1. REMOTE SENSING – SENSORS, PLATFORMS
AND SATELLITE ORBITS
Dr. Ajay Singh Lodhi
Assistant Professor
College of Agriculture, Balaghat
Jawahar Lal Krishi Vishwa Vidyalaya, Jabalpur (M.P.)

2. Remote Sensing Sensors
 Sensor is an electronic circuit which can record the
electromagnetic radiation incident upon it.
 A sensor is a device comprising of optical component or system
and a detector with electronic circuit. It senses a variation in
input energy to produce a variation in another or same form of
energy.
 All sensors employed on earth observation platforms use
electromagnetic radiation to observe terrain features. Various
types of sensor are employed based on requirements and purpose.
 Remote sensing sensors measure radiance of objects under study
in a given wavelength.
 Sensors comprise of several components such as-
 System to receive radiation from the pixel and a telescope (objective),
 Calibration source and spectrometer,
 Amplifier and recording system,

3. Classification of Remote Sensing Sensors
 Remote sensors can be broadly classified as passive sensors and
active sensors.
 Sensors which sense natural radiation, either emitted or reflected
from the earth are called passive sensors. This process is known
as passive remote sensing.
 It is also possible to produce electromagnetic radiation of a
specific wavelength or band of wavelengths as a part of the
sensor system. The interaction of this radiation with the target
could then be studied by sensing the scattered radiation from the
targets. Such sensors, which produce their own electromagnetic
radiation, are called active sensors. This process of remote
sensing is active remote sensing.

4. Sensors Characteristics
Sensor Parameters and Resolution
 The information collected by the remote sensors is meant to
identify and map various earth surface objects. Therefore, we
may say that the performance of the sensor is evaluated based
on its classification as well as its mapping accuracy
requirements. It is reasonable to assume that this will depend on
the instruments ability to detect small differences in the
emittance/ reflectance of the earth’s surface in a number of
spectral bands for as small an object as possible and as often as
possible.
 We may consider the sensor parameters under four domains:
(1) Spatial
(2) Spectral
(3) Radiometric
(4) Temporal.

5. Resolution
 Resolution is defined as a measure of sensor ability to distinguish
between signals. All remote sensing information is resolution
dependent. The various parameters which characterize theses
different kinds of sensor systems are described by resolution. The
quality of remote sensing data depends on its spatial, spectral,
radiometric and temporal resolution.
 Spatial resolution refers to the size of the smallest object that
can be detected in an image. The basic unit in an image is called
a pixel. One-meter spatial resolution means each pixel image
represents an area of one square meter in the ground.
 Spectral resolution refers to the number of bands and the
wavelength width of each band. A band is a narrow portion of the
electromagnetic spectrum. Shorter wavelength widths can be
distinguished in higher spectral resolution images.

6.  Radiometric resolution refers to the sensitivity of a remote
sensor to variations in the reflectance levels. The higher the
radiometric resolution of a remote sensor, the more sensitive it is
to detecting small differences in reflectance values. Higher
radiometric resolution allows a remote sensor to provide a more
precise picture of a specific portion of the electromagnetic
spectrum. Radiometric resolution is expressed in bits.
 Temporal resolution refers to how often a remote sensing
platform can provide coverage of an area. Geo-stationary
satellites can provide continuous sensing while normal orbiting
satellites can only provide data each time they pass over an area.

7. SENSORS
Optical-Infrared Sensors
 Optical infrared remote sensors are used to record
reflected/emitted radiation of visible, near-middle and far
infrared regions of electromagnetic radiation.
 They can observe for wavelength extend from 400-2000
nm. Sun is the source of optical remote sensing.
 There are two kinds of observation methods using optical
sensors:
 Visible/near infrared remote sensing and
 Thermal infrared remote sensing.

8. Visible/Near Infrared Remote Sensing
 In this observation method visible light and near infrared rays of
sunlight reflected by objects on the ground is observed. The
magnitude of reflection infer the conditions of land surface, e.g.,
plant species and their distribution, forest farm fields, rivers,
lakes, urban areas etc. In the absence of sunlight or darkness,
this method cannot be used.
Thermal Infrared Remote Sensing
 In thermal infrared remote sensing, the land surface radiate heat
due to interaction of earth surface with solar radiation. Also this
is used to observe the high temperature areas, such as volcanic
activities and forest fires. Based on the strength of radiation, one
can surface temperatures of land and sea, and status of volcanic
activities and forest fires. This method can observe at night
when there is no cloud.

9. The optical remote sensing can be classified into panchromatic
imaging system, multispectral imaging system and
hyperspectral imaging system.
Panchromatic Imaging System
 A single channel sensor with broad wavelength range is used to
detect radiation within a broad wavelength range.
 In panchromatic band, visible and near infrared are included.
 The imagery appears as a black and white photograph.
 The color of the target is not available.
 Examples of panchromatic imaging system are Landsat ETM+
PAN, SPOT HRV-PAN and IKONOS PAN, IRS-1C, IRS-1D and
CARTOSAT-2A. Spectral range of Panchromatic band of ETM+
is 0.52 µm to 0.9 µm, CARTOSAT-2B is 0.45-0.85 µm, SPOT is
0.45- 0.745 µm.

