Satellite Tracking, Telemetry and Command Design Basis

Transcription

1 Satellite Tracking, Telemetry and Command Design Basis Jyh-Ching Juang ( 莊智清 ) Department of Electrical Engineering National Cheng Kung University juang@mail.ncku.edu.tw November, 2008 SATELLITE TRACKING, TELEMETRY AND COMMAND 1

2 Purpose Understand the functions of satellite telemetry, tracking, and command (TT&C) subsystem. Understand basic communication principles and operations. Learn to perform fundamental analyses in spacecraft communication. Be prepared for the design of TT&C final project. SATELLITE TRACKING, TELEMETRY AND COMMAND 2

3 Scope Function of TT&C: provides the means of monitoring and controlling the satellite operations. Scientific Instruments antenna telemetry Data recorder Data processor data Transceiver Data handling unit Command decoder command Attitude & orbit control subsystem Power control subsystem Thermal control subsystem SATELLITE TRACKING, TELEMETRY AND COMMAND 3

4 Definition Telemetry: a system that reliably and transparently conveys measurement information from a remotely located data generating source to users located in space or on Earth. Tracking: a system that observes and collects data to plot the moving path of an object. Command: a system by which control is established and maintained. Communication: a system that enabling the transfer of information from one point to another. tracking telemetry command communication SATELLITE TRACKING, TELEMETRY AND COMMAND 4

5 Contents Satellite communication overview Techniques of radio communications Radio wave Antenna Link budget Noise Modulation Multiple access Telemetry system Telecommand system Protocol: AX.25 SATELLITE TRACKING, TELEMETRY AND COMMAND 5

6 galaxy Satellite Communication sun ionosphere troposphere SATELLITE TRACKING, TELEMETRY AND COMMAND 6

7 Characteristics Long distance: depends on satellite altitude, nadir pointing, and observer s elevation Restricted coverage in time and space Varying geometry and Doppler shift Propagation effects due to ionosphere and troposphere Environmental effects: acoustic, vibration, shock, thermal, radiation Power, weight, and volume restrictions GEO satellites LEO satellites 600 km km 6378 km SATELLITE TRACKING, TELEMETRY AND COMMAND 7

8 Communication System Transmission Antenna Coder Modulator Up converter Power Amplifier Reception Antenna desired format desired spectrum desired strength Low Noise Amplifier Down converter Demodulator Decoder SATELLITE TRACKING, TELEMETRY AND COMMAND 8

9 Electromagnetic Wave Maxwell s equation: specify the relationship between the variation of the electric field E and the magnetic field H in time and space within a medium. The E field strength is measured in volts per meter and is generated by either a time-varying magnetic filed or by a free charge. The H field is measured in amperes per meter and is generated by either a time-varying electric field or by a current. Principle of conservation of energy Electric Magnetic Faraday s Ampere s Gauss s law Gauss s law law law Maxwell s equations SATELLITE TRACKING, TELEMETRY AND COMMAND 9

10 Radio Wave Radio energy emitted in space exhibits both electric and magnetic fields. A changing magnetic field produces an electric field and a changing electric field produces a magnetic field. Direction of wave propagation: E x H E H Direction of propagation SATELLITE TRACKING, TELEMETRY AND COMMAND 10

11 Radio Wave as a Signal A radio wave is a signal whose characteristics include Amplitude: peak value or strength of the signal; measured in volts or watts Frequency: rate at which a signal repeats, measured in cycles per second or Hertz (Hz) Period: amount of time it takes for one repetition of a signal Phase: Analog versus digital signals Bandwidth and Data rate waveform amplitude time phase period SATELLITE TRACKING, TELEMETRY AND COMMAND 11

12 Frequency and Polarization Velocity, frequency, and wavelength Frequency or number of cycles per second is given the unit of the hertz (Hz). In nondispersive media, the velocity is equal to the speed of light c = 3 x 10 8 m/sec. The velocity c (in m/sec) is related to the frequency f (in Hz) and wavelength λ (in m) by c = fλ. Polarization is the alignment of the electric field vector of the plane wave relative to the direction of propagation. Linear polarization (vertical, horizontal) Circular polarization (right-hand, left-hand) Elliptical polarization H E E Horizontal polarization H Vertical polarization Right-handed circular polarization Left-handed elliptical polarization SATELLITE TRACKING, TELEMETRY AND COMMAND 12

