Transcript No Slide Title

Mission
Attrition
Gets used
Works
Launched
Not preempted
Tech Success
Stay Committed
Committed Resources
Affordable: (ROI>1)
Designs
Good Ideas
Ideas
Not yet Ideas
1s
Engineering 176 Meeting #7
10s
100s 1000s 10,000s 105
106
The Lineup
• 1 - Introduction
• 2 - Propulsion & ∆V
• 3 - Attitude Control &
instruments
• 4 - Orbits
& Orbit Determination
• 5 - Launch Vehicles
• 6 - Power
& Mechanisms
(finish tonight)
• CoDRs
(final 2 tonight)
• 7 - Radio & Comms (3/6)
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•
Engineering 176 Meeting #7
Radio Concepts
Spectrum usage
Link margin & Orbits
Modulation
Antennas
8 - Thermal / Mechanical
Design. FEA (3/20)
9 - Reliability (3/13)
10 - Digital & Software
11 - Project Management
Cost / Schedule
12 - Getting Designs Done
13 - Design Presentations
Design Roadmap
You Are
Here
Define
Mission
Solutions &
Tradeoffs
Concept
Propulsion
/ ∆V
Comms
Attitude
Determine
& Control
Parts
Specs
Materials
Fab
Launch
Ground
Station
Thermal /
Structure
Deployables
Analysis
Info
Processing
Next week
I want you
Top Level Design
to visit here
Mass
Suppliers / Budgets
Orbit
$
Power
∆V
Link
Bits
Iterate Subsystems
Detailed Design
Engineering 176 Meeting #7
Conceptual
Design
Requirements
Final Performance
Specs & Cost
Last week: Power
Engineering 176 Meeting #7
Freja: did x 8
• Definitely not moving - for a long (or too
long) time
• 1-g vs. 0-g (& vacuum) matters
• Tolerance v. launch loads
• Vacuum welds, lubricants, galling
• Creating friction - rigging
• Static strength, dynamics, resonance
• Safety inhibits (it’s physical)
Engineering 176 Meeting #7
Galileo: didn’t x 1
And...Deployables
• Flaws, cracks, delamination,
vibration loosen/tighten
• Minute population & test
experience (the Buick antenna)
• Total autonomy
• High current actuation
• Statistics - ways to work v. not
Due tonight
• Preparation: Radios & • Technical requirements:
Create a list of technical
Comms
requirements - even if it
• SMAD Chapter 13
has “TBD”s in it.
• TLOM Chapters 7,8,9
(+ revisit mission rqts)
• Systems design / CoDR:
create a good looking
“cartoon” set of the
spacecraft, orbit and
ground segments
• Something Physical
Engineering 176 Meeting #7
• Tools selection:
– Finite element
– Design and layout
– Presentation & Graphics
• Tech Design / Analysis / Suppliers:
– Structure / Thermal
– Design and layout
– Orbit / Launch
– ACS / Propulsion // $$$
The plan for March 13
• Part 1 (assignment): Radio Strategy:
- what & why & why not the other options
• Spacecraft Tx Power, modulation, antenna selection, l
• same for Ground Station
• Up and down link calcs
• Part 2: (reading on reliability):
– SMAD 19.2 (15 Pages worth reading / skimming)
– TLOM 15 (clean rooms etc.) •Tech Design / Analysis /
• Part 3: Design
– What you are going to build
– Requirements Document
Engineering 176 Meeting #7
Suppliers:
–Structure / Thermal
–Design and layout
–Orbit / Launch
–ACS / Propulsion
– $$$
Electromagnetic Spectrum Map
- SHF and some radars:
15 - 50 GHz
- UHF / L-band: Television,
spacecraft, cordless & cellular:
500 MHz to 1 GHz
- Short Wave radio:
International broadcast,
amateur “HF”, worldwide nonsatellite comms:
1.6 Mhz to 50 MHz
1 cm
10 GHz
1m
100m
Telephone / RTTY baseband:
2400 - 9600 Hz
1000 km
Engineering 176 Meeting #7
1
Ghz
100 MHz
10 km
Power transmission:
60 Hz
100 GHz
10 MHz
1
MHz
100 KHz
10 KHz
1
10
- Microwave Terrestrial &
satellite, µwave ovens, Radio
Astronomy:
1 GHz - 15 GHz
- VHF: FM radio, Taxi, Air
Traffic, Air Nav, VHF Amateur
radio, Little LEOS:
50 MHz to 500 MHz
- AM Radio, medium and long
wave: 180 KHz to 1.6 MHz
KHz
100 Hz
(l)
- (3 - 30) Millimeter wave /
blackbody radiation (10 - 100GHz)
Hz
VLF Comms (eg submarines):
100 - 5000 Hz
Some Radio Facts (?)
