ElectroScience Lab

Department of Electrical EngineeringElectroScience LaboratoryThe Ohio State University

*Department of Electrical and Computer EngineeringVirginia Tech

23rd Sept 2004

Design and Demonstration of an Interference Suppressing Microwave Radiometer

IGARSS 2004:Frequency Allocations for Remote Sensing

Joel T. Johnson, Grant A. Hampson,Steven W. Ellingson*,

ElectroScience Lab

Motivation Traditional radiometer architecture poor at rejecting RFI

“Low-level” RFI problematic in post-processing; difficult to distinguish from geophysical information

High amplitude but low duty cycle pulsed RFI (for example, microsecond radar pulses out of millisecond integration period)can appear as low-level RFI

Similarly, strong amplitude CW interferers can appear as low-level RFI

RFI localized in time and/or frequency can potentially be suppressed by simple time/frequency blanking methods

Traditional architecture can be retained by sampling data stream faster (0.1 to 1 msec) and adding analog sub-band channels; increases data rate; post-processing RFI removal, but can only go so far….

Since 2002, a digital receiver based radiometer has been under development at Ohio State to implement such methods in real-time

ElectroScience Lab

Outline

System design

Implemented L-band prototype

Local experiments– Water pool observation– Radio astronomy observations

ElectroScience Lab

Design Concept Traditional direct-detection radiometer

New design

Try to remove RFI in real time: clean data can still be integrated to retain low data rate

Antenna Filter LNA Detector LPF ADC

Antenna Filter LNA Downconvert ADC

RFI

RFI Suppression/ Filtering/Detection/Integration

Digital Hardware

ElectroScience Lab

Design including RFI Removal Stages

Antenna

Data Recording/Control

Low-noisefront end

AnalogDownconverter

DigitalDownconverterADC

Asynchronous Pulse Blanker 1024 point FFT Frequency domain

blanker

Detection/Integration

(not yet implemented)

(APB)

(DIF)

(SDP)

(FFT)

ElectroScience Lab

APB algorithm APB estimates mean/variance of incoming time domain signal; a sample >

standard deviations above the mean triggers blanker

Pre-detection samples can be blanked by including memory in the system, NWAIT parameter sets time period

“Blanked” samples replaced with zero; calibration effects can be corrected by scaling average power appropriately

Some FFT issues, but tests show minor

Threshold

NBLANK

NWAIT

NSEP

ElectroScience Lab

Frequency Domain Blanking Post-FFT, two types of blanking can be considered

– Time blanking of each FFT bin Similar to original APB, but now at higher S/N Implementation very similar to time-domain APB

– Cross-frequency blanking Requires some information on expected instrument passband Can look for rapid changes in spectrum to indicate narrow-band RFI Can also permanently blank certain bins known to contain RFI (for

example hydrogen line emissions at L-band)

Again calibration effects can be corrected by keeping track of the number of blanked samples

Rapid frequency domain blanking of type #2 perhaps not required, since narrowband interferers vary slowly; still reduces data rate though

ElectroScience Lab

Outline

System design

Implemented prototype

L-band local experiments– Water pool observation– Radio astronomy observations

ElectroScience Lab

Digital Back-End Prototype samples 100 MHz, includes Digital IF downconverter (DIF), asynchronous

pulse blanker (APB), FFT stage, and SDP operations

Implemented in FPGA’s for algorithm flexibility: – Altera "Stratix" parts: apprx 10000 LE, ~$260 each

A final prototype has been designed to combine processor components into one Stratix FPGA: apprx 30000 LE, ~$950

Microcontroller interface via ethernet for setting on-chip parameters– Possible modes:

Direct capture of time domain data, sampled every 10 nsec Integration, blanker on/off, integration lengths 0.01 to 21 msec Max-hold, blanker on/off

ADCDIF APB FFT SDP

200 MSPS 100 MSPS I/Q

AnalogDevices9410 ADC

ElectroScience Lab

DIF/APB

ADC

ADC FFT SDP Capture

Three FPGA Prototype Modular form used for processor boards: note microcontrollers EEPROM's on each card for autoprogramming of FPGA's on power-up

ElectroScience Lab

Outline

System design

Implemented prototype

L-band local experiments– Water pool observation– Radio astronomy observations

ElectroScience Lab

L-band Antenna/Front End Unit Front end Tsys approx. 200K neglecting antenna

Temperature control iscritical to maintain internalstandards; rest of system nottemperature controlled

ElectroScience Lab

L-band Dual Channel Downconverter One channel is ~1325-1375 MHz, other is ~1375-1425 MHz

Downconverter, digital receiver, computer, and thermal control systems in rack inside lab

High-compression point amplifiers used; isolators used to reduce channel coupling

Terminator Test of System StabilityTerminator Spectra After ND Stabilization

Total Power vs. Time Sensitivity vs. Integration

15hrs

+0.25 dB

-0.25 dB

ElectroScience Lab

Water Pool Observations Experiments designed to demonstrate radiometric accuracy in the presence of

interference Observations of a large water tank; external cal sources are ambient absorbers and

a sky reflector Highly accurate ground-based radiometry is tough due to contributions from objects

not under view, including reflections Keep cal targets exactly the same size as pool to reduce these effects; observations

of pool as ambient temp varies also

Initial tests in existing RFI, incl. air traffic control radar at 1331 MHz

Hei

ght (

m)

ElectroScience Lab

Pool and Cal TargetsAbsorbers: Assume Tb=Tphys Reflectors: Assume Tb=Tref~Tsky?

Water: Tb~Twat+QTref

Still working toward obtainingabsolutely calibrated data;

Can still examine effectivenessof blanking strategies in uncalibrated data

ElectroScience Lab

Relative Power Variations: Pool ObservationBlanker Off: H pol Blanker On: H pol

Noise Generator Terminator

240secs

ElectroScience Lab

Sky Observations An alternate experiment was initiated using observations of the sky;

a 3 m reflector was available – used same feed/front end Sky observations at declination angles up to 30 degrees Expect to see cold sky plus astronomical sources; minor atmospheric

influence Potential for using cold sky plus moon in a calibration Initial results use software FFT’s

and integration;low duty cycle as aresult

24 hour observationsof astronomical sources

ElectroScience Lab

Sky Observation Results: Blanker on Software FFT’s allow very high spectral resolution (~0.4 kHz); sufficient

to observe Doppler shift of neutral Hydrogen lineEl

apse

d Ti

me

(Hr)

Moon

Hydrogen lineemission around1420 MHz; “S-curve” is dueto Doppler shiftassociated withgalactic regionobserved

ElectroScience Lab

Relative Power Variations: Sky ObservationAPB Off APB On

+.25 dB

-.25 dB

Radar contributionsgreatly decreased by APB

ElectroScience Lab

Conclusions Digital receiver prototype developed and currently being applied in

L-band water pool and sky observations

Base suppression algorithm is APB, followed by post-processing narrow band removal at present; can implement spectral processing in future hardware as well

Current data shows qualitative success of this approach, although continuing to work toward a final demonstration

Goal is to demonstrate well calibrated and stable brightness measurements even in the presence of RFI

We have also deployed this backend in aircraft observations at C-band, subject of next talk…..