13 Electromagnetic Spectrum Band f (GHz) L 1 ~ 2 Designation Frequency Wavelength VLF (very low frequency) 3KHz ~ 30 KHz 100 Km ~ 10 Km LF (low frequency) 30 KHz ~ 300 KHz 10 Km ~ 1 Km MF (medium frequency) 300 KHz ~ 3 MHz 1 Km ~ 100 m HF (high frequency) 3 MHz ~ 30 MHz 100 m ~ 10 m VHF (very high frequency) 30 MHz ~ 300 MHz 10 m ~ 1m UHF (ultra high frequency) 300 MHz ~ 3 GHz 1 m ~ 10 cm SHF (super high frequency) 3 GHz ~ 30 GHz 10 cm ~ 1cm S 2 ~ 4 C 4 ~ 8 X 8 ~ 12 Ku 12 ~ 18 K 18 ~ 27 Ka 27 ~ 40 EHF (extremely high frequency) 30 GHz ~ 300 GHz 1cm ~ 1mm SATELLITE TRACKING, TELEMETRY AND COMMAND 13

14 Frequency Allocation SATELLITE TRACKING, TELEMETRY AND COMMAND 14

15 Space TT&C Spectrum Frequency band (MHz) Space operation Direction indicator Allocation status 136 ~ 137 Space-Earth Secondary 137 ~ 138 Space-Earth Primary 148 ~ Earth-Space 267 ~ 272 Space-Earth Secondary 272 ~ 273 Space-Earth Primary ~ 401 Space-Earth Secondary 401 ~ 402 Space-Earth Primary ~ Earth-Space 1427 ~ 1429 Earth-Space Primary 1525 ~ 1535 Space-Earth Primary 2025 ~ 2110 Earth-Space 2200 ~ 2290 Space-Earth 7125 ~ 7155 Earth-Space Frequency band Direction indicator Allocation status 136 ~ 137 MHz Space-Earth Secondary 137 ~ 138 MHz Space-Earth Primary ~ 401 MHz Space-Earth Primary 2025 ~ 2110 MHz Earth-Space 2200 ~ 2290 MHz Space-Earth 7190 ~ 7235 MHz Earth-Space 8450 ~ 8500 MHz Space-Earth Primary ~ 13.4 GHz Earth-space 13.4 ~ 14.3 GHz none Secondary 14.4 ~ GHz Space-Earth Secondary 14.5 ~ GHz none Secondary 31.0 ~ 31.3 GHz none Secondary 31.8 ~ 32.3 GHz Space-Earth Secondary 34.7 ~ 35.2 GHz none Secondary 65.0 ~ 66.0 GHz none Primary SATELLITE TRACKING, TELEMETRY AND COMMAND 15

16 Decibel Representation Decibel representation: a quantity P in decibels (db) is defined as P in db = [P] = 10 log 10 (P) An amplifier of gain 100 is the same as 20 db. Power is generally represented in terms of dbw or dbm. Power in dbw = 10 log 10 (power in watts/one watt). Power in dbm = 10 log 10 (power in milli-watts/one milli-watt). 0.1 watts is equivalent to -10 dbw or 20 dbm Boltzmann s constant κ = 1.38 x J/ 0 K = 1.38 x W/Hz/ 0 K = dbw/hz / 0 K. A frequency of 22 GHz is equivalent to db-hz db-hz = 10 log 10 (22 x 10 9 Hz/1 Hz) A noise temperature of K is the same as 24.8 db- 0 K 24.8 = 10 log 10 (300) SATELLITE TRACKING, TELEMETRY AND COMMAND 16

