Scanners, image resolution, orbit in remote sensing,

1. Satellite Orbits
Dr. P. K. Mani
Bidhan Chandra Krishi Viswavidyalaya
E-mail: pabitramani@gmail.com
Website: www.bckv.edu.in

2. 
Satellites generally have
either polar orbits or
geostationary orbits.

3. 

A program sponsored by 17 European weather services. It started with
the successful launch of METEOSAT 1 in 1977 and has continued
unabated. METEOSAT 7, the last in the current series was launched in
early September 1997.
They are stationed above the equator at 0° longitude above the Gulf of
Guinea and image this part of the globe every 30 minutes in three
wavebands. These are:
 the visible (0.4 to 1.1 micrometers),

the water vapour (5.7 to 7.1 micrometers) and
 the thermal infrared (10.5 to 12.5 micrometers).

The first has a resolution of 2.5 km The first has a resolution of 2.5 km at
the sub-satellite point (SSP) while the two infrared wavebands both have
one of 5 km.

4. 
The programme that started with the successful launch of
TIROS-1 in 1960 continues today with the NOAA polar
orbiters. The satellites' AVHRR instruments image the planet
in five bands:
 8 to 0.68 micrometres
 0.725 to 1.10 micrometres
 3.55 to 3.93 micrometres
 10.30 to 11.30 micrometres
 11.50 to 12.50 micrometres

5. 


This NOAA series of geostationary weather satellites
began in 1974 and continues today with the launch of a
new ‘GOES-NEXT series of more advanced satellites in
1994.
The resolution varies from 4 to 8 km.
They have five wavebands:





0.55 to 0.75micrometres
3.80 to 4.00 ”
6.50 to 7.00 “
10.20 to 11.20 ”
11.50 to 12.50 "

6. 





The space shuttle is a
manned spacecraft mission,
carrying astronauts and
scientists as well as
instruments.
The most important sensors
that have been carried on the
Shuttle for Earth imaging
are:
The Shuttle Imaging Radar
The Metric Camera.
The Large Format Camera (LFC)
The Modular Optical-Electronic
Multispectral Scanner (MOMS).

7. 
First LANDSAT satellite
launched in 1972
 Lasted until 1978

Six Landsat satellites to
date
 LANDSAT-4 and -5 still
operational
 LANDSAT -6 experienced
launch failure

8. Technical Information
 Landsat 4 (launched 16.07.82)


Landsat 5 (launched 01.03.84)
Orbit
 near polar sun-synchronous
 complete orbit every 99 mins
 Altitude- 705 km, 438 miles

Re-visit - 16 days

9. Landsat 1, 2 and 3
• Landsat was the first satellite to
be designed specifically for
observing the Earth’s surface.
• Landsat 1, 2 and 3 carried a
multispectral scanner (MSS)
system which records reflected
energy from the Earth’s surface
or atmosphere across four
wavebands; three visible
channels and one near infrared.
• The MSS has a pixel resolution
of 80 metres.

10. Landsats 4 & 5 - Thematic Mapper
• LANDSATs 4 and 5 have a
more refined multispectral
scanner, the Thematic
Mapper (TM), instrument on
board.
• TM is a multispectral sensor
since it detects energy
across seven wavelength
bands.
• The pixel resolution of six of
the wavelength bands is 30
metres but the thermal
infrared band has a pixel
resolution of 120 metres.
On this Landsat TM true colour image of west and central London

11. Landsat 7
• After a successful launch on April 15 1999, Landsat 7 is up and running.
Visit http://www.eurimage.com/ to see "Quick Looks" of some of the
first acquired images..
• LANDSAT 7 carries a TM sensor developed from that on Landsat 5. Key
differences are: (1) extra 15m panchromatic band co-registered with the
multi-spectral, and (2) band 6 resolution at 60m.
The fires of Dili, east Timor
The Turkish earthquake

