active and passive remote sensing basic terminology

1. Remote Sensing – An Earth Perspective

2. Remote Sensing – Definition
● Acquisition of Earth information about the state and condition of
an object and/or a process through sensors that are not in physical
contact with it
– RS also includes analysis and interpretation of the acquired
information
– RS: an extension of eye
● Raise our 'eyes' above the surface for synoptic view
● Extend these 'eyes' in to different spectrum to 'see‘ through 'clouds'
and to 'see' at night
● Therefore it is an energy interaction between the target and the
sensor
– Energy: Sun (Passive)
– Energy: Artificial such as Radar, Lasers (Active)

3. Human Eyes vs. RS
● Human Eyes has 3 major limitations
– Limited to only Visible spectrum
– Dependent on external energy source (Sun)
– Limitation to our own height causing
restricted observation capabilities

4. Remote Sensing - History
● Hot Air Balloon – 1858 (First aerial photography
above 80m height in Bievre, France)
● Kites – 1880s by British
● Carrier Pigeon camera – Early 1900s
● Airplane photography – first time in 1909 in Italy
● From 1915-1960s: extensive use of aerial
photography for military and civilian use
● First aerial multispectral photograph – 1930 in
Russia
● 1931 – first non-visible (in NIR) film developed by
Kodak research laboratories
● 1947 - First space-borne photo, about 200 km above
space in New Mexico
● Sputnik by Soviet Union: 1957

5. ● First Space program launched: April 1960 by NASA for global
cloud patterns, named Television and Infrared Observation Satellites
● First Satellite RS: Landsat 1- July 23, 1972
● First Indian RS Satellites: IRS–1 in March 1988
● First Commercial satellites IKONOS – 1999
● Quickbird – 2001 (in April 2011 it was raised from from an orbit
of 450km to 482km & on jan 2015 quick bird reenters in earth’s
atmosphere)
Remote Sensing – History (cont.)

6. RS – Some Basic
● Albedo (for a given surface) = Exitance (all outgoing) /
Irradiance (all incidence)
● Spectral signature: behaviour of an object over various
wavelength of EM, e.g. Snow appears
– 'White' in 'Visible' region (white is produced due to high
and equal amount of Blue, Green and Red
– 'Black' in 'SWIR' (reflectance is near-zero in longer
wavelength) region
● Pixel – radiance measurement at every n meters (unit
area covered by the sensor in a digital image) – a basic
unit of information
● DN: Digital number - a brightness value of the pixel (volts
converted in to intensity value from 0-255 in 8-bit format

7. Resolution
● Resolution – an ability to distinguish between two
closely space points
– Spectral (VIS, NIR..)
– Spatial (10cm, 1m, 30m, 200m, 1km..)
– Radiometric (8- or 16-bit) - an ability of an imaging
system to differentiate very slight variation in the
energy, i.e. it denotes the sensitivity of a sensor
– Temporal (1972, 2012) – revisiting period of a
sensor
– Angular resolution: sensor's capacity to make
observation of the same area from different angles

8. Pixel size and Resolution; Intensity
● Pixel size and Resolution:
– Sensor's Resolution Cell = IFOV * A
● Sensor's Resolution Cell is fixed at a particular time of recording
at a particular place
● But Pixel resolution may vary depending on the resampling unit
● Intensity: Total quantity of reflected energy received by a cell in a
given spectral band
– Therefore, intensity value changes in different EM spectrum
– 'Brightness' relates to intensity

9. Mapping Scale
● Mapping Scale: it shows how many times items represented in a
map are smaller than in reality
– Large scale (1:10,000): low reduction in reality, more detailed
information
– Small scale (1:50,000): large reduction in reality, less detailed
information
● Large mapping scale requires high spatial resolution images and
vice-verse

10. RELIEF
● A quantitative measurement of vertical elevation change in a
landscape
● It is the difference between maximum and minimum elevations
within a given area
– The relief of a landscape can change with the size of the area over
which it is measured, making the definition of the scale over which it
is measured very important
– Because it is related to the slope of surfaces within the area of
interest and to the gradient of any streams present, the relief of a
landscape is a useful metric in the study of the Earth's surface

11. SLOPE
● Slope: The slope is commonly taught as "rise over run"
or rise/run between two points on a line.
● In mathematics, the slope or gradient of a line describes
its steepness, incline, or grade. A higher slope value
indicates a steeper incline.

12. SLOPE Cont..
● The rise of a road between two points is the difference between the altitude
of the road at those two points, say y1 and y2, or in other words, the rise is
(y2 − y1) = Δy.
● For relatively short distances - where the earth's curvature may be
neglected, the run is the difference in distance from a fixed point measured
along a level, horizontal line, or in other words, the run is (x2 − x1) = Δx.
● Here the slope of the road between the two points is simply described as
the
ratio of the altitude change to the horizontal distance between any two points
on the line. In mathematical language,
the slope m of the line is m = Δy/Δx
● The concept of slope applies directly to grades or gradients in geography
and civil engineering.
● Through trigonometry, the grade m of a road is related to its angle of
incline
θ by m = tanθ; The larger the absolute value of a slope, the steeper the line.
● θ= arctan m

13. Contours
● In cartography, a contour line joins points of equal elevation
(height) above a given level, such as mean sea level
● A contour map is a map illustrated with contour lines, for
example a topographic map, which thus shows valleys and hills,
and the steepness of slopes
● The contour interval of a contour map is the difference in
elevation between successive contour lines
● When the lines are close together the magnitude of the gradient
is large: the variation is steep. A level set is a generalization of a
contour line for functions of any number of variables
● Contour lines are curved or straight lines on a map describing
the intersection of a real or hypothetical surface with one or
more horizontal planes

