scanners, image resolution, orbit in remote sensing, pk mani

Dr. P. K. ManiBidhan Chandra Krishi Viswavidyalaya

E-mail: pabitramani@gmail.com Website: www.bckv.edu.in

Satellite Orbits

Satellite Orbits Satellites generally

have either polar orbits or geostationary orbits.

Meteosat

A program sponsored by 17 European weather services. It started with the successful launch of METEOSAT 1 in 1977 and has continued unabated. METEOSAT 7, the last in the current series was launched in early September 1997.

They are stationed above the equator at 0° longitude above the Gulf of Guinea and image this part of the globe every 30 minutes in three wavebands. These are: the visible (0.4 to 1.1 micrometers), the water vapour (5.7 to 7.1 micrometers) and the thermal infrared (10.5 to 12.5 micrometers).

The first has a resolution of 2.5 km The first has a resolution of 2.5 km at the sub-satellite point (SSP) while the two infrared wavebands both have one of 5 km.

NOAA (National Oceanic & Atmospheric Administration) Polar Orbiters

The programme that started with the successful launch of TIROS-1 in 1960 continues today with the NOAA polar orbiters. The satellites' AVHRR instruments image the planet in five bands:

8 to 0.68 micrometres 0.725 to 1.10 micrometres 3.55 to 3.93 micrometres 10.30 to 11.30 micrometres 11.50 to 12.50 micrometres

Geostationary Operational Environmental Satellites (GOES)

This NOAA series of geostationary weather satellites began in 1974 and continues today with the launch of a new ‘GOES-NEXT series of more advanced satellites in 1994.

The resolution varies from 4 to 8 km. They have five wavebands:

0.55 to 0.75micrometres 3.80 to 4.00 ” 6.50 to 7.00 “ 10.20 to 11.20 ” 11.50 to 12.50 "

The Space Shuttle

The space shuttle is a manned spacecraft mission, carrying astronauts and scientists as well as instruments.

The most important sensors that have been carried on the Shuttle for Earth imaging are:

The Shuttle Imaging Radar The Metric Camera. The Large Format Camera (LFC) The Modular Optical-Electronic

Multispectral Scanner (MOMS).

LANDSAT Satellite Series First LANDSAT satellite

launched in 1972 Lasted until 1978

Six Landsat satellites to date LANDSAT-4 and -5 still

operational LANDSAT -6

experienced launch failure

Landsat 4 (launched 16.07.82) Landsat 5 (launched 01.03.84) Orbit

near polar sun-synchronous complete orbit every 99 mins

Altitude - 705 km, 438 miles Re-visit - 16 days

Technical Information

Landsat 1, 2 and 3

• Landsat was the first satellite to be designed specifically for observing the Earth’s surface.

• Landsat 1, 2 and 3 carried a multispectral scanner (MSS) system which records reflected energy from the Earth’s surface or atmosphere across four wavebands; three visible channels and one near infrared.

• The MSS has a pixel resolution of 80 metres.

Landsats 4 & 5 - Thematic Mapper• LANDSATs 4 and 5 have a

more refined multispectral scanner, the Thematic Mapper (TM), instrument on board.

• TM is a multispectral sensor since it detects energy across seven wavelength bands.

• The pixel resolution of six of the wavelength bands is 30 metres but the thermal infrared band has a pixel resolution of 120 metres.

On this Landsat TM true colour image of west and central London

Landsat 7

• After a successful launch on April 15 1999, Landsat 7 is up and running. Visit http://www.eurimage.com/ to see "Quick Looks" of some of the first acquired images..

• LANDSAT 7 carries a TM sensor developed from that on Landsat 5. Key differences are: (1) extra 15m panchromatic band co-registered with the multi-spectral, and (2) band 6 resolution at 60m.

The fires of Dili, east Timor

The Turkish earthquake

1. 0.45 - 0.52 mm blue R 2. 0.52 - 0.60 mm green R 3. 0.63 - 0.69 mm red R 4. 0.76 - 0.90 mm near IR R 5. 1.55 - 1.75 mm mid IR R 6. 10.4 - 12.5 mm thermal IR E 7. 2.08 - 2.35 mm mid IR R and E

B G R Near IR Mid IR Thermal IR

1 2 3 4 5 7 6

LANDSAT’s Thematic Mapper Sensor

Collect 7 different types of light

SPOT (Systeme Probatoire de l’Observation de la Terre)

•SPOT satellites are placed in near-polar, sun-synchronous orbits at an altitude of 832 km. •The satellites have a pushbroom scanner system which can view the surface immediately below them (nadir view), or can be directed so that they view the surface to the side. This is important because it means that two views of the same area can be collected within a short time of each other, by two adjacent over-passes. Because the system can take two images of the same area with different look angles, it is possible to create stereoscopic (3-D) images. This is similar to having two eyes, enabling us to view in three dimensions because each eye views the same scene from a slightly different position, the brain then creates a 3-D picture.