12. LANDSAT’s Thematic Mapper Sensor
Collect 7 different types of light
B G R Near IR
1 2







1.
2.
3.
4.
5.
6.
7.
3
4
0.45 - 0.52 mm
0.52 - 0.60 mm
0.63 - 0.69 mm
0.76 - 0.90 mm
1.55 - 1.75 mm
10.4 - 12.5 mm
2.08 - 2.35 mm
Mid IR
5
7
Thermal IR
6
blue
R
green
R
red
R
near IR
R
mid IR
R
thermal IR
E
mid IR
R and E

13. SPOT (Systeme Probatoire de l’Observation de la Terre)
•SPOT satellites are placed in near-polar, sun-synchronous
orbits at an altitude of 832 km.
•The satellites have a pushbroom scanner system which can
view the surface immediately below them (nadir view), or can
be directed so that they view the surface to the side. This is
important because it means that two views of the same area
can be collected within a short time of each other, by two
adjacent over-passes. Because the system can take two
images of the same area with different look angles, it is
possible to create stereoscopic (3-D) images. This is similar to
having two eyes, enabling us to view in three dimensions
because each eye views the same scene from a slightly
different position, the brain then creates a 3-D picture.
A computer can create a Digital Elevation Model (DEM) from
two stereoscopic images, which means the shape of the land

14. SPOT
Images with two different spatial resolutions
can
be produced from SPOT data. The first is a
panchromatic (black and white) image with a
spatial resolution of 10 m. The second is an
image from the multi-spectral sensor (SPOT
XS) which collects 3 bands of data, with a
resolution of 20 m.

15. Active Sensors: ERS-1 and ERS-2
• In July 1991, the first European Remote Sensing Satellite (ERS1) was launched by the European Space Agency (ESA). It had a
sun-synchronous circular orbit (near polar) at 770 km. Its
purpose was to collect information on areas of the world that
are difficult to observe from the surface, such as oceans and
ice-covered areas. It also produces images of the land surface
in all weather conditions, 24 hours a day.
• ERS-1 was so successful that the ERS-2 was launched in April
1995, carrying additional equipment called Global Ozone
Monitoring Experiment (GOME).

16. Active Sensors: Radarsat
• RADARSAT-1 spacecraft was launched in November 1995,
by the Canadian Space Agency. The satellite carries a
Synthetic Aperture Radar system providing all-weather, day
and night observations of land and sea surfaces.The
satellite’s SAR system has been designed to make it useful
for a range of applications including:
• crop investigations
• coastal zone mapping
• ship detection
• hydrological applications
• geological applications
• ice monitoring
• oil spill detection
• ocean applications
– e.g. mapping the ocean depths/shape of the sea floor)

17. E. Transmission, Reception, and Processing
All remote sensing systems have some method of transmitting,
receiving, and processing the data. Some satellites actually drop
film canisters to Earth using parachutes. Most remote sensing is
now done digitally, and the data is transmitted using radio waves.

18. Remote Sensor Types:
non-imaging
non-scaning
passive
imaging Camera
imaging
Sensor
types
Microwave radiometer
Magnatic ensor
Gravimeter
Fourier spectrum
scanning
Monochrom
IR
TV camera
image plane scanning
object plane scanning
Solid scanner
Optical mechanical scan.
Microwave radiometer
non-scaning non-imaging Microwave radiometer
Microwave altimeter
active
image plane scanning
scanningimaging
object plane scanning
Passive phased
array radar
Real aperture radar
Synthetic aperture radar

21. Satellite sensor characteristics
The basic functions of most satellite sensors is to collect
information about the reflected radiation along a pathway,
also known as the field of view (FOV), as the satellite orbits
the Earth.
The smallest area of ground that is sampled is called the
instantaneous field of view (IFOV). The IFOV is also
described as the pixel size of the sensor. This sampling or
measurement occurs in one or many spectral bands of the EM
spectrum. The data collected by each satellite sensor can be
described in terms of spatial, spectral and temporal resolution.

