Remote sensing technology

1. Remote Sensing{Elective-III}
Kathmandu Engineering College
E-mail: shankar62099@gmal.com
Shankar Gangaju

2. Chapter 3: Remote Sensing Technology
Remote sensing may be classified from many perspectives like
based on:
•Platform
•Source of energy
•Regions of electromagnetic spectrum
•Number of bands
•Imaging media
Classification based on platform:
a.Ground Based Remote Sensing
•This RS technique is used to record detailed information about the surface
that is compared with information collected from aircraft or satellite sensors.
•In some cases this can be used to better characterize the target that is being
imaged by these other sensors making it possible to better understand the
information in the imagery.
•Sensors may be placed on a ladder, scaffolding tall building and crane etc.
Fig: Ground based RS

3. b. Airborne Remote Sensing
•If the remotely sensed data are collected from the platforms within
the air is called aerial or airborne remote sensing.
•Different aerial platforms are balloons, kites, aircrafts etc. are early
platform
•Currently aircrafts are the main aerial platforms.
•The airborne remote sensing may be more susceptible to imaging
geometry problems. They are flexible in their capability to collect
data from different look angles and look directions.
•By acquiring imaging from more than one look direction these
effects may be reduced.
•They are susceptible to variations in velocity and other motions of
the aircraft as well as to the environment (weather ) conditions.

4. • In order to avoid geometrical positing errors due to random variations in
motion of the aircraft the radar system must use sophisticated navigation
and positioning equipment and advanced image processing to
compensate their variations.
Advantages:
• Repetitive look is not needed.
• Surveys can be scheduled for specific purpose, time and locations
• Higher image resolutions than space borne platform
• Low environment loss than space borne platform
C. Space borne Remote Sensing
•Space borne remote sensing is mainly conducted from satellite and also
called satellite remote sensing.
•Satellites are objects which revolve around the another object. E.g.
moon is natural satellite of earth.
•A space borne remote doesn’t have ability to collect data anywhere and at
any time or such degree of flexibility as its viewing geometry and data
acquisition scheduled is controlled by the pattern of its orbit.
•However space borne remote sensing do have the advantages of being able
to collect imaginary over a larger area quicker than an airborne remote
sensing and provide consistent viewing geometry.

5. • The frequency of coverage may not be possible as often as that with
an airborne platform but depending in the orbit parameters, the
viewing geometry flexibility and geographic area of interest a space
borne remote sensing may have revisit period as short as one day.
• Space borne remote sensing are capable of avoiding imaging
geometry problems since they operate at altitude up to 100 times
higher than airborne remote sensing. It also have comparable swath
width.
• Disadvantages
• Space borne remote sensing include the inability of many sensors
to obtain data and information through cloud cover and the
relatively low spatial resolution achievable with many sensing
instruments.
• It also creates large quantities of data sets. Typically requires
extensive processing as well as storage and analysis.

6. Classification based on energy source:
The sun provides a very convenient source of energy for remote
sensing. The sun’s energy is either reflected, as it is for visible
wavelength or absorbed and then re-emitted (for thermal infrared
wavelength)
Remote sensing can be either passive or active. It is conceptually easy
to understand the difference between the two by considering the
following example.
If you look outside the window and see that the sky is blue, you are
making a passive remote sensing observation. On the other hand, if
you walk into a dark room and shine a flashlight onto the wall and
notice that the wall is blue, you are making an active remote sensing
observation.
Camera takes photo as example, no flash and flash

7. A. Passive Remote Sensing
• Remote sensing systems which measure energy that is
naturally available are called passive sensors.
• This can only take place when the sun is illuminating the
earth. There is no reflected energy available from the sun at
night.
• Energy that is naturally emitted can be detected day and night
provided that the amount of energy is large enough to be
recorded.
• Much simpler and less expensive.
Examples:
• Passive microwave radiometers that detects naturally emitted
microwave energy.
• Radiometers that measure reflected ( or backscattered ) sun
light from the atmosphere and ocean.

8. Optical (visible /near infrared) technique for passive remote
sensing
•Passive remote sensing technique
•Uses the IR parts of the electromagnetic spectrum
•Cameras, telescope etc. are used.
•Uses solar radiation as a direct energy source, is a passive remote
sensing method covering visible and near infrared region.
•Relies on ambient light sources (e.g. sunlight)
•Problems with cloud, rain, fog, snow, smoke and smog etc.
•Only from surface, cannot penetrate soil, vegetation, snowpack etc.
Microwave Remote sensing
•Uses the microwave and RF parts of the spectrum
•Radars and Radiometers are used
•Is largely immune to cloud, precipitation, smoke etc. so enables
observation in all weather conditions without any restriction. This is
an advantage that is not possible with the visible/infrared remote
sensing.
•Penetrates sand, soil, rock, vegetation, dry snow, ice etc.