A computer can create a Digital Elevation Model (DEM) from two stereoscopic images, which means the shape of the land can be measured.

Images with two different spatial resolutions can be produced from SPOT data. The first is a panchromatic (black and white) image with a spatial resolution of 10 m. The second is an image from the multi-spectral sensor (SPOT XS) which collects 3 bands of data, with a resolution of 20 m.

SPOT

Active Sensors: ERS-1 and ERS-2

• In July 1991, the first European Remote Sensing Satellite (ERS-1) was launched by the European Space Agency (ESA). It had a sun-synchronous circular orbit (near polar) at 770 km. Its purpose was to collect information on areas of the world that are difficult to observe from the surface, such as oceans and ice-covered areas. It also produces images of the land surface in all weather conditions, 24 hours a day.

• ERS-1 was so successful that the ERS-2 was launched in April 1995, carrying additional equipment called Global Ozone Monitoring Experiment (GOME).

Active Sensors: Radarsat

• RADARSAT-1 spacecraft was launched in November 1995, by the Canadian Space Agency. The satellite carries a Synthetic Aperture Radar system providing all-weather, day and night observations of land and sea surfaces.The satellite’s SAR system has been designed to make it useful for a range of applications including:

• crop investigations • coastal zone mapping • ship detection • hydrological applications • geological applications • ice monitoring • oil spill detection • ocean applications

– e.g. mapping the ocean depths/shape of the sea floor)

E. Transmission, Reception, and Processing

All remote sensing systems have some method of transmitting, receiving, and processing the data. Some satellites actually drop film canisters to Earth using parachutes. Most remote sensing is now done digitally, and the data is transmitted using radio waves.

Remote Sensor Types:

Sensor types

passive

active

non-scaning

scanning

non-scaning

scanning

non-imaging

imaging

Microwave radiometerMagnatic ensorGravimeterFourier spectrum

CameraMonochromIR

imaging image plane scanning

object plane scanning

TV camera

Solid scannerOptical mechanical scan.

Microwave radiometer

Microwave radiometerMicrowave altimeter

non-imaging

imagingimage plane scanning

object plane scanning

Passive phased array radarReal aperture radar

Synthetic aperture radar

Satellite sensor characteristics

The basic functions of most satellite sensors is to collect information about the reflected radiation along a pathway, also known as the field of view (FOV), as the satellite orbits the Earth.

The smallest area of ground that is sampled is called the instantaneous field of view (IFOV). The IFOV is also described as the pixel size of the sensor. This sampling or measurement occurs in one or many spectral bands of the EM spectrum. The data collected by each satellite sensor can be described in terms of spatial, spectral and temporal resolution.

Multispectral Scanning

• Many electronic remote sensors acquire data using scanning systems, which employ a sensor with a narrow field of view (i.e. IFOV) that sweeps over the terrain to build up and produce a two-dimensional image of the surface.

• Scanning systems can be used on both aircraft and satellite platforms and have essentially the same operating principles. A scanning system used to collect data over a variety of different wavelength ranges is called a multispectral scanner (MSS)

• There are two main modes or methods of scanning employed to acquire multispectral image data - across-track scanning, and along-track scanning

Across-track scanners

Across-track scanners• Across-track scanners scan

the Earth in a series of lines. The lines are oriented perpendicular to the direction of motion of the sensor platform. Each line is scanned from one side of the sensor to the other, using a rotating mirror (A). As the platform moves forward over the Earth, successive scans build up a two-dimensional image of the Earth´s surface.

A bank of internal detectors (B), each sensitive to a specific range of wavelengths, detects and measures the energy for each spectral band and then, as an electrical signal, they are converted to digital data and recorded for subsequent computer processing

Across-track scanners

The IFOV (C) of the sensor and the altitude of the platform determine the ground resolution cell viewed (D), and thus the spatial resolution. The angular field of view (E) is the sweep of the mirror, measured in degrees, used to record a scan line, and determines the width of the imaged swath (F).

Across-track scanners

Along-track scanners

The Along Track Scanner has a linear array of detectors oriented normal to flight path. The IFOV of each detector sweeps a path parallel with the flight direction. This type of scanning is also referred to as pushbroom scanning (from the mental image of cleaning a floor with a wide broom through successive forward sweeps).