22. Multispectral Scanning
• Many electronic remote sensors acquire data using
scanning systems, which employ a sensor with a
narrow field of view (i.e. IFOV) that sweeps over the
terrain to build up and produce a two-dimensional
image of the surface.
• Scanning systems can be used on both aircraft and
satellite platforms and have essentially the same
operating principles. A scanning system used to
collect data over a variety of different wavelength
ranges is called a multispectral scanner (MSS)
• There are two main modes or methods of
scanning employed to acquire multispectral image
data - across-track scanning, and along-track
scanning

23. Across-track scanners

24. Across-track scanners
• Across-track scanners scan
the Earth in a series of lines.
The lines are oriented
perpendicular to the
direction of motion of the
sensor platform. Each line is
scanned from one side of
the sensor to the other,
using a rotating mirror (A).
As the platform moves
forward over the Earth,
successive scans build up a
two-dimensional image of
the Earth´s surface.

25. Across-track scanners
A bank of internal detectors
(B), each sensitive to a
specific range of
wavelengths, detects and
measures the energy for
each spectral band and
then, as an electrical
signal, they are converted
to digital data and
recorded for subsequent
computer processing

26. Across-track scanners
The IFOV (C) of the
sensor and the altitude
of the platform
determine the ground
resolution cell viewed
(D), and thus the spatial
resolution. The angular
field of view (E) is the
sweep of the
mirror, measured in
degrees, used to record a
scan line, and
determines the width of
the imaged swath (F).

27. Along-track scanners

28. The Along Track Scanner has a linear array of detectors
oriented normal to flight path. The IFOV of each detector
sweeps a path parallel with the flight direction. This type of
scanning is also referred to as pushbroom scanning (from
the mental image of cleaning a floor with a wide broom
through successive forward sweeps).
The scanner does not have a mirror looking off at varying
angles. Instead there is a line of small sensitive detectors
stacked side by side, each having some tiny dimension on its
plate surface; these may number several thousand. Each
detector is a charge-coupled device (CCD). In this mode,
the pixels that will eventually make up the image correspond
to these individual detectors in the line array.

29. Remote
Sensing
Scanning
System
Sabin, 1997
Wiskbroom
Pushbroom
Field of View (FOV), Instantaneous Field of View (IFOV)
Dwell time is the time required for the detector IFOV to sweep across a ground cell.
The longer dwell time allows more energy to impinge on the detector, which creates a
stronger signal.

31. Advantages of Along-track scanners
• measure the energy from each ground resolution cell
for a longer period of time
• more energy to be detected and improves the
radiometric resolution
• smaller IFOVs and narrower bandwidths for each
detector
• cross-calibrating thousands of detectors to achieve
uniform sensitivity across the array is necessary and
complicat

36. Spatial Resolution
• The size of the area on the
ground that a sensor "sees" at
any point in time
– or more accurately, every time a
signal is captured
• Determined by size of IFOV
– function of look or cone angle
( ) and height of platform (H)
IFOV = H
The minimum distance between two objects that a sensor
can record distinctly
Ability to distinguish small adjacent objects in an image

37. Spatial Resolution
• The higher the spatial resolution:
– the smaller the ground resolution cell
– the higher the resolving power of the system
– The greater the spatial detail attainable
• Note: to discriminate a feature from its surroundings:
– It must be at least as large as the ground resolution
cell
– It should be 2x larger to ensure that it is detected
– Or, may be smaller if it is spectrally unique/overwhelming

38. Spatial resolution
 Spatial resolution
 formal definiton: a measure of smallest
angular or linear separation between two
objects that can be resolved by sensor
 Determined in large part by
Instantaneous Field of View (IFOV)
 IFOV is angular cone of visibility of the
sensor (A)
 determines area seen from a given altitude at
a given time (B)
 Area viewed is IFOV * altitude (C)
 Known as ground resolution cell (GRC) or
element (GRE)
38

