LIDAR: A Primer for Primates

 Why am I wasting your time?

Why has LIDAR (Light Detection and Ranging) become so important lately? Why should LIDAR bother you? Now why are you serious?

“All the answers lie out there in the sunshine.

You just need a magnifying glass to find out what’s a LIDAR.”

In this age of driver less cars, LIDAR is the angle that guides these to the destination and saves it from falling down over the horizon. LIDAR has become the eye while RADAR (Radio Detection and Ranging) has become the ears of today’s driver less cars.

So, let’s have a few bytes on LIDAR.


 A very brief history of time

Various Echo Location methods has been used since the evolution of life on earth. Have you wondered how a bat flies in dark night (of course without headlights) without crashing into another. This is what is called Echo Location. And since bats are serial generational illiterates, so humans just plagiarized their technology and created patents. Now the hard part is that bats have to pay royalty to humans every time they took a stroll at night.

The Echo Location is based upon this simple technique of bouncing of sound or any electromagnetic waves (like radio waves, light waves) against an object.

The RADAR, SONAR and LIDAR are based upon calculating the distance by measuring the time between transmitting and receiving the wave.

 Disadvantages of RADAR:

Master Yoda says “RADAR disadvantages following are”.

• RADARs are expensive to mobilize while LIDAR is fairly smaller in size. RADAR is not suitable for small area survey as Airborne RADAR are expensive to operate.

• RADAR cannot measure the ground elevation below the canopy cover. LIDAR is also capable of "seeing" between trees in forested areas.

• RADAR has lesser accuracy than the LIDAR.

• RADAR emits a broad range radiations while LIDAR emits a focused laser pulses.

• Differential absorption LIDAR can also determine some of the chemical properties of the target which is not possible in case of RADAR.

 LIDAR Species:

 LIDAR Family Tree

*IMU – Inertial Measurement Unit
*GPS – Global Positioning System

 What is inside LIDAR?


LIDAR, which stands for Light Detection and Ranging, is a remote sensing method that uses light in the form of a pulsed laser to measure ranges (variable distances) to the Earth.


The fundamental steps of LIDAR functionality are explained below.

Laser

• Frequency:

50,000 ~2,00,000 pulses per second (Hz)

• Wavelength:

Near Infrared (1040 ~ 1060 nm) – Terrestrial mapping

Blue Green (500 ~ 600 nm) – Bathymetry

Ultraviolet (250 nm) – Meteorology

Infrared (1500 ~ 2000 nm) – Meteorology

• Wattage:

Low wattage (<1 W)

Scanner

• Mirror spins or scans to project laser pulses to the surface.
• Scanning angles up to 75^o. Scanner measures the angle at which the pulse was fired.
• Receives reflected back pulse.

High Precision Clock

• Records the time the laser pulse leaves and returns to the scanner.

GPS (Global Positioning Systems)

• Records the x, y, z location of the scanner.
• Survey ground base station in flight area.

IMU (Inertial Measurement Unit):

• Measures angular orientation of the scanner relative to the ground (Pitch, Roll, Yaw).

Principles of Distance Calculation

Pulse Range Distance:

• The distance is calculated by comparing the time between pulse sent and the pulse received.
• Pulse Range Distance (R).

Where
C: Speed of light (299,792,458 m/s)
t: Time interval between sending and receiving pulses (ns)

Point Cloud:

• The x, y, z coordinates of each return are calculated using the location and orientation of the scanner (from the GPS, IMU), the angle of the scan mirror and the range distance of the object/surface.
• The collection of returns is called a “Point Cloud”.

Resolution:

• The number of pulses per unit area.
• Current systems capable of 20 pulses/square meter.
• Resolution determined by aircraft speed, Flying altitude, Field of View (FOV), Rate of pulse emission.
• Higher resolution and a narrow FOV is needed to penetrate dense vegetation.
• higher resolutions allow the surface and features on the surface to be better resolved, but at cost of larger data sets and slower processing times.

Accuracy:

• Vertical accuracy typically 15 to 20 cm.
• Horizontal accuracy 30 to 100 cm.
• Accuracy improved by flying low and slow, with a narrow FOV.

Spot size:

• Once the pulse leaves the laser source, based upon the distance between the targets and the source, the width of the laser increases.
• At the target, it might happen that multiple small objects will be engulfed within same pulse. This gives rise to multiple returns.

Intensity:

• Strength of returns varies with the composition of the surface object reflecting the return
• Reflective percentages are referred to as LiDAR intensity
• Can be used to identify land cover types
• Intensity values need to be normalized among flights

Reflectivity:

• A white surface will have better reflectivity than a black surface. 
• A smooth surface type will have better reflectivity than a rough surface.

Data Processing:

• Data collected based upon system vendor
• Data has to be post-processed to calibrate multiple flight lines, filter erroneous values and noise
• Returns are classified and separated by category: first returns, last (or bare-earth) returns, etc.
• LIDAR data formats can be ASCII point files or LAS (Log ASCII Standard) format. LAS format is maintained by ASPRS
• The contents of LAS format are explained below (comparing version 1.0 and 1.1)

Wavelength:

• 600-1000nm lasers:

        Used for non-scientific purposes but, as they can be focused and easily absorbed by the eye, the maximum power has to be limited to make them 'eye-safe'. 

• 1550nm lasers: 

        Are a common alternative as they are not focused by the eye and are 'eye-safe' at much higher power levels. These can be used for longer range and lower accuracy purposes. 

        This wavelength does not show under night-vision goggles and are therefore well suited to military applications.

• 905nm lasers:

        Class 1 laser product and is are 'eye-safe' with 1.3 Watts.