• 100 KHz is the low end of the “useful” radio band
• 100 MHz is low end of useful satellite < --- > earth links
• Light and heat are alternatives to radio
-and no license required
• Radio is barely possible: 0.5 W @ 2000 km yields 4 x 10-14 W/m2
• Propagation goes as 1/r2
• Since E=IR, for a 50W antenna, that signal is a µV varying at >
109/second
• Bandwidth and data rate are proportional - mostly:
Shannon’s Law
R(max) = Blog2( 1 + C / N )
Engineering 176 Meeting #7
Spectrum Trades
Spectrum Region
Pros
Below 100 MHz
HF / VHF
l > 3m
( )
100 - 500 MHz
VHF / UHF
• Best link with omnis
• Low cost RF components
Cons
• Ionosphere blocks
• Limited bandwidth
• Big antennas
• Antennas are large
• Hard to provide gain
3m > l > 60 cm
• Highest h (80%)
1 - 2.5 GHz
L & S Bands
30 cm > l > 12 cm
• Commercial gear plentiful
• Good bw (Mbit/s)
• Good Tx h (60%)
• Small, low cost antennas
• Crowded!
• Usually requires
gain antennas
8 - 10 GHz
X - band
l - 3 cm
• Small, high gain antennas
• Less Crowded
• High bw (many Mbit/s)
• Higher cost
• Lower h (<50%)
• Some Wx sensitivity
25 GHz
SHF
l - 1.5 cm
• Huge bw (Gbit/s)
• Very high gain antennas
• Easiest license to obtain
• Wx sensitive
• Slant angle limited
• High Cost
• Low h (<30%)
Engineering 176 Meeting #7
• Limited bw (kbit/s)
Data Rates
• What’s in a baud?
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• Spacecraft data rates
1 to 100: basic pager
100 to 1k: messaging pager
1k to 10k: fax, email, voice
10k to 100k: web surfing,
picture phone, digital radio
– 100k to 1M: Digital LDTV
– 1M to 10M: Digital HDTV
– 10M to 1B: Data,
multiplexing and multi
channel of above
Engineering 176 Meeting #7
– Amsat: 10k
– Advanced micros: 1M
– “Small Sats” (Iridium):
10M - 100M
– Big satellites:
gigabits
per
second
Modulation
•
•
•
•
•
AM: not inherently digital, low efficiency
FM: Easy lock where power is not critical (uplinks)
BPSK QPSK: Inherently Digital and efficient
Spread Spectrum: Low efficiency, bulletproof
Phase Shift
Interpretation
0°
90°
180°
270°
360°
0,0
0, 1
1, 1
1, 0
0, 0
(Same as 0°)
Ranging:
– Round trip time measurement
• good to ~ bit rate i.e. 106 => 100 m (maybe 10m)
• Doppler: Typically 500m to 10 km
– Note: Repeated measures of range and time
allows orbit solution
•
To Avoid:
– Multiple Modulation Schemes
Engineering 176 Meeting #7
• subcarriers etc. (not info dense)
Attrition II
Field degradation
Worst case link
Eb / No (error spec)
Demodulation
Modulation
Noise(Tr, a, sky) BW: No
Receive Aperture
Absorb, polarize
4r2 ( - Gt )
Line / feed losses
Bit rate: Eb
Transmitter nWatts
100
Engineering 176 Meeting #7
102
104
106
108
1010
1012
Notes on Links and Link Margins
• The Link Equation: objective: Eb / Nb large enough to detect the signal
within a specific level of uncertainty (error rate)
Eb / Nb = Power x Lossl x Gaint x Ls x La x Gr
kx Tx R
where
Lossl = line loss
Gaint = transmit antenna gain
Ls
= space loss (spreading)
La
= path attenuation
Gr
= receive antenna gain
k = Boltzmann’s const.
