Remote Sensing Satellites
Introduction
In spaceborne remote sensing, sensors are mounted on-board a spacecraft (space shuttle or
satellite) orbiting the earth. At present, there are several remote sensing satellites providing
imagery for research and operational applications.
Spaceborne remote sensing provides the following advantages:
• Large area coverage
• Frequent and repetitive coverage of an area of interest
• Quantitative measurement of ground features using radiometrically calibrated sensors
• Semiautomated computerised processing and analysis
• Relatively lower cost per unit area of coverage
Satellite imagery has a generally lower resolution compared to aerial photography. However, very
high resolution imagery (up to 1-m resolution) is now commercially available to civilian users with
the successful launch of the IKONOS-2 satellite in September 24, 1999.
Each of these satellite-sensor platform is characterised by
• the wavelength bands employed in image acquisition,
• the spatial resolution of the sensor,
• the coverage area and
• the temporal coverge, i.e. how frequent a given location on the earth surface can be imaged by
the imaging system.
Classification
In terms of the spatial resolution, the satellite imaging systems can be classified into:
• Low resolution systems (approx. 1 km or more)
• Medium resolution systems (approx. 100 m to 1 km)
• High resolution systems (approx. 5 m to 100 m)
• Very high resolution systems (approx. 5 m or less)
Classification
In terms of the spectral regions used in data acquisition, the satellite imaging systems can be
classified into:
• Optical imaging systems (include visible, near infrared, and shortwave infrared systems)
• Thermal imaging systems
• Synthetic aperture radar (SAR) imaging systems
Optical/thermal imaging systems can be classified according to the number of spectral bands used:
• Monospectral or panchromatic (single wavelength band, "black-and-white", grey-scale image)
systems
• Multispectral (several spectral bands) systems
• Superspectral (tens of spectral bands) systems
• Hyperspectral (hundreds of spectral bands) systems
Classification
Low Resolution
• Geostationary Satellites
• Polar Orbiting Met. Satellites
• NOAA-AVHRR DMSP-OLS
• Orbview2-SeaWiFS
• SPOT4-Vegetation
High Resolution
• LANDSAT
• SPOT1,2,4
• MOS
• IRS
• RESURS
Very High Resolution
• IKONOS2
• Quickbird
• SPOT 5
Satellites
Satellites are objects which revolve around another object - in this case, the Earth. For example,
the moon is a natural satellite, whereas man-made satellites include those platforms launched for
remote sensing, communication, and telemetry (location and navigation) purposes. Because of
their orbits, satellites permit repetitive coverage of the Earth’s surface on a continuing basis.
Satellite Orbits
Satellites are placed into orbits tailored to match the objectives of each satellite mission and the
capabilities of the sensor they carry. A normal orbit forms an ellipse with the centre of the earth at
one focus, characterized by an apogee (point farthest from earth), perigee (point nearest to
earth), ascending node(the point the satellite crosses equator on passing north to south (the
shadowed side of the earth), descending node (the point the satellite crosses equator on passing
south to north (the sunlit side of the earth).
Inclination is the angle between the orbital plane and the equatorial plane.
Figure:
Geostationary Satellite
• A geostationary satellite is launched in such a way that it follows an orbit parallel to the
equator and travels in the same direction as the earth’s rotation with the same period of 24
hours. Thus, it appears stationary with respect to the earth surface. A satellite following a
geostationary orbit always views the same area on the earth.
• Revolve at an angular rate at altitudes of approximately 36000 km that matches the earth’s
rotation
• Weather satellites, communication satellites designed to maintain a constant position with
respect to a specific region on the earth’s surface.
Geostationary Satellite
• GMS (Geostationary Meteorological Satellites) (Japan), over the Asia-Pacific region (1400E)
• FY-2 (Fengyun-2) (China), over the Asia-Pacific region (1050E)
• INSAT (Indian National Satellite System) (India), A series of geostationary satellites for
meteorological observation and telecommunication over India and the Indian Ocean.
• GOES (Geostationary Operational Environmental Satellites) (USA), over the American
continents (750W and 1350W).
• METEOSAT (Europeean Space Agency), over Europe and Africa (00E).
