Introduction to Remote Sensing –

Fundamentals of Satellite Remote Sensing

Mads Olander Rasmussen (mora@dhi-gras.com)

01.

Introduction to Remote Sensing

© DHI

• “the art, science, and technology of obtaining reliable information about

physical objects and the environment, through the process of recording,

measuring and interpreting imagery and digital representations of energy

patterns derived from noncontact sensor systems”. (Cowell 1997)

• “Satellite remote sensing refers to the reception, preprocessing and later

analyses of data obtained from Earth orbiting satellites” (Campbell, J.B.,

2001)

• Other terms: Earth Observation

What is remote sensing?

Components of a remote sensing system

PRE-PROCESSING

Remote Sensing

Platform

IMAGE ANALYSIS END USER INFORMATION

1858 1900 1950 19901970

Balloon

Plane

Space

Program

Landsat-4

SpaceShuttle

Landsat-1

Sputnik

Meteorological

Satellites

Space Station

2010

ERS-1UnitedSsatte

Commercial

Satellites

Pigeon camera

Historical development of remote sensing systems

2014

Historical development of remote sensing systems

Landsat

NOAA AVHRR

Quickbird

• Airborne (lidar, hyperspectral, thermal etc.).

• Unmanned (UAV)

• Ground sensors

Other sources of Remote Sensing data

…but focus of this course is Satellite Remote Sensing

02.

The fundamentals

© DHI

• Satellite and Sensor types

• Satellite orbit

− Spatial resolution

− Temporal resolution

− Spectral resolution

• From reflected ’light’ to a satellite image

− The electromagnetic spectre

− Spectral signatures

• Digital image processing

Key parameters

Satellite orbits

• Polar-orbiting satellites (200-1000km)

Most satellites are polar orbiting. They

circle the Earth in a certain track that

allows them to monitor all areas within a

certain time interval (e.g. Landsat, SPOT,

WorldView, Sentinel)

• Geo-stationary satellites (36000 km)

Geo-stationary satellites orbit Earth in

app. 36.000 km altitude. Monitors the

same area of the Earth at all times (e.g.

Meteosat, NOAA GOES)

• Video

Satellite orbits

• Passive

− Sun’s energy which is reflected (visible) or

− Absorbed and re Absorbed and re-emitted as

emitted as thermal infrared wavelengths

− Landsat, SPOT, WorldView, Pleiades

• Active

− Emit radiation

− Radiation reflected is detected and measured

− SAR, LIDAR, and SONAR

− SRTM (Shuttle Radar Topography Mission)

Types of Satellite Sensors

© DHI

There is no such thing as bad weather…

Active/

SAR data

Passive/

Optical data

An image type for all weather conditions:

• Ikonos (1 / 4 m) - 1999

• QuickBird (0,6 / 2.4 m) - 2001

• WorldView-1 (0,5 m) – 2007

• GeoEye-1 (0,5 / 2 m) - 2008

• WorldView-2 (0,5/2 m) – 2009

• Pleiades (0,5/2 m) – 2011/2012

• SkyBox

• …etc!

Very High Resolution

• Landsat (15 - 90 m) - 1973

• SPOT (2.5-20 m) - 1981

• IRS (5 – 23 m) - 1988

• Aster (15-90 m) – 1999

• RapidEye (5 m) – 2009

• DMC2 (22m) - 2009

• SPOT 6-7 (1.5m) – 2012/2014

• Sentinel 2 (10/20/60m) - 2015

• PlanetLabs (5-8m) - 2015…

• …

High/Medium Resolution

• NOAA AVHRR (1 km) - 1998

• MODIS (250 – 1000 m) - 1999

• MERIS (300 – 1000 m) – 2002

• NPP VIIRS (750m) - 2011

• Geostationary (Meteosat/MSG/GOES,MTSAT etc) (3km – 8 km)

• …

Low Resolution

© DHI

A. Archive imagery

− On/offline archives

− What has been imaged in the past

B. New acqusition

− Possibility of controlling the time and area to be imaged

− Area of 2400 sqkm can be collected in a few minutes!

Getting access to imagery with high resolution

• Spatial …is defined as the area of

ground represented by one

pixel in the image

Key parameter: RESOLUTION

• Spectral …is a measure of the number

(and width) of bands in which a

given sensor records

information

…is defined as the frequency at

which images are recorded in a

specific place on the earth

• Temporal

• Spatial …is defined as the area of

ground represented by one

pixel in the image

Key parameter: RESOLUTION

• Spectral …is a measure of the number

(and width) of bands in which a

given sensor records

information

…is defined as the frequency at

which images are recorded in a

specific place on the earth

• Temporal

• The registered energy for every band is written to an image file; everyobservation becomes a pixel value.

