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Optimizing Satellite Communication With
Adaptive and Phased Array AntennasMary Ann Ingram, Robert Romanofsky, Richard Q. Lee, Felix Miranda, Zoya
Popovic, John Langley, William C. Barott, M. Usman Ahmed, Dan Mandl
John LangleyHalf Moon Bay, [email protected] 30, 04
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Background
• Satellites with Earth Sciencemissions are in low-altitude,polar orbits
• Cost of past ground nodedesigns has allowed only asmall number of groundnodes• Ground nodes must be
located at high latitudes tomaximize satellite visibility
• High latitude locations oftenhave inhospitable climaticconditions
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A New Approach to LEO Ground Nodes
• Many Low-cost GroundNodes• Build from adaptive arrays of
small antennas• Electronically scanned array
elements would have nomoving parts
• Network Large Number ofLow-cost Ground Nodes toProvide Near-continuousCoverage
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Adaptive Arrays as LEO Ground Nodes
Antenna directionalsensitivity
•Antenna patterns adjusted electronically thousands of times per second tofollow users and avoid interference
•Array can be built out of phased array elements or conventional antennaelements
•Capability for multiple access on a single channel
•Data rates vary per link according to the configuration of the adaptive array
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Adaptive Array Concepts
• The outputs of multipleapertures are combinedcoherently to provide thesignal to be demodulated
• An adaptive combinationalgorithm changes inresponse to changes inthe propagationenvironment
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Propagation Environment
• LOS
• Multipath fromterrestrial reflections
• High Doppler, butlow Doppler spread
• Interference?
• Tracking errorsamong arrayelements
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How an Adaptive Algorithm Works
• A feedback control loop iterates on the adaptiveweights 4000 times a second to try to drive theerror to zero
• A training sequence is what the ideal combineroutput should look like
w1
Σ
trainingsequence
RF-to-BB and
PLL
w2RF-to-BB and
PLL
w3RF-to-BB and
PLL
w4RF-to-BB and
PLL
Σ+
- error
adaptivealgorithm
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Adaptive Array Ground Node Configurations
An Adaptive Array Composed of EightMechanically Scanned 1.0 Meter Parabolas
An Adaptive Array Composed of EightElectronically Scanned 500 Element Space
Fed Lens Antennas
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Array Elements
• Adaptive Arrays can be built of manytypes of elements
• The first S-band array is built of fixedelements
• Two electronically scanned arrayelements are under development atGlenn Research Center for use at X-band
• Space Fed Lens
• Ferroelectric Reflectarray
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Focal Surface
Feed Side RadiatingSide
Space-Fed Lens Approach:• Based on discrete lens arraydesign.• Switches used to achievebeam scan• Polarization: LHCP• Frequency: X-band• Scan coverage: ± 55 o
What is it?
• A space-fed lens array is composed of a feed array and a radiating arraywith each corresponding element pair interconnected by transmission lines ofdifferent lengths to radiate a plane wave in the forward direction
• As in the case of reflector antennas, the feed array is space-fed with feedslocated on a focal surface.
Space Fed Lens
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Feed side
Radiating side
Unit Element and Interconnects
• 4 substrate layers, 2 ground planes
• Coplanar strip delay lines
• Metallized via interconnects
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Different Layers of the Lens Array
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Reflectarray Front End
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Reflectarray Front End Testing
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Adaptive Array Demonstration
• The EO-1 was scheduled to transmit 4 kbps(coded) on 7 April 2004 as it passed overAtlanta, GA
• During the pass, we recorded the signalthrough four channels
• Later, we performed adaptive combining ofthe four channels using digital signalprocessing
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Four-Channel Recording of EO-1
7 April 2004
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7 April 2004
Four radio receiversconvert the signalfrom a continuouswaveform, centeredat 2.27GHz, down tocomplex basebanddigitized samples
Four-Channel Reception of EO-1
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Some Initial Results
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Some Initial Results
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Summary & Next Steps
• Adaptive Arrays of small antennas offer a low-cost way to construct ground nodes for LEOEarth Science data reception
• Exploiting multi-path propagation permits lowerelevation acquisition and longer tracking times
• Good agreement with theory has beendemonstrated for strong multipath channels
• Several electronically and mechanically steeredarray elements are possible
• Next year a prototype X-band Adaptive Arraywill be demonstrated
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Ultimately, We Needn’t Stop on Earth...
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This work is being funded through ESTOAIST NRA research on Ground Hybrid
Phased Array Antennas,
PI: Dan Mandl, NASA/GSFC, Code 584