14. Optical Remote Sensing
● Imaging in visible, NIR and SWIR of the earth's surface by
detecting the solar radiation reflected from targets on the ground
● Different materials reflect and absorb differently at different
wavelengths
– Thus, the targets can be differentiated by their spectral reflectance
signatures in the remotely sensed images

15. Optical Remote Sensing
Source: http://www.crisp.nus.edu.sg/~research/tutorial/intro.htm

16. Optical RS – Types
● Depending on the number of spectral bands used in the
imaging process, Optical RS systems are classified into
● Panchromatic imaging system
● Multispectral ” ”
● Superspectral ” ”
● Hyperspectral ” ”

17. Active vs. Passive Remote Sensing
● A passive RS system records the EM energy naturally reflected
(in the visible spectrum) or absorbed and remitted (thermal IR)
from an object (Passive sensors)
● Passive Sensing is possible only when
– Naturally occurring energy is available either during day light as
in the case of visible spectrum or
– During night as long as the thermal IR is available in the quantity
enough to record
● Passive Sensors: Landsat, IRS-series, IKONOS

18. Active vs. Passive Remote Sensing
Source:http://www.oneonta.edu/faculty/baumanpr/geosat2/RS-Introduction/RS-
Introduction.html
Source: http://www.nrcan.gc.ca/earth-
sciences/geographyboundary/
remote-sensing/fundamentals/1212

19. Active vs. Passive RS Cont..
● An Active RS system supplies its own source of EM energy,
which is directed at the object in order to measure the returned
energy (Active sensors)
– Flash photography is active Remote Sensing in contrast to
available light photography, which is passive
● Active Sensors: RADARSAT, LiDAR, SRTM
● RADAR, which provides its own source of EM energy in the
microwave region
● LASER or LiDAR is a relatively new form of active Remote
Sensing, operating in the visible and NIR wavelength bands

20. ● It encompasses both active and passive RS
● Microwave wavelength spectrum: 1cm to 1m
● Microwave wavelength long- special properties, not susceptible to
atmospheric scattering which affects shorter optical wavelengths
● It penetrate through cloud cover, haze, dust, and all but the heaviest
rainfall
● Detection of microwave energy under almost all weather and
environmental conditions so that data can be collected at any time
Microwave or RADAR Remote Sensing

21. Passive Microwave Sensing
● All objects emit microwave energy of some magnitude, but the
amounts are generally very small
● A passive microwave sensor (radiometers or scanners) detects
the naturally emitted microwave energy within its field of view
by its antenna
● This emitted energy is related to the temp. and moisture
properties of the emitting object or surface

22. Passive Microwave Sensing
Source: http://www.nrcan.gc.ca/earth-sciences/geography-boundary/remote-
sensing/fundamentals/2021

23. Application of Passive Microwave RS
● Meteorology, hydrology, and oceanography
● By looking "at", or "through" the atmosphere, depending on the
wavelength, Meteorologists can use passive microwaves to measure
atmospheric profiles and to determine water and ozone content in the
atmosphere
● Hydrologists use passive microwaves to measure soil moisture
since microwave emission is influenced by moisture content
● Oceanography - mapping sea ice, currents, and surface winds as
well as detection of pollutants, such as oil slicks

24. Active Microwave or RADAR RS
● RADAR - Radio Detection And Ranging
● Active radar sensors emit microwave radiation in a series of
pulses from an antenna, looking obliquely at the surface
perpendicular to the direction of motion
● When the energy reaches the target, some of the energy is
reflected back towards the sensor. This backscattered microwave
radiation is detected, measured, and timed
● The time required for the energy to travel to the target and return
back to the sensor determines the distance or range to the target
● By recording the range and magnitude of the energy reflected
from all targets as the system passes by, a two dimensional image of
the surface can be produced
● Also, microwave energy is able to penetrate through clouds and
most rain, making it an all-weather sensor

25. Microwave Imaging
Source: http://www.crisp.nus.edu.sg/~research/tutorial/intro.htm

26. ● Active microwave sensors divided into two
distinct categories:
– imaging and non-imaging
● RADAR - RAdio Detection And Ranging: The
most common imaging active MW sensor
● Altimeters and scatterometers - Non-imaging
microwave sensors
● They are profiling devices which measure in
one linear dimension (as opposed to the twodimensional
representation of imaging sensors)
Active Microwave Sensors

27. Non-imaging Active MW sensors
● RADAR altimeters transmit short MW pulses (Nadir looking) and
measure the round trip time delay to targets for the target-sensor
distance
● RADAR altimetry is used for altitude finding, topographic
mapping and sea surface height estimation
● Scatterometers are used to measure the amount of energy
backscattered from targets
– The amount of energy backscattered is dependent
on the surface properties (roughness) and
– The angle at which the MW energy strikes the target

28. Application of Scatterometry MW
● Scatterometry measurements over ocean surfaces can be used to
estimate wind speeds based on the sea surface roughness
● Ground-based scatterometers are used to characterize different
materials and surface types (analogous to the concept of spectral
reflectance curves in the optical spectrum)

29. Reference Books
● Remote sensing: principles and interpretation
Floyd F. Sabins (Publisher: W.H. Freeman and Company,
1997)
● Introduction to remote sensing, James B. Campbell and
Randolph H. Wynne (5th ed., 2011)
● Remote Sensing And Image Interpretation, Thomas M.
Lillesand, Ralph W. Kiefer, Jonathan W. Chipman, 6th Ed.,
2008
● An Introduction to Geographical Information Systems,
Heywood Ian, Pearson Education India, 2010
● A Primer of GIS: Fundamental Geographic and
Cartographic Concepts, Francis Harvey, The Guilford.