# Presentation: 479319

## Satellite Remote SensingWhat's it all about?

- John M. Morrison
- Department of Marine, Earth and Atmospheric Science
- North Carolina State University
- Raleigh, NC 27695-8208
- John_Morrison@ncsu.edu

## Why is remote sensing 

- Why is remote sensing
- important to us?

## What is Satellite Remote Sensing?

- Satellite remote sensing involves gathering information about features on the Earth's surface from orbiting satellites.
- One of the fundamental problems faced by oceanographers is the sheer size of the oceans. Oceans cover 70 per cent of the Earth's surface. Remote sensing allows measurements to be made of vast areas of ocean repeated at intervals in time.

## Geostationary Satellites remain over the same point above the Earth, traveling at the same speed as the rotation of the Earth.Polar Orbiting Satellites circle/orbit round the Earth in a polar direction, and with each orbit pass over the equator about 30 degrees west of the previous orbit. This is because the Earth continues to rotate below the satellite.

## Why do we collect Remotely Sensed Data for Earth Observations?

- Regular revisit capabilities
- road regional coverage
- Good spectral resolution
- Good spatial resolution
- bility to manipulate/enhance digital data
- bility to combine satellite with other digital data
- Cost effective data
- ap-accurate data

## Satellite Remote Sensing

- Satellite sensors detect electromagnetic energy and thus, acquire data on the way various earth features emit and reflect electromagnetic energy. Energy radiates in accordance with basic wave theory: electromagnetic energy travels in harmonic, sinusoidal manner at the *speed of light*, (c). The distance from one wave peak to the next is the *wavelength (****)* and the number of peaks passing a fixed point in space per unit time is the *wave frequency* (). Waves obey the general equation c= . Since the speed of light is essentially constant, frequency and wavelength are inversely related.

## Radiative PartitioningThe atmosphere affects electromagnetic energy through absorption, scattering and reflection. How these processes affect radiation seen by the satellite depends on the path length, the presence of particulates and absorbing gases, and wavelengths involved.  

## Electromagnetic Spectrum

***   ******Continuum         of all types of electromagnetic radiation in which each type of radiation is ordered according to its wavelength.******Note: 1 micrometer (μm)= 10******-6******  meters            (m).***

## Electromagnetic Spectrum  

*Visible (VIS)* : 		0.4 μm to 0.7 μm                                      								(400 to 700 nanometers)
*Infrared (IR)*: 		0.7 μm to 14 μm
*Microwave* 	: 		1 mm to 1 m
*    **In the visible, features are observed by virtue of reflected solar energy. By contrast, in the infrared portion, sensing of emitted energy predominates. The dividing line between sensing reflected and emitted radiation is the wavelength of ~3 μm. *

## Two Generic Types of Sensors Available

- A "passive" system generally consists of an array of small sensors or detectors which record the amount of electro-magnetic radiation reflected and/or emitted from the Earth's surface.
- An "active" system propagates its own electro-magnetic radiation and measures the intensity of the return signal.
- The digital data acquired by the satellites is transmitted to ground stations and can be used to reconstitute an image of the Earth's surface not too dissimilar to an aerial photograph.

## Atmospheric Windows

*Atmospheric windows* define wavelength ranges in which the atmosphere is particularly *transmissive* of energy.
- V*isible* region of the electromagnetic spectrum resides within an atmospheric window with wavelengths of about 0.3 to 0.9 μm
- Emitted energy from the earth's surface is sensed through windows at 3 to 5 μm and 8 to 14 μm.
- Radar and passive microwave systems operate through a window region of 1 mm to 1 m.

## Atmospheric Windows

-    The dominant windows in the atmosphere are in the visible and radio frequency regions, while X-Rays and UV are very strongly absorbed and Gamma Rays and IR are somewhat    less strongly    absorbed.

## Examples of Passive Sensors:

- Advanced Very High Resolution Radiometer (AVHRR) Sea Surface Temperature
- Sea-viewing Wide Field-of-View Sensor (SeaWiFS) Ocean Color

## Why Sea-Surface-Temperature?

- Sea surface temperature (SST) is a key parameter for energy transfer between the oceans and the atmosphere and AVHRR-derived SST can deliver important data to meteorological and climatological models.
- AVHRR imagery is also useful for meteorological applications (for example, providing detailed imagery of cloud systems), and land applications such as vegetation mapping and monitoring.

## Infared Satellite Imagery

- These images were obtained with an AVHRR sensor (Advanced Very High Resolution Radiometer) carried on a NOAA Polar Orbiting Environmental Satellite (POES).
- Measures the amount of thermal infared radiation given off by the surface of the ocean.
- The amount of thermal infared radiation given off by an object is related to its temperature (dying embers give off less radiation than a hot fire and a person gives off even less).
- Thus by measuring the amount of radiation given off by the ocean we can calculate its temperature.

