The Signal Processor System For The NASA Dual...
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The Signal Processor System For The NASA Dual-Frequency Dual-Polarized Doppler Radar NASA D3R System: Waveform Design Challenges Kumar Vijay Mishra 1 , V. Chandrasekar 1 , Cuong Nguyen 1 and Manuel Vega 1,2 1 Colorado State University 2 NASA Goddard Space Flight Center • D3R is the Ground Validation radar for Global Precipitation Measurement program [Chandrasekar et. al., “Scientific and engineering overview of the NASA Dual-frequency Dual- polarized Doppler Radar (D3R) system for GPM ground validation”, IGARSS’2010]. • Currently operational and has been deployed for field campaigns. • Sensitivity: -10 dBZ at 15 km to enable snow measurements. • Maximum unambiguous range = 30 km. • Precipitation measurements at frequencies. Ku-band: 13.91±.25GHz, Ka-band: 35.56±.25GHz. • Dual linear polarizations with capability for both polarization agility and diversity. Figure 1. D3R deployed at ARM Southern Great Plains site during GPM Midlatitude Continental Convective Clouds Experiment (MC3E) (05/28/2011). This was the first mechanical deployment of D3R in a field campaign. D3R Waveform Design • D3R employs solid-state transmitters with duty cycle up to 30% to achieve higher sensitivity with lower peak transmit power and enhance system stability in extreme climatic conditions. • A programmable multi-channel pulse-compression digital receiver implements a novel multi-pulse wideband frequency-diversity waveform to achieve desired sensitivity for D3R [Mishra et. al., “Waveform design and implementation for the solid-state NASA Dual-frequency Dual-polarized Doppler Radar”, IGARSS’2011]. Figure 3. Expected sensitivity of the Ku- and Ka- band radars for different sampling frequency f s , chirp bandwidth B and range resolution (Δr). Noise figure values are 4.6 dB (Ku) and 5.5 dB (Ka). Figure 2. Time (top) and frequency (bottom) plots of the D3R waveform. 140 MHz is the central IF. The digital receiver samples this signal at 200 MHz. Unambiguous Doppler Requirement • The unambiguous velocity requirement of 25 m/s is met with the staggered PRT 2/3 scheme. • Simultaneous mode staggered PRTs: 400/600 us. Alternate mode uniform PRT: 500 us. • Equivalent PRF = 5 kHz gives unambiguous Doppler of ~27 m/s while also achieving the maximum unambiguous range. • Time-domain clutter suppression for non-uniform sampling [Nguyen et. al., “Time domain GMAP clutter filter for weather radars”, 34 th AMS Radar Conference, 2009]. • Estimates of spectrum width are obtained using second lag autocorrelation for low SNR regions. Signal Processor Architecture 3 4 2 1 Figure 4. The digital receivers and the signal processing architecture in the context of D3R system. • Separate digital receivers and waveform generators for each frequency. • Digital receivers also sample the transmit pulse from the calibration channel for each PRT. • Digital receivers and waveform generators are mounted on top of the slip rings at the back of the antenna • High-end signal processing servers and storage are mounted on the trailer below the slip rings. • Raw I-Q data for all the subpulses is archived for all range gates • A single range profile obtained by merging data from different subpulses is provided for all meteorological products. • Two separate NetCDF files and displays are available for each frequency. Results: Pulse Compression Filter Performance 5 Figure 5. Pulse compression filter outputs when the sampled Ku-band transmit pulse is passed through the digital receiver filter chain comprising of digital down-conversion, decimation and pulse compression filters (Jan 03, 2012). This output shows the combined effect of the transceiver and digital filters on the side-lobe performance. • The design of the pulse compression filters in the digital receiver gives suppression of range side-lobes close to 50 dB. • The theoretical side- lobe suppression design value for L p - norm filter is 60 dB. • The effect of the down-conversion filters on the side-lobes is of the order of 1 dB only. Results: Comparison Of Individual Subpulse Profiles 6 • Reflectivity profiles are aligned after range calibration • As expected, long pulse has the highest sensitivity followed by the medium and short pulses. • The long pulse and medium pulse data are matched after their respective blind ranges: ~9 km and ~6 km. Figure 6. Comparison of the Ku-band Z h profiles across the 3 subpulses. Results: Comparison Of Velocity Estimates In Staggered 2/3 PRT Mode With The CHILL Radar 7 CHILL Ku Ka Z h V Figure 7. Simultaneous observations by CSU-CHILL S- Band radar and D3R for a rain event on Nov 1, 2011. Ka-Band system was operating in only H- polarization with the placeholder 1W transmitter. D3R data from different subpulses has been merged