9. • Provides unique information on some aspects that can’t be observed by
visible/infrared sensors. For instance, sea wind and wave direction.
• Does not rely on sunlight- radar provides its own illumination.
• Sensing can be active as well as passive
• Active microwave RS is performed either from satellite or aero planes
whereas passive RS is performed from satellite.
• Passive microwave sensors are generally used for collecting the data
about the atmosphere.
Active Remote Sensing
•Remote sensing systems, which provide their own source of energy for
illumination are known as active sensors.
•These sensors have the advantage of obtaining data any time of day
•The sensor emits radiation which is directed toward the target to be
investigated. The radiation reflected from that target is detected and
measured by the sensor.
•More complex and more expensive.
•Examples:
RADAR (Radio Detection and Ranging)
LIDAR (Light Detection and Ranging)

10. RADAR (Radio Detection and Ranging)
•Radar is an acronym for Radio Detection and Ranging.
•Radar system has three primary function.
•It transmit microwave (radio) signals towards a scene.
•It receives the portion of the transmitted energy backscattered from
the scene.
•It observes the strength (detection) and the time delay (ranging) of
the return signals.
•Radar provides its own energy source and therefore can operate both
day or night and through cloud cover.
•This type of system is known as an active remote sensing system.

11. • The radar antenna illuminates the target with a microwave signal
which is then reflected and picked up by a receiving device.
• The electrical signal picked up by the receiving antenna is called
echo or return.
• The radar signal is generated by a powerful transmitter and received
by a higher sensitive receiver.
• All targets produce a diffuse reflection. i.e. it is reflected in a wide
number of directions.
• The reflected signal is also called scattering/Backscatter is the term
given to reflections in the opposite direction to the incident rays.
• Radar signals can be displayed on the traditional plan position
indicator (PPI) or other more advanced radar display systems.
• A PPI has a rotating vector with the radar at the origin, which
indicates the pointing direction of the antenna and hence the bearing
of targets.
• Radar is an object detection system that uses electromagnetic wave
to identify the range, altitude, direction or speed of both moving and
fixed objects such as aircraft, ships, motor vehicles, weather
formations and obstacles (mountain, tree etc)

12. Radar applications/uses include:
•Air traffic control
•Weather forecasting
•Measuring ocean surface waves
•Police detection of speeding traffic
•Satellite
What information RADAR can give?
•Target range (distance)
•Target height( altitude)
•Target speed
•Target identity
•Target features (imaging)
•Target size
Power
Supply
Synchronizer
Transmitter
Display
Duplexer
(Switching Unit)
Receiver
Antenna
Antenna Bearing or Elevation
Video
Echo
RF

13. Basic Radar Equation
{
1/4

14. • If Rmax is to be double, Pt must be increases to 16 fold which is not
an ideal solution.
• For a fixed lamda, Rmax varies as the square root of antenna gain so it
is require to increase the radar range by increase in the antenna
gain than to increase the transmitter power.
• So the dominant parameter to increase range of radar is anteena
gain among transmitted power, area of reflecting object, effective
observing area of radar antenna, echoing area and received power

15. Factors That Affect Radar
Performance
• Signal Reception
• Receiver Bandwidth
• Pulse Shape
• Power Relation
• Beam Width
• Pulse Repetition Frequency
• Antenna Gain
• Radar Cross Section of
Target
• Signal-to-noise ratio
• Receiver Sensitivity
• Pulse Compression
• Scan Rate
– Mechanical
– Electronic
• Carrier Frequency
• Antenna aperture

16. Pulse Shape Sharp a rise as possible Better range accuracy Require infinite bandwidth, more complex
Tall as possible More power /longer range Requires larger equipment/more power
Pulse Width Short as possible Closer minimum range Reduces maximum range
More accurate range
Pulse Repetition Freq. Short Better range accuracy Reduces maximum range
Better angular resolution
Better detection probability
Pulse Compression Uses technique Greater range More complex circuitry
Shorter minimum range
Power More Greater maximum range Requires larger equipment & power
Beam Width Narrow Greater angular accuracy Slow antenna rate, Detection time
Carrier Frequency High Greater target resolution Reduces maximum range
Detects smaller targets
Smaller equipment
Receiver Sensitivity High Maximizes detection range More complex equipment
Receiver Bandwidth Narrow Better signal-to-noise ratio Distorts pulse shape
Factor Desired Why Trade-off Required