## Advanced Very High Resolution Radiometer (AVHRR)

- _Channel_ _ Bandwidth (μm)_
-       1 	0.58-0.68 (visible) 	2 	0.725-1.10(near IR)	3a 	1.58-1.64 (infrared) 			3b	3.55-3.93 (infrared) 	4 	10.30-11.30 (infrared) 	5 	11.50-12.50 (infrared)

## Sea Surface Temperature from AVHRR

- Radiative transfer theory is used to correct for the effects of the atmosphere by utilizing "windows" where little or no atmospheric absorption occurs.
- Channel radiances are transformed (through the use of the Planck function) to units of temperature, then compared to a-priori temperatures measured at the surface.
- This comparison yields coefficients which, when applied to the global AVHRR data, give estimates of  surface temperature which are nominally            accurate to 0.3° C.

## False Color AVHRR SST

- North Atlantic June 1984.
- Blue and purple represent cold water while green represents warmer water. Just as Canada is cooler than Florida, water off Canada is cooler than water off of Florida.
- Note the current that brings warm water from the south up to the north. This current is called the                                                                                                             		Gulf Stream

## AVHRR SST

- Biggest problem with using the infrared
- bands to measure SST is clouds.  Sensor
- cannot “see” through clouds

## Why Ocean Color?

- Light reflects off particulate matter in the water, and absorption is primarily due to photosynthetic pigments (chlorophyll) present in phytoplankton.
- The net result of these interactions is light radiating from the ocean surface (water-leaving radiance).
- The measured radiance may be quantitatively related to various constituents in the water column that interact with visible light, such as chlorophyll.
- Chlorophyll is used to calculate the carbon produced by photosynthesis -- primary productivity.

## Ocean Color Viewed from Space

- What color is the ocean?
- The ocean reflects the color of the sky, but even on cloudless days the color of the ocean is not a consistent blue. Phytoplankton may alter the color of the water.
- Taken by astronauts aboard the space shuttle looking down at the coast of Angola.

## Sea-viewing Wide Field-of-view Sensor (SeaWiFS) 

- SeaWiFS is a spectroradiometer (measures radiance in specific bands of the visible light spectrum).
- The advantage of a space-based spectroradiometer is the global coverage.
- The disadvantage is that interfering optical effects, primarily light scattering in the atmosphere.

## SeaWiFS -- Clouds and Dust !!!!

## SeaWiFS Bands 

**    ****Wavelength (nm)****	****Primary Use**
-   1      412 (violet)	Dissolved organic matter
-   2	 443 (blue)		Chlorophyll absorption
-   3	 490 (blue-green)	Pigment absorption(Case 2), K(490)
-   4	 510 (blue-green)	Chlorophyll absorption
-   5	 555 (green)	Pigments,optical prop, 	sediment
-   6	 670 (red)		Atmospheric corr (CZCS heritage)
-   7      765 (near IR)	Atmospheric corr, aerosol radiance
-   8	 865 (near IR)	Atmospheric corr, aerosol radiance

## SeaWiFS : Producing a "True Color" Image

## The intensities of light are collected in 8 bands            in the visible and near infrared.  
 When these data are displayed visually, the result is a series “gray-scale" images where different features have different intensities in each bands (in the blue and purple bands,clouds and water are bright - land is dark. In the red and infrared bands, land is bright - water is dark.) 
3 bands closest to red, green, and blue (RGB) are displayed in a monochromatic scale corresponding to its appropriate color and mixed on a computer screen to produce an image that is fairly close to what         the human eye and brain would perceive. 

-  When these data are displayed visually, the result is a series “gray-scale" images where different features have different intensities in each bands *(in the blue and purple bands,clouds and water are bright - land is dark. In the red and infrared bands, land is bright - water is dark.) *
- 3 bands closest to red, green, and blue (RGB) are displayed in a monochromatic scale corresponding to its appropriate color and mixed on a computer screen to produce an image that is fairly close to what         the human eye and brain would perceive.

## Ocean Color Viewed from Space

- Ocean and land plants are green because of the chlorophyll in plant cells.
- Chlorophyll a absorbs mainly blue-violet and red and reflects green; chlorophyll b absorbs mainly blue and orange and reflects yellow-green.
- Satellite instruments measure the amount of reflected light of different wavelengths.
- These amounts allow scientists to estimate the productivity of Earth's     land masses and oceans.

## SeaWiFS Imagery

- 	El Nino			 Non-El Nino
- 		  Galapagos Islands

## SeaWiFS

-       “True Color”		“False Color” Chlorophyll
-                         Charleston Bump

## Ocean Color:

- Exciting and extremely complex picture of the coastal zone of Oman, October 4,2002 from Ocean Color
- Remote sensing portrays the full complexity of the      ocean environment.

## Active Sensors

- TOPEX/POSEIDON Altimeter
- SeaWinds Scatterometer

## Why Satellite Altimetry? 

- Movement of water in the sea on spatial scales exceeding ~30 km and temporal scales exceeding ~1 day are manifested by changes in mean sea level associated with the strength and direction flow.
- This change in topography varies from the 1 m increase in mean sea level in 100 km over the Gulf Stream to a 10 cm change over 1000 km over an El Nino event in the tropical Pacific.
- Given the strength of the sea surface topography signature, one can infer the magnitude and  direction of the oceanic water movement.

