The LiDAR is a device used in many applications including:
- measuring distances
- calculating the speed of a moving object
- environmental sensing
- estimating the location of an object in space
- and others
The domain we are interested in here is the remote sensing and more specifically when this technique is used embedded on drones.
In the remote sensing field, one of the most frequently asked questions is: how does a LiDAR work?
And more precisely: how does a drone LiDAR work?
The name LiDAR comes from Light Detection And Ranging. In other words, this defines a system that makes remote distance measurements and sensing thanks to lights emissions, which enables to qualify it as an active sensor, unlike cameras for example.
Principle of drone LiDAR operation
The drone LiDAR system is mainly composed of the following parts:
- Laser sensor
- Inertial management unit (IMU)
- GNSS receiver
- Embedded computer
The laser sensor of a LiDAR
The LiDAR’s laser sensor is composed by a light transmitter and a receiver.
The LiDAR emits pulses of light at high frequency. We speak here about several hundreds of thousands shots per second, and even more in some set-ups.
On a drone embedded LiDAR, each of these pulses is sent by the laser transmitter to the ground. When one of these pulses encounters an object, its returned echo is captured by the LiDAR light receiver and transformed into a digital signal.
The time that this light takes to travel between the transmitter and the obstacle on which it was reflected is used to measure the distance between the sensor and the reached object.
A LiDAR is usually able to sense more than one echo produced by the same pulse.
The first echo will be reflected by the object located the closest to the sensor, often also the highest, such as the roof of a house or the top of a tree for instance.
The following echoes will be returned by objects that are usually located lower, such as the leaves and lower branches of a tree, or the ground.
The number of echoes that a LiDAR is able to sense is generally two or three, or more in some cases.
Generally, the more echoes a LiDAR has, the heavier it becomes and therefore it is more difficult or even impossible to embed it on a drone. It is then mandatory to use a manned aircraft to bring this kind of systems in the air, which induces all the disadvantages of such a solution while losing at the same time all of the drone benefits.
The inertial management unit (IMU)
Calculating the distance between the LiDAR and the reached object is obviously not sufficient in order to position the impact point in space.
As the drone and the LiDAR are always moving, the position of the sensor also moves constantly.
One of the essential information necessary for the calculation of the position of each of the reflected points is the precise location of the LiDAR at the moment of the shot.
This is done thanks to the information provided by the inertial management unit (IMU) which contains the orientation data of the sensor at the precise moment of the pulse and its echo.
The data transmitted by the inertial management unit (IMU) thus makes possible to calculate the relative position of the sensor at a precise moment in time for the pitch, roll and yaw axis.
This information includes the angular velocities for each of these three axis, the linear speed of the sensor and the estimation of its current position related to the one calculated just before, within the time line.
The GNSS receiver
The Global Navigation Satellite System (GNSS) receiver is an essential component of the UAV-embedded LiDAR. It is used to calculate the geographical position of the system and the precise time at the moment of the transmission of each laser pulse, as well as for the reception of its echo.
In addition to the precise positioning, the GNSS receiver also acts as a metronome to synchronize the LiDAR sensor with the inertial management unit (IMU) .
The accuracy of the GNSS receiver directly influences the airborne LiDAR measurements.
GNSS receivers provide very precise positioning today by using several satellite constellations, such as GPS, Glonass, Galileo, Beidou, QZSS etc.
Depending on the region in which the receiver is used, it can also benefit from precision augmentation systems such as DGPS (Differential Global Positioning System) and EGNOS (European Geostationary Navigation Overlay Service).
The positioning information generated by this receiver will mostly be combined with correction data transmitted by ground reference stations in order to achieve RTK (Real Time Kinematics) or PPK (Post Process Kinematics) positioning. This will further increase the quality of the data and therefore improve the accuracy of the airborne LiDAR operation.
The embedded computer
Fundamental component of the system, the embedded computer is the masterpiece of the drone LiDAR as it contains all of the system’s intelligence.
Among its functions, the LiDAR on-board computer controls and coordinates the operations of the system’s peripherals such as the laser sensor, the inertial management unit and the GNSS receiver.
It collects the data provided by these subsets in order to perform the positioning calculations.
Its role is extremely important and without it, telemetry operations wouldn’t be possible.
Files produced by the drone embedded LiDAR
The drone embedded LiDAR generates LAS type files, which is a well known public standard format. These files are used to generate dense point clouds.
These dense point clouds are used to produce digital terrain models (DTM), three-dimensional renderings, and so on.
The benefits of the LiDAR solution on drones
Drone LiDAR remote sensing has several advantages:
Light conditions independence:
LiDAR is an active sensor that doesn’t depends on ambient light conditions. This means that it is able to operate day and night unlike other passive sensors such as cameras.
Whether a place is well lit or in the shade, or when the light condition vary during the survey, the LiDAR operation will not be affected.
Measurement precision:
The measurements made by a drone LiDAR are among the most accurate in the field of remote sensing.
Scanning through the vegetation:
Depending on the density of the vegetation, the drone LiDAR is able to cross it and thus sense what is below (soil, buildings, etc.)
Data acquisition speed:
Whether in the implementation or the information process, the drone LiDAR allows to have accurate data very rapidly.
Information quality:
The low altitude flight performed thanks to the drone makes possible to precisely scan the targeted location and to increase the density of the collected points. This is achieved without being intrusive nor disturbing the environment around the surveyed area.
The proximity, the ease of the operation and the simplicity of the navigation increase the quality of the collected information which precisely focus the target of the survey.