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Satellite Tracking, Telemetry and Command Design Basis. Jyh-Ching Juang ( 莊智清 ) Department of Electrical Engineering National Cheng Kung University email@example.com. November, 2008. Purpose. Understand the functions of satellite telemetry, tracking, and command (TT&C) subsystem.
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Satellite Tracking, Telemetry and CommandDesign Basis Jyh-Ching Juang (莊智清) Department of Electrical Engineering National Cheng Kung University firstname.lastname@example.org November, 2008
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.
Scope • Function of TT&C: provides the means of monitoring and controlling the satellite operations. Scientific Instruments antenna telemetry data Data recorder Data processor Transceiver Command decoder Data handling unit command Thermal control subsystem Power control subsystem Attitude & orbit control subsystem
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. telemetry command tracking communication
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 Communication galaxy sun ionosphere troposphere
GEO satellites LEO satellites 36000 km 600 km 6378 km 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
Communication System • Transmission Antenna Power Amplifier Coder Modulator Up converter desired format desired spectrum • Reception desired strength Antenna Low Noise Amplifier Down converter Demodulator Decoder
Principle of conservation of energy Electric Gauss’s law Magnetic Gauss’s law Faraday’s law Ampere’s law Maxwell’s equations 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.
H 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 Direction of propagation
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 period phase
Right-handed circular polarization Left-handed elliptical polarization 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 108 m/sec. • The velocity c (in m/sec) is related to the frequency f (in Hz) and wavelength l (in m) by c = fl. • 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 H E Horizontal polarization Vertical polarization
Space TT&C Spectrum • Space operation
Decibel Representation • Decibel representation: a quantity P in decibels (dB) is defined as P in dB = [P] = 10 log10(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 log10(power in watts/one watt). • Power in dBm = 10 log10(power in milli-watts/one milli-watt). • 0.1 watts is equivalent to -10 dBW or 20 dBm • Boltzmann’s constant k = 1.38 x 10-23 J/0K = 1.38 x 10-23 W/Hz/0K = -228.6 dBW/Hz /0K. • A frequency of 22 GHz is equivalent to 103.4 dB-Hz 103.4 dB-Hz = 10 log10(22 x 109 Hz/1 Hz) • A noise temperature of 300 0K is the same as 24.8 dB-0K 24.8 = 10 log10(300)
Communication Link Analysis EIRP • Quantities in link analysis • Transmit power P (dBW) • Antenna gain G (dBi) • Received carrier power C (dBW) • Noise temperature T (0K) • Dissipative loss L (dB) • Slant range r (m) • Frequency f (Hz) or wavelength l (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/ N0: carrier to noise power (density) ratio, a measure of received signal quality. • G/T: gain to temperature ratio, figure of merit of the receiver. • Eb/ N0: energy per bit to noise power density, a measure related to the bit error rate in digital transmission. C/N0 G/T Eb/ N0
Antenna Types • Dipole • Horn • Helical • Yagi • Parabolic • Antenna array
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: 0.13l2 • Efficiency h: 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 = 4phA/l2 where A is the aperture, h 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
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
Equivalent Isotropic Radiated Power • Let Pt be the transmitter power and Gt be the transmitter antenna gain, then the equivalent isotropic radiated power (EIRP) is the product of Pt and Gt, i.e., EIRP = Pt x Gt. • In terms of dB, [EIRP] = [Pt] + [Gt].