## Satellite Radar Altimetry 

- Satellite radar altimeters work on the principle of active remote sensing.
- Transmit very 'sharp' electromagnetic pulses of a predetermined wavelength and frequency (a typical radar altimeter operates at 13.5 GHz)
- The satellite's altitude (i.e. absolute height above the surface) is obtained by measuring the time required for the pulse to travel from the altimeter antenna to the earth's surface         and back to the satellite's receiver.

## Dynamic Sea Surface Topography - (from Altitude)

- A measure of sea level relative to earth's geoid
- from which oceanographers can calculate the
- speed and direction of winds.
- First, the height of the satellite above the reference ellipsoid using orbit dynamic equations.
- Second, the height of the satellite above the closest ocean surface is measured with a microwave radar altimeter. The difference between the height above the reference ellipsoid and the altitude above the ocean surface is approximately equal to the geoid height.

## How TOPEX/POSEIDON works?

## Ocean Surface Topography

- 	To find the surface caused by ocean circulation, we mathematically remove the sea height caused by gravity (the geoid) and create maps of ocean topography. We then calculate the speed and direction of ocean currents similar to how meteorologists use atmospheric pressure maps to calculate winds.

## Dynamic Sea Surface Topography 

- TOPEX/Poseidon monitored large-scale features like _[Rossby](http://www.ae.utexas.edu/courses/ase389/sensors/alt/alt3.html)_ and _[Kelvin waves,](http://www.ae.utexas.edu/courses/ase389/sensors/alt/alt3.html)_ tracked the great _[El Niño](http://www.ae.utexas.edu/courses/ase389/sensors/alt/alt3.html)_ of 1997-1998 and the subsequent _[La Niña,](http://www.ae.utexas.edu/courses/ase389/sensors/alt/alt3.html)_ and has begun to explore possible long-term changes such as the Pacific Decadal Oscillation, of which El Niño and La Niña are but a part.

## Dynamic Sea Surface Topography

- TOPEX/Poseidon monitored  the great _[El Niño](http://www.ae.utexas.edu/courses/ase389/sensors/alt/alt3.html)_ of 1997-1998 and the subsequent _[La Niña,](http://www.ae.utexas.edu/courses/ase389/sensors/alt/alt3.html)_ and has begun to explore possible long-term changes such as the Pacific Decadal Oscillation, of which El Niño and La Niña are but a part.

## SST vs Sea Surface TopographyEl Nino/La Nina

## Watching for the Next El Niño 

## El Niño Continues to Grow: Pacific Ocean Shows Higher Than Normal Sea Surface Heights 

-     December 2, 2002, shows the Pacific dominated by two significant areas of higher-than-normal sea level (warmer ocean temperatures).

## What is a Scatterometer?

- A scatterometer is a microwave radar sensor used to measure the reflection or scattering effect produced while scanning the surface of the earth from an aircraft or a satellite.

## SeaWinds Scatterometer

- Wind speed
- Wind direction
- Spatial Resolution
-  Location Accuracy
- Coverage
- Mission Duration
- Transmit Frequency
-  Transmit Power

- 2m/s :3-20m/s-10%:20 30m/s
- 20 Deg‹(RMS)3-30m/s
- 50km
- 15km(RMS)
- 90% of oceans in 2days
- 3year(5year goal)
- 13.402GHz
- 120W

## Why Remote Sensing of Ocean Winds?

- As the largest source of momentum for the ocean surface, winds affect the full range of ocean movement - from individual surface waves to current systems.
- Winds over the ocean modulate air-sea exchanges of heat, moisture, gases, and particulates. This modulation regulates the interaction between the atmosphere and the ocean, which establishes and maintains        both global and regional climates.

## SeaWinds Science Objectives

- Acquire all-weather, high-resolution measurements of near-surface winds over global oceans.
- Determine atmospheric forcing, ocean response, and air-sea interaction mechanisms on various spatial and temporal scales.
- Combine wind data with measurements from scientific instruments in other disciplines to help us better understand the mechanisms of global climate change and weather patterns.

## SeaWinds Operational Objectives

- Improve weather forecasts near coastlines by using wind data in numerical weather- and wave-prediction models.
- Improve storm warning and monitoring.

## SeaWinds Scatterometer 

- NASA's SeaWinds Scatterometer provides high accuracy wind speed and direction measurements over at least 90% of the ice-free global oceans every 2 days.
- SeaWinds is providing a continuing set of long term wind data for studies of ocean circulation, climate, air-sea interaction and weather forecasting.
- SeaWinds will use a one meter diameter dish antenna with two beams rotated about the satellite nadir axis at 18 RPM. SeaWinds radiates and receives microwave pulses at a frequency of 13.4 GHz across on 1800km wide swath.

## SeaWinds Scatterometer

- Scatterometers use a highly indirect technique to measure wind velocity over the ocean. Changes in the wind velocity cause changes in ocean surface roughness, modifying the radar cross section of the ocean and the magnitude of the backscattered power. Multiple, collocated, measurements acquired from several directions can be used to solve wind speed and direction simultaneously.

## SeaWinds Animation

## Six (6) years of the biosphere from SeaWiFS!!!!!!!

## Then End