Sun-synchoronous Satellite
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 nearpolar orbits, so named for the inclination of the orbit
relative to a line running between the North and South poles. Many of these satellite orbits are
also sun-synchronous such that they cover each area of the world at a constant local time of day
called local sun time.In this way, the same solar illumination condition (except for seasonal
variation) can be achieved for the images of a given location taken by the satellite.
Figure:
Figure:
Sun-synchoronous Satellite
Sun-synchronous orbits are designed to reduce one important source of variation in illumination
which is caused by differences in time of day. The hour angle(h) describes the difference in
longitude between a point of interest and that of the direct solar beam. Because h varies with
longitude, to maintain uniform local sun angle, it is necessary to design satellite orbits that acquire
each scene at the same local sun time. The nodes of the satellite’s orbit will move eastward 10
each day, so that over a year, the orbit will move through the complete 3600 cycle. A satellite
placed in such a orbit will observe each part of the earth within its view at the same local sun time
each day.
Figure:
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(shadowed side)and descending passes(sunlit
side), respectively.
Swath
Figure:
• 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 spaceborne sensors generally vary between tens and hundreds of kilometres
wide.
• As seen from the Earth, it seems that the satellite is shifting westward because the Earth is
rotating (from west to east) beneath it. This apparent movement allows the satellite swath to
cover a new area with each consecutive pass.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.
Orbit cycle
Figure:
If we start with any randomly selected pass in a satellite’s orbit, an orbit cycle will be completed
when the satellite retraces its path, passing over the same point on the Earth’s surface directly
below the satellite (called the nadir point) for a second time. The exact length of time of the
orbital cycle will vary with each satellite.
The interval of time required for the satellite to complete its orbit cycle is not the same as the
"revisit period". Revisit period is the time required to capture the image of a specified area in a
repetitive manner. Using steerable sensors, an satellite-borne instrument can view an area
(off-nadir) before and after the orbit passes over a target, thus making the ’revisit’ time less than
the orbit cycle time.The revisit period is an important especially when frequent imaging is required
(for example, the extent of flooding).
Landsat
Landsat is the first Earth Resources Technological Satellite launched in 1972 and
jointlymanaged by NASA and the U.S. Geological Survey.
Figure:
IRS
IRS-1 is India’s dedicated Earth resources satellite system operated by ISRO and
the National Remote Sensing Agency (NRSA). The primary objective of the
IRSmissions is to provide India’s National Natural Resources Management
System (NNRMS) with remote sensing data.
Figure:
IRS-1C Launced in 1995 has the following characteristics:
• Type: Sun-Synchronous
• Altitude: 817 km
• Inclination: 98.69 deg
• Period: 101 min
• Repeat Cycle: 24 days
Sensors:
• a single-channel panchromatic (PAN) high resolution camera
• a medium resolution four-channel Linear Imaging Self scanning Sensor (LISS-III)
• a coarse resolution two-channel Wide Field Sensor (WiFS).
IRS-1C
IRS-1C LISS and PAN Sensor Characteristics
Sensor Band Wavelength(µm) Resolution(m) Swath width(Km)
LISS 1 0.52-0.59(Green) 23.5 142
LISS 2 0.62-0.68(Red) 23.5 142
LISS 3 0.77-0.86(NIR) 23.5 142
LISS 4 1.55-1.75(SWIR) 70 142
PAN 0.5-0.9 <10 70.5
WIFS characteristics
Band Wavelength(µm) Resolution(m) Swath width(Km)
Red 0.62-0.68 189 774
NIR 0.77-0.86 189 774
Sensor
There are two types of sensors:
• Active Sensor
• Passive Sensor
Passive Sensor:
The sun provides a very convenient source of energy for remote sensing. The sun’s energy is either
reflected, as it is for visible wavelengths, or absorbed and then reemitted, as it is for thermal
infrared wavelengths. Remote sensing systems which measure energy that is naturally available are
called passive sensors. Passive sensors can only be used to detect energy when the naturally
occurring energy is available. For all reflected energy, this can only take place during the time
when the sun is illuminating the Earth.
Active Sensor:
Active sensors, on the other hand, provide their own energy source for illumination. 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. Advantages for active sensors include
the ability to obtain measurements anytime, regardless of the time of day or season.
Sensor
Figure:
