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Transcript
ELEC-E4240
Satellite systems
Satellite
communication
and
communication
subsystem
Jari Hänninen, Jaan Praks
Telstar 1 satellite
History
History
• October 1945 Arthur C. Clarke: the idea of geostationary satellites,
the Wireless World magazine ("Extra-Terrestrial Relays", pp. 305–8)
• October 1957 Sputnik I: the first satellite
• January 1958 Explorer I: the first US satellite
• August 1960 Echo I: a passive reflector balloon
• July 1962 Telstar I: the first communications satellite (MEO, C band)
• July 1964: Intelsat is established
• August 1964 Syncom 3: the first geostationary satellite
• April 1965 INTELSAT I (Early Bird): the first geostationary satellite for
cross-Atlantic communications
Why do we communicate?
• Platform
• Telecommand (orbit control, system configuration)
• Telemetry (satellite health monitoring)
• Payload
• Telecommunications payload
• Scientific or observation payload
• Usually downlink only
Typical customer requirements for telecom
system
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Type of relayed signals
Capacity
Coverage area
Signal strength and quality; types of ground terminals
Connectivity (switchboarding)
Availability
Lifetime (LEO 7 / MEO 12 / GEO 12–15/20+ years)
Flexibility
Security
Communication satellite orbits
Roughly 420 satellites
Sat TV
MEO constellations
Iridium
LEO Constellations
LEO communication has significant advantages as satellites are
closer to Earth
Pros
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Less free space losses
Smaller antennas
Smaller terminals
Two way communication with hand held terminals
Smaller delays
Cons
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Long revisit time
Large Doppler shift
Tracking is usually needed
A typical
satcomms
system
Credit: Fortescue et al.:
Spacecraft Systems Engineering
Technical constraints
• The usual suspects:
• Transmitter power
• Receiver sensitivity
• Interference
• Environment
• Available components etc.
• Frequency allocations (politics…)
• But in satellite systems also
• Mass & dimensions
• Available power
• Survivability (launch, space environment)
The challenges
Free space loss & atmospheric effects
• Radiation power density is inversely proportional to the square of
the distance from the source (in the far field)
• Satellite communication links are very long compared to terrestrial
microwave links—huge free space loss
• Additional attenuation due to atmospheric losses: a big issue for
frequencies above 20 GHz (K-Band), can be a problem for X-band
in heavy thunder storms and low elevations
•
(band names:
http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?reload=true&punumber=8332)
• The major absorbing medium is water (water vapor and rain)
Atmospheric attenuation
Diverse effects of rain (1)
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Attenuation
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On most satellite links above 10 GHz rain attenuation limits the availability of the system and needs to be
calculated
The long-term behaviour of rainfall rate (mm/h, a key quantity) is described by an exceedance curve: it gives the
percentage of time (usually a of year) that the rainfall rate exceeds a given value
ITU developed rainfall rate exceedance contour maps; for example, ITU-R P.837 includes contours for rain rate
exceeded for 0.01 % of the average year
To compute the attenuation for a given time percentage (for example, for 0.01 % of a year = 53 minutes of a year)
determine the rainfall rate (mm/h) for the time percentage
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calculate the specific attenuation of the signal at this rainfall rate (dB/km)
find the effective length of the path with this specific attenuation [difficult]
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The specific attenuation γR = k (R0.01)α (dB/km), where R0.01 is the rainfall rate measured for 0.01 % of the
average year and k and α are polarisation-dependent coefficients, please see ITU-R P.838
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The rain attenuation A = (specific attenuation) × (effective path length in rain) (dB) – The ITU-R
procedure for predicting slant-path rain attenuation for geostationary earth orbit (GEO) satellite uplink
paths is contained in Section 2.2.1.1 of ITU-R P.618; in addition one needs
• ITU-R P.839 ITU-R P.837 ITU-R P.838, ITU-R P.678 and possibly some others, please see ITU-R P.618
• For non-GEO satellites additional computations are required
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Diverse effects of rain (2)
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Addition of noise
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At frequencies below 50 GHz, rain attenuation is mostly caused by absorption (rather than
by scattering of the signal energy out of the path)
Any absorber will act as a blackbody radiator (emitting white Gaussian noise at frequencies
below 300 GHz)
Rain will cause signal attenuation (above), depolarisation (below), and an increase in sky
temperature
Depolarisation
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Successful orthogonal polarisation frequency sharing (dualpolarisation frequency reuse)
requires sufficient isolation between two orthogonal polarisations at the receiving antenna
(the wanted polarisation is copolarisation and the unwanted is crosspolarisation)
Signals are never purely polarised: energy is converted from one polarisation to another
(depolarisation occurs)
If asymmetric rain drops or ice crystals exist in the transmission medium, depolarisation
will happen
Spacecraft RX/TX limitations
Basic Communications/Data Rate
Constraints (RF)
Signal to Noise Ratio Available signal to noise ratio may limit ability of
ground station to track the signal or limit data rate. Signal to noise ratio
may be limited by:
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Power of transmitter
Antenna size on board and/or on ground
System temperature of receiving antenna
Bandwidth (the more bandwidth, the higher data rate) Limits depend on:
• Type of mission (near Earth or deep space)
• Frequency band
• Technical implementation (e.g. modulation scheme)
Flux Density RF flux density may not exceed certain values on Earth
surface
Carrying digital signals: keying
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For transmitting bits (or groups of bits = symbols) over a radio link we need a carrier
(sine wave) and a keying scheme (modulation)
We must combat noise and intersymbol interference (at least)
Intersymbol interference (= consecutive symbols affect each other) arises from
filtering of a signal; a bandwidth of at least half the symbol rate (baseband
transmission) or at least equal to the symbol rate (double-sideband passband
transmission) is required
Selecting the proper modulation method, using matched filters ([square-]root-raisedcosine filters), and finding a suitable multiple access method are an art
In satellite communications constant-envelope modulations are preferred, especially
phase-shift keying (PSK) is used
Multiple access: link capacity sharing by allocating bandwidth, time, or codes to
users
Modulation and keying
Noise
• All warm matter (T> 0 K) emits noise (blackbody radiation)
• The noisiness of amplifiers and receivers can be expressed with
• noise temperature Tn (the [equivalent] temperature of a noisy resistor at
the input of a noiseless amplifier, such that the output noise power of the
amplifier equals the true one)
• noise factor F (a factor needed to multiply a reference temperature to
obtain the “true” equivalent temperature of a resistor at the input of a
noiseless amplifier), F = 1 + Tn/T0, usually T0 = 290 K
Noise
• The first amplifier of an amplifier chain largely determines the
overall noise performance of a system
• Attenuators attenuate a signal and add noise
• The environmental noise received by an antenna is added to the
system noise
Eb/N0 and probability of error
• Eb: energy per bit; N0: noise density
• There is a function that relates the probability of an error (BER) to Eb/N0
for certain modulation schemes
• The communications link designer must decide how much error he can
accept on the link and find the corresponding Eb/N0
Example: BPSK BER
Using coding
It becomes possible
to obtain coding
gain, i.e. to use a
noisier signal to get
the same BER than
without coding
Antennas
Antenna—the crucial part
•
A component for converting free-space electromagnetic waves into guided
waves to feed circuits (and vice versa)
Low Gain Antennas
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Coverage of ½ sphere
Gain -3dB to 0 dB
Isoflux Antennas
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Coverage of Earth with increase at low elevations
Gain X-band about 6 dBi (lower gain in S-band due to size limit)
Medium Gain Antennas MGA)
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Mild directional coverage
Simple construction (e.g. horn), 1-3 m in diameter
Gain (S or X-band) 15 dBi to 20 dBi
High Gain Antennas (HGA)
•
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Parabolic dish, 1-3 m in diameter
Gain (X-band) 30 to 45 dBi
The Friis transmission formula
The link budget
• Writing the signal-to-noise-power-density ratio in decibels to a table
gives us the link budget
• Decibels express power ratios: PdB = 10 lg(P/P0)
• Using a spreadsheet programme allows experimenting with the system
parameters: antenna gain (size), transmitter power, coding gain &c.
• For large aperture or reflector antennas
• G = 4πηAA/λ2 ≈ 33000/(Δϴq3 dB,degrees) 2,
• where ηA is the aperture efficiency and A is the area of the antenna
• 3-dB beamwidth Δϴq3 dB,degrees ≈ 75 λ /D,
• where D is the diameter of a (circular) antenna
A geostationary link budget
GPS L1 Link budget
Communication and pointing budget
Satellite communication strategy
affects strongly to satellite
pointing and stabilization strategy
and antenna design.
Magellan
Magellan used the same
antenna for radar imaging and
Earth communication and had
to turn the satellite every orbit.
Regulations
(frequency coordination)
Frequency usage of any satellite should be coordinated with
ITU
• ITU coordinates frequency usage with all countries in the world
• The coordination process can take years (no more than 7 years)
• Satellite communication has special reserved frequency bands
Amateur satellites can use special bands dedicated to Radio
Amateurs
• Radio Amateur bands are coordinated by International Radio
Amateur Union (IARU)
• Amateur bands licences are easier and quicker to get
• IARU notes ITU on frequency usage
Ground segment
Ground segment availability defines
satellite operations strategy.
• Polar orbits have best ground station
location near the pole
Ground segment
K-sat antenna cluster at Swalbard
ESA Payload Data Ground Segment
(PDGS)
PDGS schematic (Credit: ESA)
Redundant
UHF COM
Redundant
OBC on Linux
Spectral Imager AaSI
ADCS Star tracker
Aalto-1
S-band COM
GPS
EPS
3U CubeSat
4 kg
3-axis stabilization
Plasma Brake
Electronguns
RADMON
Communication system
2 cold-redundant UHF radios based on
CC1125 radio chip + PA
Sensitivity: -120dBm
Max TX power. 1.2W
Mass: 56.4g
Power consumption: 0.1W (RX), 3.5W (TX)
UHF - 437.220 MHz
Beacon
Morse code (CW)
9600 baud FSK with G3RUH
recursive scrambler and AX.25
protocol
Command link
Bidirectional, half-duplex
9600 baud FSK with additive
scrambler
S-band - 2.402 GHz
Downlink
500 kilobaud MSK
Aalto-1
The Finnish Student Satellite
iADCS 100
BST, Hyperion Technologies
Mass: 330 g
Power: 1.4 W nominal
Integrated star tracker
30 arcsecond determination accuracy
Aalto-1
The Finnish Student Satellite
Radio Licences
Aalto-1 has all radio licences
Aalto-1
The Finnish Student Satellite
Ground Station (OH2AGS)
Aalto-1
The Finnish Student Satellite
Future
Currently FCC is reviewing 21 applications to
operate global broadband satellite constellations!
Keep eye on:
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SpaceX
Boeing
LeoSat Enterprises
O3b Networks
Spire Global
ViaSat
Audacy
OneWeb
Karousel LLC
Oneweb
Has ordered 890 (sic!) Ku band
nanosatellites from Airbus
Defense and Space
Optical communication
Because of increasing traffic in radio
frequencies and slow regulatory process
increasing amount of communication
concepts move to optical
communication.
Pros and cons
• Less restrictions
• High throughput
• Less risks of eavesdropping
• Needs less energy
• Needs very accurate attitude
control