Download Deep Space Communication

Deep Space Communication
The further you go, the harder it gets
D. Kanipe, Sept. 2013
Deep Space Communication
Introduction
Obstacles: enormous distances, S/C mass and power limits
International Telecommunications Union (ITU)
Volume of space at distances from the earth 2 million km (about 5
moon radii)
Internationally allocated frequency bands for deep space comm
S-band: ≈ 2GHz (used in early days)
X-band: ≈ 7-8GHz
Ka-band: ≈ 32-34GHz (higher data rates, but weather sensitive)
NASA’s Deep Space Network (DSN)
Three locations ≈ 120° apart
Large and very high power
Overcome losses and spacecraft’s low power antennas and transmitters
One 70m steerable antennas with 20 KW transmitters
Several 34m beam waveguide antenna – current standard
Deep Space Network
Canberra, Australia
DSN Coverage
Transponder
Heart of the S/C Communication System
Requires the most reliability and longevity
Function
Ground Station
Transmits a high frequency carrier signal with encoded commands - UPLINK
Spacecraft
Transponder receives the uplink signal from Earth
Amplifies the signal and decodes commands
Converts to appropriate frequency and transmits signal to Earth - DOWNLINK
Ground Station
Receives and demodulates the return signal
Measures round trip delay time
May also perform other functions
Signal detection, Demodulation, De-multiplexing, Re-modulation, Message routing
Elements of Deep Space Comm
Telemetry
Send data and spacecraft health status to earth
Tracking
Estimating spacecraft trajectory
Commanding
Send commands/data to spacecraft
Radio Science
Byproduct of tracking spacecraft near or behind celestial objects
Atmospheric dynamics
Atmospheric density
Gravity field mapping
The Long Distance Problem
Comm performance α 1/distance2
Barriers
Troposphere
Ionosphere
Solar plasma, etc.
NASA’s Deep Space Network
70m steerable antennas with 20 KW transmitters
34m beam waveguide antenna – current standard
Deep Space missions operate very close to communication efficiency
limits: typically within 1 db
For a spacecraft designed to operate with a 70m antenna
If performance is degraded 1 db, an additional 32m antenna
would be required to make up the difference
Signal Latency
Signal latency: minutes to hours to cover intervening space
Critical decisions often cannot wait – requires S/C autonomy
Spacecraft are usually “sequenced”
Programmed to operate autonomously for long periods of time
Spacecraft must also manage the data they acquire while waiting
to send it to earth
In case of emergencies
“Safing” algorithms
Configure spacecraft to a safe, low energy mode
Deep Space Navigation
No GPS
Requires ground stations
Use precision measurements of the radio signals
Ranging: measurement of the distance to the spacecraft
Doppler: measurement of relative spacecraft motion
Interferometric techniques: use multiple ground antennas to
measure spacecraft angle in the sky.
Augmented by on-board sensors
Gyros
Star trackers
Sun sensors
Photographs of known objects against stellar background
Future Requirements
Deep Space communications requirements
Expected to grow by factor of 10 per decade
About 100 times increase by 2030
Current transponders approaching their performance limit
New transponders are being designed
Development to use can be a long time
What’s coming
Optical Communication: Lasercom
Higher frequency compared to RF and narrower beam
Space missions could return 10-100 times more data with 1% of the
antenna area
Performance increase without increasing mass or power
Virtually unlimited bandwidth
No regulation – other than eye safety
Autonomous and Cognitive Radios
Operational efficiencies
Antenna Arraying
Creates a virtual antenna of a size equal to the sum of the constituents
DSN Facts
Command Power
The DSN puts out enough power in commanding Voyager that it
could easily provide high quality commercial TV at Jupiter.
Transmitted power = 400 kW
Dynamic Range
The ratio of the received signal power to the DSN transmitting
power is like comparing the thickness of a sheet of tissue paper to the
diameter of the earth
Ratio = 1027
Reference Clock Stabilities
The clocks used in the DSN are so stable that they would drift only
about 5 minutes if operated over the age of the universe
1 part in 1015