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SNR EXPT < SNR THEORY TARGET A ¹ TARGET B RESULTS CLR DATA ¹ OTHER SENSORS’ DATA

This paper discusses the factors of two that need to be considered in Coherent Lidar measurements, such as atmospheric loss, target reflectance, and optical aberrations.

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SNR EXPT < SNR THEORY TARGET A ¹ TARGET B RESULTS CLR DATA ¹ OTHER SENSORS’ DATA

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  1. Coherent Lidar:Factors Of Two Among FriendsMichael J. KavayaNASA/Langley Research CenterJuly 16, 2002NOAA Working Group on Space-Based Lidar Winds

  2. The Situation • SNREXPT < SNRTHEORY • TARGET A ¹ TARGET B RESULTS • CLR DATA ¹ OTHER SENSORS’ DATA

  3. 1. SNREXPT < SNRTHEORY • Brandewie & Davis, Appl. Opt. 11, 1526 (1972) • [CW, 10.6 mm, 2 x 5.8 cm, -6 dB, HT-D] • Huffaker, NCAR Atmospheric Technology, 71 (Winter 74-75) • [Pulsed, 10.6 mm, 30.5 cm, -20-30 dB, collected particles/theory] • Schwiesow & Cupp, Appl. Opt. 19, 3168 (1980) • [CW, 10.6 mm, 30 cm, -5 dB, HT-D] • Post, Richter, Hardesty, Lawrence, & Hall, Jr., Proc. SPIE 300, 60 (1981) • [Pulsed, 10.6 mm, 28 cm, -7 dB, HT-D] • Renhorn, Steinvall, Letalick, et al, Proc. SPIE 415, 39 (1983) • [CW, 10.6 mm, 11 cm, -3 dB, HT-G] • Foord, Jones, Vaughan, & Willetts, Appl. Opt. 22, 3787 (1983) • [CW, 10.6 mm, 20 cm, -0.8 dB, HT-D] • Shapiro, Appl. Opt. 24, 1245 (1985) • [Corrects Foord et al to –6.4 dB] CW = Continuous Wave, HT = Hard Target, D = Diffuse, G = Glint

  4. Coherent Lidar SNR Equation • SNRSB = E A (LSE)TA2 bc hMIS,S hnB 2 R2 n = c/lA = pD2/4 LIDARATMOSPHERE SB – Search Band f – frequency V – velocity A – atmosphere MIS – misalignment S – spacecraft LSE – Lidar System Efficiency (photon) Frehlich and Kavaya, Appl. Opt. 30, 5325 (1991). Eqs. (92, 117, 119)

  5. Lidar System Efficiency (LSE) • 0.707 Lidar portion of T/R angle misalignment factor hMIS,L • 0.578 T/R intensity transmission factor • 0.931 T/R aberration sub product • 0.319 Detection sub product • ________________________________________ • 0.122 Lidar System Efficiency = LSE • (12/01 input to GSFC ISAL/IMDC mission design teams)

  6. SNREXPT < SNRTHEORY SUSPECTS • Lidar system overestimated • Atmosphere loss underestimated • Lidar target reflectance overestimated

  7. 2. TARGET A ¹ TARGET B RESULTS • Jarzembski, Srivastava, & Chambers, Appl. Opt. 35, 2096 (1996): • LSEEXPT,HT ~ 2 x LSEEXPT,AER (1.7-2) • Hypothesis : Assume LSEEXPT,AER correct Þ LSEEXPT,HT x2 too high Þ rHT x2 too low Þ bCLR x2 too low

  8. 3. CLR DATA ¹ OTHER SENSORS’ DATA • Post, Grund, Wang, & Deshler, J. Geophys. Res. 102, 13535 (1997): • Used Mt. Pinatubo well studied spherical aerosols at 18 km • bEXPT,OPC ~ 2 x bEXPT,CLR (1.7-2.2) • Hypothesis: AssumebEXPT,OPC is correct • ÞbEXPT,CLR is x2 too low • ÞLSEEXPT,HT is x2 too high ÞrHT is x2 too low

  9. TARGET CALIBRATION METHODOLOGY? • Haner & Tratt, 11th Coherent Laser Radar Conf., Malvern, UK, p. 64 (2001) • Discussed Jarzembski et al and Post et al papers • Hypothesized a factor of 2 difference in reflectivity of diffuse dispersed target vs. diffuse rigidly connected target