to form a single range profile. Unfolded velocity estimates for D3R are based on 2/3 staggered PRT. Results: Comparison of Dual-Polarimetric Variables 8 CHILL ρ hv Ψ dp D3R Ku Figure 8. Simultaneous observations by CSU- CHILL S-Band radar and Ku-Band system during an intense rain storm on Jul 19, 2011. In the plots, Ψ dp range for D3R is 3 times larger than that of CHILL. Data is plotted for the 40μs pulse after noise subtraction. Results: Comparisons With C-Band Radar During GCPEx Campaign 9 Figure 10. D3R deployed at Environment Canada site in Egbert, Canada during the GPM Cold Season Precipitation Experiment (GCPEx) (01/14/2012). Figure 9. Comparison between D3R and C- Band King City Radar (WKR) for a light rain event (01/14/2012). The sign of D3R velocity has been reversed since the two radars are looking towards each other. Results: Observations Of Passing Rain-band And Weakening Melting Layer 10 Figure 11. The minimum operational range of D3R is 500 m. Here, the radar signal processor records a melting layer as close as ~2.2 km. These RHIs were recorded looking at Skydive site of Environment Canada on Jan 23, 2012. Results: First Observations Of Weaker Snow Echoes 11 Figure 12. Ten-minute snapshots of over-the-head RHI scans of D3R for a light snow event on Jan 28, 2012. The RHIs are in the azimuthal direction of the King City Radar (WKR). Echoes weaker than - 5 dBZ are detected by the Ku-band. Results: Observations Of Mixed-Phase Precipitation 12 Figure 13. 20-minute snapshots of RHI scans of D3R for a cold rain event on Jan 26, 2012. The RHIs are in the azimuthal direction of the Environment Canada Skydive site (Az = 87.8 deg). The reflectivities shown here are not corrected for the attenuation in either of the bands. The chronological observation of mammatus clouds, brightband formation and freezing rain on ground is shown here. Summary and Future Work • A multi-channel digital receiver and real-time signal processor was designed, developed, tested and deployed using an advanced frequency-diversity waveform. • Comparison with S-band and C-band radars indicates that correct product estimation and good calibration. • Future work: Phase-coding, alternate mode of transmission, real-time clutter suppression and attenuation correction. 13 Acknowledgement This research is supported by the NASA PMM program. The authors acknowledge the contribution of Dr. Mathew Schwaller during the development and integration phase of this project. 14 • Maximum unambiguous Doppler = 25 m/s.
Transcript of The Signal Processor System For The NASA Dual...
The Signal Processor System For The NASA Dual-Frequency Dual-Polarized Doppler Radar
NASA D3R System: Waveform Design Challenges
Kumar Vijay Mishra1, V. Chandrasekar1, Cuong Nguyen1 and Manuel Vega1,2
1Colorado State University 2NASA Goddard Space Flight Center
• D3R is the Ground Validation radar for Global Precipitation Measurement program [Chandrasekar et. al., “Scientific and
engineering overview of the NASA Dual-frequency Dual-polarized Doppler Radar (D3R) system for GPM ground
validation”, IGARSS’2010]. • Currently operational and has been deployed for field campaigns. • Sensitivity: -10 dBZ at 15 km to enable snow measurements. • Maximum unambiguous range = 30 km. • Precipitation measurements at frequencies. Ku-band: 13.91±.25GHz, Ka-band: 35.56±.25GHz. • Dual linear polarizations with capability for both polarization agility and diversity.
Figure 1. D3R deployed at ARM Southern Great Plains site during GPM Midlatitude Continental Convective Clouds Experiment (MC3E) (05/28/2011). This was the first mechanical deployment of D3R in a field campaign.
D3R Waveform Design
• D3R employs solid-state transmitters with duty cycle up to 30% to achieve higher sensitivity with lower peak transmit power and enhance system stability in extreme climatic conditions. • A programmable multi-channel pulse-compression digital receiver implements a novel multi-pulse wideband frequency-diversity waveform to achieve desired sensitivity for D3R [Mishra et. al., “Waveform design and implementation for the
solid-state NASA Dual-frequency Dual-polarized Doppler Radar”, IGARSS’2011].
Figure 3. Expected sensitivity of the Ku- and Ka-band radars for different sampling frequency fs, chirp bandwidth B and range resolution (Δr). Noise figure values are 4.6 dB (Ku) and 5.5 dB (Ka).
Figure 2. Time (top) and frequency (bottom) plots of the D3R waveform. 140 MHz is the central IF. The digital receiver samples this signal at 200 MHz.
Unambiguous Doppler Requirement
• The unambiguous velocity requirement of 25 m/s is met with the staggered PRT 2/3 scheme.
• Simultaneous mode staggered PRTs: 400/600 us. Alternate mode uniform PRT: 500 us.
• Equivalent PRF = 5 kHz gives unambiguous Doppler of ~27 m/s while also achieving the maximum unambiguous range.
