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/oratory The Ohio State University *NOAA Environmental Technology Laboratory +Department of Electrical and Computer Engineering Virginia Tech 23rd Sept 2004
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
Outline System design Experiment plan Deployment in SMEX04 SMEX04 initial results
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 subchannels, 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)
Polarimetric Scanning Radiometer (PSR) System PSR/A: 10.7, 18.7, 21.5, 37, 89 GHz Polarimetric PSR/CX: 6-7.3, 10.6-10.8 GHz Polarimetric with Interference Mitigation PSR/S PSR/L: 1.4 GHz V IR 18/21 89 118 340 37 50-57 183 380 424 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
Digital Receiver Module Includes additional channel separation stage: 100 MHz IF at 125 MHz to 2 50 MHz IF s at 150 MHz
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
Outline System design Experiment plan Deployment in SMEX04 SMEX04 initial results
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 Digital receiver acq. Interface to PSR 23 msec 15 msec 38 msec 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.
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
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
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)
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 min This plot includes scan and flight location effects; no huge RFI effects seen in these wide channels 6.0 6.5 6.92 7.32 Frequency (GHz) 0.5 Calibrated data shows low-level corruption Narrower channels should be more sensitive to in-band RFI.
Sample Data: PSR 10/100 MHz Log Channels 100 MHz (swept) 10 MHz (swept) 1.02 67 min 0.98 Frequency (GHz) Frequency (GHz)
Digital Receiver: H pol, Ch 12 (6.6-6.7 GHz) 2 24 min 0
Digital Receiver: H pol, Ch 18 (7.2-7.3 GHz) 2 24 min 0
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