17. http://weather.noaa.gov/radar/radinfo/radinfo.html

18. Radar Band
L band radars :
15-30 cm, 1-2 GHz
Clear air turbulence studies.
S band radars :
8-15 cm, 2-4 GHz
Not easily attenuated (far range)
Large antenna dish
C band radars :
4-8 cm, 4-8 GHz
Easily attenuated (short range)
Small antenna dishX band radars :
2.5-4 cm, 8-12 GHz
Easier attenuated (shorter range)
detect smaller particles

20. LIDAR (Light Detection And Ranging)
• Lidar is an acronynm for Light Detection and Ranging
• An active imaging technology very similar to RADAR
• LASER (Light Amplification by Stimulated Emission of
Radiation) is a key component of the LIDAR.
• Pulses of LASER light are emitted from the sensor and
energy reflected from a target is detected.
• The time required for the energy to reach the target and return
to the sensor determines the distance between the two.
• Lidar is used effectively of measuring heights features, such
as forest canopy height relative to the ground surface, and
water depth relative to the water surface.
• Lidar is also used in atmospheric studies to examine the
particle content of various layers of the earth’s atmosphere
and acquire air density reading and monitors air currents

21. LIDAR Equations

22. LIDAR compared to RADAR
• Lidar uses laser radiation and a telescope/scanner similar to the
way radar users radio frequency emissions and a dish antenna.
• Optically thick cloud and precipitation can attenuate the lidar
beam, but radar signals can penetrate heavy clouds ( and
precipitation)
• In optically clear air, radar return signals may be obtained from
insects and birds and from air refractive index variations due to
humidity, temperature or pressure fluctuations.
• Lidar beam divergence is two to three orders of magnitude
smaller compared to conventional 5 and 10 cm wavelength
radars.
• The combination of the short pulse (of the order of 10-8
Sec)
and the small beam divergence (about 10-3
to 10-4
radiant) gives
a small volume illuminated by a lidar (about a few m3 at ranges
of tens of Km)

23. Satellite and Sensors
• For a sensor to collect and record energy reflected or emitted
from a target or surface, it must reside on a stable platform
removed from the target or surface being observed.
• Platforms for remote sensors may be situated on the ground, on
an aircraft or balloon (or some other platform within the
Earth’s atmosphere) or on a spacecraft or satellite outside of the
earth’s atmosphere.
• Ground-based sensors are often used to record detailed
information about the surface which is compared with
information collected from aircraft or satellite sensors.
• In some cases, this can be used to better characterize the target
which is being imaged by these other sensors making it
possible to better understand the information in the imagery.
• In space, remote sensing is sometimes conducted from the
space shuttle or more commonly from satellites.
• Satellites are objects which revolve around another objects.

24. Cond..
• For example, the moon is a natural satellite whereas man-made
satellite include those platforms launched for remote sensing,
communication and telemetry (location and navigation )
purpose.
• Because of their orbits, satellites permit repetitive coverage of
the earth’s surface on a continuing basis.
• Cost is often a significant factor in choosing among the various
platform options.

25. Cond….
• Although ground-based and aircraft platforms may be used for
remote sensing.
• Satellite provide a great deal of the remote sensing imagery
commonly used today.
• Satellites have several unique characteristics which make them
particularly useful for remote sensing of the earth’s surface.
• The path followed by a satellite is referred to as its orbit.
• Orbit selection can vary in terms of altitude (their height above
the Earth’s surface) and their orientation and rotation relative to
the earth.
• Satellites at very high altitudes which view the same portion of
the earth’s surfaces at all times have geostationary orbits.
• These geostationary satellites at altitudes of approximately
36000 km, revolve at speeds which match the rotation of the
earth so they seem stationary relative to the earth’s surface.

26. Cond…
• This allows the satellites to observe and collect information
continuously over specific areas.
• Weather and communications satellites commonly have these
types of orbits.
• Due to their high altitude, some geostationary weather satellites
can monitor weather and cloud patterns covering an entire
hemisphere of the earth.
• Many remote sensing platforms are designed to follow an orbit
(basically north-south) which in conjunction with the earth’s
rotation (west-east) allows them to cover most of the earth’s
surface over a certain period of time.
• These are near polar orbits, so named for the inclination of the
orbit relative to a line running between the north and south
poles.

