Williams_FR1_TO4_5_2011_07_29v1.ppt

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Williams_FR1_TO4_5_2011_07_29v1.ppt

  1. 1. Developing a Dual-Frequency FM-CW Radar to Study Precipitation <ul><li>Christopher R. Williams </li></ul><ul><li>Cooperative Institute for Research in Environmental Sciences (CIRES) </li></ul><ul><li>University of Colorado at Boulder, </li></ul><ul><li>and </li></ul><ul><li>Physical Sciences Division (PSD) </li></ul><ul><li>NOAA Earth System Research Laboratory (NOAA ESRL), </li></ul>This work is supported: University of Colorado at Boulder CIRES Innovative Research Program With collaborations from Paul Johnston and David Carter
  2. 2. Motivation <ul><li>Technically, pulse radars can not observe rain at close ranges as they switch from transmit to receive modes. </li></ul><ul><ul><li>Monostatic pulse radars have a “cone of silence” </li></ul></ul><ul><li>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 ). </li></ul><ul><ul><li>Bistatic FM-CW radar technology is well suited </li></ul></ul><ul><li>Scientifically, raindrops are not uniformly distributed. Raindrops cluster at small spatial and temporal scales due to dynamics and turbulence. </li></ul><ul><ul><li>“ Cat paws” of raindrops falling on lakes </li></ul></ul><ul><li>Mathematically, at what scales should we treat rain as a continuum of raindrops or as discrete objects? </li></ul><ul><ul><li>Need to sample to small scales to observe the discrete nature of individual objects </li></ul></ul>
  3. 3. Technology & Methodology <ul><li>Utilize technologies developed for the mobile phone, the police radar, and the video gaming industries. </li></ul><ul><ul><li>One radar operated in the frequency band used for point-to-point Internet service (5.8 GHz). </li></ul></ul><ul><ul><li>The other radar operated in the police radar frequency band (10.5 GHz). </li></ul></ul><ul><ul><li>A Sony Playstation 3 was used as the numerical workhorse. Installed Linux as Other OS. </li></ul></ul><ul><li>The radar hardware for both radar systems cost less than $12k. (no labor costs were included) </li></ul>
  4. 4. Presentation Outline <ul><li>Hardware Layout </li></ul><ul><li>Radar Block Diagram </li></ul><ul><li>FMCW Signal Processing </li></ul><ul><ul><li>Range Equation </li></ul></ul><ul><ul><li>Doppler Processing </li></ul></ul><ul><li>Observations </li></ul>
  5. 5. Hardware Layout
  6. 6. Antennas in my backyard (an understanding wife) C-band Antennas – 5.8 GHz X-band Antennas – 10.4 GHz Antennas are designed for point-to-point Internet service
  7. 7. Hardware Layout Costs for C-band Radar ~$US 6k
  8. 8. Simplified Tx and Rx Schematic
  9. 9. 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
  10. 10. Timing Equations <ul><li>T sweep -Duration the DDS is linearly sweeping from f 0 to f 1 </li></ul><ul><li>T wait -Duration after sweep before next sweep </li></ul><ul><ul><li>Allows system to stabilize </li></ul></ul><ul><li>T IPP - “Inter-Pulse Period”, time between sweeps </li></ul><ul><ul><li>T IPP = T sweep + T wait </li></ul></ul><ul><ul><li>T ipp = 310 us + 10 us = 320 us </li></ul></ul><ul><li>T dwell -Duration the DAS is collecting data </li></ul><ul><ul><li>T dwell = 256 us </li></ul></ul><ul><li>T delay -Delay until first collected data point data </li></ul><ul><ul><li>Want to wait for sweep signal to reach maximum range </li></ul></ul><ul><ul><li>T delay > 2R max / c </li></ul></ul><ul><ul><li>Set R max = 1.5 km, T delay = 10 us </li></ul></ul>
  11. 11. Range Equations <ul><li>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. </li></ul>Target Tx Rx R
  12. 12. Range Equations <ul><li>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. </li></ul>Target Tx Rx R f 0 f 1 B = f 1 - f 0 T sweep Tx
  13. 13. 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.
  14. 14. Range Equations <ul><li>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. </li></ul>Target Tx Rx R f 0 f 1 B = f 1 - f 0 T sweep Tx Rx
