NCAR/TN-444 NCAR TECHNICAL NOTE _· i May 1999 Pulse Compression for Phased Array Weather Radars R. Jeffrey Keeler Charles A. Hwang Ashok S. Mudukutore ATMOSPHERIC TECHNOLOGY DIVISION i NATIONAL CENTER FOR ATMOSPHERIC RESEARCH BOULDER, COLORADO - Pulse Compression for Phased Array Weather Radars NCAR Technical Report I R. Jeffrey Keeler 1, Charles A. Hwang1 and Ashok Mudukutore 2 1National Center for Atmospheric Research* PO Box 3000, Boulder, Colorado 80307 USA 2Colorado State University Fort Collins, Colorado 80369 USA E-mail: keeler@ucaredu Tel: 303-497-2031 Fax: 303-497-2044 *NCAR is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation Preface This Technical Report is a reprint of the Final Report from NCAR's Atmospheric Technology Division on work per- formed from 1991 through 1995 for the FAA Terminal Area Surveillance Systems Program. It details the application of pulse compression waveforms to weather radar, the importance of range time sidelobes, special considerations for FM waveforms, simulations of fluctuating weather targets, and a validation study using the NCAR ELDORA testbed radar. The report was originally written in 1995, but not published until now. A few relevant references have been added when they amplify the work originally performed. rJK May 15, 1999 List of figures Figure 1.1. Advanced high resolution radarsystem using pulse compression waveform and phased array electronic scanned antenna . ..................................................... ................................................. .................................................. 2 Figure 2.1. Graphicaldescription of optimal sidelobe suppressionfilter design. The desiredoutput response, dk, is an impulse, but the actual output, yk, has sidelobes. ........ 6.................................................................................................6 Figure 2.2. The integratedsidelobe levels (ISL)for a Barker 13 code with inversefiltering decrease with longerfilter length for zero Doppler. ..... ... .................. ....................................................................................................................... 7 Figure2.3. Waveforms usedfor compressionfilter tests: Barker 13 (B-13), Pseudo-Noise (PN-15), Linear FM 63 (LinFM), and Nonlinear FM 39 (TanFM). The bandwidth or frequency sweep of each waveform is 1 MHz and the durations are as shown. .................................. 7 Figure 2.4. Impulse responses of MF and Inv lx/2x/3x/5x compressionfilters to a B-13 biphase coded waveform. ...8 Figure2.5. Compressionfilter responsesto B-13 waveformfor MF and Inv-lx/2x3x/S5xfilters. Note reduced sidelobes and extended response as filter length increases .......................................................................................................... 9 Figure2.6. Ambiguityfunction for B-13 and matchedfilter. Sidelobes are uniformly high at -22 dB and main response peak is constant showing negligible Doppler sensitivity ................. ........................................................................... 9 Figure 2.7. Ambiguity function of B-13 and Inv-Sx filter. Both ISL and PSL are much lower than the MF response but show extreme Doppler sensitivity. Peak sidelobes at zero velocity are -60 dB. ......................................................... 10 Figure 2.8. Integrated sidelobe level, peak sidelobe level and mismatch lossfor Barker-13 waveform. Longerfilters suppress sidelobes, increase loss and show greater Dopplersensitivity. The labels at left are ISL and PSL curves and labels are right are Lmm curves. .................... 1.............................0 Figure 2.9. ISL, PSL and Lmm vs. Dopplerfor Pseudo-Noise bi-phase waveform of length 63 for MF, Inv-lx and Inv- 5x fi lters........................................................................................................................................................................ 11 Figure 2.10. Ambiguityfunction ofLinFM-63 waveform and Inv-5x filter. Peak sidelobes are 45 to 50 dB down at zero velocity. ................................................................................................................................................................. 12 Figure 2.11. ISL, PSL and Lmm vs. DopplerforLinFM (BT=63) with MF, Inv-lx and Inv-5x compressionfilters. Data are oversampledoversapled by 2B . ................................................................................................................................................2B are Figure 2.12. Ambiguityfunction of TanFM waveform and Inv-5xfilter. Peak sidelobes are 70 dB down at zero velocity. Co pression ratio is 39 ............................................................................................................. .. ... 