Tracking, Telemetry and Command

Transcription

1 Tracking, Telemetry and Command Jyh-Ching Juang ( ) Department of Electrical Engineering National Cheng Kung University juang@mail.ncku.edu.tw 1 November, 2004

2 Purpose Understand the functions of telemetry, tracking, and command (TT&C) system in spacecraft engineering. Understand basic communication systems and operations. Learn to perform fundamental analyses in spacecraft communication. Understand various telemetry and telecommand designs and standards. Be prepared for the design of pico-sat TT&C 2

3 Scope Function of TT&C: provides the means of monitoring and controlling the satellite operations. Scientific Instruments Data recorder Data processor telemetry data Spacecraft antenna Data handling unit Command decoder command Attitude & Power Thermal orbit control control control subsystem unit unit 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 4

5 Contents Satellite communication overview Techniques of radio communications Radio wave Antenna Propagation Link budget Noise Modulation Multiple access Telemetry system Telecommand system Tracking system Protocol: AX.25 5

6 Satellite Communication galaxy sun ionosphere troposphere 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 7

8 Communication System Transmission Antenn Coder Modulator Up converter Power Amplifier Reception Antenna desired format desired spectrum desired strengt Low Down Noise Demodulator converter Amplifier Decoder 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 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 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 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 H Horizontal polarization Vertical polarization Right-handed circular polarization Left-handed elliptical polarization 12

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

14 Frequency Allocation 14

15 Space TT&C Spectrum Frequency band (MHz) 136 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 7155 Space operation Direction indicator Space-Earth Space-Earth Earth-Space Space-Earth Space-Earth Space-Earth Space-Earth Earth-Space Earth-Space Space-Earth Earth-Space Space-Earth Earth-Space Allocation status Secondary Primary Secondary Primary Secondary Primary Primary Primary Frequency band 136 ~ 137 MHz 137 ~ 138 MHz ~ 401 MHz 2025 ~ 2110 MHz 2200 ~ 2290 MHz 7190 ~ 7235 MHz 8450 ~ 8500 MHz ~ 13.4 GHz 13.4 ~ 14.3 GHz 14.4 ~ GHz 14.5 ~ GHz 31.0 ~ 31.3 GHz 31.8 ~ 32.3 GHz 34.7 ~ 35.2 GHz 65.0 ~ 66.0 GHz Direction indicator Space-Earth Space-Earth Space-Earth Earth-Space Space-Earth Earth-Space Space-Earth Earth-space none Space-Earth none none Space-Earth none none Allocation status Secondary Primary Primary Primary Secondary Secondary Secondary Secondary Secondary Secondary Primary 15

16 Propagation of Radio Waves Propagation properties Reflection Refraction Diffraction Absorption Scattering Transmission path Ground wave Direct (line-of-sight) wave Sky wave 16

17 Atmosphere and Beyond 17

18 Tropospheric Effects Due to the presence of gas molecules (water vapor) in the air, the propagation velocity is less than the speed of light, giving the tropospheric refraction. The refractivity depends on the atmospheric pressure, partial pressure of water, and dry temperature. Effects on radio signals Gaseous absorption Attenuation due to hydrometers Attenuation from cloud and fog Tropospheric scintillation Depolarization 18

19 Ionospheric Effects Formation of the ionosphere (50 ~ 600 km above the Earth) Construction of the ionosphere D layer (50 ~ 90 Km) E layer (90 ~ 130 Km) sun F layer (130 Km upwards) Variations Diural: Earth s rotation Seasonal: revolution Geographic Cyclic: sunspot activity day Effects on radio signals Fading Group delay Polarization rotation night Phase advance Angular refraction Amplitude and phase scintillation F2 layer 300 Km F1 layer 200 Km E layer 110 Km D layer 60 Km 19

20 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) 20

21 Communication Link Analysis Quantities in link analysis EIRP 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) G/T Frequency f (Hz) or wavelength l (m) E Bit rate R (bps, bit per second) b / N 0 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. 21

22 Antenna Types Dipole Horn Helical Yagi Parabolic Antenna array 22

23 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 l 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 23

24 Gain and Effective Area Type of antenna Isotropic Gain 1 Effective area l 2 /4p Elementary dipole (l 2 /4p) Halfwave dipole Horn Parabolic reflector Broadside array A/l 2 ( ) A/l 2 4pA 2 /l (l 2 /4p) 0.81 A ( ) A A 24

25 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 4p steradians (square radians). Isotropic antenna directional antenna 25