Interference between sensors:

• The beams from multiple LIDARS in the same orientation would cause interference. 
• This can be avoided by placing sensors in different orientations.
• Multiple sensors can have synchronization between them before sending out the pulses to avoid clashing.

Returns per pulse:

• Laser pulses emitted by LIDAR is reflected back due to objects like vegetation, buildings, roads, bridges etc.
• Each pulse may return to the LIDAR sensor as one or many returns.
• The first return is the most significant return. May be due to tallest feature in landscape like canopy or building top.
• The last return is generally due to bare earth.

Advantages of LIDAR:

• Day or night operation
• Efficient acquisition of millions of elevation points per hour
• Faster coordinate acquisition than traditional methods
• All digital: no intermediate steps to generate digital XYZ
• Rapid turnaround: Capable of “overnight” processing. Ability to cover large areas quickly.
• Captures multiple returns per pulse with intensity information
• Dense data
• Accurate: Elevation +/- 10 cm (or better)
• Airborne: Easy to mobilize and demobilize
• Non-Intrusive method of survey (airborne) capable of accessing remote areas. Can collect data in steep terrain and shadows.
• Quicker turnaround, less labour intensive, and lower costs than photogrammetric methods
• Can produce DEM (Digital Elevation Models) and DSM (Digital Surface Models).

Disadvantages of LIDAR:

• LIDARs cannot penetrate dense canopy.
• LIDARs has very large dataset which require high cost to store, process and interpret.
• LIDAR cannot penetrate cloud cover.
• Loose performance at bad weather condition (like rain, fog, snow etc.) 

Level of Autonomous - 5 Levels and Role of LIDARs

Sensors in Automotive Vehicles:

Applications of LIDAR in Automotive:

Adaptive Cruise Control (ACC):

• The LIDARs are used in Adaptive Cruise Control (ACC) for automobiles.
• The LIDARs are mounted on the cars to monitor the distance between vehicle and any other device.
• In the event the vehicle in front slows down or is too close, the ACC applies the brakes to slow the vehicle. When the road ahead is clear, the ACC allows the vehicle to accelerate to a speed pre-set by the driver.

Collision Avoidance System:

• A collision avoidance system is an automobile safety system designed to reduce the severity of a collision
• It uses radar (all-weather) and sometimes laser (LIDAR) and camera (employing image recognition) to detect an imminent crash.
• Once the detection is done, these systems either provide a warning to the driver when there is an imminent collision or take action autonomously without any driver input (by braking or steering or both).
• Collision avoidance by braking is appropriate at low vehicle speeds (e.g. below 50 km/h), while collision avoidance by steering is appropriate at higher vehicle speeds.
• Volvo introduced the first cyclist detection system. All Volvo automobiles now come standard with a LIDAR laser sensor that monitors the front of the roadway, and if a potential collision is detected, the safety belts will retract to reduce excess slack. 

Pedestrian Recognition:

• The LIDAR data can be used to determine whether the pedestrians are on road or out of road.
• The pedestrian recognition and tracking system is integrated with the autonomous vehicle platform which provides timely prediction of pedestrian motions.
• The data from a camera sensor can be fused with LIDAR data, to enhance detection performance and avoid false alarms.

Automotive Speed Detection and Law Enforcement:

• LIDAR is being used instead of RADAR since 2000 for Speed limit enforcement. 
• Current devices are designed to automate the entire process of speed detection, vehicle identification, driver identification and evidentiary documentation
• Lidar has a narrow beam, and easily targets an individual vehicle, thereby eliminating the need for visual estimation, and records an image of the license plate at the same instant as recording the speed violation.

LIDAR Manufacturing: Automotive TIER1 and Sensor Manufacturers

And the Award goes to!!!!!

1. http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snaa123
2. http://velodynelidar.com/docs/datasheet/63-9194%20Rev-E_HDL-64E_S3_Spec%20Sheet_Web.pdf
3. https://www.extremetech.com/extreme/176715-new-optical-chip-will-sharpen-aerial-mapping-and-autonomous-car-vision
4. http://www.princetonlightwave.com/products/geigercruizer/
5. http://spectrum.ieee.org/tech-talk/semiconductors/optoelectronics/mit-lidar-on-a-chip
6. http://www.laserfocusworld.com/articles/2016/09/ford-and-baidu-lead-150m-investment-in-velodyne-lidar-for-self-driving-cars.html
7. http://www.laserfocusworld.com/articles/print/volume-51/issue-05/features/photonics-applied-transportation-lidar-advances-drive-success-of-autonomous-vehicles.html
8. http://denso-europe.com/denso-invests-in-semiconductor-laser-startup-trilumina/
9. http://www.10tv.com/interactive-radar-columbus-ohio
10. http://desktop.arcgis.com/en/arcmap/10.3/manage-data/las-dataset/types-of-lidar.htm
11. https://www.e-education.psu.edu/geog481/l1_p4.html
12. http://www.robotics.org/content-detail.cfm/Industrial-Robotics-Industry-Insights/Intelligent-Robots-A-Feast-for-the-Senses/content_id/5530
13. http://www.liblas.org/development/format_elements.html
14. https://www.nae.edu/Publications/Bridge/133842/134343.aspx
15. http://www.valeo.us/valeo-in-north-america/company-regional-profile/business-groups.html
16. http://docs.leddartech.com/doc/leddartech-publications/the-automotive-lidar-magazine/2016082601/#2
17. https://projectmaxcpm.wordpress.com/page/2/
18. http://www.yole.fr/AutonomousVehicles_Functions.aspx#.WIOlBFN97IU

Ok Bye!!!