Light Detection and Ranging (LiDAR) is an active sensor which uses electromagnetic waves in the optical and infrared wavelengths. This post will discuss about what is Light Detection and Ranging (LiDAR), it’s types, architecture, how it works, applications, advantages and disadvantages.
What is LiDAR (Light Detection and Ranging)
Light Detection and Ranging (called as LiDAR in short) is used as a Range Finder or tracking device that uses light in the form of Laser. It consists of a laser source, sensor and Global Positioning System (GPS) receiver. Laser pulses are directed towards the Earth’s surface and the light pulses are reflected back to the sensor.
Fig. 1 – Introduction to Light Detection and Ranging (Li-DAR)
Light Detection and Ranging operates on both terrestrial platform and aerial platform. However, this post emphasizes on its operation from aerial platform i.e. using spacecraft like satellite or aircraft like helicopter, airplanes, drones etc. It is helpful in making 3-D representations of the target area.
Fig. 2 – 3-D Image using Light Detection and Ranging (Li-DAR)
Types of LiDAR (Light Detection and Ranging)
There are two types of Li-DAR, namely:
- Airborne Light Detection and Ranging
- Terrestrial Light Detection and Ranging
As the name suggests, Airborne LiDAR operates laser scanning from an Aircraft, Spacecraft, Helicopter or Drones.
It is categorized into two types:
- Topographic Light Detection and Ranging
- Bathymetric Light Detection and Ranging
It uses near-infrared light to generate Topographic maps of the scanned area.
It scans through water and mainly used to gather information of hydrographic data. It is used for coastal and inland water projects.
It is used for scanning on the earth’s surface and performs High Definition surveying. It is also used in Digital Terrain Models.
It is categorized into two types:
- Mobile Lidar
- Static Lidar
The system is mounted on moving vehicles on land like cars and trains.
This type of system is fixed on an immovable tripod.
Architecture of LiDAR (Light Detection and Ranging)
The components include:
- Optical System
- Control and Processing Unit
Fig. 3 – Schematic Diagram of LiDAR Architecture
Fig. 3 above shows the Architecture of Light Detection and Ranging (LiDAR) System. Mostly the system uses diode laser or diode-pumped solid-state laser to transmit the light pulse. Diode Lasers have broad beam and are inexpensive. Transmitter Optics helps the system to focus the light on to the target area. Some of the light reflects off the objects like buildings, trees etc.
The Reflected light or the back scattered light is focused into the Photo-detector. Light Detection and Ranging (LiDAR) Receiver can be a single detector or an array of detectors. The photo-detector is connected to Control and Data Acquisition unit that measures the amount of back scattered light as a function of distance.
It helps in transmitting the Laser light in the form of pulses.
It receives the reflected light or back scattered light.
The Optical System in the Transmitter helps to focus the light on the target area and Receiver Optical system helps to focus the reflected light into Photo-detector.
Control and Processing Unit
This unit receives the reflected light and processes the Data using specialized software’s. This unit has:
- GPS Receiver
- IMU (Inertial Measurement Unit)
Aircraft’s onboard GPS Receiver determines the aircraft’s position in terms of latitude, longitude and altitude with the help of Sensor.
IMU (Inertial Measurement Unit)
Inertial Measurement Unit referred as IMU is responsible for the orientation of an aircraft or aerospace vehicle with respect to an inertial frame of reference such as the celestial sphere. It helps in determining the attitude of the aircraft when the sensor is taking measurements continually. Horizontal and vertical position of the aircraft helps the system to transmit laser pulses effectively.
How does LiDAR (Light Detection and Ranging) Work
Light Detection and Ranging (LiDAR) device is mounted on aircraft or spacecraft and as it flies over the ground, laser pulses are directed towards the Earth’s surface. The light travels fast to the earth’s surface, reflecting off Photons from obstacles like buildings, mountains, trees etc. The reflected light (Photons) reaches the LiDAR Sensor and Receiver records the time delay between the emission of the light pulse and the detection of the reflected signal also called as “Echo”. The Receiver records repeated measurements of the reflected signal.
The Light which reflects from the earth’s surface is in the form of a wave. Peak detection of this Waveform is a prerequisite for further processing. The areas on the earth which reflects more Photons are referred as “Peak Points” or “Peaks”.
Fig. 4 – (a) Representation of Pulse Measurement Principle (b) Block Diagram of LiDAR System
With the help of GPS Receiver and IMU (Inertial Measurement Unit) , the angular orientation of the sensor with respect to target area is measured and “Range Distance” between the sensor and target area is calculated by the Receiver and records the x, y, z coordinates of each pulse which helps the system to correct the aircraft’s position which helps in generating output accurately.
Millions of recorded data points are generated by the system. System Control and Data Acquisition unit helps in processing of “Point Cloud” of x, y, z data coordinates with the help of computer interface. Data is analysed and processed to obtain Digital Surface Model of the target area.
Few systems use iterative algorithms, scan-matching algorithms for “Point Cloud” matching. Some of the systems use “LiDAR Odometry and Mapping method (LOAM)” which offers accurate mapping in Real-Time.
Pulse Laser Measurement Principle is explained below:
Applications of LiDAR
The applications of Light Detection and Ranging (Li-DAR) include:
- Advanced Surveying like survey of Dams and Reservoirs, topography of land surveying has become easier with this technology.
- This technology is widely used by the Military to generate 3-D Maps.
- It is also used in Archaeology, Geology, Agriculture, Mining etc.
- It is also used in Contour Mapping which is helpful in estimating surface elevations, shows steep valleys and hills.
- It is extensively used in Space exploration programs.
- Atmospheric study has become easy with this technology.
- Virtual Reality Scans has helped Navy engineers to create Virtual Ship environment which is used for training and testing purposes.
- This technology is used for wildlife habitat analysis.
Advantages of LiDAR
The advantages of Light Detection and Ranging (Li-DAR) include:
- It offers wide detection range.
- Data acquisition is accurate and precise.
- Temperature stability of the system is high.
- It can be operated during day and night.
- Data is error free and precise.
- Operates at high speed.
- Data Density is high i.e. it uses 1,67,000 pulses per second.
Disadvantages of LiDAR
The disadvantages of Light Detection and Ranging (Li-DAR) are:
- Operating cost is high.
- Penetration of LiDAR pulses in densely vegetated areas is not adequate.
- Processing of LiDAR data requires specialized software.
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