Copyright 2016 by Robert Stengel. All rights reserved. For educational use only. http://www.princeton.edu/~stengel/MAE342.html Communications Space System Design, MAE 342, Princeton University Robert Stengel • Antenna characteristics • Power transmission and reception • Signals, information, and noise • Analog and digital modulation • Communication link budgets • Laser communications C bits/s ( ) = BW log 2 S N + 1 ! " # $ % & 1 Typical Spacecraft Communications Architecture 2 Pisacane High frequency signals Low frequency signals
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Copyright 2016 by Robert Stengel. All rights reserved. For educational use only.http://www.princeton.edu/~stengel/MAE342.html
Communications! Space System Design, MAE 342, Princeton University!
Robert Stengel•! Antenna characteristics•! Power transmission and
reception•! Signals, information, and noise•! Analog and digital modulation•! Communication link budgets•! Laser communications
C bits/s( ) = BW log2SN
+1!"#
$%&
1
Typical Spacecraft Communications Architecture
2
Pisacane
High frequency signals Low frequency signals
Antenna Gain•! Isotropic (uniform) radiation of power, P, from the center of a sphere of radius, r•! Power per unit area (power density) of the sphere s surface
p = P 4!r2
•! Power received from isotropic radiator over area, S
PS = Sp
•! Power 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
= 4!r2
S
! = beamwidth half-angle
3
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
4
Pisacane
Communications Geometry•! Ground station communication and tracking limited
by its minimum elevation angle, !!•! 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
5
Pisacane
Beamwidth Coverage
•! Broad or narrow coverage may be desired
! (cone) " 21fd
, deg
f = carrier signal frequency, GHzd = reflector diameter, m
Beamwidth of reflector antenna
6Pisacane
One-Way Radio Communication Calculation Nomogram (GE, 1960)
7https://en.wikipedia.org/wiki/Nomogram
Relationship of Antenna Area and Signal Wavelength to Antenna Gain
Geff =4!Aeff
"2
Aeff = effective antenna area, m2
! = c / f = carrier signal wavelength, mc = speed of light " 3#108 m/sf = carrier signal frequency, Hz
Effective antenna gain (transmitting or receiving)
8
Relationship of Antenna Area and Signal Wavelength to Antenna Gain
Power received from the transmitter
Pr = prAr = GtPtAr
4!r2
pr = power density at receiving antennaAr = effective area of receiving antennaGt = gain of transmitting antenna
Pt = transmitted powerr = distance between transmitting antenna and receiving antenna
Power ratioPr (watts)Pt (watts)
= GtAr4!r2
9
Antenna Characteristics
10
Pisacane
Electric and Magnetic Fields of a Dipole Antenna
http://en.wikipedia.org/wiki/Antenna_(radio) 11
Linear and Circular Polarization of Waves
Transmit and receive antennas must be aligned for best communication
Left or right helical rotation of signal
12
Characteristics of Typical Spacecraft Antennas
Conical log spiral
antenna
Gain(dBi) = 10 log Antenna GainIsotropic Antenna Gain
7 data bits plus parity bitWidely used in computers 94 printable characters
40
Manchester Code for Telecommunication
•! Each data bit has one transition and occupies the same amount of time
•! Self-clocking•! Widely used on Ethernet
http://en.wikipedia.org/wiki/Manchester_code 41
Error Detection and Correction•! Parity check (simple)
–! Transmitter•! n Data bits added up•! Parity bit added to make sum
even•! n+1 bits transmitted
–! Receiver•! Check to determine if word is
odd or even•! If odd, odd number of errors has
been detected (but not corrected)–! Column-wise parity check of m
words determines where bits are corrupted
•! Additional bits must be transmitted
•! Turbo code (complex): see
http://en.wikipedia.org/wiki/Turbo_code 42
43
Typical Command and Telemetry Characteristics
Larson, Wertz
Communications Carrier Frequencies
DSCS-3
44Larson, Wertz
Typical Communication Satellite Transponder
Characteristics
45Larson, Wertz
46
GOES Communication Sub-System
Free-Space Laser Communication
Lesh, JPL, 1999
•! Diffraction limit of electro-magnetic beam is proportional to !/d•! ! = Wavelength•! d = aperture (diameter) of beam source•! Radio frequency wavelengths: cm – m•! Optical wavelengths: "m•! Up to 106 less beam spread for optical
communication
47
Lesh, JPL, 1999 48
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 Lesh, JPL, 1999
49
Good News/Bad News for Optical Communication #
Lesh, JPL, 1999
•! 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
50
51
52
LADEE Lunar LaserCom Space Terminal
53
LADEE LaserCom Components
54
Deep Space Network•! Command and telemetry•! 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
55
DSN 64-m Antenna
56
Deep Space Network Coverage
JPL Control Center
57
58
Tracking and Data Relay Satellites
59
Next Time:!Telemetry, Command, Data
Handling & Processing!
60
!!uupppplleemmeennttaall MMaa""rriiaall
61
Signal-to-Noise Ratio per Bit, Eb/No
S = received signal powerN = received noise powerBW = bandwidth of receiver
R = data bit rate
Eb : energy per bitNo :noise power spectral density