26 Antenna Pattern and Beamwidth The antenna gain is a function of the pointing angle. The half power (3dB) beamwidth of an antenna is given by θ 3dB = 70 λ/d (degree). The antenna gain and beamwidth depend on the frequency. 26

27 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 ]. 27

28 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 ) 28

29 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 (4pr/l) where r is the distance of travel and l 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) 29

30 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 ] 30

31 Atmospheric Attenuation 31

32 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 Free-space gain in db loss in db EIRP in dbw Other losses in db 32

33 Link Budget Example A transmitter with power 6 W and antenna gain 35 db. Its EIRP in dbw is [EIRP] = 10 log = 42.8 dbw. For a 2-GHz (S-band) ground antenna with 1 m in diameter with 50% efficiency, what is its gain? The wavelength is λ = c/f = (3.0 x 10 8 )/(2.0 x 10 9 ) = 0.15 m. The gain is G r = 4πAη/(λ 2 ) = 4π x 1 2 x 0.5/( ) = 279 = 24.5 db. The range between a ground station and a satellite is km. The free space loss at a frequency of 2 GHz is 20 log(4πr/λ) = 20 log 10 (4π x 3.6 x 10 7 /0.15) = db Neglecting other attenuations, what is the received power for the above setup? [C] = [EIRP] + [G r ] 20 log 10 (4πr/λ) = = dbw 33

34 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. 34

35 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 35

36 Equivalent Noise Temperature For an amplifier of gain G, The input noise energy coming from the antenna is N 0,ant = kt 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 = Gk(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 = k(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 36

37 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 k (T ant + T E1 ) + k T E2 The noise energy referred to amplifier 1 input is N 0,1 = N 0,2 /G 1 = k (T ant + T E1 +T E2 /G 1 ) A system noise temperature T S is defined as N 0,1 = k 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. 37

38 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 Cable Receiver 50 db -20 db K 0 K K 38

39 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)

40 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 [C/ N 0 ] = [EIRP] + [G/T] [L]-[κ] or [C/ N 0 ] = [EIRP] + [G/T] [L]

41 Link Budget Calculation Transmitter output power Multiple carrier loss Transmitter circuit loss Transmitted carrier power Transmitting antenna gain EIRP Space loss Polarization loss Atmospheric and multi-path losses Total transmission loss Ground terminal G/T Boltzmann s constant Received C/ N dbw db db dbw db dbw db db db db db/ 0 K dbj/ 0 K db-hz 41

42 Link Budget Calculation The downlink frequency is 11.7 GHz and the wavelength λ is λ = 3.0 x10 8 / 11.7 x10 9 = m. The minimum elevation angle is 5 0. For a geostationary satellite at an altitude km, the slant range r is km. The free space loss is thus [L S ] = 20 log 10 (4πr/λ) = 20 log 10 (4πx4.1338x10 7 /0.256) = db Nadir angle Elevation angle Local horizon km 6378 km 42

43 Link Budget Calculation Received C/ N db-hz 3.89 x10 5 Transmitted C/ N db-hz 8.32 x10 6 Resultant C/ N db-hz 3.72 x10 5 Carrier to intermod density ratio C/I db-hz 5.01 x10 6 Overall C/ N db-hz 3.47 x10 5 Required C/ N db-hz Link margin 0.1 db 1/ 3.89 x10 5 = 1/ 8.32 x / 3.72 x

44 Exercise on Link Calculation Data and assumptions: Ground transmitter frequency: 6 GHz Ground transmitter power: 300 watts Ground antenna diameter: 5 m Ground antenna efficiency: 0.55 Ground receiver noise temperature: K Ground receiver bandwidth: 40 MHz Spacecraft transmitter frequency: 4 GHz Spacecraft transmitter power: 200 watts Spacecraft antenna diameter: 1 m Spacecraft antenna efficiency: 0.5 Spacecraft receiver noise temperature: K Spacecraft receiver bandwidth: 40 MHz Distance between the ground station and the spacecraft: km Determine the downlink and uplink carrier-to-noise ratios 44

45 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 45

46 Analog Modulation Modulation: baseband signal RF waveform RF waveform: A cos(wt+f) where w is the carrier frequency. Amplitude modulation (AM): vary A with baseband signal Frequency modulation (FM): vary df/dt with baseband signal Phase modulation (PM): vary f with baseband signal 46

47 Digital Modulation Methods: ASK (Amplitude shift Keying) FSK (Frequency shift keying) PSK (Phase shift keying) QPSK (Quadrature phase shift keying)

