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

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

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 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 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, Tant vastly 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?

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