17 Communication Link Analysis Quantities in link analysis Transmit power P (dbw) Antenna gain G (dbi) Received carrier power C (dbw) Noise temperature T ( 0 K) C/N Dissipative loss L (db) 0 Slant range r (m) Frequency f (Hz) or wavelength λ (m) Bit rate R (bps, bit per second) Bandwidth B (Hz) Parameters EIRP: equivalent isotropic radiated power, a measure of transmitter power in the direction of the link. C/N or C/ N 0 : carrier to noise power (density) ratio, a measure of received signal quality. G/T: gain to temperature ratio, figure of merit of the receiver. E b / N 0 : energy per bit to noise power density, a measure related to the bit error rate in digital transmission. EIRP G/T E b / N 0 SATELLITE TRACKING, TELEMETRY AND COMMAND 17

18 Antenna Types Dipole Horn Helical Yagi Parabolic Antenna array SATELLITE TRACKING, TELEMETRY AND COMMAND 18

19 Antenna Parameters Aperture A: the area that captures energy from a passing radio wave. Dish: size of the reflector Horn: area of the mouth Dipole: 0.13λ 2 Efficiency η: a function of surface/profile accuracy, physical size, focal length, aperture blockage, mismatch effects, and so on. Dish: typically 55% Horn: 50% Gain G: amount of energy an isotropic antenna would radiated in the same direction when driven by the same input power. G = 4πηA/λ 2 where A is the aperture, η is the efficiency, and λ is the wavelength. Polarization: must be compatibly with the radio wave. 3dB loss for linear/circular mismatch 25 db loss (or greater) for right/left mismatch Infinite loss for vertical/horizontal mismatch SATELLITE TRACKING, TELEMETRY AND COMMAND 19

20 Directive Gain An antenna does not amplify. It only distributes energy through space to make use of energy available. Isotropic antenna: equal intensity in all directions Normally, the gain is a function of the elevation and azimuth. The entire sphere has a solid angle 4π steradians (square radians). Isotropic antenna directional antenna SATELLITE TRACKING, TELEMETRY AND COMMAND 20

21 Equivalent Isotropic Radiated Power Let P t be the transmitter power and G t be the transmitter antenna gain, then the equivalent isotropic radiated power (EIRP) is the product of P t and G t, i.e., EIRP = P t x G t. In terms of db, [EIRP] = [P t ] + [G t ]. SATELLITE TRACKING, TELEMETRY AND COMMAND 21

22 Signal or Carrier Power At a distance r from the transmitter, the power flux density is S = EIRP/(4πr 2 ) = P t G t /(4πr 2 ) If atmospheric attenuation results in power loss by a factor L A, then the flux density at the receiver is S = P t G t /(4πr 2 L A ) Let A r be the effective aperture of the receiving antenna with efficiency η, then the received power is C = S A r η = (EIRP)A r η/(4πr 2 L A ) As the antenna gain is G r = 4πA r η/(λ 2 ) where λ is the wavelength Thus, the signal power at the input to the receiver is C = EIRP G r (λ/(4πr)) 2 (1/L A ) SATELLITE TRACKING, TELEMETRY AND COMMAND 22

23 Free Space Loss Free space loss: loss due to the spreading of electromagnetic wave. The free space loss is L S = (4πr/λ) 2 In terms of db, the free space loss is [L S ] = 20 log 10 (4πr/λ) where r is the distance of travel and λ is the wavelength. Let f be the frequency (in GHz) and r be the distance (in km), then [L S ] = log 10 (f) + 20 log 10 (r) For example, for a geostationary satellite, r = km, the free space loss in db is [L S ] = log 10 (f) SATELLITE TRACKING, TELEMETRY AND COMMAND 23

24 Losses in Communication Link The free-space loss [L S ] = 20 log 10 (4πr/λ) is quadratically proportional to the distance between the transmitter and the receiver. The loss depends on the wavelength (frequency) used. In addition to the free-space spreading loss, there are Receiver feeder loss Antenna pointing loss Faraday rotation loss Atmospheric and ionospheric absorption loss Rain attenuation Polarization mismatch loss Multipath loss Random loss All these make up the L A term, that is [L A ] = [L feeder ] + [L pointing ] + [L atmosphere ] + The overall loss is thus [L] = [L S ] + [L A ] SATELLITE TRACKING, TELEMETRY AND COMMAND 24

