond P. Jefferis III 58 Beamwidth of Ku-Band Antenna • • • • Antenna diameter: 1.8 [m] Frequency: 12 [GHz] Wavelength: 0.025 [m] Beamwidth ≈ 75/(1.8/0.025) ≈ 1.05˚ Lect 03 © 2012 Raymond P. Jefferis III 59 Example - Ku-Band antenna gain • 3dB beamwidth = 3˚ • D/ = 25 = 0.63 • G = 3886 • Gdb = 36 Lect 03 © 2012 Raymond P. Jefferis III 60 Sample Calculation of Antenna Gain eff = 0.63; beamw = 3; f = 12*10^9; c = 2.99792458*10^8; lam = c/f; app = 75.0/beamw diam = app*lam G = eff*p^2*app^2 lG = 10*Log[10, G] Lect 03 © 2012 Raymond P. Jefferis III 61 Phased Array Antennas • For N antenna sources phased ϕ degrees apart in an array of aperture radius, a • Physical spacing, typically /4 • Resulting beam intensity and angle, (from Wikipedia) are: sin a I I0 a Lect 03 2 N sin N sin 4 2 sin sin 4 2 © 2012 Raymond P. Jefferis III 2 2 sin N 1 62 Transmitter Antenna Gain For a circular antenna (parabolic dish), Ae A (d / 2) 4 G 2 Ae d G A Lect 03 2 2 where, Ae = Effective aperture [m2] A= aperture efficiency d = aperture diameter [m] G = aperture antenna gain = operating wavelength [m] © 2012 Raymond P. Jefferis III 63 Reliability • Satellites cannot easily be maintained • Reliability methods: – – – – Lect 03 Component qualification Burn-in (100 - 1000 hours) Redundancy Component switching © 2012 Raymond P. Jefferis III 64 Component Qualification Conditions • Components manufactured with 100% tested materials • Raw material tracked to component lots • Component failure rates characterized – Specified operating conditions (-85 to +125 ˚C) – Many components tested (some destructively) – Failure rates calculated • Lot numbers qualified for further use Lect 03 © 2012 Raymond P. Jefferis III 65 Burn-in • Most component failures occur early on • Running under power (burn-in) causes weak components to fail early • Used to catch systematic problems - bad lots • Does not reduce life of most components • Burn-in times of 100 - 1000 hours is considered optimal Lect 03 © 2012 Raymond P. Jefferis III 66 Measures Used To Provide Redundancy • Multiple redundant pathways (Repair by ground command) • Median voting (Self-repair) • Switched alternative circuits (Repair by ground command) Lect 03 © 2012 Raymond P. Jefferis III 67 Multiple Redundant Pathways • • • • All components operate simultaneously Results can be rescaled for correct values There is a common point of failure at output Assumes advantageous failure modes! Lect 03 © 2012 Raymond P. Jefferis III 68 Parallel Amplifier Example Output of C will either be double or half of its correct value, assuming A or B fails in OFF mode (advantageous failure mode) Note: Permits repair by ground station command (Gain change) Lect 03 © 2012 Raymond P. Jefferis III 69 Median Voting Circuit • Its rules: – Reject largest value – Reject smallest value – Take median value as true • Rejects up-scale and down-scale failures • Expensive! • A voter circuit is a common point of failure Lect 03 © 2012 Raymond P. Jefferis III 70 Median Voting Schematic Note: Self-repairing Lect 03 © 2012 Raymond P. Jefferis III 71 Simple Probability Calculations • Given that a single channel has a failure probability (p = 10-6), per unit time, the failure probability is p fail p 106 • For three equal channel failure probabilities, p, the probability of two simultaneous failures for (p = 10-6) is, p fail 3p2 (1 p) 3*1012 *(1 106 ) 3*1012 Lect 03 © 2012 Raymond P. Jefferis III 72 Conclusion • The triple redundancy system failure probability with voting is nearly the square of the single-element system failure • The voter circuit is a common point of failure to be considered • Up-scale or down-scale failure (advantageous failure mode) is assumed Lect 03 © 2012 Raymond P. Jefferis III 73 Switched Alternative Circuits • Two-way redundant paths built into signal path • Switching between paths are provided to select preferred component • Both outputs analyzed on the ground • Switching is effected to select the chosen component • Cheaper – Uses fewer components – Saves power Lect 03 © 2012 Raymond P. Jefferis III 74 Switching Circuit Example Note: permits repair by ground station command, when either Amplifier A or Amplifier B fails. Lect 03 © 2012 Raymond P. Jefferis III 75 End Lect 03 © 2012 Raymond P. Jefferis III 76