LIDAR for Remote Sensing and CALIPSO M Maarrcchh 3311 22000044 PPrreesseenntteedd bbyy D Drr CCaarrll W Weeiim meerr ooff BBaallll AAeerroossppaaccee TTeecchhnnoollooggiieess CCoorrpp Summary of this presentation written by John Marshall with all pictures and slides provided by Dr Weimer The LIDAR System In his presentation on LIDAR for Remote Sensing and CALIPSO Dr Weimer introduced to us LIDAR light detection and ranging and its use in Remote Sensing followed by an introduction to CALIPSO a new space based LIDAR to be launched next year currently undergoing integration in France with Alcatel Dr Weimer points out that LIDAR is very similar to radar in principle but with light instead of radar waves Essentially LIDAR can do two things First it can be used to gain a picture of topography by measuring the time it takes for light pulses emitted from the sensor to bounce off the target surface and return and the second which is the case of CALIPSO is to analyze the return signal to see how the atmosphere affected it which in turn tells us what is in the atmosphere LIDAR lasers have some unique characteristics such as being very monochromatic another words the laser is very pure having a very small spectral range which will illuminate an area from orbit about a 70m wide which is limited to prevent eye injury to animals or humans on the surface As it shines down three things can happen to the light from a LIDAR It can be scattered absorbed or transmitted straight through the atmosphere Only a limited number of wavelengths are available that can work from space for laser applications One problem is passing the laser light through the atmosphere which will have an affect on the laser yet this is also advantageous By receiving the signal back and analyzing it a vertical snapshot of the atmosphere directly below the satellite can be obtained And because the signal is constantly transmitted and received in a line a vertical curtain can be obtained of the atmosphere This will lead to a better of understanding of earth s radiation balance which is critical in understanding global warming Typical LIDAR System Geometry Coaxial LASER Laser transmitted ATMOSPHERIC SCATTERERS Or ABSORBS DETECTOR OPTICS Receiver Subsystem Backscattered Radiation Received One of the advantages of a LIDAR sensor Dr Weimer explains is that we know the exact properties of the laser so when the signal is received back to the sensor we can see accurately how the signal was changed or affected by the atmosphere The following is a list of the main types of interactions between the laser light and the atmosphere Optical Interactions of Relevance to Laser Remote Sensing z z z z z z Rayleigh scattering laser radiation elastically scattered from atoms and molecules no change in wavelength Mie scattering laser radiation elastically scattered from particles aerosols clouds of sizes comparable to the wavelength of radiation no change in wavelength Raman scattering laser radiation inelastically scattered from molecules wavelength shift that is a characteristic of the molecule Resonance scattering laser radiation matched to specific atomic transition scattered by large cross section no change in wavelength Fluorescence laser radiation matched in frequency to specific electronic transition of an atom or molecule is absorbed with subsequent emission at longer wavelength Absorption attenuation of laser radiation Can be non resonant e g black soot or resonant when wavelength is matched to the absorption band of a given molecule or atom An example of Raman scattering would be the interaction of a laser with carbon dioxide since the light when it is reemitted will be shifted Fluorescence Dr Weimer explained is caused by molecules rattling down through the quantum states causing different light to come out LIDAR for Remote Sensing LIDAR Equation z For Elastic Scattering the LIDAR Equation can be written as R Pr R C h kb R exp 2 k e r dr R 2 2 4 0 Here Pr is the power returned to the lidar at the laser wavelength C lidar constant includes Pt laser power receiver Area filter bandwidths and other instrument factors R range to target Ar kb 4 km 1sr 1 is the backscattering factor or lidar backscatter coefficient ke km 1 is the volume extinction coefficient CU Seminar 3 31 04 Light scattering particles h R Range Lidar for Remote Sensing CALIPSO Scientists Dr Weimer points out spend most of there time on the backscatter coefficient 6 Ranging Equation z For all lidars the Range Equation for R is simply R z LIDAR for Remote Sensing c t 2 Where c is the Speed of Light t is the round trip time for the light from lidar to target and back measured using precision oscillators Solutions z Two measurements roundtrip time signal strength z Three unknowns Range backscatter coefficient extinction coefficient z CU Seminar 3 31 04 Extensive work on finding solutions see Applied Optics e g work by Klett Fernald Kovalev Lidar for Remote Sensing CALIPSO If you want 1ft resolution your instrument needs to be able to resolve 1 nanosecond A typical roundtrip for light could be around 4 5 milliseconds 7 The CALIPSO Satellite Mission z CALIPSO will provide key measurements of aerosol and cloud properties needed to improve climate predictions by reducing uncertainties in z radiative forcing from clouds and aerosols aerosol cloud interactions cloud climate feedbacks CALIPSO will fly a 3 channel backscatter lidar and passive VIS IR instruments in formation with Aqua CloudSat PARASOL Dr Weimer explained that so far there has not been a LIDAR that has completely mapped the earth With CALIPSO there are 3 instruments on board and it will send down 4 GB of data a day Because the atmosphere is its own reference validation is simple he says Saharan Dust Transport LITE Lidar 1994 Strong backscatter from aerosols z z z Above is displayed a color modulated plot of LITE Level 1 532 nm profile data over the Sahara during orbit 146 at approximately 23 GMT on September 18 1994 The image displays 5 minutes of LITE data The color assigned to each pixel represents the intensity of the return signal in digitizer counts The count values range from zero for no return to a value less than 4000 for the strongest return Prominently featured at the start of this orbit transect is the Atlas Mountain range near 31N 8W This mountain range approximately separates a more optically thick aerosol air mass to the southeast from a relatively cleaner air mass to the northwest http www lite larc nasa