25 Atmospheric Attenuation SATELLITE TRACKING, TELEMETRY AND COMMAND 25

26 Link Budget Recall that the received signal power is C = EIRP G r (λ/(4πr)) 2 (1/L A ) In terms of db, [C] = [EIRP] + [G r ] [L S ] [L A ] Received power in dbw Antenna gain in db EIRP in Free-space loss in db Other losses in db dbw SATELLITE TRACKING, TELEMETRY AND COMMAND 26

27 Link Budget Example A transmitter with power 2 W and antenna gain 3 db. Its EIRP in dbw is [EIRP] = 10 log = 6.01 dbw. Suppose that the satellite is flying at 600 km in altitude, with an elevation limit of 10 o, what is the maximum transmission distance? The slant range is km Suppose that the frequency is 430 MHz, the free space loss is [L S ] = log 10 (f) + 20 log 10 (r) = db Suppose that the receiver antenna gain is 6 db, the received carrier power is [C] = [EIRP] + [G r ] [L S ] = = dbw Elevation angle Nadir angle 600 km 6378 km SATELLITE TRACKING, TELEMETRY AND COMMAND 27

28 Noise Noise is defined as the unwanted form of energy that tends to interfere with the reception and accurate reproduction of wanted signals. The thermal noise power is given by P n = κtb where T is the equivalent noise temperature (in 0 K), B is the equivalent noise bandwidth (in Hz), and κ = 1.38 x J/ 0 K is Boltzmann s constant. The noise power spectral density N 0 = P n /B = κt. The bandwidth B depends on the design of the receiver. The temperature T (noise temperature) is a function of the environment. It is customary to use temperature as a measure of the extent of noise. SATELLITE TRACKING, TELEMETRY AND COMMAND 28

29 Noise Sources Contributions of system noise: sky, ground, galaxy, circuit, and medium. Non-thermal noises are characterized in terms of noise temperature. Sun: ( K) communication is effectively impossible with sun in the field of view. Moon: reflected sunlight Earth: (254 0 K) Galaxy: negligible above 1 GHz Sky: (30 0 K) Atmosphere: noise radiated by O 2 and H 2 O, less than 50 0 K Weather: clouds, fogs, and rain Electronics noise: receiving equipment SATELLITE TRACKING, TELEMETRY AND COMMAND 29

30 Equivalent Noise Temperature For an amplifier of gain G, The input noise energy coming from the antenna is N 0,ant = κt ant. The output noise energy N 0,out is the sum of GN 0,out and the noise induced in the amplifier. N 0,out = Gκ(T ant + T E ) where T E is the equivalent input noise temperature for the amplifier. The total noise referred to the input is N 0,in = κ(t ant + T E ). The typical value of T E is in the range 35 to K. T ant N 0,in Amplifier power gain G N 0,out SATELLITE TRACKING, TELEMETRY AND COMMAND 30

31 System Noise Temperature T ant N 0,1 Amplifier N 0,2 Amplifier N 0,out G 1, T E1 G 2, T E2 The total noise energy referred to amplifier 2 input is N 0,2 = G 1 κ (T ant + T E1 ) + κ T E2 The noise energy referred to amplifier 1 input is N 0,1 = N 0,2 /G 1 = κ (T ant + T E1 + T E2 /G 1 ) A system noise temperature T S is defined as N 0,1 = κ T S. Hence, T S = T ant + T E1 + T E2 /G 1 The noise temperature of the second stage is divided by the power gain of the first stage when referred to the input. Thus, in order to keep the overall system noise as low as possible, the first stage (usually an LNA) should have high power gain as well as low noise temperature. SATELLITE TRACKING, TELEMETRY AND COMMAND 31