• Each value represents the average brightness for a portion of the surface,

represented by the square unit areas in the image. In computer terms the

grid is commonly known as a raster, and the square units

are cells or pixels

• Satellite Images are typically stored as GeoTiff images (.tif)

Digital Image

Spatial Resolution

• Very High Resolution

0.5 - 1 m

• High/Medium Resolution

2.5 - 90 m

• Low Resolution

250 m – 8 km

Spatial Resolution

Spatial resolution is controlled by the specifications of the radiometer, view

angle geometry and the orbit altitude

Spatial Resolution

Spatial Resolution - Landsat 8

MODIS (250m)

Balance between coverage and resolution

MODIS vs. Landsat

Landsat (30m)

Landsat (30m)

Landsat (30m)

VHR (50cm)

VHR (50cm)

VHR (50cm) vs Landsat (30m)

Landsat (30m)

Balance between coverage and resolution

• Spatial …is defined as the area of

ground represented by one

pixel in the image

Key parameter: RESOLUTION

• Spectral …is a measure of the number

(and width) of bands in which a

given sensor records

information

…is defined as the frequency at

which images are recorded in a

specific place on the earth

• Temporal

24h 48h

Around the world in 90 minutes

Around the world in 90 minutes

• Temporal resolution is important when choosing imagery

− If the goal is to monitor a process that changes rapidly (e.g. floods, fires), several images may be needed per day

− If the goal is to document a slow process (e.g. Vegetation trends, Glacier movement or urban growth) then one image per year would suffice

Temporal Resolution

• Use time series of satellite data to include seasonal variations of

wetness/flooding

Importance of time series

Importance of time series

Importance of time series

• Spatial …is defined as the area of

ground represented by one

pixel in the image

Key parameter: RESOLUTION

• Spectral …is a measure of the number

(and width) of bands in which a

given sensor records

information

…is defined as the frequency at

which images are recorded in a

specific place on the earth

• Temporal

• 2 relevant parts of the electromagnetic spectrum:

− Visible-IR (Optical)

− Microwave (SAR)

• Not all parts of the spectrum are ’transparent’

• Electromagnetic radiation blocked primarily by: water vapour, Ozone, CO2

and aerosols (dust etc.)

The electromagnetic spectre

Digital Colour Image

Pan-c

rom

atisk

bill

ede

Fals

e-c

olo

r im

age

Spectral resolution

400 500 600 700 800 900 1000 1100

MS

WV2 Pan

WV1 Pan

MS

QB Pan

Wavelength (nm)

400 500 600 700 800 900 1000 1100

MS

WV2 Pan

WV1 Pan

MS

QB Pan

Wavelength (nm)

Blue Green Red Near-Infrared

• Optical remote sensing is based on the detection of reflected and emitted

energy at different wavelenghts

Different materials will have varying reflective/emmissive proportions at

different wavelengths.

Radiation principles

Landsat bands 7 and 8

Landsat 8 - Band Combinations

Natural Color 4 3 2

False Color (urban) 7 6 4

Color Infrared (vegetation) 5 4 3

Agriculture 6 5 2

Atmospheric Penetration 7 6 5

Healthy Vegetation 5 6 2

Land/Water 5 6 4

Natural With Atmospheric

Removal

7 5 3

Shortwave Infrared 7 5 4

Vegetation Analysis 6 5 4

Image Classification

Image Classification

Image Classification

Image Classification

Spectral signatures

Spectral signatures

0.4 0.5 0.6 0.7 0.9 1.6

Vegetation

Soil

Wavelength (µm)

Ref

lect

an

ce (

%)

50

Blu

e

Gre

en

Red

Nea

r-in

fra

red

Mid

-in

fra

red

• E.g. NDVI

Vegetation Index

NDVI?

NDVI

• Spatial …is defined as the area of

ground represented by one

pixel in the image

Key parameter: RESOLUTION

• Spectral …is a measure of the number

(and width) of bands in which a

given sensor records

information

…is defined as the frequency at

which images are recorded in a

specific place on the earth

• Temporal

• Radiometric …refers to the effective bit-

depth of the sensor (number of

grayscale levels)

Key parameter: RESOLUTION

…what is the cost?• Economic