Airborne Radio Frequency Interference Studies at C-band Using a Digital Receiver IGARSS 2004: Frequency Allocations for Remote Sensing Joel T. Johnson, A. J. Gasiewski*, G. A. Hampson, S. W. Ellingson+, R. Krishnamachari, M. Klein* Department of Electrical Engineering/ElectroScience Laboratory The Ohio State University *NOAA Environmental Technology Laboratory +Department of Electrical and Computer Engineering Virginia Tech 23rd Sept 2004 ElectroScience Lab
Motivation As seen in this session, RFI at C-band is a major concern for ● radiometry; AMSR-E, WindSAT showing data corruption Designing effective RFI mitigation strategies for C-band requires ● detailed knowledge of RFI environment Some flexibility in choosing center frequency/bandwidth; can add-on ● more rapid sampling and/or multiple analog sub-channels to traditional radiometer to improve RFI rejection as post-processing Use of digital receiver provides much more rapid sample rate and ● larger number of channels; can implement real-time suppression GOAL: demonstration of interference suppressing radiometry at C- ● band in airborne experiments using the prototype from the last talk GOAL: improve knowledge of RFI environment using prototype ● ElectroScience Lab
Outline System design ● Experiment plan ● Deployment in SMEX04 ● SMEX04 initial results ● ElectroScience Lab
System Design The digital receiver prototype can sample a 100 MHz channel at IF ● frequency 150 MHz Front end/downconversion for these experiments provided by the ● PSR/CXI radiometer of NOAA/ETL – highly successful C-band radiometer deployed in many previous campaigns – excellent antenna and calibration systems – collaboration eliminates need for redundant development efforts “Spectrometer” modification to PSR/CX implements downconverter; ● located inside PSR/CXI scanhead PSR/CXI includes both traditional direct detection analog sub- ● channels, as well as tuned 10/100 MHz direct detection channels Use of both analog and digital receivers allows comparison of results ● Combined system: C-band Interference Suppressing Radiometer ● (CISR) ElectroScience Lab
Polarimetric Scanning Radiometer (PSR) System PSR/A: PSR/CX: 10.7, 18.7, 21.5, 6-7.3, 10.6-10.8 GHz 37, 89 GHz Polarimetric Polarimetric with Interference Mitigation PSR/S 89 118 380 PSR/L: 18/21 340 1.4 GHz 424 IR 50-57 37 183 V IPO Briefing - PSR and WB-57F July 12, 2004 Silver Spring, MD
Basic PSR Spectrometer Schematic Antenna located in aircraft bomb-bay, steered in Az/El ● Spectrometer includes 10/100 MHz tuned analog channels ● Mixer rejects upper-side-band; LO tuned to allow observations from ● 5.5 to 7.7 GHz ElectroScience Lab
Digital Receiver Module Includes additional channel separation stage: 100 MHz IF at ● 125 MHz to 2 50 MHz IF’s at 150 MHz ElectroScience Lab
Digital Receiver Module Channel selection, PC-104 computer, digital receiver mounted in ● cabin-rack box 120 GB internal storage -> data rate ~ 8 GB/hr to capture detailed RFI ● information; USB 2.0 interface to external hard drives for archiving ElectroScience Lab
Outline System design ● Experiment plan ● Deployment in SMEX04 ● SMEX04 initial results ● ElectroScience Lab
Interface to PSR PSR scanhead computer controls LO tuning, as well as internal ● switches; digital receiver waits for LO settling before acquiring data Simple 1 bit interface: PSR “pulses” this bit when digital receiver ● measurement should begin LO tune PSR trigger pulse 23 msec 15 msec 38 msec Digital receiver acq. 20 msec 10 msec Digital receiver measures for ~ 20 msec, then observes noise diode for ● ~ 10 msec; “pulse” comes every ~38 msec Scanhead and digital receiver computers have internal clock systems, ● synchronized through IRIG-B standard; should be within 1-2 msec Time synchronization allows later alignment of data in LO freq., etc. ● ElectroScience Lab
Experiment Plan Spectrometer/digital receiver observe 100 MHz channel, tuned through ● 22 states 5.5-7.7 GHz Each state = 38 msec; complete sweep of channels in ~ 836 msec ● PSR antenna rotates ~ every 3 secs, so we get 3.6 sweeps per ● rotation Digital receiver operates in 5 modes: ● – Direct capture of data: 1.31 msec, sampled every 10 nsec – Integration, blanker on : integration length is 1.31 msec – Integration, blanker off – Max hold, blanker on – Max hold, blanker off Duty cycle of capture is low due to large amount of data collected; ● integration/max-hold operations approx 60% duty cycle neglecting tuning delays, etc. Digital receiver mode switched approximately every antenna rotation; ● data stored in a large RAM buffer, no observation when writing out ElectroScience Lab
Deployment in SMEX04 The PSR/CX spectrometer system first deployment was the Soil ● Moisture Experiment 2004 SMEX04 flights occurred 13 days (54 flight hours) from August 4 th -27 th , ● 2004, including transit to/from Patuxent River Naval Base in Maryland SMEX04 flights were based out of Tucson, AZ, and included flights ● over characterized soil moisture sites in AZ and northern Mexico Navy P-3 ElectroScience Lab
Spectrometer/CISR Performance: SMEX04 First flight of PSR/CXI + CISR: a few issues encountered ● – Spectrometer LO feed through to direct detection channels ● Results in internal RFI to standard PSR/C channels ● Mostly eliminated by adding absorber inside scanhead ● Further mitigations planned post-SMEX04 ● Consequence: Direct detection channels and spectrometer/CISR could not be operated simultaneously for most of SMEX04 – Cabling issues into CISR ● IRIG-B synchronization lost ● Can still synchronize in post-processing, but more effort required ● Stability of system impaired: failed to operate some flights Spectrometer/CISR data sets of interest acquired only on 8/14 (partial ● flight), 8/24 (full flight), 8/26 (partial), and 8/27 transit to PAX (no CISR) ElectroScience Lab
Sample Data: PSR H-Pol 400 MHz Channels, 8/26 1.5 Uncalibrated voltages versus time for four 400 MHz channel Each channel normalized by its mean 67 This plot includes scan and flight min location effects; no huge RFI effects seen in these wide channels Calibrated data shows low-level corruption Narrower channels should be more sensitive to in-band RFI…. 0.5 6.0 6.5 6.92 7.32 Frequency (GHz) ElectroScience Lab
Sample Data: PSR 10/100 MHz Log Channels 100 MHz (swept) 10 MHz (swept) 1.02 67 min 0.98 Frequency (GHz) Frequency (GHz) ElectroScience Lab
Digital Receiver: H pol, Ch 12 (6.6-6.7 GHz) 2 24 min 0 ElectroScience Lab
Digital Receiver: H pol, Ch 18 (7.2-7.3 GHz) 2 24 min 0 ElectroScience Lab
Conclusions CISR/PSR spectrometer combination should provide useful ● quantitative information on the effectiveness of alternative RFI mitigation strategies Detailed information on the C-band RFI environment should also be ● obtained Further examination of SMEX04 data in progress; improvements to ● combined system for future flights as well Next deployment: Oct 2004 in Antarctic campaign; RFI data to be ● observed on transit flights ElectroScience Lab
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