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19th Coherent Laser Radar Conference Analytical Feasibility Study of Wind Lidar with Long-Duration Frequency-Modulated Pulse Ei-ichi Yoshikawa Japan Aerospace Exploration Agency Tomoo Ushio 〇 Hiroshi Yamasuge Tokyo Metropolitan University 19 June, 2018, Okinawa. Outline. Background Concept

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  1. 19th Coherent Laser Radar Conference Analytical Feasibility Study of Wind Lidar with Long-Duration Frequency-Modulated PulseEi-ichi Yoshikawa Japan Aerospace Exploration AgencyTomoo Ushio 〇Hiroshi YamasugeTokyo Metropolitan University19 June, 2018, Okinawa.

  2. Outline Background Concept Hardware and software Theoretical performance Practical issues Summary

  3. Background • Today, wind lidars are working in many kinds of applications • Climate and weather researches • Surveillance of wind power resources • Airport windshear/turbulence detection • Clear air turbulence detection by airborne wind lidar • Non-specialists, however, say that... • Lidar is WIND RADAR • (Compared with rain radar) maximum range is very short • (Compared with rain radar) scan is very slow • And, its size, weight, and cost are similar to RAIN RADAR • Pulse compression to wind lidars? • Pulse compression is a radar technique to use long-modulated pulses to improve SNR and range resolution. • Can pulse-compression-like technique be applied to wind lidars? • How good performances can be achieved?

  4. Concept Long-modulated pulse,to emit high power Blank for roundtrip(practically unnecessary) Signal processing to extract signals with resolution and high SNR

  5. Hardware Modulated signal is generated in IF Beat signal is extracted Discretized and stored in IF

  6. Software • Multi-Reference Matched Filter Approach Wavelet Time-Frequency Analysis Range-VelocityField Reference signal 1 Filtered signal 1 Reference signal 2 Filtered signal 2 Received signal ・・・ ・・・ ・・・ ・・・ Reference signal N Filtered signal N Reference signal : ( :Doppler shift caused by) Filtered signal :

  7. Performance • Resolution and SNRare analytically evaluated by using ambiguity function.(Random 8-PSK signal is used in this simulation) Random-phaser sum at a range-velocity results in Peak gain by T is cancelled by the velocity resolution. Therefore, long-modulated pulse does not give power gain but increase number of samples in a Doppler spectrum. Range ambiguity Peak is proportional to pulse duration T. Velocity ambiguity

  8. Performance • RangingDetectability of received power estimate (the zeroth moment of a Doppler spectrum) (-27, 0.5) means that, in received power estimate, its lower 0.5σ is equal to noise level when SNR is -27 dB. T=5usec T=500usec 100 times longer pulse duration (Number of Doppler spectral samples x100) => 10-dB lower signals can be detected with equivalent detectability. *400-square-summed Doppler spectra was supposed.

  9. Performance • VelocimetryAccuracy of mean velocity estimate (first moment of a Doppler spectrum) (-30, 1) means that, in velocity estimate, its standard deviation is 1 m/sec when SNR is -30 dB. T=5usec T=500usec 100 times longer pulse duration (Number of Doppler spectral samples x100) => 10-dB lower signals can be estimated with equivalent accuracy. *400-square-summed Doppler spectra was supposed.

  10. Practical Issues • Accuracy on Pulse Shaping • Poor accuracy will result in range-velocity sidelobes increasing. • The range sidelobes can be suppressed by a combination of frequency hopping and short-time divided pulse compression for the hopping frequencies. • Direct Coupling • Depend on isolation performance of a T/R circulation. • Bi-static optical antenna configuration is probably needed to accomplish a sufficient T/R isolation. • Time-Variation of Scattering Coefficient of an Aerosol • We need to examine by an experiment using a prototype model.

  11. Summary and Future Work • Long-modulated pulsed lidar was proposed. • Modulated signal is generated in IF then mixed with laser light. • In receiver side, beat signal is digitally sampled then signal processing (multi-reference matched filter) is performed. • Theoretical performance was analytically evaluated. • Power of Doppler spectrum is not gained by expanding pulse duration. • Number of Doppler spectral samples increases proportionally to pulse duration. • Wind ranging and velocimetry improve in proportion to the square root of pulse duration; that is, a 100 times longer pulse is equivalent to a 10-dB amplification • Experiments with prototype • The prototype is now under development. • Practical issues will be evaluated by using the prototype.

  12. Contact yoshikawa.eiichi@jaxa.jp Thank you for attention. In more details, please check; E. Yoshikawa and T. Ushio, “Wind ranging and velocimetry with low peak power and long-duration modulated laser,” Optics Express, Vol. 25, Issue 8, pp. 8845-8859 (2017). https://doi.org/10.1364/OE.25.008845

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