• Time-domain clutter suppression for non-uniform sampling [Nguyen et.
al., “Time domain GMAP clutter filter for weather radars”, 34th AMS Radar Conference, 2009].
• Estimates of spectrum width are obtained using second lag autocorrelation for low SNR regions.
Signal Processor Architecture 3 4
2 1
Figure 4. The digital receivers and the signal processing architecture in the context of D3R system.
• Separate digital receivers and waveform generators for each frequency.
• Digital receivers also sample the transmit pulse from the calibration channel for each PRT.
• Digital receivers and waveform generators are mounted on top of the slip rings at the back of the antenna
• High-end signal processing servers and storage are mounted on the trailer below the slip rings.
• Raw I-Q data for all the subpulses is archived for all range gates
• A single range profile obtained by merging data from different subpulses is provided for all meteorological products.
• Two separate NetCDF files and displays are available for each frequency.
Results: Pulse Compression Filter Performance 5
Figure 5. Pulse compression filter outputs when the sampled Ku-band transmit pulse is passed through the digital receiver filter chain comprising of digital down-conversion, decimation and pulse compression filters (Jan 03, 2012). This output shows the combined effect of the transceiver and digital filters on the side-lobe performance.
• The design of the pulse compression filters in the digital receiver gives suppression of range side-lobes close to 50 dB.
• The theoretical side-lobe suppression design value for Lp-norm filter is 60 dB.
• The effect of the down-conversion filters on the side-lobes is of the order of 1 dB only.
Results: Comparison Of Individual Subpulse Profiles 6
• Reflectivity profiles are aligned after range calibration • As expected, long pulse has the highest sensitivity followed by the medium and short pulses. • The long pulse and medium pulse data are matched after their respective blind ranges: ~9 km and ~6 km.
Figure 6. Comparison of the Ku-band Zh profiles across the 3 subpulses.
Results: Comparison Of Velocity Estimates In Staggered 2/3 PRT Mode With The CHILL Radar
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CHILL Ku Ka
Zh
V
Figure 7. Simultaneous observations by CSU-CHILL S-Band radar and D3R for a rain event on Nov 1, 2011. Ka-Band system was operating in only H-polarization with the placeholder 1W transmitter. D3R data from different subpulses has been merged to form a single range profile. Unfolded velocity estimates for D3R are based on 2/3 staggered PRT.
Results: Comparison of Dual-Polarimetric Variables 8
CHILL
ρhv
Ψdp
D3R Ku
Figure 8. Simultaneous observations by CSU-CHILL S-Band radar and Ku-Band system during an intense rain storm on Jul 19, 2011. In the plots, Ψdp range for D3R is 3 times larger than that of CHILL. Data is plotted for the 40µs pulse after noise subtraction.
Results: Comparisons With C-Band Radar During GCPEx Campaign
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Figure 10. D3R deployed at Environment Canada site in Egbert, Canada during the GPM Cold Season Precipitation Experiment (GCPEx) (01/14/2012).
Figure 9. Comparison between D3R and C-Band King City Radar (WKR) for a light rain event (01/14/2012). The sign of D3R velocity has been reversed since the two radars are looking towards each other.
Results: Observations Of Passing Rain-band And Weakening Melting Layer
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Figure 11. The minimum operational range of D3R is 500 m. Here, the radar signal processor records a melting layer as close as ~2.2 km. These RHIs were recorded looking at Skydive site of Environment Canada on Jan 23, 2012.
Results: First Observations Of Weaker Snow Echoes 11
Figure 12. Ten-minute snapshots of over-the-head RHI scans of D3R for a light snow event on Jan 28, 2012. The RHIs are in the azimuthal direction of the King City Radar (WKR). Echoes weaker than -5 dBZ are detected by the Ku-band.
Results: Observations Of Mixed-Phase Precipitation 12
Figure 13. 20-minute snapshots of RHI scans of D3R for a cold rain event on Jan 26, 2012. The RHIs are in the azimuthal direction of the Environment Canada Skydive site (Az = 87.8 deg). The reflectivities shown here are not corrected for the attenuation in either of the bands. The chronological observation of mammatus clouds, brightband formation and freezing rain on ground is shown here.
Summary and Future Work
• A multi-channel digital receiver and real-time signal processor was designed, developed, tested and deployed using an advanced frequency-diversity waveform. • Comparison with S-band and C-band radars indicates that correct product estimation and good calibration. • Future work: Phase-coding, alternate mode of transmission, real-time clutter suppression and attenuation correction.
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Acknowledgement
This research is supported by the NASA PMM program. The authors acknowledge the contribution of Dr. Mathew Schwaller during the development and integration phase of this project.
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• Maximum unambiguous Doppler = 25 m/s.