Signal or Carrier Power • At a distance r from the transmitter, the power flux density is S= EIRP/(4pr2)= PtGt /(4pr2) • If atmospheric attenuation results in power loss by a factor LA, then the flux density at the receiver is S= PtGt /(4pr2 LA) • Let Ar be the effective aperture of the receiving antenna with efficiency h, then the received power is C= S Arh = (EIRP)Arh/(4pr2 LA) • As the antenna gain is Gr = 4pArh/(l2) where l is the wavelength • Thus, the signal power at the input to the receiver is C= EIRPGr (l/(4pr))2 (1/LA)
Free Space Loss • Free space loss: loss due to the spreading of electromagnetic wave. • The free space loss is LS = (4pr/l)2 • In terms of dB, the free space loss is [LS] = 20 log10(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 [LS] = 92.45 + 20 log10(f) + 20 log10(r) For example, for a geostationary satellite, r = 36000 km, the free space loss in dB is [LS] = 183.58 + 20 log10(f)
Losses in Communication Link • The free-space loss [LS] = 20 log10(4pr/l) 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 LA term, that is [LA] = [Lfeeder] + [Lpointing] + [Latmosphere] + … • The overall loss is thus [L] = [LS] + [LA]
Link Budget • Recall that the received signal power is C= EIRP Gr (l/(4pr))2 (1/LA) • In terms of dB, [C] = [EIRP] + [Gr] – [LS] – [LA] Received power in dBW Other losses in dB Antenna gain in dB Free-space loss in dB EIRP in dBW
Link Budget Example • A transmitter with power 2 W and antenna gain 3 dB. Its EIRP in dBW is [EIRP] = 10 log10 2 + 3 = 6.01 dBW. • Suppose that the satellite is flying at 600 km in altitude, with an elevation limit of 10o, what is the maximum transmission distance? • The slant range is 1932.3 km • Suppose that the frequency is 430 MHz, the free space loss is [LS ] = 92.45 + 20 log10 (f) + 20 log10 (r) = 150.84 dB • Suppose that the receiver antenna gain is 6 dB, the received carrier power is [C] = [EIRP] + [Gr] – [LS] = 6.01 + 6 – 150.84 = -138.83 dBW Elevation angle Nadir angle 600 km 6378 km
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 Pn = kTB where T is the equivalent noise temperature (in 0K), B is the equivalent noise bandwidth (in Hz), and k = 1.38 x 10-23 J/0K is Boltzmann’s constant. • The noise power spectral density N0 = Pn/B = kT. • 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.
Noise Sources • Contributions of system noise: sky, ground, galaxy, circuit, and medium. • Non-thermal noises are characterized in terms of noise temperature. • Sun: (104 -10100K) communication is effectively impossible with sun in the field of view. • Moon: reflected sunlight • Earth: (254 0K) • Galaxy: negligible above 1 GHz • Sky: (30 0K) • Atmosphere: noise radiated by O2 and H2O, less than 50 0K • Weather: clouds, fogs, and rain • Electronics noise: receiving equipment
Equivalent Noise Temperature • For an amplifier of gain G, • The input noise energy coming from the antenna is N0,ant = kTant. • The output noise energy N0,out is the sum of GN0,out and the noise induced in the amplifier. N0,out = Gk(Tant + TE) where TE is the equivalent input noise temperature for the amplifier. • The total noise referred to the input is N0,in = k(Tant + TE) . • The typical value of TE is in the range 35 to 100 0K. Tant N0,in N0,out Amplifier power gain G
System Noise Temperature Tant • The total noise energy referred to amplifier 2 input is N0,2 = G1 k(Tant + TE1) + kTE2 • The noise energy referred to amplifier 1 input is N0,1 = N0,2/G1 =k(Tant + TE1 +TE2/G1) • A system noise temperature TS is defined as N0,1 = k TS. Hence, TS = Tant + TE1 +TE2/G1 • 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. N0,1 N0,2 N0,out Amplifier G1 , TE1 Amplifier G2 , TE2