  10. More Hypotheses

  11. Atmospheric Refractive Turbulence Loss Underestimated • Yura, Optica Acta 26, 627 (1979) • Clifford & Wandzura, Appl. Opt. 20, 514 (1981) • Frehlich & Kavaya, Appl. Opt. 30, 5325 (1991) • Belen’kii, Appl. Opt. 32, 5368 (1993) • Frehlich, Appl. Opt. 39, 4237 (2000) • Yes, would cause SNREXPT < SNRTHEORY; but a continuumof difference values, not X2

  12. Optical Aberrations Underestimated • Rye, Appl. Opt. 21, 839 (1982) • Spiers, 10th Coherent Laser Radar Conf., p. 170 (1999) • Delautre, Breugnot, & Laude, Opt. Comm. 160, 60 (1999) • Yes, would cause SNREXPT < SNRTHEORY; but a continuum of difference values, not X2 • Sub-Optimum Detection • Hunt, Holmes, & Amzajerdian, Appl. Opt. 27, 3135 (1988) • Wilson, Constant, Foord, & Vaughan, Infrared Phys. 31, 109 (1991) • Holmes & Rask, Appl. Opt. 34, 927 (1995) • Amzajerdian, 11th Coherent Laser Radar Conf., p. 176 (2001) • (“Optimization of the lidar receiver will gain up to 3 dB … in SNR”) • Yes, would cause SNREXPT < SNRTHEORY; but a continuum of difference values, not X2

  13. Opposition Effect Causes X2 Hard Target Reflectance Underestimation • Egan & Hilgeman, Appl. Opt. 16, 2861 (1977) [indep. of incident angle] • Mendez & O’Donnell, Opt. Commun. 61, 91 (1987); [hard target effect seen] • McGurn, Surface Science Reports 10, 357 (1990) • Neito-Vesperinas, Opt. & Photonics News, 50 (12/1990) • Corey, Kissner, & Saulnier, Am. J. Phys. 63, 560 (1995) • Pitter, Jakeman, & Harris, Opt. Lett. 22, 393 (1997); [3 dB, 1-3 mrad, het. det.] PR PR 0 180 0 180 PI PI

  14. Opposition Effect Causes X2 Hard Target Reflectance Underestimation • Lab target calibration FOV large; misses OE • Lidar FOV to field target small; within OE • ÞrHT is x2 too low • ÞLSEEXPT,HT is x2 too high • ÞbEXPT,CLR is x2 too low Close target; laboratory calibration FOV Far target; lidar FOV PR PR 180 0 180 180 0 180

  15. Observation/Suspect Compatibility Matrix SNREXPT < TARGET A ¹ CLR DATA ¹ OTHER SNRTHEORYTARGET B RESULTS SENSORS’ DATA X2 X2 Ref. Turb. ü Aberrations ü Detection ü Alignment ü Opp. Effect üüü Rigid Target üüü

  16. Notes & Recommendations • These phenomena do NOT affect space performance prediction IF prediction is scaled from experimental performance • Develop theory of diffuse dispersed target vs. diffuse rigid target reflectance; connect mean values to first principles of scatterers and to each other • Measure opposition effect of lidar diffuse hard targets and target calibration primary and transfer targets • Repeat Jarzembski et al experiments with several types of particle targets

  17. BACK UP CHARTS

  18. LOTS OF SUSPECTS • Atmospheric refractive turbulence • Optical aberrations • Sub-optimum heterodyne detection • Misalignment • Opposition effect: X2 • Dispersed vs. rigid diffuse target • If LO shot noise = all other noise, rather than >>, then LSE is ½ of ideal • If backscattered light is assumed linear polarized, but is actually randomly polarized, then the SNR will be X2 below expected • If a 50/50 beam splitter is used, SNR falls by X2 (vs. dual det.) • Diffuse target SNR factor of 0.46 (vs. specular) [Goodman, April 1965; “diffuse target loss”?] • The laser pulse moves forward at speed c, while the measured air volume moves forward at speed c/2, X2 • The laser pulse instantaneously illuminates ct length of aerosols, but the instantaneous heterodyne signal is from ct/2 length of aerosols; X2 • Experiments with light choppers and synchronous detection can easily make a X2 error in laser power • Beam diameter vs. radius: X2 • Beam amplitude vs. intensity: X2 in area • Beam 1/e vs. 1/e2: X2 in area

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