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

Questions?

mleech@ripnet.comhttp://www.keoscientific.comhttp://www.science-radio-labs.com