10. Multispectral imaging system
 The multispectral imaging system uses a multichannel detectors
and records radiation within a narrow range of wavelength. Both
brightness and color informations are available on the image.
LANDSAT, LANDSAT TM, SPOT HRV-XS and LISS etc. are
the examples.
Hyperspectral imaging system
 Hyperspectral imaging system records the radiation of terrain in
100s of narrow spectral bands. Therefore the spectral signature of
an object can be achieved accurately, helps in object
identification more precisely. Example, Hyperion data is recorded
in 242 spectral bands, and AVIRIS data is recorded in 224
spectral bands.

11. Microwave Sensors
 The region Microwave sensors receive microwaves, which are longer
wavelength than visible light and infrared rays, and observation is not
affected by day, night or weather.
 Microwave portion of the spectrum includes wavelengths within the
approximate range of 1 mm to 1m. Thus, the longest microwaves are
about 2,500,000 times longer than the shortest light waves.
 There are two types of observation methods using microwave sensor:
active and passive.
 Active sensor: The sensor emits microwaves and observes
microwaves reflected by land surface features. It is used to observe
mountains, valleys, surface of oceans wind, wave and ice conditions.
 Passive sensor: This type of sensor records microwaves that
naturally radiated from earth surface features. It is suitable to observe
sea surface temperature, snow accumulation, thickness of ice, soil
moisture and hydrological applications etc.
 RISAT is an Indian remote sensing satellite provides microwave
data.

12. SCANNING MECHANISM
 Multispectral scanner images are acquired by means of along
track or across track scanning system. Depending on the way of
scanning, remote sensing scanner can be categorized as: across
track (whiskbroom) and along track (push broom) scanning.
Across-track scanning
 Multispectral scanning systems make two dimensional images of
terrain for swath beneath the platform. Swath is width of the
strip of a scene along the across track direction.
 Across track scanners scan the earth terrain in a series of lines.
The lines are oriented in the direction perpendicular to the
motion of the sensor platform (i.e. across the swath). Hence in
across track scanning an optical-mechanical scanner (also known
as whiskbroom scanner) scans along the swath from one side to
another.

13. Along-track scanning
 To achieve better spatial / spectral
resolution than those provided by opto-
mechanical scanners, push broom
imaging systems are currently being
used, referred as along track scanning.
 In along track scanning the detector
scans the terrain of earth in the flight
direction directly beneath the platform.
The detector scans the whole terrain
equal to width of swath in a strip or line.
To build up a two dimensional image by
recording successive scan lines, the
detectors are oriented at right angles to
the flight direction.

14. REMOTE SENSING PLATFORMS
 Platforms: For remote sensing applications, sensors should
be mounted on suitable stable platforms.
 These platforms can be ground based air borne or space borne
based.
 As the platform height increases the spatial resolution and
observational area increases. Thus, higher the sensor is
mounted; larger the spatial resolution and synoptic view is
obtained.
 The types or characteristics of platform depend on the type of
sensor to be attached and its application.
 Depending on task, platform can vary from ladder to satellite.
 For some task sensors are also placed on ground platforms.
Though aircrafts and satellites are commonly used platforms,
balloons and rockets are also used.

15.  Three types of platforms are used to mount the remote
sensors –
 Ground Observation Platform
 Airborne Observation Platform, and
 Space-Borne Observation Platform
Ground Observation Platform
 Ground observation platforms are used to record detailed
information about the objects or features of the earth’s surface.
 These are developed for the scientific understanding on the
signal-object and signal-sensor interactions. Ground observation
includes both the laboratory and field study, used for both in
designing sensors and identification and characterization of land
features.
 Ground observation platforms include – handheld platform,
cherry picker, towers, portable masts and vehicles etc. Portable
handheld photographic cameras and spectroradiometers are
largely used in laboratory and field experiments as a reference
data and ground truth verification.

16. Air Borne Based Platform
 Aircraft remote sensing system may also be referred to as sub-
orbital or airborne, or aerial remote sensing system. At present,
airplanes are the most common airborne platform.
 Other airborne observation platforms include balloons, drones
(short sky spy) and high altitude sounding rockets. Helicopters
are occasionally used.
Space-borne Observation Platforms
 In spaceborne remote sensing, sensors are mounted on-board a
spacecraft (space shuttle or satellite) orbiting the earth.
 Space-borne or satellite platform are onetime cost effected but
relatively lower cost per unit area of coverage, can acquire
imagery of entire earth without taking permission.
 Space borne imaging ranges from altitude 250 km to 36000 km.

