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
P ayload
OBC
E ncryp tio n
M o d ulato r
A m p lifier
EPS
TCS
D ata storage
C&DH
S ubsystem
TX
C om m
S ubsystem
RX
A nte nna

E rror D etection and
C orrection (E D A C )
throughout
G round station
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
• Carrier signal typically a sinusoid
- Easy to recreate
A  signal amplitude
V t   A sin  t  

  signal frequency
  signal phase angle
Period, P  
S in u s o id
1 .5
V  t   A s in  t   
1
Amplitude, A
A m p litu d e
0 .5
0
0
0 .5
1
1 .5
-0 .5
-1
-1 .5
Phase shift, 
T im e (s e c )
2
2 .5
3
 rad 
 sec 


rad 
Amplitude Modulation
A m p litu d e M o d u la tio n (A M )
2 .5
1
0
1
2
1 .5
1
A m p litu d e
0 .5
0
0
0 .5
1
1 .5
2
2 .5
-0 .5
-1
-1 .5
-2
V
1
t 

A 1 s in

t  

V
0
t 

A
0
s in

t  

-2 .5
T im e (s e c )
1  V 1 t   A1 sin  t  

0  V 0 t   A 0 sin  t  

3
Frequency Modulation
F re q u e n c y M o d u la tio n (F M )
1 .5
1
0
1
1
A m p litu d e
0 .5
0
0
0 .5
1
1 .5
2
2 .5
-0 .5
-1
V
1
t 

A s in

1
t  

V
0
t 

A s in

0
t  

-1 .5
T im e (s e c )
1  V 1 t   A sin  1 t  

0  V 0 t   A sin  0 t  

3
Phase Modulation
P h a s e M o d u la tio n (P M )
1 .5
1
0
1
1
A m p litu d e
0 .5
0
0
0 .5
1
1 .5
2
-0 .5
-1
V
1
t 

A s in

t  
1

V
0
t 

A s in

t  
0

-1 .5
T im e (s e c )
1  V 1 t   A sin  t   1 
0  V 0 t   A sin  0 t   0 
2 .5
3
Modulation
Binary Phase Shift Keying
Quadriphased Phase Shift Keying
Frequency Shift Keying
Multiple (8) Frequency Shift Keying
Link Design
• Signal to Noise
sign al
RM S + bias
D C n oise
Pulse shape for illustration purposes
only – would use sinusoidal waveform
Frii’s Transmission Formula (ratio of received energy-perbit to noise-density):
Eb
N0

Pt L l G t L s L a L r L p G r
kT S R
Signal to Noise
Eb
Pt L l G t L s L a L r L p G r

N0
Eb
kT S R
 ratio of energy/bit
received
to noise density (generally
N0
Pt  transmit
Eb
 10 dB)
N0
power
W 
L l  line losses between tr ansmitter
G t  transmit
need 5 
antenna
and antenna
gain
L s  free space path loss
L a  atmospheri
c attenuatio
n
L r  rain loss
L p  pointing
error loss
G r  receive antenna
k  Boltzmann'
gain
s constant
W  sec 

- 23 J

 1.38  10

K
K


T s  system noise temperatu re K 
 bits 
R  data rate 

 sec 
SNR = Eb R / (No)
dB Language
• dB or Decibels are power ratios

Pout  10 log 


Pout 
Pref 

• Pref = 1 W or 1 mW (dBW or dBm respectively)
• P(dBm) = P(dB) +30
• Examples
– 1W
– 1000W
=
=
0 dBW =
30 dBW =
30 dBm
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
Frii’s Transmission Formula
Given Frii’s Transmission Formula:
Eb

Pt L l G t L s L a L r L p G r
N0
kT S R
a) Write equation in terms of transmit power
Pt 
E b kT S R
N 0 Ll G t L s La Lr L p G r

Eb
kT S R
N 0 Ll G t L s La Lr L p G r
b) Express in logarithmic (dB) form
E

Pt  10 log  b
  10 log k  10 log T S  10 log R  10 log Ll  10 log G t  10 log L s  10 log L a
N
0

 10 log L r  10 log L p  10 log G r
Comm Subsystem—Design
Transmitter Link Contributions

Effective Isotropic Radiated Power:

Antenna gain
EIRP  Pt L l G t

Measure of how well antenna concentrates the power
density
 Ratio of peak power to that of an isotropic antenna
Isotropic
G = 1
.
D irected
G > 1
.

