Fahmy Asal 
  
involved in measuring the distance between the sensor and an object by determining the time delay between the 
transmission of a light signal towards the object and the reception of it by the sensor on its return from being reflected 
by the object. LiDAR employs Laser light for profiling and is based on the ranging method. It can be operated from the 
ground and from moving platforms such as a boat, a helicopter, and an aircraft as well as from a spacecraft. The output 
from airborne use of this technique is a digital surface model (DSM) which is useful in many environmental 
applications. 
3.1 Laser Background 
Laser is an established technology and acquired its name from the first letters of the expression "Light Amplification by 
Stimulated Emission of Radiation". It is light but more intensive and more concentrated than any natural light. While 
white light from different sources such as the sun, lamps or flash light spreads out as it travels, a laser does not disperse 
much as it travels. Also, laser is a monochromatic light with a theoretical unique wavelength and refers to a specific 
colour of the electromagnetic spectrum. It can be focused to a very thin beam of diameter reaching 0.001 inch. 
Moreover, it can be controlled as a continuous beam or pulses with widely varied abilities ranging from boring holes in 
the hardest materials to being used in performing very soft surgery operations (NASA, 1999). 
Lasers are classified in different ways (Measures, 1984). They can be distinguished with respect to the duration of 
emission into pulsed or continuous lasers. They are also, categorized by their wavelength into infrared lasers, visible 
lasers and ultraviolet lasers. In addition, they vary due to the power of the beam. Moreover, a traditional classification 
is due to the type of the material of the gain medium, the energized substance that can amplify the light beam. This 
leads to having solid state lasers, gas lasers, liquid or dye lasers and semiconductor lasers. 
32 Airborne Laser Scanning 
Airborne laser scanning exploits the LiDAR technology in measuring ranges from an airborne platform to ground 
objects which reflect the emitted laser beam. The laser pulse used can be sufficiently sensitive to pick up tops of trees 
and the ground surface, sending back a ‘double’ pulse that has obvious advantages in some applications such as forestry. 
The system includes a scanning unit, which has the ability to change and control the direction of the emitted laser that 
pulses a large number of times per second. There is also a timer unit with the system which has the capability to 
measure the time of delay of each signal starting from the moment of emission of the laser pulse till the moment of the 
reflected signals reception. This time is accurately measured with a resolution of 1.0nsec. For the profiling purposes, 
the ground object co-ordinates have to be determined and this is needed at every time of pulsing. This process requires 
the laser position and the attitude angles of each laser pulse to be accurately measured. Therefore, the system has to 
include a positioning unit and a unit for measuring attitudes beside the laser system. The whole system comprises of the 
following main units (figure: 1): 
1) A Laser scanning and cooling unit. 
2) Global Positioning System (GPS) receivers with antennas mounted on the platform and at ground stations. 
3) An Inertial Navigation System (INS) unit. 
4) Data storage and processing unit. 
32.1 The Laser Scanning and Cooling Unit. The main function of this unit is to generate and emit laser pulses 
with a specified frequency. A timer is included in the unit to assess accurately the time difference between pulsing of 
laser and receiving the reflected signal as described previously. By recording this time (t), from sending out the laser 
pulse to receiving the return, the distance or range (R) to the object can be computed using: 
R=c. (V2) (1) 
Where: c = the light speed in air, which is known. 
A typical scanning unit contains a mirror, mounted in front of the laser-emitting point. This mirror rotates with a 
frequency equal to the laser pulsing rate through a scanning angle similar to the field of view angle in the case of the 
imaging systems. The pulses of few nanoseconds duration are directed by the mirror, through an opening or a window, 
towards the ground. Additionally, a cooling system is provided to absorb the temperature released from the lasing 
action. 
  
46 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 
(€ EE EE 
  
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