39. Spatial resolution
 Problem with this concept is:
 Unless height is known IFOV will
change
 e.g. Aircraft, balloon, ground-based sensors
 so may need to specify (and measure) flying
height to determine resolution
 Generally ok for spaceborne
instruments, typically in stable orbits
(known h)
 Known IFOV and GRE
39

40. Spatial resolution
40

41. IFOV and ground resolution element (GRE)
IFOV
H
GRE
GRE = IFOV x H
41
where IFOV is
measured in radians

42. Total field of view
H
Image width = 2 x tan(TFOV/2) x H
where TFOV is measured in degrees
GIFOV
42
IFOV
2H tan
2

43. Spatial resolution is a measure of the smallest object that can
be resolved by the sensor, or the linear dimension on the
ground represented by each pixel or grid cell in the image
1 m resolution
2 m resolution
5 m resolution
10 m resolution
20 m resolution
30 m resolution
Image 20: Camp Randall Stadium, University of Wisconsin, viewed at different
resolutions (Image: University of Wisconsin, Institute for Environmental Studies)

44. The resolutions of today's satellite systems vary from a few
centimetres (for example military usage) to kilometres.
Application:
•Low resolution: larger than 30 m
•Medium resolution: 2 - 30 m
•High resolution: under 2 m
•A lower resolution usually coincides with a higher repetition rate,
meaning that within a short interval (METEOSAT-8 , every 15 min)
the satellite reflects the same area. A "coarse" resolution is used to
record large or global areas for climate-related inquiries, for
example the radiation budget of the earth and for weather
monitoring. Additional applications include earth observation of land
use and oceans, the ocean's ice cover and the surface temp.
For measurements based on the distinguishability between two point
targets, Rayleigh criterion is used. According to Rayleigh, the two objects
are just resolvable if the maximum (centre) of the diffraction pattern of
one falls on the first minimum of the diffraction pattern of the other.
Consequently the smallest angle resolvable is
θm = 1.22 (λ/D) radians

45. The Rayleigh Criterion
The Rayleigh criterion is the generally accepted criterion for the
minimum resolvable detail - the imaging process is said to be
diffraction-limited when the first diffraction minimum of the image
of one source point coincides with the maximum of another.
Single slit
Circular aperture

47. •Satellites with medium resolution such as LANDSAT 7 are
used for the global observation of land surfaces. Tropical
rainforests and their deforestation have been observed by the
LANDSAT satellites for more than 30 years.
•High resolution data is mainly used for smaller areas of the
earth's surface. Only recently such data has become available
commercially and privately. Satellites such
as IKONOS or QuickBird send data for topographic and
thematic mapping of for example the land use, vegetation, or
as planning resources for cities, large projects etc. Information
can also be "ordered" in advance, because the turning of the
satellite sensors can reduce the repeat rates and can monitor
the desired areas earlier.

49. Spatial Resolution = 30 m
Spatial Resolution = 15 m
Which image has better spatial resolution?
What’s the difference in the scale of the two images?

50. How does the spatial resolution
and scale of this image compare?

52. Spectral Sensitivity

54. What is spectral resolution, and when is it needed?
Spectral resolution is the ability to resolve
spectral features and bands into their separate
components.
The spectral resolution required by the analyst or
researcher depends upon the application involved.

57. Spectral Resolution
• The accuracy with which slight variations in the wavelength
can be recorded
• How narrow a portion of the EMS a sensors “sees”
– Determined by the width of each individual band
Spectral resolution describes
the ability of a sensor to define
fine wavelength intervals.
Which has better spectral
resolution?
The finer the spectral resolution, the narrower the wavelength range
for a particular channel or band.

