Deep Space Communication The further you go, the harder it gets. D. Kanipe, Sept. 2013

Transcription

1 Deep Space Communication The further you go, the harder it gets D. Kanipe, Sept. 2013

2 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

3 Deep Space Network

4 Canberra, Australia

5 DSN Coverage

6 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

7 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

8 The Long Distance Problem Comm performance α 1/distance 2 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

9 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

10 No GPS Deep Space Navigation 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

11 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 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

12 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 = 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 10 15