Pulse compression for phased array weather radars.

207 Pages·2015·11.62 MB·English

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 Keeler1, Charles A. Hwang1and Ashok Mudukutore2
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: [phone] Fax: [phone]
*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 radars ystem using pulse compression waveform and phased array electronic
scanned antenna ...................................................... ................................................. .................................................. 2
Figure 2.1. Graphicald escriptiono f optimal sidelobe suppressionf ilter design. The desiredo utput response, dk, is an
impulse, but the actual output, yk, has sidelobes. ........ 6.................................................................................................6
Figure 2.2. The integrateds idelobe levels (ISL)f or a Barker 13 code with inversef iltering decrease with longerf ilter
length for zero Doppler. ..... ... .................. ....................................................................................................................... 7
Figure2 .3. Waveforms usedfor compressionf ilter 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 compressionf ilters to a B-13 biphase coded waveform. ...8
Figure2 .5. Compressionf ilter responsest o 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 Dopplers ensitivity. 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. DopplerforL inFM (BT=63) with MF, Inv-lx and Inv-5x compressionf ilters. Data
are oovveerrssaampplleedd bbyy 2B .........................................................................2..B........................................................a..r.e.. .........
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 compressionf ilters.
Data are oversampled by 2B ............................ ....................................1..3.....................................................................
Figure2 .14. Ambiguityfunction of a CC-10 code pairu sing MF with a) no cross waveform leakage, and b) 20 db cross
w aveform leakage. ....................................................................................................................................................... 14
Figure3 .1. Inv-5x SLSfilter response to a linearc hirp 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 compressionf ilters 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 andf or the ZPSLS
filter is -5.8 dB. ........................................................................... 18
Figure 3.3. Probabilityd istributiono fISLsfor 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 compressionf ilters. Dashed lines represent the correspondingD oppler tolerant
filters. ........................................................................................................................... 22
Figure 4.4. Range profile of reflectedp ower 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 reflectedp owerf rom 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 selectedf ilter. ...................................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 targetw ith sv=2.5 m/s. .......................................................... 24
Figure 4.8. ISL vs. Doppler velocity for afluctuating targetw ith 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. Schema tic 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 receivedpowera nd velocityfor simplef requency 90 m pulse. Nyquist velocity
( 0.5) corresponds to 8 m/s. Ranges greatert han 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. Gradienti s 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 discerniblel eakage 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 degrees is detailed in Figure 5.8 ................................................................. 32
Figure 5.8. Range plot of the simple pulse (top pair)a nd B-13/Inv-5x pulse compression (bottom pair) received echo
power and velocity data takenfrom the ELDORA testbed radars howing the weather in Figure 5.7 at 332 degrees and
5-2 3 km .. ...................................................................................................................................................................... 33
Figure5 .9. Spectra of range gate at 18 km showing contaminationo f storm velocity ingressing through the sidelobes
of the coded pulse. Velocity estimate for the Inv-Sxfiltered pulse is near 0.1 (- 0.8 m/s) butfor the Inv-3x filtered pulse
is near -0.15 (- -1.2 m/s) caused by greaters idelobe leakage. ................................................... 34
Table of Contents
1 0 Tnftroductionn.. . . . . . . . . ....I...... ........... I1
1.1 Motivation for weather radar pulse compression ..... .............................. . 1
1.1.1 Weather target characteristics .............................. 1........
1.1.2 Di gital technology.................................................................................................1
1.2 Pulse compression defined ............. ..................................................................2
1.3 H igh resolution radar system ............................................................................................... 2
1.4 FAA/TASS program description ............................ ............ 2
2.0 Waveform and filter design.......................................................................................................... 4
2.1 Coded w aveform design.....................................................................................
2.2 Com pression filter design.............................................................................................. ... 5
2.3 Waveform analysis .............................................................................................................. 6
2.3.1 Bi-phase waveform s..............................................................................................8
2.3.2 FM waveforms ...... ................................. .................... 11
2.4 C om plem entary codes................................................................... ...........1.4........................