13 Figure 2.13. ISL, PSL and Lmm vs. Dopplerfor TanFM (BT=39) with the MF, Inv-lx and Inv-5x compressionfilters. Data are oversampled by 2B ............................ ...........................................................................................................13 Figure2.14. Ambiguityfunction of a CC-10 code pairusing MF with a) no cross waveform leakage, and b) 20 db cross w aveform leakage. ....................................................................................................................................................... 14 Figure3.1. Inv-5x SLSfilter response to a linearchirp waveform under top) optimum sampling conditions (zero phase) and (bottom) the same filter response to all phase shifts (all phase) simulated by 8 times oversampling ................. 16 Figure 3.2. ISL values for Inv-5x compressionfilters with sample-phase offset (shift) for zero-phase SLS filters and all- phase SLSfilters at a Nyquist (IN) sampling rate. The expected ISLfor the APSLS filter is -5.9 dB andfor the ZPSLS filter is -5.8 dB. ........................................................................... 18 Figure 3.3. Probabilitydistribution ofISLsfor both APSLS and ZPSLS Inv-5x filters with IN, 2N, and 5N sampling rates. Expected AP/ZP ISLs are (-5.9/-5.8) for IN, (-20.6/-21.9) for 2N, and (-32.5/-33.3) dBfor 5N sample rates. 18 Figure 4.1. ISL as function of Doppler shift for a point target using the B-13 bi-phase code and MF, IFxl, xS, x7 compressionfilters including two Doppler Tolerant (DT) processing variants. ......................................................... 21 Figure 4.2. ISL vs. Doppler shiftforfluctuating reflectivity (sv =1 ms, SNR =50 dB) "spike " 100 dB greater than any adjacent range sample. Waveform is B-13 code and filters are the MF, IFxl, x5 and x7, and two DT variants. ...... 21 Figure 4.3. ISL vs. SNRfor various compressionfilters. Dashed lines represent the correspondingDoppler tolerant filters. ........................................................................................................................... 22 Figure 4.4. Range profile of reflectedpower from a 100 dB reflectivity notch of a hard target with zero velocity. The power in the notch represents the ISL for the B-13 waveform and the selected compressionfilter. ............................23 Figure4.5. Range profile of reflectedpower from a 100 dB notch in afluctuating target having zero velocity and width of 2.5 m/s. Power in the notch represents ISLfor the B-13 waveform and the selectedfilter. ................................... 23 Figure 4.6. ISL vs. Doppler velocity for a hard targetfor the B-13 waveform and the selected filter. .......................24 Figure 4.7. ISL vs. Doppler velocity for afluctuating targetwith sv=2.5 m/s. .......................................................... 24 Figure 4.8. ISL vs. Doppler velocity for afluctuating targetwith sv=5.0 m/s. ........................................................... 25 Figure 4.9. Range profiles of reflected power. The input has a 50 dB reflectivity step. ............................................. 25 Figure 4.10. Range profiles of Doppler velocity. The input has a 20 n/s velocity step. ............................................. 26 Figure 5.1. Schematic of H PA test. .............................................................................................................................. 27 Figure 5.2. Phase plot of HPA's simple pulse phase response after drift correction. ............................................... 28 Figure 5.3. Ground clutter data showing receivedpowerand velocityfor simplefrequency 90 m pulse. Nyquist velocity ( 0.5) corresponds to 8 m/s. Ranges greaterthan 150 are noise only.........................................................................29 Figure 5.4. Ground clutter target using B-13 pulse and Inv-5x filter. Note 11 dB SNR increase and the sidelobe response. Simple and coded peak powers are equal.................................................................................................29 Figure 5.5. High reflectivity gradient weather using simple 90 m pulse. Gradientis 50 dB over 2 km range at front and rear of cell. Nyquist velocity is 8 m/s. Radial velocities within the cell are aliased and between 5-12 m/s ................ 30 Figure5.6. Weather echo using B-13 waveform and the Inv-5xfilter. There is discernibleleakage of the sidelobe energy from the strong precipitation echo to outside the cell in both reflectivity and velocity. .............................................. 30 Figure 5.7. Snapshot color display of weather data takenfrom the ELDORA testbed radar. Data between 5-23 km at azimuth 332 degrees and elevation 18.5