BY

K.SAI SANTOSH

GIST,CSIT DEPARTMENT,

JNTUH.

Introduction to laser ranging

LIDAR and its principle

Components of LIDAR system

Range measurements

LIDAR error sources

Advantages and applications

In satellite laser ranging (SLR) a global network of observation stations measures the round trip time of flight of ultrashort pulses of light to satellites equipped with retroreflectors.

This provides instantaneous range measurements of millimeter level precision which can be accumulated to provide accurate measurement of orbits and a host of important scientific data.

It is the most accurate technique currently

available to determine the geocentric

position of an Earth satellite, allowing for

the precise calibration of radar altimeters

and separation of long-term

instrumentation drift from secular changes

in ocean topography.

The capability to monitor vertical motion in an absolute system, makes “Laser ranging” unique for modelling and evaluating long-term climate change by:

Providing a reference system for post-glacial rebound, sea level and ice volume change

Determining the temporal mass redistribution of the solid Earth, ocean, and atmosphere system

Monitoring the response of the atmosphere to seasonal variations in solar heating.

Lidar (also written LIDAR or LiDAR) is

a remote sensing technology that measures

distance by illuminating a target with

a laser and analyzing the reflected light.

Lidar uses ultraviolet, visible, or near

infrared light to image objects. It can target a

wide range of materials, including non-

metallic objects, rocks, rain, chemical

compounds, aerosols, clouds and even

single molecules.

In addition to ranging, Lidar systems

can provide:

• Additional information about the target (for classification).

• Information about the transmission path (e.g.Atmospheric

lidar to measure concentration of elements in the

atmosphere)

Talk will focus on lidar system for obtaining

spatial information about a target i.e.

mapping and imaging systems

reflectance or backscattering coefficient (Rayleigh, Mie, Raman,

fluorescence) extinction coefficient (absorption, scattering)

Emulsion of earth features:

There are four major components of LIDAR system. They are

1. Laser2. Scanners and optics3. Inertial

measurement unit(IMU)

4. Global positioning system(GPS)

Laser — 600–1000 nm lasers are most common for non-scientific applications. They are inexpensive, but since they can be focused and easily absorbed by the eye, the maximum power is limited by the need to make them eye-safe. A common alternative, 1550 nm lasers, are eye-safe at much higher power levels since this wavelength is not focused by the eye, but the detector technology is less advanced and so these wavelengths are generally used at longer ranges and lower accuracies. They are also used for military applications as 1550 nm is not visible in night vision goggles, unlike the shorter 1000 nm infrared laser. Airborne topographic mapping lidars generally use 1064 nm diode pumped YAG lasers.

Scanner and optics — How fast images can be developed is also affected by the speed at which they are scanned. There are several options to scan the azimuth and elevation, including dual oscillating plane mirrors, a combination with a polygon mirror, a dual axis scanner . Optic choices affect the angular resolution and range that can be detected. A hole mirror or a beam splitter are options to collect a return signal.

Inertial Measurement Unit

An inertial measurement unit, orIMU, is an electronic device thatmeasures and reports on a craft'svelocity, orientation, andgravitational forces, using acombination of accelerometers andgyroscopes, sometimes alsomagnetometers. The IMU is themain component of inertialnavigation systems used in aircraft,spacecraft, watercraft, and guidedmissiles among others. In thiscapacity, the data collected fromthe IMU's sensors allows acomputer to track a craft's position

Global Positioning System (GPS) is a space-based satellite

navigation system that provides location and time information in all

weather conditions, anywhere on or near the Earth where there is an

unobstructed line of sight to four or more GPS satellites. The system

provides critical capabilities to military, civil and commercial users

around the world. It is maintained by the United States government

and is freely accessible to anyone with a GPS receiver.

Lasers can be used in various ways to measure distances or

displacements without physical contact. In fact they allow for

the most sensitive and precise length measurements, for

extremely fast recordings (sometimes with a bandwidth of

many megahertz), and for the largest measurement ranges,

even though these qualities are usually not combined by a

single technique. Depending on the specific demands, very

different technical approaches can be appropriate. They find a

wide range of applications, for example in architecture,

inspection of fabrication halls, criminal scene investigation

(CSI), and in the military.

RANGE MEASUREMENTS :

Some of the most important techniques used for laser distance measurements

are as follows

Triangulation is a geometric method, useful for distances in the range of

≈ 1 mm to many kilometers.

Time-of-flight measurements (or pulse measurements) are based on

measuring the time of flight of a laser pulse from the measurement device to

some target and back again. Such methods are typically used for large

distances such as hundreds of meters or many kilometers. Using advanced

techniques, it is possible to measure the distance between Earth and the

Moon with an accuracy of a few centimeters. Typical accuracies of simple

devices for short distances are a few millimeters or centimeters.

The distance between point A and B is given by

where c is the speed of light in the atmosphere and t is the

amount of time for the round-trip between A and B.

Interferometers allow for distance measurements with an accuracy which is

far better than the wavelength of the light used.

The phase shift method uses an intensity-modulated laser beam.Compared

with interferometric techniques, its accuracy is lower, but it allows

unambiguous measurements over larger distances and is more suitable for

targets with diffuse reflection.

Note that the phase shift technique is sometimes also called a time-of-flight

technique, as the phase shift is proportional to the time of flight, but the term

is more suitable for methods as described above where the time of flight of a

light pulse is measured.

Lidar Transceiver - Generates laser beam and captures laser energy

scattered/reflected from target

Scanner- Moves laser beam across aircraft track

POS - Measures “sensor” position and orientation

Operator I/F - Permits operator interaction (control/monitor) with system

Data Storage - Captures all AIRBORNE system data required for generation of x,

y, z “target” coordinates

Computer - Integrates/controls interaction of all of the above

SCANNERLIDAR

TRANSCEIVER

POS

(IRS & GPS)

OPERATOR

I/F

DATA

STORAGE

SYSTEM CONTROL AND DATA

ACQUISITION COMPUTER

The various sensor components fitted in

the LiDAR instrument possess different

precision.

The final data accuracy is affected by

several sources in the process of LiDAR

data capture.

A few important sources are as follows...

Error due to sensor position due to error

in GPS, INS and GPS-INS integration.

Error due to angles of laser travel as the

laser instrument is not perfectly aligned

with the aircraft’s roll, pitch and yaw

axis. There may be differential shaking of

laser scanner and INS.

There may be error in the laser range

measured due to time measurement error,

wrong atmospheric correction and

ambiguities in target surface which results

in range walk.

Error is also introduced in LiDAR data due

to complexity in object space, e.g., sloping

surfaces leads to more uncertainty in X, Y

and Z coordinates.

Accuracy

Maximum altitude

Area coverage (swath width)

Spot distribution/scan pattern

Laser pulse repetition frequenc

Spot size (footprint)

Multiple return pulse capability

y

ALTM

Gemini 367

sq km/hr

LIDAR offers advantages over more conventional means of survey that include: 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

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

There are a wide variety of applications oflidar.

1. Agriculture2. Archaeology3. Biology and conservation4. Geology and soil science5. Atmospheric Remote Sensing and Meteorology6. Military7. Mining8. Physics and astronomy9. Robotics10. Spaceflight11. Surveying12. Transport etc……

EXAMPLE :

ALTM 3100 has the distinct feature of recording 1st,

2nd, 3rd, and Last returns + Intensity for each pulse

Very useful for forestry studies

THANK

YOU