Post on 25-Apr-2020
transcript
5/7/19
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Seminar 12The Future of Space Flight
Telemetry, Communications, & TrackingRobert Stengel
FRS 148, From the Earth to the Moon Princeton University
Copyright 2019 by Robert Stengel. All rights reserved. For educational use only.http://www.princeton.edu/~stengel/FRS.html
EntrepreneurshipGovernment-Funded Projects
Cost GrowthMarkets
Antennas and Signal PropagationSignal Detection and Noise
Deep Space Network1
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Commercial Space Transportation:Industry Trends, Government Challenges, and International Competitiveness Issues, GAO 12-836T, 2013 [https://www.gao.gov/assets/600/591728.pdf]
Pathways to Exploration: Rationales and Approaches for a U. S. Program of Human Space Exploration, National Research Council, 2014 [https://www.nap.edu/catalog/18801/pathways-to-exploration-rationales-and-approaches-for-a-us-program]
Resources
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https://en.wikipedia.org/wiki/Arianespacehttps://en.wikipedia.org/wiki/Asteroid_impact_avoidancehttps://en.wikipedia.org/wiki/B612_Foundationhttps://en.wikipedia.org/wiki/Bigelow_Commercial_Space_Stationhttps://en.wikipedia.org/wiki/Blue_Originhttps://en.wikipedia.org/wiki/Chinese_space_programhttps://en.wikipedia.org/wiki/Colonization_of_the_Moonhttps://en.wikipedia.org/wiki/Commercial_Crew_Developmenthttps://en.wikipedia.org/wiki/Future_of_space_exploration
Resources
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https://en.wikipedia.org/wiki/Japanese_space_programhttps://en.wikipedia.org/wiki/List_of_government_space_agencieshttps://en.wikipedia.org/wiki/List_of_private_spaceflight_companieshttps://en.wikipedia.org/wiki/Private_spaceflighthttps://en.wikipedia.org/wiki/Space_debrishttps://en.wikipedia.org/wiki/Space_industry_of_Russiahttps://en.wikipedia.org/wiki/Space_tourismhttps://en.wikipedia.org/wiki/SpaceXhttps://en.wikipedia.org/wiki/The_Planetary_Societyhttps://www.nasa.gov/about/whats_next.htmlhttps://www.nasa.gov/exploration/systems/sls/overview.html
Resources
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Discussion Topics§ Perpetuating the human race throughout
the solar system/galaxy§ Colonization of Moon and Mars§ International cooperation and competition§ AI§ Business opportunities§ Space tourism§ Aerospace planes§ Astronomical satellites§ Deep space probes
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Discussion Topics§ Critical programs
§ Weather/Earth resources satellites§ Communications satellites§ Asteroid defense§ Surveillance
§ Launch vehicles§ Expendable§ Reusable§ Commercial
§ Rocket propulsion§ CubeSats§ Space junk
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Discussion Topics§ Federal space programs
§ Civilian§ Military
§ Weaponization of space§ Commercial space development
§ Mining asteroids and the Moon§ Sub-orbital hypersonic transportation§ Risk-taking§ Human spaceflight
§ Travel to Moon and Mars§ Farther?§ Extended weightlessness§ Radiation exposure
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Telemetry, Communications, & Tracking
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Antenna Gain• Isotropic (uniform) radiation of power, P, from the center
of a sphere of radius, r• Power per unit area (power density) of sphere�s surface
p = P 4πr2
• Power received from isotropic radiator over area, S
PS = Spψ = beamwidth half-angle
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Antenna GainPower received over area, S, if all power is
focused uniformly on that area by antenna with gain, G
�
PS = GSpS = P
• Power density in S with idealized focused antenna
�
pS = P GS• Idealized antenna gain
�
G = PSpS
= 4πr2
S10
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Relationship of Antenna Area and Signal Wavelength to Antenna Gain
�
Geff =4πAeff
λ2
c = speed of light ≈ 3×108m / sf = carrier signal frequency,HzAeff = effective antenna area,m
2
λ = carrier signal wavelength,m= c / f
Effective antenna gain (transmitting or receiving)
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Relationship of Antenna Area and Signal Wavelength to Antenna Gain
Power received from the transmitter
�
Pr = prAr = GtPtAr
4πr2
pr = power density at receiving antennaAr = effective area of receiving antennaGt = gain of transmitting antenna
Pt = transmitted powerr = distance between transmitting and receiving antennas
Power ratio
�
Pr(watts)Pt (watts)
= GtAr
4πr212
Power Ratio (decibels, dB) = 10 log10Pr (watts)Pt (watts)
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Antenna Characteristics
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Typical Antenna Pattern• Gain vs. angle from boresight axis (2-D)• Geff is average gain over beamwidth• Beamwidth variously defined as –3 dB cone
angle or half-angle
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Characteristics of Typical Spacecraft Antennas
Conical log spiral antenna
Gain(dBi) ! 10 log Antenna GainIsotropic Antenna Gain
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Alternative Expressions for Power Ratio
Pr (watts)Pt (watts)
= GtAr4πr2
= AtArλr( )2
=
AtAr f2
cr( )2= GtGrλ
2
4πr2= GrAt4πr2
Power ratio in decibels
10 log10PrPt
⎛⎝⎜
⎞⎠⎟(dB) =
Gt (dB)+10 log10 Ar (dB)−10 log10 4π (dB)− 20 log10 r(dB)16
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Detected Power
�
PrPt(dB) = Pr
Pt
⎛
⎝ ⎜
⎞
⎠ ⎟ ideal
(dB) − Absorbtion(dB) − Rainfall(dB)
± Multipath(dB) −CrossPolarization(dB)
Pr = Pcarrier + Pinformation ≈ PcarrierPd = Pr + Pn
• Receiver�s detected power from– transmitter�s carrier signal– information signal– noise
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Noise Sources
Receiver thermal and �front end� noiseAtmospheric, cosmic, solar, and man-
made noisePn =
Pnreceiver + Pnatmosphere + Pnsolar + Pncosmic + Pnman−made18
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Receiver NoisePn = kTW (watts)
T = 290 10NF (dB)/10 −1( )= 290 F −1( )
�
k = Boltzmann' s constant =1.38 ×10−23w − s /°KT = effective receiver temperature,°KW = bandwidth,HzNF = receiver noise figureF = receiver noise factor
Power and temperature
Power density No = Pn /W= kT (watts /Hz) 19
Receiver Noise
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Solar NoiseNoise proportional to (wavelength)n or 1/(frequency)n
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Cosmic and Atmospheric
NoisePn ∝λ n
∝ 1 f n
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Signal-to-Noise Ratio and Information Content
�
SN
= Pr(watts)Pn (watts)
�
SN
dB( ) = Pr(dB) − Pn (dB)
�
Pr(dB) ≡10logPr(watts)1watt
⎛
⎝ ⎜
⎞
⎠ ⎟ Channel capacity
C bits / s( ) =W log2S + NN
⎛⎝⎜
⎞⎠⎟
=W log2SN
+1⎛⎝⎜
⎞⎠⎟
�
W = bandwidth,Hz23
Information Bandwidthfc = carrier frequency,Hz
W = Δf = f2 − f1 = information signal bandwidth,Hz
• Low-frequency information signal superimposed on (i.e., modulates) high-frequency carrier radio signal for transmission
• Information signal formats– Analog (continuous)– Digital (discrete)– Digitized analog (i.e.,
A/D conversion)
Power spectral density of transmitted signal
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Signal-to-Noise Ratio per Bit, Eb/No
�
S = received signal powerN = received noise powerW = bandwidth of receiverR = data bit rate
�
Eb : energy per bitNo : noise power spectral density
�
Eb
No
= SNWR
How would you express this in decibels?