32 Noise Temperature Example 35 0 K Determine the system noise temperature at the input to the LNA when Antenna noise temperature T ant = 35 0 K Waveguide feeder gain = db (0.944), temperature = K LNA gain = 50 db (10000), temperature = 75 0 K Cable gain = -20 db (0.01), temperature = K Receiver noise temperature = K The system noise temperature T S is T S = 35 x x ( ) / /(10000 x 0.01) = K Waveguide db K T S LNA 50 db 75 0 K Cable -20 db K Receiver K SATELLITE TRACKING, TELEMETRY AND COMMAND 32

33 Carrier-to-Noise Density Ratio, C/N 0 The performance of a satellite link is often measured in terms of [C/N] or [C/ N 0 ]. The carrier-to-noise ratio is defined as the difference between the received carrier power and the noise power in db [C/N] = [C] - [P n ] The carrier-to-noise density ratio is [C/ N 0 ] = [C] [N 0 ]. Thus, [C/ N 0 ] = [C/N] [B] in db-hz. For a system temperature T S, the noise power density referred to the receiver input is N 0 = κt S and the noise power P n = κt S B. Recall that [C] = [EIRP] + [G r ] [L S ] [L A ].Thus, [C/N] = [EIRP] + [G r ] [L S ] [L A ] [κ] [T S ] [B] and [C/ N 0 ] = [EIRP] + [G r ] [L S ] [L A ] [κ] [T S ] The signal-to noise power-density ratio is indeed C/N 0 = EIRP (λ/(4πr)) 2 (1/L A ) (G r /T S ) (1/κ) If only spreading loss is considered, [C/ N 0 ] = [EIRP] + [G r ] [T S ] - 20 log 10 (4πr/λ) SATELLITE TRACKING, TELEMETRY AND COMMAND 33

34 Gain-to-Temperature Ratio, G/T The G/T ratio (gain-totemperature ratio) is a key parameter in specifying the receiving system performance. [G/T] = [G r ] - [T] Although the temperature may different at different reference point, the G/T ratio is independent of the reference point. Accordingly, the carrier-to-noise density ratio is related to the gain-to-temperature ratio via or [C/ N 0 ] = [EIRP] + [G/T] [L] - [κ] [C/ N 0 ] = [EIRP] + [G/T] [L] SATELLITE TRACKING, TELEMETRY AND COMMAND 34

35 Modulation Modulating baseband (low frequency or digital) signal Modulator Carrier (high frequency) Modulated waveform Modulation can either be analog modulation or digital modulation. Trends Analog modulation Digital modulation Multiple access Digital modulation More information capability Compatibility with digital data services Higher data security Better quality communication Quick system availability SATELLITE TRACKING, TELEMETRY AND COMMAND 35

36 Analog Modulation Modulation: baseband signal RF waveform RF waveform: A cos(ωt+φ) where ω is the carrier frequency. Amplitude modulation (AM): vary A with baseband signal Frequency modulation (FM): vary dφ/dt with baseband signal Phase modulation (PM): vary φ with baseband signal SATELLITE TRACKING, TELEMETRY AND COMMAND 36

37 Digital Modulation Methods: ASK (Amplitude shift Keying) FSK (Frequency shift keying) PSK (Phase shift keying) QPSK (Quadrature phase shift keying) SATELLITE TRACKING, TELEMETRY AND COMMAND 37

38 Data Rate and Bit Energy clock Digital data Bit period T b The bit energy E b is the energy of the signal over one bit period. It is the product of received carrier (signal) power and the bit period. In db, [E b ] = [C] + [T b ] The data rate R b in bit per second is the inverse of bit period T b. Thus, [E b ] = [C] - [R b ] SATELLITE TRACKING, TELEMETRY AND COMMAND 38

39 Bit Energy to Noise Ratio, E b / N 0 For a digital system, the bit energy-to-noise ratio is related to the carrier-to-noise density as follows [E b / N 0 ] = [C/N] + [B] [R b ] = [C/ N 0 ] [R b ] where R b is the bit rate and B is the noise bandwidth of the receiver. The ratio E b / N 0 is crucial in determining the bit error rate, which depends also on the digital modulation technique. In practice, The bit error rate is specified The modulation scheme is determined and the corresponding E b / N 0 is computed The implementation margin is specified The carrier-to-noise density ratio [C/ N 0 ] is determined SATELLITE TRACKING, TELEMETRY AND COMMAND 39

