Seminar “LIDAR” - Comisin Nacional de Actividades ... en Aplicaciones Espaciales de alerta y Respuesta Temprana a Emergencias Comisin Nacional de Actividades Espaciales Seminar “LIDAR” Author: Molina Wladimir

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  • Maestra en Aplicaciones Espaciales de alerta y Respuesta Temprana a Emergencias

    Comisin Nacional de Actividades Espaciales

    Seminar

    LIDAR

    Author: Molina Wladimir

    September, 2014

  • INDEX

    LIDAR .......................................................................................................................... 2

    How LIDAR works ...................................................................................................... 2

    Scaterring....................................................................................................................... 4 Rayleigh scattering ................................................................................................. 4 Mie Scattering ........................................................................................................ 5 Raman scattering .................................................................................................... 5 Fluorescence ........................................................................................................... 6

    Detectors........................................................................................................................ 6 Photomultipliers ..................................................................................................... 6 Photo-Diode............................................................................................................. 7 Solic Stated Detectors.............................................................................................. 8

    Detection........................................................................................................................ 8 Heterodyne detection............................................................................................... 8

    Desing ........................................................................................................................... 9 Laser ....................................................................................................................... 10 Scanner and optics .................................................................................................. 11 Photodetector and receiver electronics ................................................................... 11 Position and navigation systems.............................................................................. 11

    Most important Applications.......................................................................................... 12Geology and soil science......................................................................................... 12Atmospheric Remote Sensing and Meteorology..................................................... 12Agriculture............................................................................................................... 13

    LIDAR software generalities......................................................................................... 14Free LiDAR Tools................................................................................................... 14ENVI LiDAR ......................................................................................................... 15

    Bibliography .................................................................................................................. 15

  • INTRODUCTION

    The LIDAR technology is worldwide used to a lot of applications in topography studies, agriculture, archeology, Atmospheric Remote Sensing and Meteorology, Physics and astronomy, geology and soil science and most important Physical of Remote Sensing and astronomy due to its capacity to generate a reliable estimation to the distance between a point to another, just known the time of travel of the light from the source of LIDAR laser to a target object and the return thereof, the basics involve LIDAR operation is complex, especially the discretization and detection operation, but in this summary will be try to do a explication of all LIDAR procedures, in a pleasant and concisely way.

  • LIDAR

    LIDAR is a technology that allows to estimate the distance by illuminating a target with a laser and analyzing the reflected light. The term LIDAR was created as a portmanteau of "light" and "radar"[1].

    LIDAR is popularly used as a technology to make high-resolution maps (figure 1), with applications in geomatics, archeology, geography, geology, geomorphology, seismology, forestry, remote sensing, atmospheric physics, airborne laser swath mapping (ALSM), laser altimetry, and contour mapping [2].

    Figure 1. LIDAR land-scan, in this example the LIDAR is incorporated on a airplane to obtain a hight resolution map.

    LIDAR was developed in the early 1960s, shortly after the invention of the laser. Its first applications were in meteorology, where it was used to measure clouds by the National Center for Atmospheric Research. The general public became aware of the accuracy and usefulness of LIDAR systems in 1971 during the Apollo 15 mission, when astronauts used a laser altimeter to map the surface of the moon [3].

    How LIDAR works

    LIDAR basically combined a laser focused imaging with the radar ability to calculate distances by measuring the time for the signal to return. That is considering the constant nature of electromagnetic waves velocity in a determined propagating space.

    In a determinate space, a electromagnetic wave is moving with a speed V=C, where C=2,9979x10 m/s in vacuum, If considered this speed and a simplified movement (uniform rectilinear movement) of the electromagnetic wavefront, a x distance can be considered with the simplified relation:

    x = C(t)

  • Now, the x distance is a combination of two electromagnetic wave travel movements, first the movement of LIDAR laser source to object target (x'), and second the return movement of scatter radiation from object target to the LIDAR's photodetector (x''), the above equation can be rewrite like a component of two covered distances:

    (x'+x'') = C(t)

    However, the speed of a electromagnetic wave propagating in any space can be considered much greater than any other object in movement beside it (like an airplane, a satellite, etc), so that in the travel of electromagnetic wavefront to the object target and its return to LIDAR source, the LIDAR position can be regarded undisturbed, in this context the distance x' can be considered equal to x'', so the two covered distances equation can be rewrite like:

    2x'= C(t) or 2x''= C(t)

    The recovered distance to target object to LIDAR source (in this simplified example) is:

    x''= C(t)/2

    In this context, it becomes obvious that if the source position remains invariant and is measured the time of electromagnetic wavefront traveled, can be estimate the traveled distances of the electromagnetic wavefront, in this case the LIDAR uses a laser system by the electromagnetic source of radiation to obtain information of the scanned object and the traveled distance is the height from the LIDAR source to target objects scanning in land, this is the foundation of LIDAR detection process (figure 2).

    Figure 2. Foundation of LIDAR detection process. The laser signal rebound from target objects and return to source (a), a lot of signals are use to form a image from target objects (b).

    The laser use is propitiated by the electromagnetic wave source of LIDAR because Its coherent and monocromatic nature, a laser beam do not disturb in its propagation, its keep narrow-beam

  • shape so that if one takes security of the site which is pointing, can be sure that the return signal throw information of focused spot and not another point about this.

