spacecraft comm data

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


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


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:


Communication Structures


Possible Communication Links


Inter-Orbit

Link IOL

LEO

GEO

Inter-Satellite

Link ISL

Communication-Architectures

Molnija

Store & Forward

USA: TDRSS (Tracking &

Data Relay Satellite System)


Example of an complex Communication Link


Example of Complex Communication Link


Atmospheric Windows to Electromagnetic Radiation


Frequency Bands


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


Frequency Allocations


Satellite Frequencies


GEO Satellites – Frequency Bands



Link Budget

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


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.


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


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


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


Atmospheric and Rain Attenuation

Case A is with rain

Influence of rain fall

and fog


Satellite Downlink Characteristics

BPt


Bandwidth B

Antennae


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


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


Antenna Types – Patch Array Antenna

Phased Array Antenna:

Beam can be directed electronically. Very narrow beams possible.

Very complex fabrication (Electronics for active PAA)


Interference of Waves


Phased-Array Antenna - Example TerraSar

Phase Shifters

Active Phased Array Antenna (GEO Comm. Satellite)

Spot-Beam Antennen

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


• 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


ADC – Sampling, Quantization, Modulation


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


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.


Data Rates

Pulse Code Modulation

Compressed Rates


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.


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


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.


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

+

.

.

.

.


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.


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


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):


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