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5. Communication and
Data Handling
© LRT 2011 5.1 Spacecraft Technology II
•Communication Systems
•Link Budgets
•Data Handling
Spacecraft Technology II
Communication & Data Handling
COMM – (Tele-) Communication
OBDH – On-Board Data Handling
CDS – Command & Data Subsystem
C&DH – Command & Data Handling Subsystem
TT&C Telemetry, Tracking and Command
© LRT 2011 5.2 Spacecraft Technology II
Literature
• Charles D. Brown, Elements of Spacecraft Design AIAA Education Series, ISBN, 1-56347-524-3
• Spacecraft Systems Engineering, 2nd edition, P. Fortescue, J. Stark (eds.) John Wiley & Sons, ISBN 0-471-95220-6
• Space Mission Analysis & Design, 3rd edition, J.R.Wertz, W.J.Larson (eds.) ISBN 1-881883-10-8
• E.Messerschmid, S. Fasoulas Raumfahrtsysteme Springer Verlag, 2000, ISBN 3-540-66803-9
• M.D. Griffin, J.R.French, Space Vehicle Design ISBN 0-930403-90-8
• V.L.Pisacane, R.C.Moore, Fundamentals of Space Systems Oxford University Press, 1994, ISBN 0-19-507497-1
Special Literature
Space Engineering – Radio frequency and modulation, European Cooperation for Space
Standardization (ECSS), ECSS-E-50-05A, 24 Januar 2003
Space Engineering – Ground Systems and operations – Telemetry and telecommand packet
utilisation, European Cooperation for Space Standardization (ECSS), ECSS-E-70-41A, 30 Januar
2003
© LRT 2011 5.3 Spacecraft Technology II
Tasks of the Data-handling & Communication-Subsystem
• Telemetry, Tracking & Command = TT&C
• Telemetry = data collection, storage, multiplexing, RF modulation, transmitting/receiving
• Tracking = determination of orbital data of the satellite
• Command = performing of Up-link Commands (e.g. Satellite-reconfiguration, reorientation/deployment of antennas or solar arrays)
• Storage of payload data (data collection)
• Transmitting data to other users and satellites (data relay) = Transponder Service
The Data-handling & Communication Subsystem comprises the following three tasks:
© LRT 2011 5.4 Spacecraft Technology II
Possible Communication Links
© LRT 2011 5.6 Spacecraft Technology II
Inter-Orbit
Link IOL
LEO
GEO
Inter-Satellite
Link ISL
Communication-Architectures
Molnija
Store & Forward
USA: TDRSS (Tracking &
Data Relay Satellite System)
© LRT 2011 5.7 Spacecraft Technology II
RF Modulation – Frequency Bands
To pass the ionosphere: Frequencies > 0,1 GHz.
O2-Absorption at 60 GHz, hence, no
Jamming from Earth possible.
Satellite communication on one of the following bands has to be applied for and has to be approved by the ITU
(International Telecommunications Union).
Telemetry
Crosslink
Above 20 GHz: H2O + O2 attenuation
© LRT 2011 5.12 Spacecraft Technology II
N Sr
Communication Link
Pt
Gt
Transmitter
TX
Ll
Receiver
RX
Gr Ls
24
rLs
Tsys
BPt
Lc
Lt
L = Loss
G = Gain
r
2
4
SurfaceLs
© LRT 2011 5.17 Spacecraft Technology II
BTk
LGLLP
BTk
LGLLGLP
N
S
sysB
crtsEIRP
sysB
crtstltr 1111
1111
Figure of Merit
(Quality) of RX
Figure of Merit
(Quality) of TX
Space Loss
BTkN sysB
tt AG2
4
Link Budget
N noise signal = N0B
N0 noise signal density = kTsys
Pt transmitter power (typ.: for every Watt Pt, four times the electrical power)
B bandwidth = range of frequencies included in signal
R data rate [bps]
Gt transmit-antenna gain
Gr receive-antenna gain
Ls space loss
Lt transmission path loss (rain, atmospheric absorption)
Ll transmitting antenna line loss
Lc receiving antenna cable loss
Tsys system noise temperature
PEIRP effective isotropic radiated power
inm
dBbpsdBKsysdBcdBrdBtdBsdBtdBldBWt
dB
b
GdB
RTLGLLGLPN
E
arg
,,,,,,,,
0
105
60.228
dBEIRPP ,
received energy per bit
dBBk ,1
see Bit Error Rate
!
< 10 GHz: 4-5 dB, > 10 GHz: 6-20 dB (Atmosphere & Rain!)