  15. 15. Range Equations <ul><li>The range equation is give by: </li></ul><ul><ul><li>B = Sweep frequency bandwidth </li></ul></ul><ul><ul><li>T sweep = Duration of sweep </li></ul></ul><ul><ul><li>R = Distance to Target </li></ul></ul><ul><ul><li>C = speed of light in free air </li></ul></ul><ul><li>Frequency resolution determined by Digital Acquisition System (DAS) </li></ul><ul><ul><li>T dwell = time data is collected </li></ul></ul><ul><ul><li>n = number of samples </li></ul></ul><ul><ul><li>∆ t = time between samples </li></ul></ul><ul><li>Rearranging the range equation, the range resolution is given by </li></ul>
  16. 16. Range Equations <ul><li>The range resolution: </li></ul><ul><ul><li>B = 36.328 MHz </li></ul></ul><ul><ul><li>T sweep = 310 us </li></ul></ul><ul><ul><li>n = 128 </li></ul></ul><ul><ul><li>∆ t = 2 us </li></ul></ul><ul><li>Frequency resolution determined by Digital Acquisition System (DAS) </li></ul><ul><li>n R=(n-1)∆R f IF =(n-1)∆f IF </li></ul><ul><li>1 0 DC </li></ul><ul><li>2 5 m 3.9 kHz </li></ul><ul><li>3 10 m 7.8 kHz </li></ul><ul><li>11 50 m 39 kHz </li></ul><ul><li>64 315 m 245.7 kHz </li></ul>∆ R = 5 m ∆ f IF = 3.9 kHz
  17. 17. Doppler Processing <ul><li>Two FFTs generate Doppler velocity spectra at each range </li></ul><ul><li>First FFT is the range-FFT and is applied to the 128 voltages collected during T dwell </li></ul><ul><ul><li>This range-FFT converts the n real valued voltages into complex intermediate frequencies </li></ul></ul><ul><ul><li>Range: –f Nyquist < f IF < +f Nyquist (f Nyquist = (2∆t) -1 = 250 kHz) </li></ul></ul><ul><ul><li>Spacing: 3.9 kHz </li></ul></ul><ul><ul><li>Spectrum is symmetric </li></ul></ul><ul><ul><li>Drop negative frequencies which are “Behind” the radar </li></ul></ul><ul><ul><li>Rename real and imaginary components “I” and “Q” </li></ul></ul><ul><li>Second FFT is the Doppler-FFT and is applied to time series of I’s & Q’s at each range </li></ul><ul><ul><li>Similar to pulse radar processing </li></ul></ul><ul><ul><li>Time between sweep is same as Inter-pulse Period (T ipp ) </li></ul></ul>
  18. 18. Sample Observations <ul><li>20 to 300 m height coverage </li></ul><ul><ul><li>Need to put the antennas closer together </li></ul></ul><ul><li>5 m resolution </li></ul><ul><li>Doppler velocity spectra at each range </li></ul><ul><li>System is not calibrated (need to deploy with a disdrometer for absolute calibration) </li></ul><ul><li>65,536 consecutive sweeps </li></ul><ul><li>21 second dwell period </li></ul>
  19. 19. 21 Second Dwell during Rain Clutter Downward Motion 5 m resolution 300 m 0 m
  20. 20. 21 Second Dwell during Rain Clutter Downward Motion 300 m 0 m
  21. 21. 21 & 10.5 Second Dwells Time: 0-10.5 sec Time: 10.5-21 sec Time: 0-21 sec
  22. 22. 5 second Dwells Time: 0-5 sec Time: 10-15 sec Time: 5-10 sec Time: 15-20 sec
  23. 23. 21 Second Dwell, Mean Reflectivity & Doppler Velocity
  24. 24. 21 Second Dwell processed into 1 second intervals 21 Seconds
  25. 25. 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.
  26. 26. Key Design Elements <ul><li>There are 3 key design elements </li></ul><ul><li>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 </li></ul><ul><li>The FM bandwidth and DAS sampling frequency control the range resolution allowing the DAS sampling frequency to be only 500 kHz </li></ul><ul><li>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 </li></ul>
  27. 27. Concluding Remarks <ul><li>Key Result: </li></ul><ul><ul><li>Proof of Concept was a Success </li></ul></ul><ul><li>Separate the two radars so that they have their own data acquisition system </li></ul><ul><li>Remove the Sony Playstation 3 (SP3) as the numerical workhorse </li></ul><ul><ul><li>Sony prohibits the installation of Linux on the SP3 (since 2010) </li></ul></ul><ul><ul><li>GPU’s can be used if intense signal processing is needed </li></ul></ul><ul><ul><li>Plan to use FPGA for range-FFT </li></ul></ul><ul><li>Need to calibrate system with a disdrometer </li></ul><ul><li>Acquired funds to develop a 915 MHz wind profiler to measure winds in the lowest 300 meters </li></ul>

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