an sdr riometer marcus leech, keo scientific (under contract from science radio laboratories)
TRANSCRIPT
An SDR Riometer
Marcus Leech, Keo Scientific(Under contract from Science Radio Laboratories)
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
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.
Typical QDC
Instrumentation: Traditional
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
Instrumentation: Digital
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
Front End Response
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”.
Gnu Radio Signal Graph
FFT Filter
Implements combined-mode band-pass and multi-notch filter
Further define pre-detector bandpass Notches out RFI based on RFI analyser
feedback
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”
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
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
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.
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.
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
Measured Linearity
Measured Stability
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.
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
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