40 Bit Error Rate and E b / N 0 SATELLITE TRACKING, TELEMETRY AND COMMAND 40

41 Link Budget Analysis 400 MHz MHz Transmitter power 1W 30 dbm 30 dbm Modulation loss -1.0 db -1.0 db Spacecraft cable & filter losses -0.5 db -0.5 db Spacecraft antenna gain 15.9 db 15.9 db Path loss db db Polarization loss -0.5 db -0.5 db Receiver power db db Receiver antenna gain 0.0 db 0.0 db Bit rate, 1000 bps 30.0 db-hz 30.0 db-hz Energy/bit, E b dbmj dbmj Noise density, N dbm/hz dbm/hz Received E b / N db 1.8 db E b / N 0 required for 10-5 bit error 9.6 db 9.6 db Typical implementation loss 1.4 db 1.4 db Required margin 3.0 db 3.0 db Transmitter power shortage -3.8 db db Total required power 33.8 dbm (2.4W) 42.2 dbm (17W) SATELLITE TRACKING, TELEMETRY AND COMMAND 41

42 Link Design Earth station Geographical location e rain fades, look angle, path loss Transmit antenna gain and power e earth station EIRP Receive antenna gain e G/T of the earth station Inter-modulation noises e C/N Equipment characteristics e additional link margin Satellite Satellite orbit e coverage region and earth station look angle Transmit antenna gain and radiation pattern e EIRP and coverage area Receive antenna gain and radiation pattern e G/T and coverage area Transmitted power e satellite EIRP Transponder gain and noise characteristics e EIRP and G/T Inter-modulation noise e C/N Channel Operating frequency e path loss and link design Modulation/coding characteristics e required C/N Propagation characteristics e link margin and modulation/coding design Inter-system noise e link margin SATELLITE TRACKING, TELEMETRY AND COMMAND 42

43 Telemetry System Telemetry system: Collect data at a place (say microsatellite) Encode, modulate, and transmit the data to a remote station (say ground) Receive the data (on the ground) Demodulate, decode, record, display, and analyze the data Data collection: sensors, signal conditioners Analog multiplexer & analog-to-digital converter Data Formatter Modulator, transmitter, antenna Time tag Digital multiplexer On-board Storage channel Data processing and display Synchronizer & Demultiplexer Antenna, receiver, demodulator SATELLITE TRACKING, TELEMETRY AND COMMAND 43

44 Telemetry Data Collection Data acquisition Sensor and transducer Signal conditioner: may be passive or active Amplification, attenuation Buffering: provide impedance Power supply Noise filtering Load protection Automatic gain control Data to collect: measurements and status of health Power functions Telemetry functions Telecommand functions Attitude control functions Propulsion functions Structure functions Antenna functions Tracking functions Payload functions Miscellaneous functions Acceleration, velocity, displacement Angular rate, angular position Pressure Temperature Density Resistance Voltage, current Intensity Electric field, magnetic field SATELLITE TRACKING, TELEMETRY AND COMMAND 44

45 Multiplexing When a series of input signals from different sources have to be transmitted along the same physical channel, multiplexing is used to allow several communication signals to be transmitted over a single medium. Frequency division multiplexing (FDM) FDM places multiple incoming signals on different frequencies. Then are they are all transmitted at the same time The receiving FDM splits the frequencies into multiple signals again Time division multiplexing (TDM) TDM slices multiple incoming signals into small time intervals Multiple incoming lines are merged into time slices that are transmitted via satellite The receiving TDM splits the time slices back into separate signals SATELLITE TRACKING, TELEMETRY AND COMMAND 45

46 FDM signal 1 carrier f 1 FM modulator Summer FDM signal signal 2 FM modulator carrier f 2 A multi-tone signal is formed Must consider Frequency plan Pre-emphasis signal N FM modulator carrier f N IRIG standard: Proportional bandwidth (PBW): peak frequency deviation of the subcarrier is proportional to the subcarrier frequency Constant bandwidth (CBW): the deviation is constant CCITT multiplexing scheme: FDM telephone signals SATELLITE TRACKING, TELEMETRY AND COMMAND 46

