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Williams_FR1_TO4_5_2011_07_29v1.ppt
 

Williams_FR1_TO4_5_2011_07_29v1.ppt

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    Williams_FR1_TO4_5_2011_07_29v1.ppt Williams_FR1_TO4_5_2011_07_29v1.ppt Presentation Transcript

    • Developing a Dual-Frequency FM-CW Radar to Study Precipitation
      • Christopher R. Williams
      • Cooperative Institute for Research in Environmental Sciences (CIRES)
      • University of Colorado at Boulder,
      • and
      • Physical Sciences Division (PSD)
      • NOAA Earth System Research Laboratory (NOAA ESRL),
      This work is supported: University of Colorado at Boulder CIRES Innovative Research Program With collaborations from Paul Johnston and David Carter
    • Motivation
      • Technically, pulse radars can not observe rain at close ranges as they switch from transmit to receive modes.
        • Monostatic pulse radars have a “cone of silence”
      • Proof of concept study to develop an inexpensive radar to observe precipitation between the surface and the first range gate of a pulse radar (~150m ).
        • Bistatic FM-CW radar technology is well suited
      • Scientifically, raindrops are not uniformly distributed. Raindrops cluster at small spatial and temporal scales due to dynamics and turbulence.
        • “ Cat paws” of raindrops falling on lakes
      • Mathematically, at what scales should we treat rain as a continuum of raindrops or as discrete objects?
        • Need to sample to small scales to observe the discrete nature of individual objects
    • Technology & Methodology
      • Utilize technologies developed for the mobile phone, the police radar, and the video gaming industries.
        • One radar operated in the frequency band used for point-to-point Internet service (5.8 GHz).
        • The other radar operated in the police radar frequency band (10.5 GHz).
        • A Sony Playstation 3 was used as the numerical workhorse. Installed Linux as Other OS.
      • The radar hardware for both radar systems cost less than $12k. (no labor costs were included)
    • Presentation Outline
      • Hardware Layout
      • Radar Block Diagram
      • FMCW Signal Processing
        • Range Equation
        • Doppler Processing
      • Observations
    • Hardware Layout
    • Antennas in my backyard (an understanding wife) C-band Antennas – 5.8 GHz X-band Antennas – 10.4 GHz Antennas are designed for point-to-point Internet service
    • Hardware Layout Costs for C-band Radar ~$US 6k
    • Simplified Tx and Rx Schematic
    • Timing Diagram: Frequency Chirp and Data Collection Trigger f 0 f 1 B = f 1 - f 0 Frequency Chirp Data Collection Trigger T delay T dwell T end T wait T IPP T sweep Sweep Trigger Data is being collected
    • Timing Equations
      • T sweep -Duration the DDS is linearly sweeping from f 0 to f 1
      • T wait -Duration after sweep before next sweep
        • Allows system to stabilize
      • T IPP - “Inter-Pulse Period”, time between sweeps
        • T IPP = T sweep + T wait
        • T ipp = 310 us + 10 us = 320 us
      • T dwell -Duration the DAS is collecting data
        • T dwell = 256 us
      • T delay -Delay until first collected data point data
        • Want to wait for sweep signal to reach maximum range
        • T delay > 2R max / c
        • Set R max = 1.5 km, T delay = 10 us
    • Range Equations
      • For a linear FM signal, a hard target located at range R will generate a delayed version of the transmitted signal t = 2 R / c seconds later.
      Target Tx Rx R
    • Range Equations
      • For a linear FM signal, a hard target located at range R will generate a delayed version of the transmitted signal t = 2 R / c seconds later.
      Target Tx Rx R f 0 f 1 B = f 1 - f 0 T sweep Tx
    • Range Equations Target Tx Rx R f 0 f 1 B = f 1 - f 0 T sweep Tx Rx For a linear FM signal, a hard target located at range R will generate a delayed version of the transmitted signal t = 2 R / c seconds later.
    • Range Equations