.
H alf
P o w er
P eak
P o w er
Comm Subsystem—Design
Frii’s Transmission Formula
Break formula into pieces…
 1
Eb Pt Ll Gt Ls La Lr L p Gr

 Pt Ll Gt Ls La Lr L p Gr 
N0
kTS R
kTS
receive 
carrier
power at
transmitte r
losses
carrier power density
at receive antenna
EIRP
antenna
gain
1
N0
1
  
 R
reciprical
of data
rate
Comm Subsystem—Design
Transmitter Link Contributions

Antenna gain:

G  4
for parabolic antenna:
  aperture efficiency
Ae

2

 D 2 
4  
 
 4

 c 
 f 


2
  Df 

 
 c 
• function of imperfections in antenna
• typical   0.55 for S/C,   0.6 – 0.7 for GS

may approximate as:
G 
27 , 000

2
 
21
f GHz D
2
deg 
Comm Subsystem—Design
Transmitter Link Contributions

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
1W
Ll
0.8
0.8
Gt
5
125
• Same EIRP
EIRP
100
100
• Much different 

75 deg
15 deg
Comm Subsystem—Design
Receiver Link Contributions

Receiver figure of merit:
Gr
Ts

System noise:
T S  T ant  T r
T ant  noise in front of antenna
T r  noise between antenna

(in FOV)
and receiver
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

Values given in SMAD Table 13-10

Comm Subsystem—Design
Typical System Noise Temperatures
Comm Subsystem—Design
Transmission Loss Contributions


Free space path loss:
  
Ls  

4

S


Pointing loss:
e
  12  
 
Lp
2
2
dB 
• Valid for e  /2 (identical antennas)
• Contributions from both antennas
T ransm it
beam
 
R eceive
beam

Comm Subsystem—Design
Transmission Loss Contributions


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 (La)
Comm Subsystem—Design
Modulation Schemes
Comm Subsystem—Design
Modulation Schemes
Data Handling
Data Handling—Intro
Driving 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—Intro
Driving 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
Data Handling—Intro
Functions
• 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—Intro
Functions
P ayload
OBC
E ncryp tio n
M o d ulato r
A m p lifier
EPS
TCS
D ata storage
CT&DH
S u b system
TX
C om m
S ubsystem
RX
A nte nna

E rror D etection and
C orrection (E D A C )
throughout
G round station
Data Handling—Intro
Functions
• 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.
Data Handling—Intro
Functions—Command Handling
• 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—Intro
Functions—Data/Telemetry 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
DH Subsystem—Design
Acquiring Analog and Digital Data
Point-to-point digital data interface
Digital In
Digital network interface
Sel
MUX
Digital Out
Flight
Computer
Op Amp
ADC
DAC
Analog In
Op Amp
Analog Out
Shared data bus
DH Subsystem—Design
Acquiring Analog Data—Op Amps
• 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—Design
Acquiring Analog Data—Op Amps
VCC
i=0
Inverting
input
Non-Inverting
input
VP
eg =0
VN
+
Zin = 
-
Vo
+
Zout =0
-VCC
DH Subsystem—Design
Acquiring Analog Data—Op Amps
R fb
R fb
V fb
R in
V out
Ri
+
V out
V in
V in
+
Inverting .OpAmp
Vo

Rf
Vi
Non .inverting .OpAmp
Ri
i1
Ri
i= 0
V1
-
+
R3
V2
+
eg= 0
i2
i3
if
+
+
Vo
-
V3
0
ii
+
+
Summer .OpAmpV
Ri
i= 0
R2
Rf
C
if
+
 1
Vi
Rf
R1
Vo
Rf
Rf
 Rf

  
V1 
V2 
V 3 
R2
R3
 R1

+
e g= 0
+
-
Vi
+
Vo
-
-
Integrator .OpAmp
V0 ( s )
Vi ( s )

