In this section we’ll see the pros and cones for the two categories of drones (Rotary Wing Vs Fixed Wing UAVs) interns of being platformed for a LiDAR system.
It’s obvious that each of them possess set of advantages and limitations interns of their contents and the service that they can offer for this system.
Here the purpose of the data planned to be captured (for what purpose we’ll use the system matters) and size of the survey should be our starting grounds to start to prefer one over the other.
Advantages of Fixed Wing UAV: It’s known that a data acquisition on a bigger area can demand a UAS with longer operation time and support larger payloads better fuel economy. In this sense as we know most fixed winged UASs have generally special capabilities to longer flying time, stability, and speed, which makes them suitable for large areas or long-distance missions while we know that rotary-winged generally struggling with inertia in a more complex way.
Fixed Wing UAV Limitations: However fixed wing UAVS also have some limitations in that they need more space at turns which results in increasing the path length employing decreasing in efficiency. Their speed can also claiming for more faster, more advanced, high performance and expensive sensors to fulfill the needed point density.
Rotary Wing UASs Leverage : These UAVs generally enables us to use low-cost sensors, which makes UAS-based laser scanning accessible to a wider user community. Specially they are good for small but object rich areas and more complex terrain survey purpose. Rotary winged UASs exceeds the fixed winged because they have comparatively excellent maneuverability and enabling the operator for densified point scanning, they don’t need larger turning area, or to takeoff or landing for they are known for their (VTOL) capability. They allows for a low or even stationary speed controlling so that it can be possible to scan most important areas with a more densified point data collection.
In summary: Because they have an excellent speed, economical fuel conception, long flying time and higher system stability during flying fixed winged UASs preferred over rotary wings for a larger data acquisition in at a larger survey area with less topographic and detail complexities where as these restrictions make a fixed-wing platform less favorable for small-area surveys or with complex terrain/objects and rich in features areas of surveys.
Industrial sensors for a UAS leaser scanning
Some industrial scanners manufactured equipped with affordable sensors suitable for UAS laser scanning. These sensors are designed for industrial applications and robotics.
The main advantages of these sensors are their low price, small size and durability,all of which are advantages for UAS-based laser scanning. But performance of these types of sensors is relatively limited in some ways like Their:
- speed of data collection,
- ranging accuracy,
- limit for maximum range and often lack of implemented or available signal processing techniques means that they are not always suitable for commercial heavy-user operations,
but they suits for many other (research) applications.
Examples: The most known sensors in this group is UAS-compatible systems that have recently emerged multi-layer laser sensors originating from the automotive industry such as Velodyne LiDAR Puck LITE and Quanergy M8-1 LiDAR.
These systems offer high data rates and some desirable features at a reasonable price. And the most important features of these sensors are Their:
- ability for Multi-layer data capture
- 360-degree field of view
Both of which plays big role to improve the along-track sampling and enables more comprehensive data collection of complex scenes such as the urban environment.
Other sensors Suit UAV leaser caning
The sensors at riegl VQ-480-U or VUX-1 variants are examples of professional sensors which are specialized for laser scanning. Even though their specialty is due to their sophistication in their
design which results in a slightly higher price in the market. And this makes it not to be explored by mapping industries in as a larger scale as they are doing with the other UAV systems. Even though their capability to operate at higher altitudes usually(500 to 1,500 meters) is an advantage, the stricted regulations on UASs make the corresponding manned aircraft conveniently preferred over them.
Low Altitude Leaser Sensors which Can Suit UAS
Because of the strict regulations on UASs it is preferable using as low altitude as possible. Examples of UASs which are very convenient in this aspect are the above mentioned Hokuyo and Velodyne LiDAR Puck LITE. These sensors can be operated lower than 100m altitude.Their 20 minutes overall flight time makes flight path length as lower as 1.5-2km. To capture an 80m wide strip they should fly with 60 degree field of view and as high as 70m Altitude.
UASs LiDAR Application in Urban Area
The remote Sensing application in urban area is generally very diverse. The following are the likely usual mapping types in urban areas:
- For planning or building purpose
- Routes like Scale detection of power-line, or maintaining of roads.
- For analyzing some social, economical, political issues.
Most of the time all applications generally needs for higher accuracy because a little error in this application results in higher risk in damaging property or even people. The figure at this side showing an example of this type of data. An autonomous UAS system used to collect this data is:
- A rotary-wing UAS equipped with a Velodyne LiDAR Puck
- Sensor package used here also included a NovAtel IGM-S1 GNSS-IMU device for observing and recording the sensor flight path and orientation.
- A display connected to the system via WLAN
Accuracy Depends On: Here one must not forget that the resulting point cloud accuracy depends not only the sensors performance but also directly depends on the flight-path solution accuracy. Therefore during post processing GNSS based station data and data from virtual network was considered for accurate (corrected) flight path computation. With this procedure one can expect 5-10cm accuracy. The point density can be thousands of point data per square-meter compared to the traditional ALS data-set (i.e usually 1-20 points per sqm). To take advantage of simultaneous capturing of the terrain, street infrastructure, building walls and roof structures using a wider FOV is preferable.
The Upcoming LiDAR Technologies
Flash LiDAR technology and development of Solid state photon counting sensors are the upcoming trends if they fully developed and be operational, they will be expected to push the field of LiDAR to a completely new era.
Flash LiDAR is really focal plane LiDAR, with an array of sensors to capture returns with the CCD sensors of today’s digital cameras.
Advantage and Limitations: Due to framed sensing, flash LiDAR allows for faster, simultaneous areal coverage and overlapping, this will really be even more advantageous in object detection
and recognition applications which follows generally multi-look principle. Their limitations will simply be their expensive price and because of their military background they are not be explored much in the civilian users.
Solid state photons counting sensors: these can be larger sensors suitable for manned aircraft or larger UASs, or can be extremely small in size based on single photon avalanche diodes
(SPAD). According to their current research lab results SPAD can perform same as the currently low-cost leaser scanning. Which indicates that there’ll come time with a great opportunity of having LiDAR sensors with highest performance with a reasonable cost.
NB: we have adopted this note from an article “The Current State of the Art in UAS-based Laser Scanning”, posted 22/06/2016, Gim International online Magazine, just with a little modifications to not to copy and pest directly. (Dr Antero Kukko, Dr Anttoni Jaakkola, Prof Juha Hyyppä) are referenced on the magazine as original authors of this article.