BSRSE rrb
NSNE rb 0
The PEIRP or short EIRP tells what
transmitter power would be needed for
an isotropically radiating antenna to
have an identical power density
compared to the main lobe of a
concentrating antenna..
Hence, the EIRP value is not real
existing power, but a mere calculated
value.
© LRT 2011 5.18 Spacecraft Technology II
Receiver Noise – Receiver Trec
001
11
1 :......
... TTkGTTGG
TkGGP recBrec
n
nBnn
1
111
21
......: T
GG
T
G
TTT
n
nrec
From figure:
Noise temperature of cascaded amplifiers
00
0
0
1:T
T
T
TT
GP
PF recrec
rec
n
Receiver Noise Number figure of merit for receiver:
typical: F=2.5 (4 dB)
low-noise 1st-stage amplifier
= Low-Noise Amplifier = LNA
© LRT 2011 5.19 Spacecraft Technology II
K290
0T
0 0 0
System Noise Temperature
rec
c
cc
c
antsys T
L
LT
L
TT
1
BTkN sysB
K
K
K
Tsys
750
500
130 Downlink (cooled Ground-station Receiver: F=1.25)
Uplink (uncooled S/C Receiver: F=2.5, Temp. of Earth!)
Tc = Cable temperature, mostly 290K
Lc = Cable attenuation at receiver
0
1T
TF rec
00
11
1T
LF
L
TTF
L
LT
L
TT
cc
ant
c
cc
c
antsys
Downlink (uncooled Ground-station Receiver: F=2.5)
For Lc 1.12
for limiting range: 2-12 GHz
01 TFTrec
0TTc for
© LRT 2011 5.20 Spacecraft Technology II
System Noise, expressed as a tempertaure, consists of
antenna noise, cable noise, receiver noise.
Antenna noise Tant is
characterized by:
System Noise Tsys
Antenna
Background noise
Objects near transmitter path
Surface of Earth (T = 290 K)
Atmosphere, rain
K
KTant
290
90 Ground station to space
S/C to Earth
The system noise comprises:
Antenna noise Tant
Cable noise Tc and
Receiver noise Trec:
rec
c
cc
c
antsys T
L
LT
L
TT
1
© LRT 2011 5.21 Spacecraft Technology II
Atmospheric and Rain Attenuation
Case A is with rain
Influence of rain fall
and fog
© LRT 2011 5.22 Spacecraft Technology II
Beam Characteristics
DdB
703
Side lobe
dB3
Note: Side lobes can pick-up noise from the Sun (up to 106 K!),
Back lobe of a ground station can pick-up noise form Earth (290 K)!
Back lobe
Valid for parabolic
dish and horn
antenna
2
3
max, 12)(
dB
dBGG
depointing loss
typical –30 dB
© LRT 2011 5.25 Spacecraft Technology II
Antenna-Types
Horn suitable for: f > 4 GHz,
high gain:
G=10(A/)2 15-20 dB
Helix suitable for: f < 4 GHz,
low gain: G<14dB
simple design, leightweight
Dishes (Parabolic) universal usage
suitable for high gains:
G7(A/)2 15-20 dB
© LRT 2011 5.26 Spacecraft Technology II
Antenna Types – Patch Array Antenna
Phased Array Antenna:
Beam can be directed electronically. Very narrow beams possible.
Very complex fabrication (Electronics for active PAA)
© LRT 2011 5.27 Spacecraft Technology II
Advantage of Multi-Spotbeam Architectur
High Antenna Gain
high EIRP und G/T small terminals
Flexible channel addition to different spots
adapt to current bandwidth usage
High Frequency Reuse Capabilities
High “effective bandwidth”
F1/2 = Frequency 1/2
P1/2 = Polarization 1/2
Telemetry, Tracking and Command
© LRT 2011 5.33 Spacecraft Technology II
• Satellite health and status, house keeping data, resources, performance
• Orbit data / synchronizing with Guidance Navigation Control GNC
• Payload Data (image data, science data, coordinates of other satellites,
etc.
• Commands to the satellite
How to transfer these data
Telemetry - Signal processing steps
ADC
Sensor Sensor Sensor Sensor i Sensor n
commutator
formatter
steps cyclically through sensor signals and generates data
frames
Analog to Digital signal Converter (8, 16, 24 , ... bit)
adds meta information: sync word, frame count, s/c ident, error
detection/correction bits, frame format ident, s/c time, ...