      • For a linear FM signal, a hard target located at range R will generate a delayed version of the transmitted signal t = 2 R / c seconds later.
      Target Tx Rx R f 0 f 1 B = f 1 - f 0 T sweep Tx Rx
    • Range Equations
      • The range equation is give by:
        • B = Sweep frequency bandwidth
        • T sweep = Duration of sweep
        • R = Distance to Target
        • C = speed of light in free air
      • Frequency resolution determined by Digital Acquisition System (DAS)
        • T dwell = time data is collected
        • n = number of samples
        • ∆ t = time between samples
      • Rearranging the range equation, the range resolution is given by
    • Range Equations
      • The range resolution:
        • B = 36.328 MHz
        • T sweep = 310 us
        • n = 128
        • ∆ t = 2 us
      • Frequency resolution determined by Digital Acquisition System (DAS)
      • n R=(n-1)∆R f IF =(n-1)∆f IF
      • 1 0 DC
      • 2 5 m 3.9 kHz
      • 3 10 m 7.8 kHz
      • 11 50 m 39 kHz
      • 64 315 m 245.7 kHz
      ∆ R = 5 m ∆ f IF = 3.9 kHz
    • Doppler Processing
      • Two FFTs generate Doppler velocity spectra at each range
      • First FFT is the range-FFT and is applied to the 128 voltages collected during T dwell
        • This range-FFT converts the n real valued voltages into complex intermediate frequencies
        • Range: –f Nyquist < f IF < +f Nyquist (f Nyquist = (2∆t) -1 = 250 kHz)
        • Spacing: 3.9 kHz
        • Spectrum is symmetric
        • Drop negative frequencies which are “Behind” the radar
        • Rename real and imaginary components “I” and “Q”
      • Second FFT is the Doppler-FFT and is applied to time series of I’s & Q’s at each range
        • Similar to pulse radar processing
        • Time between sweep is same as Inter-pulse Period (T ipp )
    • Sample Observations
      • 20 to 300 m height coverage
        • Need to put the antennas closer together
      • 5 m resolution
      • Doppler velocity spectra at each range
      • System is not calibrated (need to deploy with a disdrometer for absolute calibration)
      • 65,536 consecutive sweeps
      • 21 second dwell period
    • 21 Second Dwell during Rain Clutter Downward Motion 5 m resolution 300 m 0 m
    • 21 Second Dwell during Rain Clutter Downward Motion 300 m 0 m
    • 21 & 10.5 Second Dwells Time: 0-10.5 sec Time: 10.5-21 sec Time: 0-21 sec
    • 5 second Dwells Time: 0-5 sec Time: 10-15 sec Time: 5-10 sec Time: 15-20 sec
    • 21 Second Dwell, Mean Reflectivity & Doppler Velocity
    • 21 Second Dwell processed into 1 second intervals 21 Seconds
    • C- and X-band Observations During Snow C-Band Radar X-Band Radar A snow event passed over my house on 13 November 2009 and was observed by both the C-band and X-band radars. The X-band transmitted power was limited due to a bad amplifier which reduced its altitude coverage.
    • Key Design Elements
      • There are 3 key design elements
      • The Data Acquisition System (DAS) commands all time signals and collects all data so that the sample voltage phases are coherent from sweep-to-sweep
      • The FM bandwidth and DAS sampling frequency control the range resolution allowing the DAS sampling frequency to be only 500 kHz
      • Doppler velocity power spectra are generated at each range using 2 FFTS: one range-FFT applied to each FM sweep followed by a Doppler-FFT that detects the phase changes over several FM sweeps
    • Concluding Remarks
      • Key Result:
        • Proof of Concept was a Success
      • Separate the two radars so that they have their own data acquisition system
      • Remove the Sony Playstation 3 (SP3) as the numerical workhorse
        • Sony prohibits the installation of Linux on the SP3 (since 2010)
        • GPU’s can be used if intense signal processing is needed
        • Plan to use FPGA for range-FFT
      • Need to calibrate system with a disdrometer
      • Acquired funds to develop a 915 MHz wind profiler to measure winds in the lowest 300 meters