3.0 Sample phase aspects of FM waveforms................................. 16
3.1 Sam ple phase problem s....................................................................... .............................. 16
3.2 Phase mismatch with FM waveforms................................................................................ 16
3.3 Optimization over unknown sample phase........................................................................ 17
3.3.1 Filter design/modification .......... ............... ............................. 17
3.3.2 Performance measure......................................................................................... 17
3.4 A na lysis.............................................................................................................................17
4.0 Simulation and analysis for fluctuating targets ......................................................................... 20
4.1 Simulation of fluctuating weather targets with pulse compression ................................... 20
4.2 Doppler tolerant design .............................................................. 20
4.3 Evaluation of ISL ...................................................... ..... 222
4.4 Reflectivity and velocity steps...........................................................................................25
5.0 Data acquisition and analysis........ ............. ......................... 27
5.1 ELDORA test bed radar description ................................................................... 27
5.2 D ata quality ...................................... ................. ...................... ........ 27
5.3 Point clutter target ............................................................................................................ 29
5.4 Convective we ather target ................................................................................................. 30
5.5 Color display: Snows torm target....................................................................................... 32
6.0 Phased array configurations ........................................................................................................ 35
/7 aIt *nnelll: ^n .I
8.0 Acknowledgments ... ..... ...... ..... . 37
9.0 References ............................................................... . ...........................................3 8
Appendix A--Ambiguity functions........................................................................................................ 41ff
Appendix B--Characterization of the NCAR high power amplifier ..................................... ..... 42ff
Appendix C-Coherent wideband processing of distributed radar targets .........................„.. 43ff
Appendix D--Rapid scan Doppler radar development considerations: Part II ................................ 44ff
Appendix E--Pulse compression polarization waveforms for rapid scan Doppler radar .................. 45ff
Appendix F--Rapid scan Doppler radar: the antenna issues................................................................ 46ff
Appendix G--Pulse compression waveform analysis for weather radar............................................ 47ff
Appendix H--Pulse compression for weather radar......4..................................................................... 48ff
Appendix I--Sample phase aspects of FM pulse compression waveforms..................................... 49ff
Appendix J--Pulse compression weather radar waveforms ... .......................................................... 50ff
Appendix K--Pulse compression weather radars................................................................................... 51ff
Pulse Compression for Phased Array Weather Radar
1.0 Introduction
1.1 Motivation for weather radar pulse compression
Small, rapidly evolving aviation weather hazards, such as microbursts, strong gust fronts, and wake vortices happen
quickly and can be very dangerous to aircraft. Using weather radar to detect their presence among highly reflective
aircraft and ground clutter requires high sensitivity and high spatial resolution. Air traffic control and meteorological
communities are demanding radars that also scan the atmosphere at faster rates. Today's mechanically scanned radars
cannot track and predict these weather hazards while simultaneously mapping aircraft in a terminal area. Phased-
array radar systems can fulfill this need by electronically steering their beams, but with current technology the peak
power is so low that these radar systems have poor sensitivity. Pulse compression allows systems to increase sensitiv-
ity by transmitting larger average power in a longer coded pulse and then compressing the pulse to achieve high range
resolution and subsequently a much faster volumetric scan rate.
Pulse compression techniques are well developed for military and aviation radar applications where scattering is from
hard point, not distributed, targets. Application of pulse compression to distributed weather targets was not investi-
gated until the 1970s when Fetter (1970) demonstrated a phase coded transmit pulse and matched filter receiver at
McGill University. Gray and Farley (1973) used a phase coded waveform for ionospheric scatter observations. In
1974 Keeler (personal communication) built a Barker phase coded system for an acoustic radar echosonde system,
but it was never tested with atmospheric returns. Keeler and Passarelli (1990) have traced the evolution of pulse com-
pression techniques in the weather radar community. In Appendix H, Keeler and Hwang (1995) provide a good sum-
mary of their theoretical studies and data validation.
Doppler weather radars can accurately depict the reflectivity and velocity structure of convective storms and other
weather phenomena. The WSR-88D (NEXRAD), Terminal Doppler Weather Radar (TDWR) and other conventional
weather radar networks require relatively long dwell times (30-100 msec) to acquire enough independent samples for
accurately measuring weather parameters. For some operational systems and especially for research systems, the vol-
ume scan time and low spatial scan spacing are too coarse to capture the essential features of convective evolution
(Carbone et al 1985, Wolfson 1993). Future operational, as well as research, systems will likely require higher space
and time resolution measurements than are presently available.