17. PLATFORMS
 Ground-based platforms: Ground, vehicles and/or towers
=> up to 50 m
Examples:
 DOE ARM (Atmospheric radiation Program): http://www.arm.gov/
 NASA AERONET (AErosol Robotic NETwork):
http://aeronet.gsfc.nasa.gov/
 Airborne platforms: airplanes, helicopters, high-altitude
aircrafts, balloons => up to 50 km
Examples: NCAR and NASA research aircrafts

18.  Spaceborne: rockets, satellites, shuttle => from about 100
km to 36000 km
 Space shuttle: 250-300 km
 Space station: 300-400 km
 Low-level satellites: 700-1500 km
 High-level satellites: about 36000 km
Examples:
 NASA Earth Science Enterprise (ESE):
http://www.earth.nasa.gov/
 NOAA weather satellites: http://www.noaa.gov/satellites.html
 NPOESS: http://www.ipo.noaa.gov/

19. PLATFORMS
 Ground-based platforms: Ground, vehicles and/or towers
=> up to 50 m
Examples:
 DOE ARM (Atmospheric radiation Program): http://www.arm.gov/
 NASA AERONET (AErosol Robotic NETwork):
http://aeronet.gsfc.nasa.gov/
 Airborne platforms: airplanes, helicopters, high-altitude
aircrafts, balloons => up to 50 km
Examples: NCAR and NASA research aircrafts

20.  Spaceborne: rockets, satellites, shuttle => from about 100
km to 36000 km
 Space shuttle: 250-300 km
 Space station: 300-400 km
 Low-level satellites: 700-1500 km
 High-level satellites: about 36000 km
Examples:
 NASA Earth Science Enterprise (ESE):
http://www.earth.nasa.gov/
 NOAA weather satellites: http://www.noaa.gov/satellites.html
 NPOESS: http://www.ipo.noaa.gov/

21. SATELLITES
 A satellite with remote sensors to observe the earth is
called a remote sensing satellite or earth observation
satellite.
 Remote sensing satellites are characterized by their altitude,
orbit and sensors. The main purpose of the geosynchronous
meteorological satellite (GMS) with an altitude of 36,000 km is
meteorological observations, while Landsat with an altitude of
about 700 km, in a polar orbit, is mainly for land area
observation.

22. Satellite Orbits
 The path followed by a satellite is referred to as its orbit.
Satellite orbits are matched to the capability and objective of the
sensor(s) they carry. Orbit selection can vary in terms of altitude
(their height above the Earth's surface) and their orientation and
rotation relative to the Earth.
 There are several types of orbits:
 Polar
 Sun Synchronous
 Geosynchronous

23. Polar Orbits
 The more correct term would be
near polar orbits. These orbits
have an inclination near 90
degrees. This allows the satellite
to see virtually every part of the
Earth as the Earth rotates
underneath it. It takes
approximately 90 minutes for the
satellite to complete one orbit.
These satellites have many uses
such as measuring ozone
concentrations in the
stratosphere or measuring
temperatures in the atmosphere.

24. Sun Synchronous Orbits
 These orbits allows a satellite to pass over a section of the
Earth at the same time of day. Since there are 365 days in
a year and 360 degrees in a circle, it means that the
satellite has to shift its orbit by approximately one degree
per day.
 These satellites orbit at an altitude between 700 to 800
km. These satellites use the fact since the Earth is not
perfectly round (the Earth bulges in the center, the bulge
near the equator will cause additional gravitational forces
to act on the satellite. This causes the satellite's orbit to
either proceed or recede. These orbits are used for
satellites that need a constant amount of sunlight.
 A Sun-synchronous orbit is useful for imaging, spy, and
weather satellites, because every time that the satellite is
overhead, the surface illumination angle on the planet
underneath it will be nearly the same.

25.  Sun-synchronous orbit (SSO) is a
particular kind of polar orbit.
Satellites in SSO, travelling over the
polar regions, are synchronous with
the Sun. This means they are
synchronized to always be in the
same ‘fixed’ position relative to the
Sun. This means that the satellite
always visits the same spot at the
same local time – for example,
passing the city of Paris every day
at noon exactly.
 A satellite in a Sun-synchronous
orbit would usually be at an altitude
of between 600 to 800 km. At 800
km, it will be travelling at a speed of
approximately 7.5 km per second.

26. Geosynchronous (Geostationary) Orbits
 Also known as geostationary orbits, satellites in these
orbits circle the Earth at the same rate as the Earth spins.
The Earth actually takes 23 hours, 56 minutes, and 4.09
seconds to make one full revolution.
 So based on Kepler's Laws of Planetary Motion, this
would put the satellite at approximately 35,790 km above
the Earth.
 The satellites are located near the equator since at this
latitude, there is a constant force of gravity from all
directions. At other latitudes, the bulge at the center of the
Earth would pull on the satellite.

27.  Geosynchronous orbits allow the satellite to observe
almost a full hemisphere of the Earth. These satellites
are used to study large scale phenomenon such as
hurricanes, or cyclones. These orbits are also used for
communication satellites. The disadvantage of this type
of orbit is that since these satellites are very far away,
they have poor resolution.
 Such orbits are useful for telecommunications
satellites.

28. Thank You