Communications and Data Handling

Communications and Data Handling

Dr Andrew Ketsdever

MAE 5595

Lesson 10

Outline

• Communication Subsystem– Introduction– Communications Architecture (uplink/downlink)– Data Rates– Budgets and Sizing

• Data Handling Subsystem– Introduction– Requirements and design– Sampling Rates– Quantization

Communications Subsystem

• Function– Transmits data to ground station(s)– Receives commands and data from ground

station(s)

• Deals with concerns arising from– Modulation scheme– Antenna characteristics– Propagating medium– Encryption

Simple Communication Architecture

Ground station

Antenna

TX RX

Amplifier

Modulator

Encryption

Comm Subsystem

OBC

Data storage

Payload

EPS

TCS

C&DH Subsystem

Error Detection and Correction (EDAC)

throughout

Alternate Communication Architectures

Communication Architectures

Communication Architecture

Military Communications Architecture

Radio Frequency Bands• Microwaves: 1 mm to 1 m wavelength. The microwaves

are further divided into different frequency (wavelength) bands: (1 GHz = 109 Hz) – P band: 0.3 - 1 GHz (30 - 100 cm) – L band: 1 - 2 GHz (15 - 30 cm) – S band: 2 - 4 GHz (7.5 - 15 cm) – C band: 4 - 8 GHz (3.8 - 7.5 cm) – X band: 8 - 12.5 GHz (2.4 - 3.8 cm) – Ku band: 12.5 - 18 GHz (1.7 - 2.4 cm) – K band: 18 - 26.5 GHz (1.1 - 1.7 cm) – Ka band: 26.5 - 40 GHz (0.75 - 1.1 cm) – V band: 50 – 75 GHz– W band: 75 – 111 GHz

• Care required since EU and other countries may use different designations. Do not confuse with RADAR bands.

Modulation Schemes

• Modulation– Variation of a periodic waveform to convey

information

• Modulation Schemes– Pulse Modulation– Amplitude Modulation– Frequency Modulation– Phase Modulation

How can you communicate with someone on the other side of the lake?

Modulation Schemes

tAtV sin rad angle phase signal

sec

radfrequency signal

amplitude signal

A

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.5 1 1.5 2 2.5 3

tAtV sinAmplitude, A

Phase shift,

Period, P

• Carrier signal typically a sinusoid - Easy to recreate

Amplitude Modulation

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5 3

tAtV sin11

tAtV

tAtV

sin0

sin1

00

11

Frequency Modulation

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.5 1 1.5 2 2.5 3

tAtV 11 sin

tAtV

tAtV

00

11

sin0

sin1

Phase Modulation

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.5 1 1.5 2 2.5 3

11 sin tAtV 000

11

sin0

sin1

tAtV

tAtV

Modulation

Binary Phase Shift Keying

Quadriphased Phase Shift Keying

Frequency Shift Keying

Multiple (8) Frequency Shift Keying

Link Design• Signal to Noise

signal

DC noise RMS + bias

RkT

GLLLLGLP

N

E

S

rprastltb 0

Frii’s Transmission Formula (ratio of received energy-per-bit to noise-density):

Pulse shape for illustration purposes only – would use sinusoidal waveform

Signal to Noise

RkT

GLLLLGLP

N

E

S

rprastltb 0

sec

bits rate data

K re temperatunoise system

K

secW

K

J 101.38constant sBoltzmann'

gain antenna receive

losserror pointing

lossrain

nattenuatio catmospheri

losspath space free

gain antennatransmit

antenna andansmitter between tr losses line

Wpower transmit

dB) 105 need (generallydensity noise toreceived energy/bit of ratio

23-

00

R

T

k

G

L

L

L

L

G

L

P

N

E

N

E

s

r

p

r

a

s

t

l

t

bb

SNR = Eb R / (No)

dB Language• dB or Decibels are power ratios

• Pref = 1 W or 1 mW (dBW or dBm respectively)

