CHARACTERISTICS OF OPTICAL SENSORS

Course: Introduction to RS & DIP

Mirza Muhammad WaqarContact:

mirza.waqar@ist.edu.pk+92-21-34650765-79 EXT:2257

RG610

Outlines

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

Remote Sensing

Celestial RS Terrestrial RS

Meteorological

Telecommunication

Earth Observation

Optical Thermal Microwave

Earth Observation Remote Sensing

Physical Basis of Remote Sensing

Physical Basis of Remote Sensing

ConclusionInterpretationObservation

Source of illumination

EM Radiations Target Traveling

Path

Platform

PlatformCherry Lifter

Balloons Air Crafts Space Craft

Low Altitude

2 Km

Medium Altitude 2-10 Km

High Altitude

10-13 Km

Low Altitude200-00

Km

Medium Altitude 500-900

Km

High Altitude36k-45k

Km

Sensors

Active

Passive

Non-Imaging

Imaging

Non-Imaging

Framing

Non-Framing

Sensor

Sensor Type

8

Passive vs Active Remote Sensing

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

17

Pixels

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.

25

1. Spatial Resolution2. Spectral Resolution3. Radiometric Resolution4. Temporal Resolution

Resolutions –Key to Check Image Quality

26

Spatial Resolution

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)

32

Spectral Resolution

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 sensingwavelengths) adds information

In this case by “sensing” RGB can combine toget 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 Data2. Multispectral Data3. Hyperspectral Data

Hyperspectral Data

Multispectral Data

Panchromatic Data

38

Radiometric Resolution

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

Radiometric Resolution

Temporal Resolution

Each satellite revisits the same area after certain time period, which is called Temporal Resolution.

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...

1. 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.

51

Color Composite

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 CombinationTM Band – 2

Green

TM Band - 4 NIR

TM Band - 7 FIR

Band Combinations

3,2,1

4,3,2

5,4,3

Questions & Discussion