Transcript Design and Demonstration of an Interference Suppressing Microwave Radiometer IGARSS 2004: Frequency Allocations for Remote Sensing Joel T.

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*,
Department of Electrical Engineering
ElectroScience Laboratory
The Ohio State University
*Department of Electrical and Computer Engineering
Virginia Tech
23rd Sept 2004
ElectroScience Lab
Motivation
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Traditional radiometer architecture poor at rejecting RFI
“Low-level” RFI problematic in post-processing; difficult to distinguish
from geophysical information
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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
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RFI localized in time and/or frequency can potentially be suppressed
by simple time/frequency blanking methods
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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
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System design

Implemented L-band prototype

Local experiments
– Water pool observation
– Radio astronomy observations
ElectroScience Lab
Design Concept
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Traditional direct-detection radiometer
Antenna

Filter
LNA
Detector
LPF
ADC
Digital Hardware
New design
RFI
Antenna
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Filter
LNA
Downconvert
ADC
RFI Suppression/ Filtering/
Detection/Integration
Try to remove RFI in real time: clean data can still be integrated to
retain low data rate
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Design including RFI Removal Stages
(DIF)
Antenna
Low-noise
front end
Analog
Downconverter
(APB)
ADC
Digital
Downconverter
(FFT)
Asynchronous Pulse
Blanker
1024 point FFT
Frequency domain
blanker
(not yet implemented)
(SDP)
Detection/
Integration
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Data Recording/
Control
APB algorithm

APB estimates mean/variance of incoming time domain signal; a
sample > b standard deviations above the mean triggers blanker
NBLANK
NWAIT
Threshold
NSEP
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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
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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
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Prototype samples 100 MHz, includes Digital IF downconverter (DIF),
asynchronous pulse blanker (APB), FFT stage, and SDP operations
Analog
Devices
9410
ADC
DIF
FFT
SDP
ADC
200 MSPS

APB
100 MSPS I/Q
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
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Three FPGA Prototype
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Modular form used for processor boards: note microcontrollers
EEPROM's on each card for autoprogramming of FPGA's on power-up
ADC
DIF/
APB
ADC
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FFT
SDP
Capture
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 is
critical to maintain internal
standards; rest of system not
temperature controlled
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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

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Terminator Test of System Stability
Terminator Spectra
After ND Stabilization
+0.25 dB
15
hrs
-0.25 dB
Total Power vs. Time
Sensitivity vs. Integration
Water Pool Observations
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Height (m)
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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
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Pool and Cal Targets
Absorbers: Assume Tb=Tphys
Reflectors: Assume Tb=Tref~Tsky?
Water: Tb~Twat+QTref
Still working toward obtaining
absolutely calibrated data;
Can still examine effectiveness
of blanking strategies in
uncalibrated data
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Relative Power Variations: Pool Observation
Blanker Off: H pol
Blanker On: H pol
240
secs
Noise Generator
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Terminator
Sky Observations
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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 a
result
24 hour observations
of astronomical
sources
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Sky Observation Results: Blanker on
Software FFT’s allow very high spectral resolution (~0.4 kHz); sufficient
to observe Doppler shift of neutral Hydrogen line
Hydrogen line
emission around
1420 MHz;
“S-curve” is due
to Doppler shift
associated with
galactic region
observed
Elapsed Time (Hr)

Moon
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Relative Power Variations: Sky Observation
APB Off
APB On
+.25 dB
-.25 dB
Radar contributions
greatly decreased by APB
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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
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We have also deployed this backend in aircraft observations at Cband, subject of next talk…..
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