27. Cond…
• Most of the remote sensing satellite platforms today are in
near-polar orbits which means that the satellite travels
northwards on one side of the earth and then toward the
southern pole on the second half of its orbit. These are called
ascending and descending passes respectively.
• Sensors recording reflected solar energy only image the surface
on a descending pass, when solar illumination is available.
• Active sensors which provide their own illumination or passive
sensors that record emitted (e.g. thermal) radiation can also
image the surface on ascending passes.
• As a satellite revolves around the earth, the sensor “sees” a
certain portion of the earth’s surface. The area imaged on the
surface is referred to as the Swath.
• Imaging swaths for space borne sensors generally vary between
tens and hundreds of kilometers wide.

28. • The satellite’s orbit and the rotation of the earth work
together to allow complete coverage of the earth’s
surface, after it has completed one complete cycle of
orbits.

29. Ground Truth( Ground Data/In Situ Data)
• Ground truth (ground data) is defined as the observation, measurement
and collection of information about the actual conditions on the ground
in order to determine the relationship between remote sensing data and
the object to be observed.
• Ground truth refers to gathering reference data on-site and deriving
information therein that properly characterize states, conditions and
parameters associated with the surface.
• Ground data are used for verification of remote sensing data so it should
be collected at the same time as data acquisition by the remote sensor.
• Ground data are used for:
• Sensor design: special characteristics are measured by a
spectrometer to determine the wavelength range and bandwidth.
• Calibration and validation: calibration of sensor as well as captured
data and validate the data for analysis.
• Analysis and Data correction: Ground investigation at a test area to
collect training sample data and correction if required.

30. Satellite Orbits
3030
Geostationary (GEO) Low Earth Orbit (LEO)
Mean Orbital Radius: Mean Orbital Radius:
35,800 km 160-2,000 km
(22,200 mi) (100-1,240 mi)
Most Earth-orbiting satellites are in one of two
stable configurations, depending on the application:
Geostationary (GEO) or Low Earth Orbit (LEO).

31. Common types of orbits
GEO
Geostationary orbit
An orbit that has the
same Earth’s rotational
period
Appears ‘fixed’ above
earth Satellite on
equator at
~36,000 km

32. Low Earth Orbit (LEO):
Orbiting at an altitude of 160-
2,000 km.
3232
Path of Satellite
Low-Earth Orbits (LEO)

33. Ascending Orbit:
The satellite is moving
South to North when
that portion of the orbit
track crosses the
equator.
Low Earth Orbit:
Orbiting at an altitude
of 160-2,000 km.
3333
Path of Satellite
Low-Earth Orbits (LEO)

34. Descending Orbit:
The satellite is moving
North to South when
that portion of the orbit
track crosses the
equator.
3434
Low-Earth Orbits (LEO)

35. Earth-Observing Satellites
Sun-Synchronous:
The satellite is always in
the same relative position
between the Earth and Sun.
Equator-Crossing Time:
The local apparent solar time
when the satellite crosses the
equator.
Example: Terra has an
equatorial crossing time
of 10:30 am, and is called
an “AM” or morning
satellite.
3535

36. Satellite Inclination
High Inclination or
Polar Orbit (near
90º)
virtually complete
global coverage
3636
Low Inclination
Orbit (often near
57º-- Space Shuttle,
TRMM)
no polar coverage
Inclination:
The position of the orbital plane
relative to the equator. For near-polar
orbits, typically about 97º.
Equator
Orbital Inclination
N
S
97°
57°

37. 3737
The orbit is
defined as having
a cross-track and
an along-track
direction.
Direction of Satellite Motion
Along-Track Direction
Cross-Track Direction
LEO Field-of-View (FOV)

38. 3838
Direction of Satellite Motion
Satellites in Low Earth
Orbit have only an
instantaneous Field-
of-View (IFOV)
LEO Field-of-View (FOV)

39. 3939
Push-Broom Sensors
“Push Broom” sensors
provide a line array of
several sensors (e.g.,
CCD optical arrays)
which view a small
strip of the Earth’s
surface perpendicular
to the motion of the
satellite.

40. 4040
By stitching together
a continuous series of
Push Broom images, a
contiguous swath or
ribbon of data
encircling the Earth
can be achieved.
Push-Broom Sensors