1
RCs
DH Subsystem—Design
Acquiring Analog Data—ADC
• 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—Design
Acquiring Analog Data—DAC
• 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—Design
Acquiring Analog Data—DAC Sampling
Rate
Infrequent Samples
50.00
Analog
0.00
Sampled
-50.00
0
90
180
270
A*sin(angle)
A*sin(angle)
Infrequent Samples
50.00
0.00
Sampled
-50.00
360
0
Angle (deg)
Sampled
-50.00
270
Angle (deg)
360
A*sin(angle)
A*sin(angle)
Analog
0.00
180
270
360
Moderately Frequent Samples
50.00
90
180
Angle (deg)
Moderately Frequent Samples
0
90
50.00
0.00
Sampled
-50.00
0
90
180
270
Angle (deg)
360
DH Subsystem—Design
Acquiring Analog Data—ADC Quantization
• 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—Design
Acquiring Analog Data—Quantization
Quantization and Raw Data
(1 bit)
A*sin(angle)
A*sin(angle)
Quantization and Raw Data
(1 bit)
50.00
0.00
-50.00
0
90
180
270
50.0000
0.0000
-50.0000
360
0
A*sin(angle)
A*sin(angle)
50.00
0.00
-50.00
180
270
Angle (deg)
270
360
Quantization and Raw Data
(4 bit)
Quantization and Raw Data
(4 bit)
90
180
Angle (deg)
Angle (deg)
0
90
360
50.0000
0.0000
-50.0000
0
90
180 270
Angle (deg)
360
DH Subsystem—Design
Multiplexing
• Used when sharing common wire for
multiple sets of data
– Need method to sequence data into telemetry
stream
EPS
12 separate data lines
(dedicated)
…
CT&DH
OBC
1 shared data line
(multiplex data)
DH Subsystem—Design
Multiplexing
• 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).
1
2
3
4
T im e S lot 1
M essage 1
M essage 5
M essage 9
M essage 13
T im e S lot 2
M essage 2
M essage 6
M essage 10
M essage 14
T im e S lot 3
M essage 3
M essage 7
M essage 11
M essage 15
T im e S lot 4
M essage 4
M essage 8
M essage 12
M essage 16
S ubfram e 1
M essage 1
M essage 2
M essage 3
M essage 4
S ubfram e
S ubfram e
S ubfram e
S ubfram e
DH Subsystem—Design
Multiplexing Example

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:






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—Design
Multiplexing Example






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
T im e Slot 1
Subfram e
Subfram e
Subfram e
Subfram e
1
2
3
4
Subfram e 1
T im e Slot 2
T im e Slot 3
T im e Slot 4
T im e Slot 5
DH Subsystem—Design
Multiplexing Example Solution
S ubfram e
S ubfram e
S ubfram e
S ubfram e
1
2
3
4
S ubfram e 1
T im e S lot 1
M2
M2
M2
M2
T im e S lot 2
M3
M3
M3
M3
T im e S lot 3
M1
M4
M5
M6
T im e S lot 4
M2
M2
M2
M2
T im e S lot 5
M3
M3
M3
M3
DH Subsystem—Design
DH Design and Sizing
Software Engineering
DoD Software statistics (The Problem)
DOD Software Expenditures
Used as delivered
(according to one Army Study)
2%
Used after changes
3%
20%
46%
Used, but abandoned or
reworked
29%
51% of all failures are
blamed on bad requirements
(by the way, only 2% of the
working software is on time,
under budget)
Paid for, never delivered
Delivered, never used
Software Engineering
Software Growth Trends (The Need)
10000
SHUTTLE/OPERATIONAL
SKYLAB 2
APOLLO 7
B-1B
MANNED SYSTEMS
E-2C
P-3A
UNMANNED SYSTEMS
F-15
APOLLO 7
F-111
MISSILE
GALILEO
S-3A
APOLLO 11
MERCURY 3
C-17
PROJECTED
P-3A
AWACS
GEMINI 2
B-2
F-16 C/D
F-111
PERSHING 11(ED)
SKYLAB 2
TITAN 34D (IUS)
VIKING
C-5A
PERSHING 11(AD)
A-7D/E
PERSHING 1
TITAN
1960
POSEIDON C3
PERSHING 1A
SURVEYOR
65
UNMANNED
TRIDENT C4
GEMINI 3
70
UNMANNED INTERPLANETARY
VOYAGER
TITAN IIIC
Flight Date
F-15E
SHUTTLE/OFT
B-1A
GEMINI 3
10
1
SHUTTLE/OFT
APOLLO 17
GEMINI 12
1000
Thousands of
Code Memory
Locations 100
(i.e. size of
executable
software)
F-22
PROJECTED
MISSION CONTROL: GROUND STATION
MANNED A/C
MANNED SPACE
MARINER
VENUS MERCURY
75
80
MANNED SPACE CONTROL
85
90
95
Software Engineering
Software Increasingly matters
Software Engineering
What 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 components
shoved 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