47 TDM signal 1 slot sync signal 2 Commutator frame Multiplexer TDM bit stream signal N Timing Frame sync A frame of data is formed for transmission Sync word Data words (slots) Error check words Must consider Sampling rate Slow and fast measurement data Resolution and bit rate Frame rate SATELLITE TRACKING, TELEMETRY AND COMMAND 47

48 PCM Telemetry Sensor 1 Timing & frame sync Sensor 2 Commutator Sample & Hold Encoder Digital multiplexer Bit sequence Sensor N SATELLITE TRACKING, TELEMETRY AND COMMAND 48

49 PCM Frame A structure that routes the sensor data to the proper channels at the ground stations Contains: major frames and minor frames Each minor frame: sync + (N-1) data words Each major frame: M minor frames Minor frame sync * * N-1 sync M Major frame sync 1 sync 1 SATELLITE TRACKING, TELEMETRY AND COMMAND 49

50 A Typical Telemetry Frame SATELLITE TRACKING, TELEMETRY AND COMMAND 50

51 PCM Commutator Commutator: cycle through and sample each sensor Supercommutation: samples a parameter at a rate that is higher than the frame rate Subcommutation: samples a parameter at an integer submultiple of the frame rate sync a 8 sync b 8 sync c 8 sync d 8 sync e 8 supercommutation subcommutation SATELLITE TRACKING, TELEMETRY AND COMMAND 51

52 PCM Frame Synchronization Synchronization is made possible through synchronization word (sync), which is a unique sequence of 1 s and 0 s. Recommended sync word (IRIG ) Length Pattern Length Pattern SATELLITE TRACKING, TELEMETRY AND COMMAND 52

53 PCM Waveforms NRZ-L (non-return to zero level): one is represented by logic 1; zero is represented by logic 0. NRZ-M (mark): one is represented by a change in level at start of clock; zero is represented by no change in level at start of clock. NRZ-S (space): one is represented by no change in level at start of clock; zero is represented by a change in level at start of clock. BiΦ-L (biphase level): one is represented by a 1-to-0 change at mid-clock; zero is represented by a 0-to-1 change at mid-clock. BiΦ-M: one is represented by a change at mid-clock; zero is represented by no change at midclock. BiΦ-S: one is represented by no change in mid-clock; zero is represented by a change in mid-clock SATELLITE TRACKING, TELEMETRY AND COMMAND 53 data clock NRZ-L NRZ-M NRZ-S BiΦ-L BiΦ-M BiΦ-S

54 Telecommand Telecommand system: allows instruction and/or data to be sent to the spacecraft. Commands may be Relay commands Data commands Delayed commands Command system design considerations Orbit influence on link design, ground coverage Need for delayed commands, data commands Length of command message Component choices Radiation does, soft errors, latchup, shielding Redundancy Autonomy Environmental considerations SATELLITE TRACKING, TELEMETRY AND COMMAND 54

55 Telecommand System Antenna Power Command Receiver switching processor Spacecraft unit subsystems Antenna Often omni for LEOs Receiver On-board On-board Continuously on computer storage Decoder Validation of command Validation of spacecraft address Decryption Recovery of clock and data Command processor Command interpretation and validation Interface to on-board units for proper actions Power switching Interface circuitry between command logic and spacecraft subsystems SATELLITE TRACKING, TELEMETRY AND COMMAND 55

56 Telemetry Channel Coding Coding system Reed-Solomon Short Constraint Periodic NRZ-L to M Encoder and Length Convolutional Conversion Interleaver Convolutional Interleaver Encoder Modulator and RF Reed-Solomon Decoder and De-Interleaver NRZ-M to L Conversion Viterbi Decoder Periodic Convolutional Interleaver Demodulator and RF Benefits of channel coding Higher overall data throughput at the same overall quality (bit error rate) Lower overall bit error rate using the same energy per information bit Amenable to data compression, adaptive telemetry, and anomaly exclusion SATELLITE TRACKING, TELEMETRY AND COMMAND 56