1.1.1 Weather target characteristics
Like other volumetric scatterers, such as insects and refractive index gradients, weather is made up of many small
scatterers, each with its own backscatter cross section and velocity. Radar characterization of precipitation is defined
in terms of the "radar reflectivity factor" (Z), an average quantity related to the Rayleigh scattering from a dielectric
sphere, which, in turn, is related to the radar cross section (a) using the X-4wavelength dependence (Battan 1973,
Doviak and Zrnic 1993). Accurately estimating the backscatter power and velocity in a weak echo region near a
strong reflectivity gradient requires that range sidelobes in a pulse compression scheme be minimized so that power
from these extended sidelobe regions do not contaminate the desired main lobe (Appendix C: Keeler and Frush
1983a).
1.1.2 Digital technology
Rapid technology developments in digital signal processing components and associated digital filter methods have led
to new capabilities in waveform generation and pulse compression filter techniques. In the past, waveforms were
designed with the concept of using matched filters to optimize the detectability, or the signal to noise ratio, of radar
targets. However, weather radar signals are scattered from targets distributed in range, not point targets, and it is the
integrated range sidelobe response that becomes the relevant quantity to optimize rather than the signal to noise ratio.
Consequently, more sophisticated and precise designs are needed for these digital sidelobe suppression filters used for
weather measurements. Highly precise, real time filters using these techniques are readily implemented with modem
digital signal processing (DSP) chips.
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Pulse Compression for Phased Array Weather Radar
1.2 Pulse compression defined
Pulse compression (PC) is a technique of obtaining higher power and resolution from a low peak power transmitter
(Skolnik 1990). A long pulse with wide frequency content has a narrow autocorrelation and compression filter
response. The output of the compression filter typically has a high-power main lobe and low sidelobes which we
desire to minimize.
1.3 High resolution radar system
Figure 1.1 depicts the two distinct technologies required for higher space and time resolution weather radar measure-
ments (Smith 1974, Keeler and Frush 1983a): 1) a coded wide bandwidth waveform that allows higher range resolu-
tion and subsequent larger number of independent estimates within a dwell time, and 2) an electronic step scanned (e-
scan) phased array antenna that allows rapid and agile beam movement The wideband waveform allows accurate
weather measurements in a short data acquisition time and the e-scan antenna allows rapid beam movement, thereby
covering the surveillance volume without inducing scan modulation that degrades the accuracy of the measurements.
As as example from aviation weather, convective activity frequently spawns dangerous winds, such as microbursts,
that may rapidly form a lethal aviation hazard in an airport terminal area. Weather radar volume scan times of 2.5-10
minutes, characteristic of TDWR and NEXRAD radars, do not have adequate time resolution to acquire data by
which an accurate microburst hazard can be forecast. A standard mechanically scanned radar located in the airport
would have extreme difficulty in searching an airport terminal area for precursor signatures of aviation hazard and
other weather events in a timely fashion. By decreasing the beam dwell time, the volume scan time may be decreased
dramatically. Alternatively, spatial resolution of the data may be improved for the same volume scan time by spacing
the short dwell time beams more closely in azimuth and elevation.
In this report we emphasize waveform design issues of this advanced radar architecture and a few of the feasible
phased array configurations previously discussed by Keeler and Frush (1983b), Holloway and Keeler (1993), and
Keeler (1994).
Pulse c
wav
A
Phased array
antenna
Figure 1.1. Advanced high resolution radars ystem
using pulse compression waveform and phased
array electronic scanned antenna.
1.4 FAA/TASS program description
The FAA's Terminal Area Surveillance System (TASS) Program has been active in the "advanced weather radar"
arena since 1991 and has explored concepts for a multiple function, next generation air traffic and weather surveil-
lance radar. The radar is envisioned as a phased array system capable of detecting, tracking and predicting positions
May 24, 1999 Page 2 of 52
Pulse Compression for Phased Array Weather Radar
of all aircraft and hazardous weather phenomena in the airport terminal area. A solid state pulse compression e-scan
radar is a promising technology. Various antenna system configurations have been considered, ranging from a single
ID e-scan mechanically rotating phased array to multiple fixed face 2D phase scanned active arrays. Dual polariza-
tion measurements are also being studied. The FAA is working closely with American and European industry in
developing concepts and system designs. Rogers, et al (1997), Buckler (1997), and Tidwell (1997) give detailed sum-
maries and status of the TASS program.
May 24, 1999 Page 3 of 52