    It becomes obvious too that in a different space of vacuum, the above relation showed is too simplified. The speed of a electromagnetic wave is affected and it change in a space different to vacuum, calibration factors that reflect reality are necessary to establish the correct speed of any electromagnetic wave involved in the retrodispersion process, also the nature of incident and scatter radiation interacting with the different target objects becomes necessary the implementation of advanced methods to detection and discrimination of all electromagnetic wave arrived to the detector and a adaptation system to accommodate the incident wavelengths LIDAR laser in target objects.

    In this context, in a LIDAR, Laser wavelengths vary to suit the target: from about 10 micrometers to the UV (approximately 250 nm). Different types of scattering are used for different LIDAR applications: most commonly Rayleigh scattering, Mie scattering, Raman scattering, and fluorescence. Based on different kinds of backscattering, the LIDAR can be accordingly called Rayleigh LIDAR, Mie LIDAR Raman LIDAR, Na/Fe/K Fluorescence LIDAR, and so on [2]. Suitable combinations of wavelengths can allow for remote mapping of atmospheric contents by identifying wavelength-dependent changes in the intensity of the returned signal.

    About the scattering, a review of the different kinds of scattering are necessary to understand the LIDAR basics ans process, a review to next.

    Scattering

    Differents kinds of scattering obtained in LIDAR studies can be:

    Rayleigh scattering:

    Is the (dominantly) elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. After the Rayleigh scattering the state of material remains unchanged, hence Rayleigh scattering is also said to be a parametric process. The particles may be individual atoms or molecules. It can occur when light travels through transparent solids and liquids, but is most prominently seen in gases. Rayleigh scattering results from the electric polarizability of the particles. The oscillating electric field of a light wave acts on the charges within a particle, causing them to move at the same frequency. The particle therefore becomes a small radiating dipole whose radiation we see as scattered light.

    Scattering by particles similar to, or larger than, the wavelength of light is typically treated by the Mie theory, the discrete dipole approximation and other computational techniques. Rayleigh scattering applies to particles that are small with respect to wavelengths of light, and that are optically "soft" (i.e. with a refractive index close to 1). On the other hand, Anomalous Diffraction Theory applies to optically soft but larger particles.

    The size of a scattering particle is often parameterized by the ratio:

    x=2 r

  • Where r is its characteristic length (radius) and is the wavelength of the light. The amplitude of light scattered from within any transparent dielectric is proportional to the inverse square of its wavelength and to the volume of material, that is to the cube of its characteristic length. The wavelength dependence is characteristic of dipole scattering and the volume dependence will apply to any scattering mechanism. Objects with x 1 act as geometric shapes, scattering light according to their projected area. At the intermediate x 1 of Mie scattering, interference effects develop through phase variations over the object's surface. Rayleigh scattering applies to the case when the scattering particle is very small (x 1, with a particle size < 1 /10 wavelength) and the whole surface re-radiates with the same phase. Because the particles are randomly positioned, the scattered light arrives at a particular point with a random collection of phases; it is incoherent and the resulting intensity is just the sum of the squares of the amplitudes from each particle and therefore proportional to the inverse fourth power of the wavelength and the sixth power of its size [4]. In detail, the intensity I of light scattered by any one of the small spheres of diameter d and refractive index n from a beam of unpolarized light of wavelength and intensity I0 is given by

    I=Io 1+cos2 R (

    2 )

    4

    ( n1n+2 )2

    ( d2 )6

    where R is the distance to the particle and is the scattering angle. Averaging this over all angles gives the Rayleigh scattering cross-section.

    s=25

    3d6

    4 ( n1n+2 )2

    Mie Scattering:

    The Mie solution to Maxwell's equations describes the scattering of electromagnetic radiation by a sphere. The solution takes the form of an infinite series.

    It can be seen from the above equation that Rayleigh scattering is strongly dependent upon the size of the particle and the wavelengths. The intensity of the Rayleigh scattered radiation increases rapidly as the ratio of particle size to wavelength increases. Furthermore, the intensity of Rayleigh scattered radiation is identical in the forward and reverse directions.

    The Rayleigh scattering model breaks down when the particle size becomes larger than around 10% of the wavelength of the incident radiation. In the case of particles with dimensions greater than this, Mie's scattering model can be used to find the intensity of the scattered radiation. The intensity of Mie scattered radiation is given by the summation of an infinite series of terms rather than by a simple mathematical expression. It can be shown, however, that Mie scattering differs from Rayleigh scattering in several respects; it is roughly independent of wavelength and it is larger in the forward direction than in the reverse direction. The greater the particle size, the more of the light is scattered in the forward direction [5].

    Raman scattering

    Raman effect is the inelastic scattering of a photon. When photons are scattered from an atom or molecule, most photons are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency and wavelength) as the incident photons. A small fraction

  • of the scattered photons (approximately 1 in 10 million) are scattered by an excitation, with the scattered photons having a frequency different from, and usually lower than, that of the incident photons[6]. In a gas, Raman scattering can occur with a change in energy of a molecule due to a transition.

    Typically, photons from the laser beam produce an oscillating polarization in the molecules, exciting them to a virtual energy state. The oscillating polarization of the molecule can couple with other possible polarizations of the molecule, including vibrational and electronic excitations. If the polarization in the molecule does not couple to these other possible polarization, then it will not change the vibrational state that the molecule started in and the scattered photon will have the same energy as the original photon. This type of scattering is known as Rayleigh scattering.

    When the polarization in the molecules couples to a vibrational state that is higher in energy than the state they started in, then the original photon and the scattered photon differ in energy by the amount required to v...

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