Satellite Tracking, Telemetry and Command Design Basis

1
Satellite Tracking, Telemetry and CommandDesign
Basis

• Jyh-Ching Juang (???)
• Department of Electrical Engineering
• National Cheng Kung University
• juang_at_mail.ncku.edu.tw

November, 2008
2
Purpose

• Understand the functions of satellite telemetry,
  tracking, and command (TTC) subsystem.
• Understand basic communication principles and
  operations.
• Learn to perform fundamental analyses in
  spacecraft communication.
• Be prepared for the design of TTC final project.

3
Scope

• Function of TTC provides the means of
  monitoring and controlling the satellite
  operations.

Scientific Instruments
antenna
telemetry data
Data processor
Data recorder
Transceiver
Command decoder
Data handling unit
command
Thermal control subsystem
Power control subsystem
Attitude orbit control subsystem
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.

telemetry
command
tracking
communication
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

6
Satellite Communication
galaxy
sun
ionosphere
troposphere
7
Characteristics

• Long distance depends on satellite altitude,
  nadir pointing, and observers 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

8
Communication System

• Transmission

Antenna
Power Amplifier
Modulator
Up converter
Coder
desired format
desired spectrum

• Reception

desired strength
Antenna
Low Noise Amplifier
Down converter
Decoder
Demodulator
9
Electromagnetic Wave

• Maxwells 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.

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
Direction of propagation
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
period
phase
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 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
13
Electromagnetic Spectrum
Band f (GHz)
L 1 2
S 2 4
C 4 8
X 8 12
Ku 12 18
K 18 27
Ka 27 40
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
EHF (extremely high frequency) 30 GHz 300 GHz 1cm 1mm

14
Frequency Allocation
15
Space TTC Spectrum

• Space operation

Frequency band Direction indicator Allocation status
136 137 MHz Space-Earth Secondary
137 138 MHz Space-Earth Primary
400.15 401 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
13.25 13.4 GHz Earth-space
13.4 14.3 GHz none Secondary
14.4 14.47 GHz Space-Earth Secondary
14.5 15.35 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
Frequency band (MHz) Direction indicator Allocation status
136 137 Space-Earth Secondary
137 138 Space-Earth Primary
148 149.9 Earth-Space
267 272 Space-Earth Secondary
272 273 Space-Earth Primary
400.15 401 Space-Earth Secondary
401 402 Space-Earth Primary
449.75 450.25 Earth-Space
1427 1429 Earth-Space Primary
1525 1535 Space-Earth Primary
2025 2110 Earth-Space
2200 2290 Space-Earth
7125 7155 Earth-Space
16
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
• Boltzmanns 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)

17
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
18
Antenna Types

• Dipole
• Horn
• Helical
• Yagi
• Parabolic
• Antenna array

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 0.13l2
• 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

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 4p steradians
  (square radians).

Isotropic antenna
directional antenna
21
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.

22
Signal or Carrier Power

• At a distance r from the transmitter, the power
  flux density is
• S EIRP/(4pr2) Pt?Gt /(4pr2)
• If atmospheric attenuation results in power loss
  by a factor LA, then the flux density at the
  receiver is
• S Pt?Gt /(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 EIRP ?Gr ? (l/(4pr))2 ? (1/LA)

23
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)

24
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

25
Atmospheric Attenuation
26
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
27
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
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 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 Boltzmanns
  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.

29
Noise Sources

• Contributions of system noise sky, ground,
  galaxy, circuit, and medium.
• Non-thermal noises are characterized in terms of
  noise temperature.
• Sun (104 -1010 0K) 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

30
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
31
System Noise Temperature
Tant
N0,1
N0,2
N0,out
Amplifier G1 , TE1
Amplifier G2 , TE2

• The total noise energy referred to amplifier 2
  input is
• N0,2 G1 k (Tant TE1) k TE2
• 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.

32
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
Cable -20 dB 290 0K
Waveguide -0.25 dB 290 0K
TS
Receiver 2000 0K
33
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

34
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

35
Modulation
Modulating baseband (low frequency or digital)
signal
Modulator
Modulated waveform
Carrier (high frequency)

• 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

Analog modulation
Digital modulation
Multiple access
36
Analog Modulation

• Modulation baseband signal ? RF waveform
• RF waveform A cos(wtf) 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

37
Digital Modulation

• Methods
• ASK (Amplitude shift Keying)
• FSK (Frequency shift keying)
• PSK (Phase shift keying)
• QPSK (Quadrature phase shift keying)

0
1
1
0
0
38
Data Rate and Bit Energy
clock
Digital data
0
0
0
0
1
1
Bit period Tb

