ALTM-Air Bourne Laser Terrain Mapping

1. BY
K.SAI SANTOSH
GIST,CSIT DEPARTMENT,
JNTUH.

2.  Introduction to laser ranging
 LIDAR and its principle
Components of LIDAR system
Range measurements
LIDAR error sources
Advantages and applications

3.  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.

4.  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.

5. 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.

7.  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.

8.  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

9. reflectance or backscattering coefficient (Rayleigh, Mie, Raman,
fluorescence) extinction coefficient (absorption, scattering)

10. Emulsion of earth features:

11. There are four major
components of LIDAR
system. They are
1. Laser
2. Scanners and optics
3. Inertial
measurement
unit(IMU)
4. Global positioning
system(GPS)

12.  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.

13.  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.

14.  Inertial Measurement Unit
An inertial measurement unit, or
IMU, is an electronic device that
measures and reports on a craft's
velocity, orientation, and
gravitational forces, using a
combination of accelerometers and
gyroscopes, sometimes also
magnetometers. The IMU is the
main component of inertial
navigation systems used in aircraft,
spacecraft, watercraft, and guided
missiles among others. In this
capacity, the data collected from
the IMU's sensors allows a
computer to track a craft's position

15.  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.

16.  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.

17. 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.

18.  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.

19. SCANNER
LIDAR
TRANSCEIVER
POS
(IRS & GPS)
SYSTEM CONTROL AND DATA
ACQUISITION COMPUTER
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
OPERATOR
I/F
DATA
STORAGE

20. 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...

21.  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.

22.  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.

23.  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

24.  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

25. There are a wide variety of applications of
lidar.
1. Agriculture
2. Archaeology
3. Biology and conservation
4. Geology and soil science
5. Atmospheric Remote Sensing and Meteorology
6. Military
7. Mining
8. Physics and astronomy
9. Robotics
10. Spaceflight
11. Surveying
12. Transport etc……

26.  EXAMPLE :
 ALTM 3100 has the distinct feature of recording 1st,
2nd, 3rd, and Last returns + Intensity for each pulse
Very useful for forestry studies

27. THANK
YOU