{"id":361,"date":"2016-02-19T13:08:50","date_gmt":"2016-02-19T13:08:50","guid":{"rendered":"http:\/\/generic.wordpress.soton.ac.uk\/tda\/?page_id=361"},"modified":"2018-03-23T11:30:30","modified_gmt":"2018-03-23T11:30:30","slug":"airborne-laser-scanning","status":"publish","type":"page","link":"https:\/\/generic.wordpress.soton.ac.uk\/tda\/airborne-and-spaceborne-laser-scanning\/airborne-laser-scanning\/","title":{"rendered":"Airborne Laser Scanning"},"content":{"rendered":"

The objective of this unit is to introduce the various components of an airborne laser scanner and the mechanism of data acquisition, with some reference to commercial airborne LiDAR data suppliers.<\/p>\n

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A typical Airborne Laser Scanner (ALS) system has the following components:<\/p>\n

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  1. Basis laser ranging unit.<\/li>\n
  2. Electromechanical device for scanning.<\/li>\n
  3. Electronics unit containing hardware for the processing functions of the system.<\/li>\n
  4. Differential GPS\/IMU for positioning. Allows georeferencing and therefore practical use of data collected.<\/li>\n
  5. Software: recording, storage and processing of data. Post flight processing needed to produce a LiDAR point cloud.<\/li>\n<\/ol>\n

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    \"schem_MD_MAN04_2\"<\/a><\/p>\n

     <\/p>\n

    Most of the ALS system uses the NIR part of the electromagnetic spectrum. The electromechanical device generally uses a mirror located in front of the laser and sends out pulses at known angles allowing cross track scanning to the longitudinal profile (motion) of airborne platform. The scanner contains a control and orientation system which records raw navigation data (GPS coordinates) and angular encoder values. The differential GPS processing requires ground reference GPS data from one of four sources:<\/p>\n

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    1. GPS receiver at a base station.<\/li>\n
    2. Wide area service (eg. OmniSTAR) using satellite broadcast technology providing real time corrections based on network of reference stations.<\/li>\n
    3. Data from a local network of reference stations.<\/li>\n
    4. PPP (precise point positioning) technology based on precise orbits and clock corrections for satellites provided by IGS, based on worldwide network of base stations.<\/li>\n<\/ol>\n

      Differential GPS (DGPS) corrects for navigation errors by using a base station which calculates the difference between a known location and satellite signals and broadcasts this difference. Real time DGPS continually broadcasts the difference between received satellite signals and known locations as it receives the data.<\/p>\n

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      Reading Activity<\/h3>\n

      It is recommended that students read the following paper which provides an overview of the ALS system. A copy is available on the course Blackboard site.<\/p>\n

      Wehr, A. and Lohr, U. (1999) Airborne laser scanning\u2014an introduction and overview. ISPRS Journal of Photogrammetry and Remote Sensing<\/em> 54:68\u201382.<\/p>\n

      The following link provides more information on DGPS: http:\/\/www.esri.com\/news\/arcuser\/0103\/differential1of2.html<\/a><\/p>\n

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       <\/p>\n

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      Reflection<\/h3>\n

      Why do you think most Airborne Laser Scanner (ALS) systems use the NIR part of the spectrum?<\/p>\n

      This is because the NIR part of the electromagnetic spectrum is highly reflective from both ground and vegetation. Thus there is less chance of signal attenuation compared to the visible part of the spectrum.<\/p>\n<\/div><\/div>

      Incorrect<\/span><\/div><\/div><\/div><\/div>\n
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       <\/p>\n

      Scanning Mechanisms<\/h3>\n

      For airborne LiDAR scanners, the aim of the scanning mechanism is to scan fast enough to compensate for the forward movement of the platform. Currently four main scanning systems are used by operational ALS systems:<\/p>\n