CHARACTERISTICS OF OPTICAL SENSORS Course: Introduction to RS & DIP Mirza Muhammad Waqar Contact: EXT:2257 RG610

Outlines  Earth Observation Remote Sensing  Physical Basis of Remote Sensing  Platform  Sensor  Characteristics of Optical Sensor  Color Composite

Remote Sensing Celestial RSTerrestrial RS Meteorological Telecommunication Earth Observation OpticalThermalMicrowave Earth Observation Remote Sensing

Physical Basis of Remote Sensing ConclusionInterpretationObservation Source of illumination EM RadiationsTargetTraveling Path

Platform Cherry Lifter Balloons Air CraftsSpace Craft Low Altitude 2 Km Medium Altitude 2-10 Km High Altitude Km Low Altitude Km Medium Altitude Km High Altitude 36k-45k Km

Sensors Active Passive Non-Imaging Imaging Non-Imaging Framing Non-Framing Sensor

Sensor Type

Passive vs Active Remote Sensing 8 Passive Remote Sensing Active Remote Sensing

1.Discrete Detectors and scanning mirrors-MSS, TM, ETM+, GOES, AVHRR, SeaWiFS, AMS, ATLAS 2.Linear Arrays-SPOT, IRS, IKONOS, ORBIMAGE, Quickbird, ASTER, MISR 3.Liner and area arrays- AVIRIS, CASI, MODIS, ALI, Hyperion, LAC

1. Pixel 2. Spectral Bandwidth 3. Instantaneous Field of View (IFOV) 4. Field of View (FOV) 5. Dwell Time 6. Altitude 7. Resolution 8. Satellite Orbits Characteristics of Optical Sensor

Pixels 17 Pixel is picture element It contains  Address (latitude & longitude)  Digital Numbers  Size

Spectral Bandwidth of the Detector  The signal is stronger for detectors that respond to a broader bandwidth of energy.  For example, a detector that is sensitive to the entire visible range will receive more energy than a detector that is sensitive to a narrow band.  Such as visible red.

Instantaneous Field of View (IFOV)  The instantaneous field of view (IFOV) of any detector is the solid angle through which a detector is sensitive to radiation.  Both the physical size of the sensitive element of the detector and the effective focal length of the scanner optics determine the IFOV.  IFOV is defined as the angle which corresponds to the sampling unit. Information within an IFOV is represented by a pixel in the image.  A small IFOV is required for high spatial resolution but also restricts the signal strength.

Instantaneous Field of View (IFOV)

Field of View (FOV)  The maximum angle of view which a sensor can effectively detect the electromagnetic energy, is called the Field of View (FOV).  The width on the ground corresonding to the FOV is called the Swath Width.

Field of View (FOV)

Dwell Time  The time required for the detector IFOV to sweep across a ground resolution cell is the dwell time.  A longer dwell time allows more energy to exposure to the detector, which creates a stronger signal.

Altitude  For a given ground resolution cell, the amount of energy reaching the detector is inversely proportional to the square of the distance.  A greater altitudes the signal strength is weaker.

1. Spatial Resolution 2. Spectral Resolution 3. Radiometric Resolution 4. Temporal Resolution Resolutions –Key to Check Image Quality 25

Spatial Resolution 26  The spatial resolution of a satellite image is based on the pixel size or picture element.  Can only identify objects which are larger than the pixel size.  To accurately determine size and shape, object must be a few pixels long and wide.

Satellite Resolution SPOT – 5 (Pan) SPOT – 5 (XS) LANDSAT TM LANDSAT MSS NOAA IRS-1C (Pan) Quick Bird (Pan) 2.5 m 10 m 30 m 80 m 1 km 5,8 m 0.6 m Spatial Resolution

Landsat MSS  80 m x 80 m  Approximately the size of a hockey field General Detail

Landsat ETM+  30 m x 30 m  approximately 1/3rd of a hockey field Local Detail

Point Detail ASTER 15 m x 15m

Quickbird (0.6 x 0.6 m)

Spectral Resolution 32  The finer the spectral resolution  the narrower the wavelength range for a particular channel or band.

