UHF Phased Array Ground Stations for Cubesat Applications

Transcription

1 UHF Phased Array Ground Stations for Cubesat Applications Colin Sheldon, Justin Bradfield, Erika Sanchez, Jeffrey Boye, David Copeland and Norman Adams 10 August 2016 Colin Sheldon, PhD

2 Outline Extending Traditional Ground Station Capabilities Phased Array Ground Station Requirements and Architecture Key Challenges COTs Component Prototype Experimental Results Future Work 2

3 Extending Ground Station Capabilities High-rate communications with singe narrow beam Simultaneous lower-rate communications with multiple beams 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 30th AIAA/USU Conference on Small Satellites 3

4 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 Antenna Element Antenna Element Antenna Element Pattern Antenna Element 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 Multiple Simultaneous Beams Beamformer Possible Array Patterns Steerable Narrow Beam Software Beamformer RF Beamformer 4

5 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 5

6 Software-Defined Ground Station Architecture Array Element User Network Example UHF/VHF element with full duplex capability Array Element Gigabit Ethernet PC Array Element GPS Disciplined Reference Signals Electronic beam steering performed in software satisfies requirements Scalable system architecture with reference signal distribution to each element 6

7 Challenge: Cost RF Antenna Misc Example cost breakdown of a UHF receive only phased array Mount Ø Based on the cost of a 4-element proof-of-concept prototype Ø COTs parts total ~$15K SDR 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 $20K Class SDR $2K Class SDR The use of low cost SDRs introduces implementation challenges $200 Class SDR $20 Class SDR 7

8 Challenge: Low Cost SDR Performance Challenge System Limitation Potential Mitigations Notes DC Offset Frequency Offset (narrow band) Post-processing (wideband) I/Q Imbalance Useable Bandwidth Calibration and/or Post-processing Sample Synchronization Calibration Reference Signal Distribution Phase/Frequency Drift Radio Imbalance SNR Improvement COTs or Custom GPS referenced sample clock and/or local oscillator signals Component screening Real time compensation may be difficult Online calibration could be performed by injecting a calibration signal Coherent signal processing relies on stable (or known) phase offsets between channels Higher cost components may have tighter performance tolerances 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 8

9 Challenge: COTs Antenna Performance 0 Normalized Antenna Gain (dbi) Antenna Element Pattern Simulated Beam Patterns Angle (deg) Antenna Element Patterns Simulated COTs 1x4 Antenna Array Beam Patterns 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 9

10 UHF Phased Array Prototype Ettus Research +5V / 1A LNA P/S GPS Antenna M2 Antenna Systems SDR SDR MIMO Cable 8-channel 1PPS & 10MHz GPS Disciplined Reference SDR SDR MIMO Cable Gigabit Ethernet Switch PC Minicircuits 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 10

11 JHU/APL Rooftop Experiment Test setup on the roof of the Space Exploration Sector building on the JHU/APL campus in Laurel, MD 1x4 Antenna Array Linear array configuration is suitable for polar orbiting cubesats achieving a reasonably high peak elevation Ø Ø Simple sighting of the phased array along the North-South compass direction Array beam is fan shaped with the narrow dimension of the beam along the electronically steered North-South direction GPS disciplined reference 4-channel SDR RF Frontend Selected Cute-1 for RX-only experiment due to reliable detection at JHU/APL 30th AIAA/USU Conference on Small Satellites 11

12 Cute-1 ground track over JHU/APL campus May 5th N AOS W Array E LOS S

13 Offline Signal Processing Record Signals Remove Relative Sample Offsets Detect and Track Carrier Signal 3 rd Order PLL Apply Phase Shifts 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 SNR Weighting Coherent Summation Software Beamformer Phased Array Beam 13

14 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 14

15 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 15

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