The scanner does not have a mirror looking off at varying angles. Instead there is a line of small sensitive detectors stacked side by side, each having some tiny dimension on its plate surface; these may number several thousand. Each detector is a charge-coupled device (CCD). In this mode, the pixels that will eventually make up the image correspond to these individual detectors in the line array.

Field of View (FOV), Instantaneous Field of View (IFOV)Dwell time is the time required for the detector IFOV to sweep across a ground cell. The longer dwell time allows more energy to impinge on the detector, which creates a stronger signal.

Sabin, 1997

Wiskbroom Pushbroom

Remote Sensing Scanning System

Advantages of Along-track scanners

• measure the energy from each ground resolution cell for a longer period of time

• more energy to be detected and improves the radiometric resolution

• smaller IFOVs and narrower bandwidths for each detector

• cross-calibrating thousands of detectors to achieve uniform sensitivity across the array is necessary and complicat

Spatial Resolution• The size of the area on the

ground that a sensor "sees" at any point in time– or more accurately, every time a

signal is captured

• Determined by size of IFOV– function of look or cone angle

() and height of platform (H)

  IFOV = H

Ability to distinguish small adjacent objects in an image

The minimum distance between two objects that a sensor can record distinctly

Spatial Resolution

• The higher the spatial resolution:– the smaller the ground resolution cell– the higher the resolving power of the system– The greater the spatial detail attainable

• Note: to discriminate a feature from its surroundings:

– It must be at least as large as the ground resolution cell

– It should be 2x larger to ensure that it is detected– Or, may be smaller if it is spectrally unique/overwhelming

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Spatial resolutionformal definiton: a measure of smallest

angular or linear separation between two objects that can be resolved by sensor

Determined in large part by Instantaneous Field of View (IFOV)IFOV is angular cone of visibility of the

sensor (A) determines area seen from a given altitude at

a given time (B) Area viewed is IFOV * altitude (C)Known as ground resolution cell (GRC) or

element (GRE)

Spatial resolution

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Problem with this concept is:Unless height is known IFOV will

changee.g. Aircraft, balloon, ground-based sensorsso may need to specify (and measure) flying

height to determine resolutionGenerally ok for spaceborne

instruments, typically in stable orbits (known h)

Known IFOV and GRE

Spatial resolution

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Spatial resolution

IFOV and ground resolution element (GRE)

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GRE = IFOV x Hwhere IFOV is measured in radians

H

IFOV

GRE

Total field of view

42

Image width = 2 x tan(TFOV/2) x Hwhere TFOV is measured in degrees

H

GIFOV 2H tanIFOV

2

1 m resolution 2 m resolution 5 m resolution

10 m resolution 20 m resolution 30 m resolution

Image 20: Camp Randall Stadium, University of Wisconsin, viewed at different resolutions (Image: University of Wisconsin, Institute for Environmental Studies)

Spatial resolution is a measure of the smallest object that can be resolved by the sensor, or the linear dimension on the ground represented by each pixel or grid cell in the image

The resolutions of today's satellite systems vary from a few centimetres (for example military usage) to kilometres. Application:

• Low resolution: larger than 30 m• Medium resolution: 2 - 30 m• High resolution: under 2 m

•A lower resolution usually coincides with a higher repetition rate, meaning that within a short interval (METEOSAT-8 , every 15 min) the satellite reflects the same area. A "coarse" resolution is used to record large or global areas for climate-related inquiries, for example the radiation budget of the earth and for weather monitoring. Additional applications include earth observation of land use and oceans, the ocean's ice cover and the surface temp.

For measurements based on the distinguishability between two point targets, Rayleigh criterion is used. According to Rayleigh, the two objects are just resolvable if the maximum (centre) of the diffraction pattern of one falls on the first minimum of the diffraction pattern of the other. Consequently the smallest angle resolvable is

θm = 1.22 (λ/D) radians

Single slit Circular aperture

The Rayleigh CriterionThe Rayleigh criterion is the generally accepted criterion for the minimum resolvable detail - the imaging process is said to be diffraction-limited when the first diffraction minimum of the image of one source point coincides with the maximum of another.

•Satellites with medium resolution such as LANDSAT 7 are used for the global observation of land surfaces. Tropical rainforests and their deforestation have been observed by the LANDSAT satellites for more than 30 years.