58. Spectral Resolution
•
Example:
Black and
white image
- Single
sensing device
- Intensity
is sum of
0.4 m
intensity
of all
visible
Black &
Blue
wavelengths+
White
Images
0.7
Green + Red
m

59. Spectral Resolution (Con’t)
•
Example: Color image
-
Color images need
least three sensing
devices, e.g., red, green,
and blue; RGB
0.4
Color
Images
m
Blue
Green
0.7
m
Red
Using increased spectral
resolution (three sensing
wavelengths) adds
information
In this case by “sensing”
RGB can combine to
get full color rendition

60. 2. Sensors on satellites
Spectral resolution
A multispectral sensor collects data at select wavebands or
channels (e.g., AVHRR – 5; SeaWiFS – 8; MODIS – 36).
When a sensor provides a continuous spectral coverage, it is
called hyperspectral. In practice, it means that its spectral
resolution is better than 10 nm or 0.01 µm and the number of
channels >100.
IoE 184 - The Basics of Satellite Oceanography. 1. Satellites and Sensors

61. Hyperspectral Scanners
• Also called imaging spectrometers
• Acquire imagery over many (>200) very narrow (e.g. 5 –
10 μm) spectral bands
• Enables discrimination based on slight variations in
spectral reflectence (signatures) otherwise obscured
by systems withy courser spectral resolution

66. Radiometric Resolution
• A sensors ability to detect and record slight variations in the
amount of EMR reaching the sensor
• Since total EMR received is directly proportional to
spatial resolution, there is an inverse relationship
between spatial and radiometric resolution
– low spatial resolution (large ground area) means more
total energy received, so slight variations in EMR can
be detected, this results in a high signal to noise ratio
– conversely, if spatial resolution is high (small ground
area) less total energy is received, slight variations are
more difficult to detect, so lower signal to noise
ratio, poorer radiometric resolution

67. Radiometric Resolution
• Radiometric resolution is normally indicated
by the number of quantization levels used to
measure reflectance
– Also called DN values for digital numbers
– Determined by bit format of data
• 6 bits allows us to record values from 0-63, 64
quantization levels
• 7 bit data from 0-127, 128 DN values
• 8 bit data from 0-255, 256 DN values
• 11 bit data from 0 – 2074, 2048 DN values
Radiometric resolution is Number of possible brightness values in
each band of data

68. 2 bit data
8 bit data
4 quantization levels
256 quantization levels
Which image has better radiometric resolution?

69. Radiometric Resolution
The radiometric resolution specifies how well the differences in
brightness in an image can be perceived; this is measured
through the number of the grey value levels. The maximum
number of values is defined by the number of bits (binary
numbers). An 8-bit representation has 256 grey values, a 16
bits (ERS Satellites) representation 65.536 grey values.
Resolutions of different satellite systems:
•LANDSAT-MSS (from LANDSAT 1-3): 6 bits (64 grey values)
•IRS-LISS I-III: 7 bits (128 grey values)
•LANDSAT-TM (from LANDSAT 4-5) & SPOT-HRV: 8 bits (256 grey values)
•LANDSAT-ETM & ETM+ (from LANDSAT 6-7): 9 bits (only 8 bits will be
transmitted)
•IRS-LISS IV: 10 bits (only 7 bits will be transferred)
•IKONOS & QuickBird: 11 bits

70. Radiometric Resolution
• Number of
Shades or
brightness
levels at a given
wavelength
• Smallest
change in
intensity level
that can
be detected by
the
sensing system

72. It is related to the time interval between two successive visits
of a particular scene by the remote sensing satellite

73. Temporal Resolution
• The length of time between repeat coverage
– How long between consecutive images
– Determined by:
• Orbital period of satellite
• Refresh period of airborne imagery
• Pointable optics capture images on adjacent orbital paths
• Important when multitemporal images are desired
– Landcover monitoring
– Progression of an event
• Forest fire
• Disaster recovery
–Acceptable temporal resolution depends on
duration of event

76. Sensor Performance and Resolution
• Temporal resolution
– Refers to the frequency with which images of a given
geographic location can be acquired.
– The temporal resolution is determined by the
orbital characteristics and swath width, the width
of the imaged area
– Swath width is given by:
2htan(FOV/2)
where h is the spatial frequency, and
FOV is the angular field of view of the sensor