57 Coding and Decoding Coding: a technique of protecting message signals from signal impairment by adding redundancy to the message signal. In power limited link, the desired fidelity in communication quality can only be achieved through coding Coding helps minimize the error rate Coding can be used to achieve better utilization of the channel capacity k information bits Coder (k+r) coded bits k reconstructed information bits Decoder (k+r) received bits Syndrome SATELLITE TRACKING, TELEMETRY AND COMMAND 57

58 Channel Coding Performance Performance of channel coding SATELLITE TRACKING, TELEMETRY AND COMMAND 58

59 AX.25 Amateur Packet-Radio Protocol AX.25 is a set of rules defining the format and content of packets and how they are handled. AX.25 is a data link layer protocol. Application layer Presentation layer Layer Functions Session layer Transport layer Data Link Segmenter Data Link Segmenter Management Data Link Data Link Link Multiplexer Management Data Link Network layer Physical Data link layer Physical Physical layer Silicon/Radio SATELLITE TRACKING, TELEMETRY AND COMMAND 59

60 AX.25 Data Link Functions Segmenter: Accepts input from higher layer Breaks down data unit for transmission Data link: Provides all logic necessary to establish and release connections between two stations and to exchange information in a connectionless and connection-oriented manner. Management data link: Provides all logic necessary to negotiate operating parameters between two stations. Link multiplexer: Allows one or more data links to share the same physical channel. DL request DLSAP (service access point) DL indication DL confirm DL response SATELLITE TRACKING, TELEMETRY AND COMMAND 60

61 AX.25 In Action message TNC packet Radio RF wave Packet radio allows several simultaneous point to point connections to share the same frequency. Transmission TNC builds a packet (in accordance with AX.25 protocol) Wait for radio silence and transmit Reception TNC monitors incoming packets and identifies addressed packets. Examples: APRS Link layer packet radio transmissions are sent in frames. Each frame is made up of several fields. Three types of frames: S frame: supervisory link control (acknowledge) I frame: information U frame: unnumbered (establish or terminate link) SATELLITE TRACKING, TELEMETRY AND COMMAND 61

62 AX.25 Frames Frame arrangement for U or S frames flag address control FCS flag Identifies both the source and destination of the frame 112 or 560 bits Identifies frame type 8 bits Frame check sequence 16 bits Frame arrangement for I frame flag address control PID Information FCS flag Protocol identifier 8 bits Information N x 8 bits SATELLITE TRACKING, TELEMETRY AND COMMAND 62

63 Summary Satellite TT&C subsystem is an important and indeed essential subsystem in a satellite. Tracking is to know the satellite on ground Telemetry is to obtain satellite information on ground Command is to active satellite operation on ground Key parameters EIRP C/N G/T E b /N 0 Link budget analysis to ensure that satellite can communicate with the ground station Modulation is needed in satellite communication Some coding schemes and protocols have been discussed SATELLITE TRACKING, TELEMETRY AND COMMAND 63

64 Further Readings P. Fortescue and J. Stark, Spacecraft Systems Engineering, Chapters 13 & 14, John Wiley, B. Razavi, RF Microelectronics, Prentice Hall, M. Richharia, Satellite Communication Systems, McGraw- Hill, D. Roddy, Satellite Communications, McGraw-Hill, J. G. Proakis, Digital Communications, McGraw-Hill, Satellite link budget calculation can be found in AX.25 can be found in either or SATELLITE TRACKING, TELEMETRY AND COMMAND 64

65 Homework and Final Project Please do the following problems. What is the average distance between moon and the earth? Can you compute the free path loss when the frequency is 10 GHz. Suppose that the bit rate of a digital radio is 9600 bps. How long does it take to transmit a file of the size 1 Mbytes? Can the data be transmitted in one pass for a low-earth orbiting satellite at altitude 600 km in one pass? Final project will be announced by Professor Lin The module GW200B will be used to establish a two-way communication. Start the project as early. SATELLITE TRACKING, TELEMETRY AND COMMAND 65