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Link Budget for a Digital Data Link
Eb
No
=
PtLlGtLsLaGr
kTsR
�
Pt = transmitter powerLl = transmitter − to− antenna line lossGt = transmit antenna gainLs = space lossLa = transmission path lossGr = receive antenna gaink = Boltzmann' s constantTs = system noise temperature
�
Eb
No
= SNWR
Link budget design goal is to achieve satisfactory Eb/No by choice of link parameters
… in decibels?
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Typical Spacecraft System Noise Temperature
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Free-Space Laser Communication• Diffraction limit of electro-magnetic beam
is proportional to λ/d• λ = Wavelength• d = aperture (diameter) of beam source• Radio frequency wavelengths: cm – m• Optical wavelengths: μm• Up to 106 less beam spread for optical
communication
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Lesh, JPL, 1999 29
Optical Communication Advantage Compared to Ka-Band RF
(One-Way Pluto example, same power input)dB Factor Comparison13 Data Rate Increase 4.9 kbs vs. 270 bps26 Smaller Spacecraft
Aperture10 cm vs. 2 m
4 Less Transmitted Power Required
1 W vs. 2.7 W
7 Lower Transmitter Efficiency
5% vs. 28%
2 Lower System Efficiencies
24% vs. 40%
3 Atmospheric Loss -10 Smaller Ground
Station10 m vs. 34 m
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Good News/Bad News for Optical Communication
• Good news• Higher bit rates possible• Optical beams are narrower• Energy concentrated on receiver
• Bad News• Optical beams are narrower• Narrow beams must pointed more precisely• Must track intended receiver• RF may be preferred for acquisition,
command, and tracking• Effects of cloud cover
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LADEE Lunar LaserComSpace Terminal
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LADEE LaserCom Components
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Deep Space Network• Radar tracking (range,
elevation, and azimuth)• Radiated signal power drops
off as 1/r2• Reflected return signal power
drops off as 1/r2• �Skin track� return signal
power drops off as 1/r4
• Beacon (or transponder) on cooperative target– Receives radiated signal– Re-transmits fresh signal
• Known time delay• Different frequency
– Return signal power drops off as 1/r2
Goldstone 70-m Antenna
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Deep Space Network Coverage
JPL Control Center
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Communications Geometry• Ground station communication and tracking limited by its minimum
elevation angle, γ• Fixed (non-steerable) antenna must have sufficient beamwidth to transmit
or receive• Antenna gains and radiated power must be adequate, given slant range
and noise environment
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Beamwidth Coverage
• Broad or narrow coverage may be desired
�
ψ(cone) ≈ 21fd,deg
f = carrier signal frequency,GHzd = reflector diameter,m
Beamwidth of reflector antenna
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One-Way Radio Communication Calculation Nomogram (GE, 1960)
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One-Way Radio Communication Calculation Nomogram (GE, 1960)
NomogramComponents
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Atmospheric Attenuation, Multipath, and Ionospheric Effects on Space-Earth
CommunicationPrPt(dB) = Pr
Pt
⎛⎝⎜
⎞⎠⎟ ideal
(dB)− Absorbtion(dB)− Rainfall(dB)
±Multipath(dB)−CrossPolarization(dB)
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Analog Amplitude, Frequency, and Phase Modulation of Carrier Signal
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Digital Amplitude-, Frequency-, and Phase-Shift Modulation of Carrier Signal
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Bit Error Rate vs. Eb/No• Goal is to achieve
lowest bit error rate (BER) with lowest Eb/No
• Implementation losses increase required Eb/No
• Link margin is the difference between the minimum and actual Eb/No
• BER can be reduced by error-correcting codes– Number of bits
transmitted is increased
– Additional check bits allow errors to be detected and corrected
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Supplemental Material
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Electric and Magnetic Fields of a Dipole Antenna
http://en.wikipedia.org/wiki/Antenna_(radio) 47
Communications Carrier Frequencies
DSCS-3
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Typical Command and Telemetry Characteristics
TDRS
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Typical Communication Satellite Transponder Characteristics
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