data store
modulator
transmitter
stores data temporarily for data downlink
modulates & encrypts data onto RF carrier
transmits RF signal to ground station or other satellite
Data Rate = nfR frame fframe = number of modulated data
frames per second
n = bits per frame
TD
M =
tim
e-d
ivis
ion
mu
ltip
lexe
r
© LRT 2010 5.34 Spacecraft Technology II
ADC – Sampling, Quantization, Modulation
© LRT 2011 5.35 Spacecraft Technology II
NRZ-PCM =
Non-Return-to-Zero
Pulse-Code Modulation
PCM
Telemetry - Signal Processing Overview
signalsampling ff 2.2Nyquist Theorem:
modulation
onto carrier
ADC
RF
Modulation
sampling
quantization
digital
modulation
Sequence
of pro
cess
Formatting data packaging
encoding
bit representation
adding redundancy bits
for error correction
sampled
signal
conversion into
binary digits
analog signal
© LRT 2011 5.36 Spacecraft Technology II
RF Modulation - Carrier Modulation
A carrier is a RF wave: tEtE sin0
With three characteristic parameters: Amplitude E0, Frequency , Phase
Modulation = “impressing” an information on this carrier
The different ways of modulation are distinguished which of those three parameters is
modulated:
Binary PSK = BPSK
tt
t
sinsin:1
sin:0
Quadrature PSK = QPSK
two bits define the 4 carrier
–frequencies, respectively
23sin,sin
2sin,sin
tt
tt
binary
ASK is seldom used, since multi path propagation strongly influences the amplitude.
Higher data rate with increased
multi path sensitivity.
© LRT 2011 5.37 Spacecraft Technology II
Bit Error Rate RBER
To achieve a good Forward-
Error-Correction Code RBER
<10-5, you should have E/N0
5-10 dB.
(see. Link Equation)
Telemetry code standard used
by NASA and ESA
Wegen Steilheit, link margin
unbedingt nötig! For very complex digital
encodings, one
combines encodings in
Concatenations.
0
1
N
E
R
b
BER
Example Globalstar: Reed-Solomon + rate 1/2, K=7 coding). This results in RBER<10-10 with
only E/N0 = 5 dB.
© LRT 2011 5.39 Spacecraft Technology II
Formatting - Command-String
Multiple command frames are combined to a Stack, and packetized to a Command-String
packetiert. The string starts for synchronization with a pure carrier wave of 500 ms, followed by a
16-bit Bit-Synchronization-Word, followed by the Frame-Stack, and closed with ASW.
This Command-String is transmitted and initially stored in the so-called command pending queue
aboard the satellite.
The correct receipt is verified by downlink, and only executed through an additional Command-
Uplink if confirmed correct.
Most commands are executed at a desired time or condition at a later time, which will also be
transmitted. There is very few real-time commanding of satellites themselves. Payloads on teh
other hand use real-time commanding more often.
The Command Error Rate with this approach is about 10-18 to 10-22
© LRT 2011 5.40 Spacecraft Technology II
Encoding (Forward Error Correction, FEC)
Transmitting errors can be reduced by increasing the redundancy of the telemetry
information Forward Error Correction FEC
1. Convolutional encoding:
i Information-bits are shifted through a register (shift register) of length K; the contents are
correlated n-fold (typ. n=2, K=7); code-bits are generated (Rate 1/2 , K=7 Registers).
From n information bits, i·n Code-bits are generated.
© LRT 2011 5.41 Spacecraft Technology II
Encoding - Convolutional Coding: Rate 1/2, K=7
Input bits (0101101) are shifted step by step through the register.
Adding bits at correct position Output bits
+
.
.
.
.
© LRT 2011 5.42 Spacecraft Technology II
Encoding - Block Coding
2. Block-Coding:
Block coding combines
bits to blocks of equal
(or unequal) length and
a Parity Bit (even or
uneven )is added.
© LRT 2011 5.43 Spacecraft Technology II
Formatting - Telemetry Data Format
Format counter: increments value from one format to the next
Timing Channel: binary readout of onboard clock SIW: Synchronization & Identification Word =
fixed code identifying the originating satellite
Format
A typical (but variable in
length) telemetry format
© LRT 2011 5.44 Spacecraft Technology II
Formatting - Command
1. 16-bit Address & Synchronization Word (ASW): contains – Barker sync words = pseudo random words and – S/C Address Word (address of connected satellite)
2. 2x 4-bit Mode Selection Word (contains Command-type = addressed function) e.g. : On/Off, Memory Load, Computer Load, ...) (2x: MSW is repeated once)
3. 2x3 (8+4)-bit Data Words (opcode = operation code = specific action) e.g.: Relay-, Pulse-, Level-, Data-Commands (2x3: 3 different tasks, each one is repeated once; 8+4: 8-bit command, 4-bit Hamming code)
A tele-command frame for carrying out an instruction is usually 96 bits long (ESA standard):
© LRT 2011 5.45 Spacecraft Technology II