• P(dBm) = P(dB) +30

• Examples– 1W = 0 dBW = 30 dBm

– 1000W = 30 dBW = 60 dBm

• Attenuation– 1 dB attenuation implies that 0.79 of the input power is left– 10 dB attenuation implies that 0.10 of the input power is left– 1000 dB attenuation implies that 0.001 of the input power is left

refPoutP

outP log10

Frii’s Transmission Formula

Given Frii’s Transmission Formula:

RkT

GLLLLGLP

N

E

S

rprastltb 0

rprastl

Sb

rprastl

Sbt GLLLLGL

RkT

N

E

GLLLLGLN

RkTEP

00

a) Write equation in terms of transmit power

b) Express in logarithmic (dB) form

rGpLrL

aLsLtGlLRSTkNbE

tP

log10log10log10

log10log10log10log10log10log10log100

log10

Comm Subsystem—DesignTransmitter Link Contributions

tlt GLPEIRP Effective Isotropic Radiated Power:

Antenna gain Measure of how well antenna concentrates the power

density Ratio of peak power to that of an isotropic antenna

. .

Isotropic G = 1

Directed G > 1

Peak Power

Half Power

.

Break formula into pieces…

ratedata of

reciprical1gainantennareceive

antenna receiveat densitypower carrier

losses

rtransmitteatpower

carrier0

11

0

RkTGLLLLGLP

RkT

GLLLLGLP

N

E

N

Srprastlt

S

rprastltb

Comm Subsystem—Design Frii’s Transmission Formula

EIRP

2

000,27

G

Antenna gain: for parabolic antenna:

2

2

2

2

44

4

c

Df

fc

D

AG e

may approximate as:

deg 21

DfGHz

efficiency aperture• function of imperfections in antenna• typical 0.55 for S/C, 0.6 – 0.7 for GS


EIRP Tradeoff between transmitter power and antenna gain (for

same frequency and antenna size) Typical EIRPs:

100 dBW for ground station 20-60 dBW for S/C

Example:

Case 1 Case 2

Pt 25 W 1 W

Ll 0.8 0.8

Gt 5 125

EIRP 100 100

75 deg 15 deg

• Same EIRP

• Much different


Comm Subsystem—Design

Receiver Link Contributions Receiver figure of merit:

s

r

T

G

rantS TTT

Values given in SMAD Table 13-10

receiver and antennabetween noise

FOV)(in antenna offront in noise

r

ant

T

T

System noise:

Antenna noise sources: Galactic noise, Solar noise, Earth (typically 290 K),

Man-made noise, Clouds and rain in propagation path, Nearby objects (radomes, buildings), Temperature of blockage items (feeds, booms)

Receiver noise sources: Transmission lines and filters, Low noise amplifiers


Typical System Noise Temperatures


Transmission Loss Contributions Free space path loss:

2

4

SLs

Pointing loss: dB 122

e

Lp

• Valid for e /2 (identical antennas)

Transmit beam

Receive beam

• Contributions from both antennas

Atmospheric loss, La Due to molecular absorption and scattering Oxygen: 60 GHz, 118.8 GHz Water vapor: 22 GHz, 183.3 GHz (seasonal variations

as much as 20-to-1) SMAD Fig 13-10

Rain loss, Lr Strong function of elevation angle May want to accept short outages rather than design

for continuous service SMAD Fig 13-11

Comm Subsystem—Design Transmission Loss Contributions

Comm Subsystem—Design Transmission Loss Contributions (La)

Comm Subsystem—DesignModulation Schemes

Comm Subsystem—DesignModulation Schemes

Data Handling

Data Handling—IntroDriving Requirements

• Two main system requirements– Receives, validates, decodes, and distributes commands to

other spacecraft systems– Gathers, processes, and formats spacecraft housekeeping

and mission data for downlink or use by an onboard computer.