• 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

39
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

40
Bit Error Rate and Eb/ N0
41
Link Budget Analysis
400 MHz 15000 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 -176.7 dB -187.8 dB
Polarization loss -0.5 dB -0.5 dB
Receiver power 132.8 dBm 143.9 dBm
Receiver antenna gain 0.0 dB 0.0 dB
Bit rate, 1000 bps 30.0 dB-Hz 30.0 dB-Hz
Energy/bit, Eb -162.8 dBmJ -173.9 dBmJ
Noise density, N0 -173.0 dBm/Hz -175.7 dBm/Hz
Received Eb/ N0 10.2 dB 1.8 dB
Eb/ N0 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 -12.2 dB
Total required power 33.8 dBm (2.4W) 42.2 dBm (17W)
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

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

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

Acceleration, velocity, displacement Angular
rate, angular position Pressure Temperature Densit
y Resistance Voltage, current Intensity Electric
field, magnetic field
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

46
FDM
FM modulator
signal 1
Summer
FDM signal
carrier f1
FM modulator
signal 2

• A multi-tone signal is formed
• Must consider
• Frequency plan
• Pre-emphasis

carrier f2
FM modulator
signal N
carrier fN

• 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

47
TDM
sync
signal 1
Commutator Multiplexer
slot
signal 2
frame
TDM bit stream

• 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 N
Timing Frame sync
48
PCM Telemetry
Timing frame sync
Bit sequence
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 1 2 3 N-1
sync 1 2 3




sync 1
sync 1
Major frame
M
50
A Typical Telemetry Frame
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 1 2 3 1 5 6 7a 8
sync 1 2 3 1 5 6 7b 8
sync 1 2 3 1 5 6 7c 8
sync 1 2 3 1 5 6 7d 8
sync 1 2 3 1 5 6 7e 8
supercommutation
subcommutation
52
PCM Frame Synchronization

• Synchronization is made possible through
  synchronization word (sync), which is a unique
  sequence of 1s and 0s.
• Recommended sync word (IRIG 106-93)

Length Pattern Length Pattern
7 101 100 0 16 111 010 111 001 000 0
8 101 110 00 17 111 100 110 101 000 00
9 101 110 000 18 111 100 110 101 000 000
10 110 111 000 0 19 111 110 011 001 010 000 0
11 101 101 110 00 20 111 011 011 110 001 000 00
12 110 101 100 000 21 111 011 101 001 011 000 000
13 111 010 110 000 0 22 111 100 110 110 101 000 000 0
14 111 001 101 000 00 23 111 101 011 100 110 100 000 00
15 111 011 001 010 000 24 111 110 101 111 001 100 100 000
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.
• BiF-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.
• BiF-M one is represented by a change at
  mid-clock zero is represented by no change at
  mid-clock.
• BiF-S one is represented by no change in
  mid-clock zero is represented by a change in
  mid-clock.

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

55
Telecommand System
Antenna
Power switching unit
Command processor
Receiver
Spacecraft subsystems

• Antenna
• Often omni for LEOs
• Receiver
• Continuously on
• 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

On-board computer
On-board storage
56
Telemetry Channel Coding

• Coding system
• 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

Demodulator and RF
Viterbi Decoder
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
(kr) coded bits
k reconstructed information bits
Decoder
(kr) received bits
Syndrome
58
Channel Coding Performance

• Performance of channel coding

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
Functions
Layer
Presentation layer
Segmenter
Segmenter
Management Data Link
Management Data Link
Session layer
Data Link
Data Link
Data Link
Transport layer
Link Multiplexer
Network layer
Physical
Physical
Data link layer
Silicon/Radio
Physical layer
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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 response
DL confirm
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AX.25 In Action
message
packet
RF wave
TNC
Radio

• 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)

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AX.25 Frames

• Frame arrangement for U or S frames

01111110
Frame check sequence 16 bits
Identifies both the source and destination of the
frame 112 or 560 bits
Identifies frame type 8 bits

• Frame arrangement for I frame

flag
address
control
FCS
flag
Information
PID
Protocol identifier 8 bits
Information N x 8 bits
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Summary

• Satellite TTC 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
• Eb/N0
• 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

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Further Readings

• P. Fortescue and J. Stark, Spacecraft Systems
  Engineering, Chapters 13 14, John Wiley, 1995.
  
• B. Razavi, RF Microelectronics, Prentice Hall,
  1998.
• M. Richharia, Satellite Communication Systems,
  McGraw-Hill, 1995.
• D. Roddy, Satellite Communications,
  McGraw-Hill, 2001.
• J. G. Proakis, Digital Communications,
  McGraw-Hill, 1995.
• Satellite link budget calculation can be found in
  http//www.satsig.net/linkbugt.htm
• AX.25 can be found in either http//www.arrl.org/
  or http//www.tapr.org

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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 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.