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What is Remote Sensing?

Remote sensing is the science and art of obtaining information about a target

through the analysis of data acquired by a device that is not in contact with

the object, area, or phenomenon under investigation.

Remote sensing is with us every day

Eyes: we see thing around and sometimes far away from us. We even

identify them as clouds, birds, and planes. Why can we see? Because

of the sunlight reflected into our nerve cells in the retina.

What’s special for our eyes is that we only see a narrow range of

solar radiation within a large spectrum.

Camera: Except we were born with a pair of most sophisticated remote

sensing device, eyes, many of us have a remote sensing device, camera.

Bats: emit microwaves and collect the reflected wave for navigation

(the same way Radar works)

In both examples above, the medium that carry the information is the

Electromagnetic Radiation. Using various sensors, we can collect the

electromagnetic radiation in any portion of the spectrum. Based on the

source of energy, remote sensing can be broken into two categories:

Passive Remote Sensing: The source of energy collected by sensors is

either reflected solar radiation (e.g. human eye) or emitted by the

targets (thermal camera).

Active Remote Sensing: The source of energy collected by sensors is

actively generated by a man-made device. For example, radar, lidar.

Two Types of Remote Sensing

Electromagnetic Radiation from the Sun

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Landsat 7 ETM+ Spectral Bands

The number of bands and the width of spectrum that each sensor covers are

referred to spectral resolution

U-2 Aircraft

A high altitude aircraft (U-2) was built to collect information over the

soviet territory. U-2 flies at 70,000 ft beyond the range of surface-

to-air missiles and aircraft at that time. U-2 provided tremendous

amount of information about the Soviet Union during to 1950’s

and 1960’s

U-2 remains to be a valuable means of collected information remotely,

today.

1. President Bush used it during Gulf War in 1991

2. President Clinton used it in the war in Bosnia in 1998-1999.

The Missile Crisis

In October 14, 1962, President Kennedy ordered high altitude U-2

reconnaissance flights over western Cuba which identified the

deployment of Soviet missiles which was only 90 miles from US

mainland. President Kennedy then initiated a naval blockade of

Cuba. This is now known as missile crisis in history.

Figure 3-8 a of Jensen

This photograph was shown to President Kennedy on Oct. 16, 1962 by US top

photo interpreter A. C. Lundahl.

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Satellite Remote Sensing—Landsat 7

Sun-synchronous orbit:

Satellite always crossed the

equator at precisely the same

local time.

Swath Width and Field of View

185 km

Field of View

175km

scene

Landsat

Satellite

ground track

705km

Spatial Resolution

Pixel size=

(30x30m)

Temporal Resolution

The shortest time needed to repeat a ground track

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What is Digital Remote Sensing?

Digital remote sensing literally means that the remotely sensed

products are digits or numbers other than films

Information on the film: Information recorded on a film is

essentially the amount of reflected sun light back into space from

the ground surface. Different ground object reflect different

amount of energy leading to a different extent of exposure on

the film. The developed photos is the printed version of sun light

reflected from the target. The interpreter has to extract information

based on the shape, size, tune, texture to identify targets.

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1. Space is covered continuously

with cells.

2. Each cell has one number

indicating the amount of

energy received from the cell

3. The cell is called pixel

(picture element)

4. The size of the pixel is the

spatial resolution

sensor

Digital Images

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Multispectral remotely sensed data

Each band will generate a layer of remotely sensed data, usually

with the same cell (pixel) size. For Landsat satellite, we will have

6 layers of data corresponding to the 6 bands.

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Color Arithmetic

red+green=yellow

green+blue=cyan

red+blue=magenta

How to Make Photos with Numbers?

red

blue

green

white

yellow

black

cyan

magenta

Color Space

Vegetation Information

Normalized Difference Vegetation Index

dNIR

dNIR

RR

RRNDVI

Re

Re

+

−=

NDVI: [-1.0, 1.0]

Often, the more the leaves of vegetation present, the bigger the

Contrast in reflectance in the red and near-infrared spectra.

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Phenology from AVHRR

Feb 27-Mar 12

Jul 17-Jul 30

Aug 14-Mar 27

Jun 19-Jul 2

Apr 24-May 7

Nov 6-19

Orbit: 705 km,

Time to cross equator: 10:30 a.m.

sun-synchronous, near-polar, circular

Sensor Systems: Across Track Scanning

Radiometric resolution: 12 bits

Temporal resolution: 2 days

Spatial Resolution:

250 m (bands 1-2)

500 m (bands 3-7)

1000 m (bands 8-36)

Design Life: 6 years

MODIS

MODIS

Bands

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Monitoring forest fire

Pre-forest fire

Post-forest fire

Burned area identified from space

This is the image from MODIS

showing the wide sediment plume

of Yangtze River as it runs into the

East China Sea. Yangtze River,

originating from the Himalayans, is

the longest river in China (6380km),

the third longest in the world after

the Amazon, and the Nile. The

world largest dam, the Three Gorges

Dam, is now being built in Sichuan

province. Due to deforestation in the

upper reaches of the river, many

fear that the river will become the

second Yellow River in China.

Color, size, and tone are all

helpful to identify the severity

and extension of soil erosion

Land-Ocean Interactions

Sea Surface Temperature

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This is MODIS image shows temperature of Gulf Stream. The current stretches from the

Gulf of Mexico up the East Coast of the United States and departs from North America

Heading across the Atlantic to British Isles. This warm current creates a moderate

Northern European climate.

Warm core rings

rotate clockwise

Cold core rings

rotate counter clockwise

Gulf Stream

Spectral Properties of Water with Algae

Algae is the primary plant in the ocean that absorbs CO2, in the meantime initiate

the food chain in the ocean. It is very important to understand its spatial and

temporal dynamics.

Phytoplankton bloom in

the Black Sea. MODIS

band 1 (red), 4 (green)

and 3 (blue)

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Remote Sensing of Clouds

Cloud types from MODIS: pink: cold high level snow and ice clouds; neon green: low level

water clouds. Different cloud types reflect and emit radiant energy differently.

Remote Sensing of Snow

In the visible spectrum

clouds and snow look

very similar. Thus, it is

difficult to separate

them with human eyes.

But they are very

different in the mid-

infrared.

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A massive iceberg, one of the largest ever observed, broke off the Ross Ice Shelf

near Roosevelt Island in Antarctica in mid-March 2000. This Iceberg is about

40 miles wide and 300 miles long. The break off of such big iceberg may be

related to global climate change.

Change Detection

Landsat 5 TM image on Dec 10,

1988 at Shenzhen special econ.

zone, China (RGB=432)

Landsat 5 TM image on Dec 30,

1995 at Shenzhen special econ.

zone, China (RGB=432)