• The data handling (DH) subsystem has probably the least defined driving requirements of all subsystems and is usually designed last– Based on the complexity of the spacecraft and two

performance parameters: 1) on-board processing power to run bus and payloads and 2) storage capacity for housekeeping and payload data

– Meeting requirements is a function of available flight computer configurations

Data Handling—IntroDriving Requirements

• System level requirements and constraints– Satellite power up default mode– Power constraints– Mass and size constraints– Reliability– Data bus requirements (architecture and number of digital

and analog channels)– Analog interface module derived requirement– Total-dose radiation hardness requirement – Single-event charged particle hardness requirement– Other strategic radiation requirements (EMP, dose rate,

neutron flux, operate through nuclear event, etc.)– Software flash upgradeable

• Subsystem known by a variety of names– TT&C: Telemetry, Tracking, and Control (or Command)– TTC&C: Telemetry, Tracking, Command, and Communication– TC&R: Telemetry, Command and Ranging– C&DH: Command and Data Handling– CT&DH: Command, Tracking and Data Handling

• Functions– Receives, validates, decodes, and distributes commands to

other spacecraft systems– Gathers, processes, and formats spacecraft housekeeping and

mission data for downlink or use by an onboard computer.

Data Handling—IntroFunctions

Ground station

Antenna

TX RX

Amplifier

Modulator

Encryption

Comm Subsystem

OBC

Data storage

Payload

EPS

TCS

CT&DH Subsystem

Error Detection and Correction (EDAC)

throughout



• CT&DH Functions:– Aid in orbit determination (tracking)– Command S/C (command) (concerned with the uplink)– Provide S/C status (telemetry) (concerned with the downlink)

• Gather and process data• Data handling

– Make payload data available (telemetry) (concerned with the downlink)

• Sometimes, the payload will have a dedicated system rather than using the bus

– CT&DH functions often performed by OBC (On-Board Computer)

• Comm Functions:– Deals with data transmission concerns (encryption, modulation

scheme, antenna characteristics, medium characteristics) These will be discussed in Comm lessons.

• Commands may be generated by:– The Ground Station– Internally by the CT&DH computer– Another subsystem

• Types of commands– Low-level On-Off: reset logic switches in SW (computer

controlled actions)– High-level On-Off: reset mechanical devices directly (i.e.

latching relays, solenoids, waveguide switches, power to Xmitter)

– Proportional Commands: digital words (camera pointing angle, valve opening size)

Data Handling—IntroFunctions—Command Handling

• Housekeeping:– Temps– Pressures– Voltages and currents– Operating status (on/off)– Redundancy status (which unit is in use)– …

• Attitude: might need to update 4 times/sec

• Payload: case-by-case payload health and payload data

Data Handling—IntroFunctions—Data/Telemetry Handling

DH Subsystem—DesignAcquiring Analog and Digital Data

Flight Computer

Digital In Digital Out

ADC DAC

Point-to-point digital data interface Digital network interface

Op AmpOp Amp

Analog In Analog Out

MUX Sel

Shared data bus

• All real world data interfaces are analog– Sound

– EM Spectrum: light, IR, UV, Gamma rays, X-rays, etc.

– Motor speed, position

• Usually analog signal levels on the input side are weak (payload sensor, receiver, telemetry level signal)– Need to boost signal level through Operational Amplifier

otherwise known as “Op Amp”

• On the output side, must match signal levels with equipment (transmitter, actuator, etc.)– Use Op Amp to match systems

DH Subsystem—DesignAcquiring Analog Data—Op Amps


VCC

+

Vo

-

i=0

+

-

eg=0

VN

-VCCZin=Zout=0

Non-Inverting input

Inverting

input

VP


Rfb

+

Rin

Vout Vin

i

f

i

o

R

R

V

VOpAmpInverting .

+

Rfb

Vfb

Ri

Vin

Vout

i

f

i

o

R

R

V

VOpAmpinvertingNon 1..

Rf

+

R1

Vo

V1

+

+

- -

-

i1

if

i=0

+

-

eg=0 +

+ V2

V3

i2

i3

R2

R3

3

32

21

10. V

R

RV

R

RV

R

ROpAmpVSummer fff

C

+

Ri

Vo Vi

+ +

- -

-

ii

if

i=0

+

-

eg=0

RCssV

sVOpAmpIntegrator

i

1

)(

)(. 0

• Once analog data is converted to “readable” level, we must convert it for use by the flight computer

• Accomplished through Analog-to-Digital Converter (ADC)– Reverse process is Digital-to-Analog Converter (DAC)

• Changes continuous signal into 1’s and 0’s representation– Sampling: choosing how often to measure signal– Quantization: choosing how many levels to approximate signal

• Must tradeoff reconstructed signal quality versus bandwidth of data– Driven by mission requirements: accuracy, bandwidth, CPU processing

speed, data storage, etc.

DH Subsystem—DesignAcquiring Analog Data—ADC

• Sampling rate considerations– Many samples → good signal representation, but

takes lots of bits (bandwidth)– Few samples → low bandwidth, but not so good

signal representation• Nyquist Criteria for sampling: fs 2fm

– fs = sampling frequency– fm = maximum frequency of sampled signal

• Example: Human ear hears sounds in the frequency range from 20 Hz to 20 kHz. Audio compact discs represent music digitally and use a sample rate of 44.1 kHz (2.2 X human max frequency)

DH Subsystem—DesignAcquiring Analog Data—DAC

Infrequent Samples

-50.00

0.00

50.00

0 90 180 270 360

Angle (deg)

A*s

in(a

ng

le)

Sampled

Infrequent Samples

-50.00

0.00

50.00

0 90 180 270 360

Angle (deg)

A*s

in(a

ng

le)

Analog

Sampled

Moderately Frequent Samples

-50.00

0.00

50.00

0 90 180 270 360

Angle (deg)

A*s

in(a

ng

le)

Analog

Sampled

Moderately Frequent Samples

-50.00

0.00

50.00

0 90 180 270 360

Angle (deg)

A*s

in(a

ng

le)

Sampled

DH Subsystem—DesignAcquiring Analog Data—DAC Sampling

Rate

• Quantization level considerations– Many levels → good signal representation,

but lots of bits (bandwidth)– Fewer levels→ low bandwidth, but not so

good signal representation

DH Subsystem—DesignAcquiring Analog Data—ADC Quantization

Quantization and Raw Data(1 bit)

-50.0000

0.0000

50.0000

0 90 180 270 360

Angle (deg)

A*s

in(a

ng

le)


-50.00

0.00

50.00

0 90 180 270 360

Angle (deg)

A*s

in(a

ng

le)

DH Subsystem—DesignAcquiring Analog Data—Quantization


-50.00

0.00

50.00

0 90 180 270 360

Angle (deg)

A*s

in(a

ng

le)


-50.0000

0.0000

50.0000

0 90 180 270 360

Angle (deg)

A*s

in(a

ng

le)

• Used when sharing common wire for multiple sets of data– Need method to sequence data into telemetry

stream

DH Subsystem—DesignMultiplexing

EPS

CT&DH OBC

…12 separate data lines (dedicated)

1 shared data line (multiplex data)

• Frames– Rigid telemetry structure, synchronous (pre-defined)

communications.– A schedule for using the data bus, where the most

crucial information (like ADACS) is sent more frequently than slowly changing, or non-critical data (for example TCS).

Time Slot 1 Time Slot 2 Time Slot 3 Time Slot 4 Subframe 1 Message 1 Message 2 Message 3 Message 4 Subframe 2 Message 5 Message 6 Message 7 Message 8 Subframe 3 Message 9 Message 10 Message 11 Message 12 Subframe 4 Message 13 Message 14 Message 15 Message 16 Subframe 1 Message 1 Message 2 Message 3 Message 4

DH Subsystem—DesignMultiplexing

M1: Send ADACS data to payload – 1 Hz M2: Get RX’d data from Comm and send to CT&DH OBC – 8 Hz M3: Send TX data to Comm – 8 Hz M4: Get thermal data from TCS – 1 Hz M5: Get battery voltage, supply current from EPS – 1 Hz M6: Get fuel levels from Propulsion – 1 Hz

Simple GEO EM surveillance satellite that receives traffic on one frequency, encrypts and transmits on a different frequency. Consider that each subframe is 250 msec long. Define the following messages/rates:

DH Subsystem—DesignMultiplexing Example

Time Slot 1 Time Slot 2 Time Slot 3 Time Slot 4 Time Slot 5 Subframe 1 Subframe 2 Subframe 3 Subframe 4 Subframe 1

M1: Send ADACS data to payload – 1 Hz M2: Get RX’d data from Comm and send to CT&DH OBC – 8 Hz M3: Send TX data to Comm – 8 Hz M4: Get thermal data from TCS – 1 Hz M5: Get battery voltage, supply current from EPS – 1 Hz M6: Get fuel levels from Propulsion – 1 Hz

DH Subsystem—DesignMultiplexing Example

Time Slot 1 Time Slot 2 Time Slot 3 Time Slot 4 Time Slot 5 Subframe 1 M2 M3 M1 M2 M3 Subframe 2 M2 M3 M4 M2 M3 Subframe 3 M2 M3 M5 M2 M3 Subframe 4 M2 M3 M6 M2 M3 Subframe 1

DH Subsystem—DesignMultiplexing Example Solution

DH Subsystem—DesignDH Design and Sizing

Software EngineeringDoD Software statistics (The Problem)

51% of all failures are blamed on bad requirements

(by the way, only 2% of theworking software is on time,

under budget)

DOD Software Expenditures(according to one Army Study)

Software EngineeringSoftware Growth Trends (The Need)10000

100

1000

10

1

1960 95908580757065

GEMINI 3

GEMINI 12

APOLLO 7APOLLO 17

SKYLAB 2

MISSION CONTROL: GROUND STATION

MERCURY 3F-111

P-3A

AWACS

APOLLO 7

P-3A

B-1A

B-1BSHUTTLE/OFT

SHUTTLE/OPERATIONAL

F-15EB-2

GEMINI 2

APOLLO 11S-3A

SHUTTLE/OFT MANNED SYSTEMS

C-5A

F-111

F-15

F-16 C/D

A-7D/E

GEMINI 3

PERSHING 1

TITAN

E-2C

SKYLAB 2TITAN 34D (IUS)

PERSHING 11(ED)

VIKING

C-17PROJECTED

GALILEO

PERSHING 11(AD)

MISSILE

TRIDENT C4

VOYAGERTITAN IIIC

PERSHING 1APOSEIDON C3

SURVEYORMARINERVENUS MERCURY

UNMANNED

UNMANNED INTERPLANETARY

MANNED A/C

MANNED SPACE

MANNED SPACE CONTROL

UNMANNED SYSTEMS

F-22PROJECTED

Thousands of Code Memory

Locations(i.e. size of executable software)

Flight Date

Software EngineeringSoftware Increasingly matters

Software EngineeringWhat can go wrong (The Errors)

• H.M.S. Sheffield– sunk by a missile its software identified as being “friendly”

• Patriot clock drift– Missed Mach 6 scud by 0.36 sec clock drift that occurred over a continuous 4-day usage period

• NASA Mariner 1– $80 million missing comma (DO 17 I = 1 10 vs. DO17I = 110 vs. DO 17 I = 1, 10 )

• SDI laser and Space shuttle mirror– Shuttle positioned to bounce a laser positioned at 10,023 miles vs. 10,023 feet

• USS Yorktown– Zero entered as data caused a divide by 0 error, cascading errors caused complete shut down of the

ship’s propulsion system for an hour (ship was eventually rebooted)• Ariane 5

– Non-critical component failure shut down system including critical componentsshoved a 64 bit float number in a 16 bit integer space

• Mars Climate Orbiter and Polar Lander failures– English units (pounds-force seconds) used instead of metric units (Newton-seconds)– Flight software vulnerability to transient signals shut down descent engines early

• Titan IVB-32/Centaur (Milstar)– Misplaced decimal point in avionics database

Communications and Data Handling

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