Example: Black and white image - Single sensing device - Intensity is sum of intensity of all visible wavelengths Can you tell the color of the platform top? How about her sash? 0.4 mm 0.7 mm Black & White Images Blue + Green + Red Spectral Resolution

Example: Color image -Color images need least three sensing devices, e.g., red, green, and blue; RGB Using increased spectral resolution (three sensing wavelengths) adds information In this case by “sensing” RGB can combine to get full color rendition 0.4 mm 0.7 mm Color Images Blue Green Red Spectral Resolution

Spectral Response Differences TM Band 3 (Red)TM Band 4 (NIR)

Spectral Data  On the basis of spectral resolution we can divide data as follows: 1. Panchromatic Data 2. Multispectral Data 3. Hyperspectral Data

Hyperspectral Data Multispectral Data Panchromatic Data

Radiometric Resolution 38  The radiometric characteristics describe the actual information content in an image.  The radiometric resolution of an imaging system describes its ability to discriminate  very slight differences in energy

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

Temporal Resolution l Each satellite revisits the same area after certain time period, which is called Temporal Resolution. l Note that the Earth is also rotating to the East, means satellite does not pass over the same path next time.

Temporal Resolution  It is the revisit frequency of the satellite  More frequency => More Temporal resolution  Different for every satellite  Varies with the altitude of satellite  Temporal resolution is high for upper latitude but lower for equator  The temporal resolution of stereo satellite vary.  Animation in comment box

 High altitude satellite => High temporal resolution (3-4 days)  For frequent coverage => Satellite to satellite transfer coverage  Data is not on same scale Temporal Resolution

1. Geostationary Orbit 2. Polar Orbit 3. Sun Synchronous Orbit Satellite Orbits

Satellite Orbit Determines What part of the globe can be viewed. 2. The size of the field of view. 3. How often the satellite can revisit the same place. 4. The length of time the satellite is on the sunny side of the planet.

Satellite Orbits  A satellite follows a generally elliptical orbit around the earth.  The time taken to complete one revolution of the orbit is called the orbital period.  The satellite traces out a path on the earth surface, called its ground track, as it moves across the sky.  As the earth below is rotating, the satellite traces out a different path on the ground in each subsequent cycle.

Repeat Cycle  Remote sensing satellites are often launched into special orbits such that the satellite repeats its path after a fixed time interval.  This time interval is called the repeat cycle of the satellite.

Geostationary Orbit  If a satellite follows an orbit parallel to the equator in the same direction as the earth's rotation and with the same period of 24 hours.  The satellite will appear stationary with respect to the earth surface.  This orbit is a geostationary orbit.  Satellites in the geostationary orbits are located at a high altitude of 36,000 km.

Near Polar Orbit  A near polar orbit is one with the orbital plane inclined at a small angle with respect to the earth's rotation axis.  A satellite following a properly designed near polar orbit passes close to the poles and is able to cover nearly the whole earth surface in a repeat cycle.

Sun Synchronous Orbit  Earth observation satellites usually follow the sun synchronousorbits.  A sun synchronous orbit is a nearpolar orbit whose altitude is such that the satellite will alwayspass over a location at a given latitude at the same local solartime.  In this way, the same solarillumination condition (except for seasonal variation) can be achieved for the images of a given location taken by the satellite.

Color Composite 51  True Color Composite Natural color composite displays combination of visible red, green & blue bands (Landsat bands 3, 2 and 1 ).  False Color Composite False-color represent multispectral image produce by using bands other than visible red, green & blue.(Landsat bands 7,4,2 or 4,3,2)

Band Combination TM Band – 2 Green TM Band - 4 NIR TM Band - 7 FIR

Band Combinations 3,2,1 4,3,2 5,4,3

Questions & Discussion