48 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 ] 48

49 Issues in Digital Modulation Schemes Transmission rate and bandwidth efficiency QPSK is twice as efficient as BPSK in terms of bandwidth usage. Bit error rate The probability of bit error, typical in the range of 10-5 to 10-7 Depends on the modulation and detection schemes Carrier recovery Bit timing recovery Coherent and noncoherent detections 49

50 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 50

51 Bit Error Rate and E b / N 0 51

52 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) 52

53 Link Design Earth station Geographical location rain fades, look angle, path loss Transmit antenna gain and power earth station EIRP Receive antenna gain G/T of the earth station Inter-modulation noises C/N Equipment characteristics additional link margin Satellite Satellite orbit coverage region and earth station look angle Transmit antenna gain and radiation pattern EIRP and coverage area Receive antenna gain and radiation pattern G/T and coverage area Transmitted power satellite EIRP Transponder gain and noise characteristics EIRP and G/T Inter-modulation noise C/N Channel Operating frequency path loss and link design Modulation/coding characteristics required C/N Propagation characteristics link margin and modulation/coding design Inter-system noise link margin 53

54 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 Antenna, processing Synchronizer & receiver, and display Demultiplexer demodulator 54

55 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 55

56 Telemetry Data Type Signals can be classified as Analog (Continuous-time) signal Discrete-time signal: the independent variable (time) becomes discrete Digital signal: the dependent variable becomes discrete Benefits of digital data in processing, storage, and transmission Little distortion Predictability Reliability Programmability Inexpensive 56

57 Sampling and Data Conversion Sampling: Analog signal Discrete-time signal Must determine the sampling rate (samples per second) The sampling rate must satisfies the Nyquist sampling theorem. Typically, the sampling rate is ten times the signal bandwidth. Quantization: Discrete-time signal Digital signal Must consider the resolution (bits per sample) When the number of bits is low, the representation is subject to quantization errors. A high resolution, however, implies fast computation speed and large memory requirement Signal type Analog bandwidth (khz) Nyquist sampling rate (khz) Typical resolution (bits) Nyquist digital rate (kbps) Voice Audio Video (B&W) Video (color)

58 Sampling Theorem Nyquist sampling theorem: a continuous-time signal can be properly sampled, only if it does not contain frequency components above one-half of the sampling rate. Aliasing: if the sampling frequency is too small, a false frequency appears in the reconstructed signal. A sampling rate of 1000 samples/second the frequencies of the analog signal must below 500 cycles/second (Hz). Sampling rate > signal bandwidth No aliasing effects Sampling rate < signal bandwidth Alias! 58

59 Data Conversion Infinitely precise analog value Finitely precise digital value Digital precision is characterized in terms of step size Bits per sample = log 2 (Dynamic range/step size) The two most important characteristics of a converter are speed and accuracy. The resolution of an A/D is expressed as the number of bits in its digital output. For an A/D with an n-bit resolution has 2 n possible digital codes. Hence 1LSB = FSR/(2 n -1) where FSR is the full-scale range. More bits Higher resolution Less susceptible to noises 59

60 Digital Modulation of Waveform Pulse amplitude modulation (PAM): varying amplitudes of pulses, each pulse amplitude representing a portion of analog waveform (data). Pulse duration modulation (PDM): the width of each pulse is modulated in accordance with the data. Pulse position modulation (PPM): the position of each Analog pulse is measured in units waveform of time relative to a fixed reference mark; the time PAM span is in proportion to the data s magnitude. Pulse code modulation PDM (PCM): the modulating signal is sampled, the sample amplitude is PPM converted into a binary code, and the binary code is transmitted in groups as a PCM 60 train of pulses.

61 Benefits of PCM PCM process: sampling, quantization, and encoding Digital representation: a signal sample is represented by a set of data bits. Noise immunity Alphanumeric data Error checking capability Easy to transmit, record, and analyze Standard: IRIG (Inter-Range Instrumentation Group) standard

62 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 62

63 FDM signal 1 carrier f 1 FM modulator Summer FDM signal signal 2 signal N carrier f 2 FM modulator FM modulator A multi-tone signal is formed Must consider Frequency plan Pre-emphasis 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 63

64 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 64

65 Multiple Access Allows several communication signals to be transmitted over a single medium Multiple access communication Uplink multiple access: allow many different ground stations to transmit up to the satellite at the same time. Downlink multiple access: allows a ground receiver of the satellite to separate any (or all) of the downlink carriers. Techniques: Frequency division multiple access (FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA) 65

66 Frequency Division Multiple Access Advantages: Mature and low cost No need of network timing No restriction on baseband signal type or modulation Disadvantages: Intermodulation noise Lack of flexibility in channel allocation Requires uplink power control to maintain link quality In a mix of traffic containing strong and weak carriers, the weak carriers tend to be suppressed. frequency time 66

67 Time Division Multiple Access Advantages: Maximum use can be made of the available satellite power since intermodulation noise is minimal. Uplink power control is not required. Transmission plans are easier to construct and modify. The digital format of TDMA permits utilization of all the advantages of digital techniques (digital speech interpolation, source and channel coding, error correction) Disadvantages: It requires network-wide timing synchronization hence it is relatively complex. Analog signals must be converted to digital form. Interface with analog terrestrial plant is expensive. frequency time 67

68 Code Division Multiple Access A spread spectrum technique in which all carriers transmit simultaneously and hence overlap their carrier waveform on top of each other in time and in frequency. Coding techniques used to serve as satellite identifiers and avoid interference between users. Typical CDMA schemes: direct sequence, frequency hopping frequency time 68

69 PCM Telemetry Sensor 1 Timing & frame sync Sensor 2 Commutator Sample & Hold Encoder Digital multiplexer Bit sequence Sensor N 69

70 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 sync * * N-1 M Major frame sync sync

71 A Typical Telemetry Frame 71

72 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 72

73 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

74 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 data clock NRZ-L NRZ-M NRZ-S BiΦ-L BiΦ-M 74 BiΦ-S

75 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 75

76 Telecommand System ntenna 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 76

77 Telemetry Channel Coding Coding system Reed-Solomon Encoder and Interleaver NRZ-L to M Conversion Short Constraint Length Convolutional Encoder Periodic Convolutional Interleaver Modulator and RF Reed-Solomon Periodic NRZ-M to L Decoder and Viterbi Decoder Convolutional Conversion De-Interleaver 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 77

78 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 78

79 Block Code A coded block comprising n bits consists of k information bits. The Hamming distance d is the minimum possible distance between two coded blocks. A coded has the capability of detecting all coded words having less than (d-1) bits in error. A code has a capability of detecting and correcting (d-1)/2 bits of error. (7,4) block code example: 7-bit code, 4-bit information. Information Codeword Information Codeword

80 Block Code Error Correction For the (7,4) block code. The information 1010 will result in codeword Suppose that the received code is subject to 1-bit error giving During decoding, the hamming distance is evaluated. The received code is corrected as and the information 1010 is recovered. 7-bit codeword information parity check Codeword Hamming distance

81 Channel Coding Performance Performance of channel coding 81

82 Channel Coding Techniques Block code Convolutional code A rate ½, constraint length 7 convolutional code with Viterbi (maximum likelihood) decoding scheme. Consists of a shift register of length six and some exclusive OR gates that implement the two parity checks. Periodic convolutional interleaving The PIC is to maintain specified performance of the link even in the presence of pulsed radio interference. Reed-Solomon code As an outer code to account for burst error. A (255,223) Reed-Solomon code is recommended. The longest codeword length is 255 symbols A 16 Reed-Solomon symbol error correction capability Two check symbol for each error to be corrected, resulting in 32 check symbols and 233 information symbols per codeword. 82

83 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 Segmenter Segmenter Session layer Management Management Data Data Link Data Data Link Data Link Link Link Transport layer Link Multiplexer Network layer Data link layer Physical layer Physical Physical Silicon/Radio 83

84 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. DLSAP (service access point) DL request DL indication DL confirm DL response 84

85 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) 85

86 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 Frame arrangement for I frame Identifies frame type 8 bits Frame check sequence 16 bits flag address control PID Information FCS flag Protocol identifier 8 bits Information N x 8 bits 86

87 Tracking Tracking stations are required to Receive spacecraft telemetry and route to control center Uplink commands from control centers Obtain Doppler range rate data and spacecraft azimuth and elevation for orbit determination Available stations and systems have widely varying locations, capabilities, and availability. Frequency bands supported may be S, C, X, Ku, or Ka bands. Tracking networks GSFC STDN GSFC TDRSS JPL DSN DMA TRANET USAF SGLS USN NAG NORAD SPADATS 87

88 TDRSS An orbiting communications satellite, developed by NASA, used to relay data from satellite sensors to ground stations and to track the satellites in orbit. 88

89 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, Other relevant lecture notes are Jer-Nan Juang, Tracking, Telemetry, and Command, 2001, Spacecraft Communication Systems, 2002, CubeSat Spacecraft Telemetry, Tracking, and Command Subsystem Design, 2002 Satellite link budget calculation can be found in AX.25 can be found in either or 89