Noise Temperature Example • Determine the system noise temperature at the input to the LNA when • Antenna noise temperature Tant = 35 0K • Waveguide feeder gain = -0.25 dB (0.944), temperature = 290 0K • LNA gain = 50 dB (10000), temperature = 75 0K • Cable gain = -20 dB (0.01), temperature = 290 0K • Receiver noise temperature = 2000 0K • The system noise temperature TS is TS = 35 x 0.944 + 290 x (1-0.944) + 75 + 290/10000 + 2000/(10000 x 0.01) = 126 0K 35 0K LNA 50 dB 75 0K TS Waveguide -0.25 dB 290 0K Cable -20 dB 290 0K Receiver 2000 0K
Carrier-to-Noise Density Ratio, C/N0 • The performance of a satellite link is often measured in terms of [C/N] or [C/ N0] . • The carrier-to-noise ratio is defined as the difference between the received carrier power and the noise power in dB [C/N] = [C] - [Pn] • The carrier-to-noise density ratio is [C/ N0] = [C] – [N0]. Thus, [C/ N0] = [C/N] + [B] in dB-Hz. • For a system temperature TS, the noise power density referred to the receiver input is N0 =kTS and the noise power Pn =kTS B. • Recall that [C] = [EIRP] + [Gr] – [LS] – [LA].Thus, [C/N] = [EIRP] + [Gr] – [LS] – [LA]–[k] – [TS] – [B] and [C/ N0] = [EIRP] + [Gr] – [LS] – [LA]–[k] – [TS] • The signal-to noise power-density ratio is indeed C/N0 = EIRP(l/(4pr))2 (1/LA) (Gr /TS) (1/k) • If only spreading loss is considered, [C/ N0] = [EIRP] + [Gr] – [TS] - 20 log10(4pr/l) + 228.6
Gain-to-Temperature Ratio, G/T • The G/T ratio (gain-to-temperature ratio) is a key parameter in specifying the receiving system performance. [G/T] = [Gr] - [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/ N0] = [EIRP] + [G/T] – [L] - [k] or [C/ N0] = [EIRP] + [G/T] – [L] + 228.6
Modulation Modulating baseband (low frequency or digital) signal Modulator Modulated waveform • Modulation can either be analog modulation or digital modulation. • Trends • Digital modulation • More information capability • Compatibility with digital data services • Higher data security • Better quality communication • Quick system availability Carrier (high frequency) Analog modulation Digital modulation Multiple access
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
Digital Modulation • Methods: • ASK (Amplitude shift Keying) • FSK (Frequency shift keying) • PSK (Phase shift keying) • QPSK (Quadrature phase shift keying) 0 1 0 0 1
Data Rate and Bit Energy clock • The bit energy Eb 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, [Eb] = [C] + [Tb] • The data rate Rb in bit per second is the inverse of bit period Tb. Thus, [Eb] = [C] - [Rb] Digital data 0 0 0 0 1 1 Bit period Tb
Bit Energy to Noise Ratio, Eb/ N0 • For a digital system, the bit energy-to-noise ratio is related to the carrier-to-noise density as follows [Eb/ N0] = [C/N] + [B] – [Rb] = [C/ N0] –[Rb] where Rb is the bit rate and B is the noise bandwidth of the receiver. • The ratio Eb/ N0 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 Eb/ N0 is computed • The implementation margin is specified • The carrier-to-noise density ratio [C/ N0] is determined
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
Analog multiplexer & analog-to-digital converter Data Formatter Digital multiplexer Data collection: sensors, signal conditioners Data processing and display Modulator, transmitter, antenna Antenna, receiver, demodulator On-board Storage Time tag channel Synchronizer & Demultiplexer 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
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
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
FDM signal 1 FM modulator Summer FDM signal carrier f1 • 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 signal 2 FM modulator • A multi-tone signal is formed • Must consider • Frequency plan • Pre-emphasis carrier f2 signal N FM modulator carrier fN
TDM sync signal 1 Commutator Multiplexer slot • 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 signal 2 frame TDM bit stream signal N Timing Frame sync
Sensor 1 Sensor 2 Commutator Sample & Hold Digital multiplexer Encoder Sensor N PCM Telemetry Timing & frame sync Bit sequence
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 Major frame M