30th AIAA/USU Conference on Small Satellites SSC16-IX-01
Colin Sheldon, PhD 240-228-8519 [email protected]
UHF Phased Array Ground Stations for Cubesat Applications
Colin Sheldon, Justin Bradfield, Erika Sanchez, Jeffrey Boye, David Copeland and Norman Adams
10 August 2016
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Outline § Extending Traditional Ground Station Capabilities
§ Phased Array Ground Station Requirements and Architecture
§ Key Challenges
§ COTs Component Prototype
§ Experimental Results
§ Future Work
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Extending Ground Station Capabilities
Traditional ground stations and phased array ground stations offer complementary capabilities § Current Challenge: Single visible cubesat tracking and high rate communications
Ø Solution: Traditional ground station with single beam and high G/T* § Future Challenge: Managing small satellite constellations in an increasingly crowded sky
Ø Solution: Software-defined phased array with multiple simultaneous beams and modest G/T *G/T is the ratio of antenna gain to noise temperature, an antenna figure of merit
High-rate communications with singe narrow beam
Simultaneous lower-rate communications with multiple beams
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Antenna Element Pattern
What is a Phased Array? § Phased arrays are antenna
arrays that combine signals from individual antenna elements to achieve an enhanced radiation pattern with respect to a single antenna
§ The enhancement may take the form of a narrower main beam and/or the ability to electrically steer the direction of the array’s main beam
§ Digital (software) beamforming offers multiple simultaneous beams pointed in different directions
Antenna Element
Antenna Element
Antenna Element
Possible Array Patterns
S
Steerable Narrow Beam
Multiple Simultaneous Beams
IRAD Focus
Possible Array Patterns
S
Steerable Narrow Beam
Multiple Simultaneous Beams
IRAD Focus
Possible Array Patterns
S
Steerable Narrow Beam
Multiple Simultaneous Beams
IRAD Focus
Beamformer
Possible Array Patterns
S
Steerable Narrow Beam
Multiple Simultaneous Beams
IRAD Focus
Possible Array Patterns
S
Steerable Narrow Beam
Multiple Simultaneous Beams
IRAD Focus
Possible Array Patterns
Software Beamformer RF Beamformer
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Ground Station Architecture Requirements Services at all levels are entirely software defined!
§ Beams formed within the phased array can be requested and instantiated on demand limited only by available processing
§ Signal flow to/from the beam former and all subsequent processing uses standards based streaming transport such as VITA-49 over IP
§ Beams are independent: the instantiation of a new beam to track an additional target does not disturb the conduct of any passes already in progress
§ Each processing step required for a service is dynamically instantiated
§ The software architecture supports different levels of service and different user interfaces transparently
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Software-Defined Ground Station Architecture
§ Electronic beam steering performed in software satisfies requirements § Scalable system architecture with reference signal distribution to each element
Array Element
Array Element
Array Element
Example UHF/VHF element with full duplex
capability
Gigabit Ethernet
GPS Disciplined Reference Signals
PC
User Network
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Challenge: Cost § Example cost breakdown of a
UHF receive only phased array Ø Based on the cost of a 4-element
proof-of-concept prototype Ø COTs parts total ~$15K
§ SDR cost exceeds combined cost of additional components Ø Prototype uses $2K class SDR Ø RF component cost could
overtake SDR cost if additional capabilities are required
§ The use of low cost SDRs introduces implementation challenges
Antenna
RF
SDR
Mount
Misc
$2K Class SDR $20K Class SDR
$200 Class SDR $20 Class SDR
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Challenge: Low Cost SDR Performance
Reducing SDR cost introduces implementation challenges
§ Higher cost SDRs generally reduce implementation complexity Ø Example: Ettus SDRs can be GPS reference locked using connectorized COTs parts
§ Alternative system architectures include Ø Digital downconversion – requires higher sample rate (high cost) ADCs Ø Sub-sampling downconversion – requires wide front end analog bandwidth and sharp filters
Challenge System Limitation Potential Mitigations Notes
DC Offset
Useable Bandwidth
Frequency Offset (narrow band) Post-processing (wideband)
Real time compensation may be difficult
I/Q Imbalance Calibration and/or Post-processing Online calibration could be performed by injecting a calibration signal Sample
Synchronization Calibration
Reference Signal Distribution
Phase/Frequency Drift
COTs or Custom GPS referenced sample clock and/or local oscillator signals
Coherent signal processing relies on stable (or known) phase offsets between channels
Radio Imbalance SNR Improvement Component screening Higher cost components may have tighter performance tolerances
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Challenge: COTs Antenna Performance
§ Ideal antenna element pattern Ø Compensates for variation in link range vs elevation Ø Attenuates terrestrial interferers with low gain near the horizon
§ Example COTs antenna pattern attenuates array beams beyond +/-50 deg § Helical antenna could be designed to approximate the desired gain pattern
Angle (deg)-100 -50 0 50 100
Nor
mal
ized
Ant
enna
Gai
n (d
Bi)
-15
-10
-5
0
Antenna Element PatternSimulated Beam Patterns
Antenna Element Patterns Simulated COTs 1x4 Antenna Array Beam Patterns
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UHF Phased Array Prototype
§ 1x4 phased array prototype fully composed of connectorized COTs components Ø M2 Antenna Systems – Antennas and Supporting Hardware Ø Minicircuits – RF Components Ø Ettus Research – SDRs and Supporting Components
SDR
+5V / 1A LNA P/S
PC
SDR
SDR
SDR
8-channel1PPS & 10MHz GPS Disciplined
Reference
Gigabit Ethernet Switch
GPS Antenna
MIMO Cable
MIMO Cable
M2 Antenna Systems
Minicircuits
Ettus Research
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JHU/APL Rooftop Experiment
§ Test setup on the roof of the Space Exploration Sector building on the JHU/APL campus in Laurel, MD
§ Linear array configuration is suitable for polar orbiting cubesats achieving a reasonably high peak elevation
Ø Simple sighting of the phased array along the North-South compass direction
Ø Array beam is fan shaped with the narrow dimension of the beam along the electronically steered North-South direction
§ Selected Cute-1 for RX-only experiment due to reliable detection at JHU/APL
1x4 Antenna Array
RF Frontend 4-channel SDR
GPS disciplined reference
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Cute-1 ground track over JHU/APL campus May 5th 2016
AOS
LOS
Array
N
S
EW270
225
180
135
90
0
70 50 30 10
45 315
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Offline Signal Processing
§ Signal processing performed offline in Matlab on recorded signals
§ Phase shifts can be calculated from expected cubesat trajectory
§ Acquisition and tracking algorithms could be used to point the phased array beams
Record Signals
Detect and Track Carrier Signal
Apply Phase Shifts
Remove Relative Sample Offsets
SNR Weighting
Software Beamformer
3rd Order PLL
Phased Array Beam
Coherent Summation
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Experimental Results
§ Example PLL Output § Characteristic Doppler
shift is evident § Carrier tracking
maintained for the length of the pass
§ Measured SNR for individual elements and tracking beam
Ø Differences in element level SNR due to variations in radio performance and multipath
§ Maximum theoretical SNR improvement over a single channel for a 4-element array is 6 dB
Ø Signal power increases 12 dB and noise power increases 6 dB
§ SNR of the coherent sum stays relatively flat despite single channel fades
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Potential Future Work
§ Develop infrastructure for on-demand user services
§ Incorporate lower cost SDRs to reduce overall system cost
§ Extend capability to full duplex VHF/UHF operation