•High resolution data is mainly used for smaller areas of the earth's surface. Only recently such data has become available commercially and privately. Satellites such as IKONOS or QuickBird send data for topographic and thematic mapping of for example the land use, vegetation, or as planning resources for cities, large projects etc. Information can also be "ordered" in advance, because the turning of the satellite sensors can reduce the repeat rates and can monitor the desired areas earlier.

Spatial Resolution = 30 m Spatial Resolution = 15 m

What’s the difference in the scale of the two images?

Which image has better spatial resolution?

How does the spatial resolution and scale of this image compare?

Spectral Sensitivity

What is spectral resolution, and when is it needed?

Spectral resolution is the ability to resolve spectral features and bands into their separate components. 

The spectral resolution required by the analyst or researcher depends upon the application involved. 

Spectral Resolution• The accuracy with which slight variations in the wavelength

can be recorded• How narrow a portion of the EMS a sensors “sees”

– Determined by the width of each individual band

Which has better spectral resolution?

Spectral resolution describes the ability of a sensor to define fine wavelength intervals.

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?

Spectral Resolution

0.4 mm 0.7 mm

Black & White Images

Blue + Green + Red

Spectral Resolution (Con’t)

• 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

IoE 184 - The Basics of Satellite Oceanography. 1. Satellites and Sensors

2. Sensors on satellites Spectral resolution

A multispectral sensor collects data at select wavebands or channels (e.g., AVHRR – 5; SeaWiFS – 8; MODIS – 36).

When a sensor provides a continuous spectral coverage, it is called hyperspectral. In practice, it means that its spectral resolution is better than 10 nm or 0.01 µm and the number of channels >100.

Hyperspectral Scanners

• Also called imaging spectrometers• Acquire imagery over many (>200) very narrow

(e.g. 5 – 10 μm) spectral bands• Enables discrimination based on slight variations in

spectral reflectence (signatures) otherwise obscured by systems withy courser spectral resolution

Radiometric Resolution• A sensors ability to detect and record slight variations in the

amount of EMR reaching the sensor

• Since total EMR received is directly proportional to spatial resolution, there is an inverse relationship between spatial and radiometric resolution

– low spatial resolution (large ground area) means more total energy received, so slight variations in EMR can be detected, this results in a high signal to noise ratio

– conversely, if spatial resolution is high (small ground area) less total energy is received, slight variations are more difficult to detect, so lower signal to noise ratio, poorer radiometric resolution

Radiometric Resolution

• Radiometric resolution is normally indicated by the number of quantization levels used to measure reflectance– Also called DN values for digital numbers– Determined by bit format of data

• 6 bits allows us to record values from 0-63, 64 quantization levels

• 7 bit data from 0-127, 128 DN values• 8 bit data from 0-255, 256 DN values• 11 bit data from 0 – 2074, 2048 DN values

Radiometric resolution is Number of possible brightness values in each band of data

2 bit data

4 quantization levels

8 bit data

256 quantization levels

Which image has better radiometric resolution?

Radiometric Resolution

The radiometric resolution specifies how well the differences in brightness in an image can be perceived; this is measured through the number of the grey value levels. The maximum number of values is defined by the number of bits (binary numbers). An 8-bit representation has 256 grey values, a 16 bits (ERS Satellites) representation 65.536 grey values. 

Resolutions of different satellite systems:

•LANDSAT-MSS (from LANDSAT 1-3): 6 bits (64 grey values)•IRS-LISS I-III: 7 bits (128 grey values)•LANDSAT-TM (from LANDSAT 4-5) & SPOT-HRV: 8 bits (256 grey values)•LANDSAT-ETM & ETM+ (from LANDSAT 6-7): 9 bits (only 8 bits will be transmitted)•IRS-LISS IV: 10 bits (only 7 bits will be transferred)•IKONOS & QuickBird: 11 bits

Radiometric Resolution

• Number of Shades or brightness levels at a given wavelength

• Smallest change in intensity level that can be detected by the sensing system

It is related to the time interval between two successive visits of a particular scene by the remote sensing satellite

Temporal Resolution• The length of time between repeat

coverage– How long between consecutive images– Determined by:

• Orbital period of satellite• Refresh period of airborne imagery• Pointable optics capture images on adjacent orbital paths

• Important when multitemporal images are desired– Landcover monitoring– Progression of an event

• Forest fire• Disaster recovery

–Acceptable temporal resolution depends on duration of event

Sensor Performance and Resolution

• Temporal resolution

– Refers to the frequency with which images of a given geographic location can be acquired.

– The temporal resolution is determined by the orbital characteristics and swath width, the width of the imaged area

– Swath width is given by:

2htan(FOV/2) where h is the spatial frequency, and FOV is the angular field of view of the sensor