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An SDR Riometer

Marcus Leech, Keo Scientific (Under contract from Science Radio Laboratories ). An SDR Riometer. What is a Riometer?. R elative I onospheric O pacity M eter Use galactic background radiation as a “standard candle” to measure ionospheric absorption. Operates in the 20Mhz to 50Mhz region

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An SDR Riometer

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  1. Marcus Leech, Keo Scientific (Under contract from Science Radio Laboratories) An SDR Riometer

  2. What is a Riometer? • Relative Ionospheric Opacity Meter • Use galactic background radiation as a “standard candle” to measure ionospheric absorption. • Operates in the 20Mhz to 50Mhz region • Variable bandwidths, depending on conditions • Two types • Broad-beam • Imaging

  3. Quiet Day Curve • Standardized diurnal power curve • Earth rotation causes roughly 2dB power variation in Riometer output on a daily basis • Averaged to provide reference for measuring absorption events • Absorption events are measured against the QDC. • Antenna temperatures of 6000K to 9000K are typical during normal ionospheric conditions • Absorption can cause 10dB or more level decrease from QDC.

  4. Typical QDC

  5. Instrumentation: Traditional

  6. Instrumentation: Traditional • Traditional Riometer • Conventional analog superhet receiver • Ryle-Vonberg switching with synchronous detector • Measures the error-voltage between noise source and antenna • Largely immune to gain variations

  7. Instrumentation: Digital

  8. Instrumentation: Digital • Similar front-end to traditional • Band-limiting filters: approx 25Mhz to 45MHz • Low-noise gain • Switching between antenna and 50Ohm load • Entire 25Mhz to 45Mhz “swath” digitized using USRP2 SDR digitizer • Actually all of DC to 50MHz digitized • Analog filtering removes all but 25Mhz to 45Mhz. • 14-bit ADC provides over 80dB SFDR • Approx. 3dB DR improvement due to filtering

  9. Front End Response

  10. Digital Signal Chain • Desired center frequency and bandwidth tuned digitally in USRP2 • Complex (I and Q) base-band (0Hz centered) delivered to host PC via Gigabit Ethernet. • Signal processed using Gnu Radio “flowgraph”.

  11. Gnu Radio Signal Graph

  12. FFT Filter • Implements combined-mode band-pass and multi-notch filter • Further define pre-detector bandpass • Notches out RFI based on RFI analyser feedback

  13. Detector+Low Pass Filter • Simple square-law detector • I*I + Q*Q • Extremely large dynamic range • Linearity determined largely by ADC linearity • Low pass filter • FIR filter • 500Hz cutoff • Samples delivered to external “data slicer”

  14. Data Slicer • Switching (if enabled) isn't synchronous to Gnu Radio flow-graph • Use data-slicing to distinguish sky samples from reference samples • Sort into two populations • Discard outliers • Average populations separately • Output delta of two averages • Originally suggested by Ken Tapping • We refer to it as the Tapping Technique

  15. RFI Analysis • External (to Gnu Radio flow-graph) spectral analysis • Locates areas of persistent narrowband RFI using FFT output • Adjustable threshold • Provides feedback to flow-graph to adjust combination-mode FFT-based bandpass/notch filter • Dynamic RFI mitigation nearly impossible in traditional receiver • Nearly-trivial in SDR receiver

  16. Audio Demodulation • Pre-detector bandwidth can be channeled to audio demod • NBFM • USB/LSB • AM • Helps in identifying RFI sources • Allows for sanity and gross-sensitivity testing using distant HF/Low-VHF transmissions.

  17. Sensitivity • A “naked” USRP2 with BASIC_RX receiver card has very poor noise figure • Dominated by ADC equivalent noise figure • Front-end LNA/filter improves equivalent noise figure to approximately 2.7dB (251K). • Short integration times are the norm • Bandwidths from 25KHz to 500Khz are typical. • Ant. temperatures of 6000K to 9000K are typical • Usually, Tantvastly exceeds Tsys. • Absorption events bring Tant to near Tsys.

  18. Linearity • System must be close to linear to allow high-quality estimation of absorption magnitude • All analog components operated well within their linear range • ADC has 0.6lsb linearity over entire range • Detector is entirely digital • No “square-law region” issues • No detector thermal issues • No detector linearity issues

  19. Measured Linearity

  20. Measured Stability

  21. Dynamic Range • ADC has a practical power range of approximately -75dBm to +5dBm in input power. • Front-end arranges for “normal sky” to appear at ADC at approx -45dBm. • Adequate margins • Deep absorption events are approx 15-18dB • Solar radio bursts may produce large 30dB transients.

  22. Field Testing • Limited field-testing so far • Operated for several months in semi-urban setting • Local noise environment not conducive to determining local QDC • Was able to copy distant HF stations using audio demod on a daily basis. • Phase II testing will likely move to quiet site

  23. Future Plans • Multi-channel support • Conceptually like multiple riometers in a single “envelope” • Dual-channel already prototyped, using dual-DDC feature in latest USRP2 FPGA/firmware. • Multi-channels on adquately-beefy platform should be no problem. • Field testing • Quiet site in Northern Ontario and Alberta • Determine high-quality QDC

  24. Questions? mleech@ripnet.com http://www.keoscientific.com http://www.science-radio-labs.com

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