T = Temp (K)
R = Data Rate
But...
Gr is not real - it is defined as the ratio of real aperture
to aperture of an isotropic antenna, l2/4 So really it is a
measure of area ratio, not gain. Rx Antenna as noise reducer.
Question: Why is R in the denominator (the No part?)
Engineering 176 Meeting #7
(more)
Notes on Links and Link Margins
• How much margin do you need?
- what’s the actual local horizon?
- what’s the penalty for losing lock sometimes?
• Local obstructions are a factor - especially when snow covered
• Variable data rate
• Some tradeoffs:
- sun vs. earth pointing
- sun tracking PV array vs. earth tracking antenna
- data rate vs. contact duration (& # of GSs)
- GS vs. satellite gain
- compression (CPU cost) vs. downlink rate
• It’s amazing the link works at all...
Engineering 176 Meeting #7
Orbit Implications for Comms
• LEO has 1600x easier
link:
– 3x (10dB) smaller
antenna (50x lighter)
– 10x lower power
– 3x (10dB) smaller GS
antenna
Engineering 176 Meeting #7
• But:
– Need 10x more
satellites,
5x more launches
– + Reconstitution
hassles
– Global Coverage!
(whether you want it
or not)
– Constellation
Management
– Cross Linking,
switching, handoffs
Rec’v Gain
Path Distance
Path Loss
Target Signal
Sensitivity
Inspector
Ground
Station
15 dB
55 dB
100 m
36,000 km
51 dB
163 dB
1.5 mW
36,000 km
Engineering 176 Meeting #7
31 W
100 m
Escort Link
Sensor
The costs:
of bit rate, small user terminals and
large coverage area
Availability
- Global Mobile
- Many Locations
- Video
- CD radio
- Telephone
Spacecraft Cost
Engineering 176 Meeting #7
Bandwidth
- Fixed Location
- Paging
Power a baud (bit rate)
Power a (1/antenna dia)2
Power a Service Area
Power a obstacles
(windows, roofs)
• Service Area a orbit
altitude
• Mass a (Antenna dia)3
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•
•
Ground
Engineering 176 Meeting #7
Elements
Station
Antenna Strategies
• Omni (Sputnik)
- 0 dB gain (or less)
- Requires >1 antenna
- Interference fringes
- Downlink power?
Directional (Pioneer)
- huge gain: 24 dB typ.
- requires >1 just in case
- Major ACS impact
Engineering
176 Meeting #7
- Steerable?
• Sector (HETE)
* 3 - 6 dB gain (or less)
* Requires >1 antenna
* Active Control
* but no ACS impact
Link design
• Start with Spacecraft
• What’s the critical link
– Up or down?
• What data rate required
• Frequency considerations
• GS limitations (power, gain)
• Eventually, pin down all but one or
two variables e.g.:
– Space antenna gain
– Modulation method
• Then do a trial link and iterate
• Note: All user links need lots of
margin - 10 dB good, 20 dB better
Engineering 176 Meeting #7
• Some tricks:
– How reliable does the link
need to be? What error rate?
– Coding requires only
computation
– How close to the horizon can
your GS see?
– Is one link critical, the other
not?
– Differentiate master GS from
user terminals
– Burst mode power can be
higher - use batteries
– Scanning a high-gain
antenna
– Spread spectrum - hurts link
but helps sharing and
security (what’s in rqts?)