Remote Sensing

Remote Sensing Electro-optical Sensors Vicki Drake

• DEFINITIONS FOR COMMON REMOTE SENSING TERMS

• Band (channel): A band is a section/grouping of wavelengths from the electromagnetic spectrum. – Landsat ETM+ has eight bands which collect radiation from different parts of the electro-magnetic spectrum • three bands are sensitive to visible light • one band is panchromatic (b/w film sensitive to all visible light wavelengths) • three bands are sensitive to near-infrared • one band is sensitive to thermal infrared.

• Geostationary orbit: An orbit in which a satellite is always in the same position with respect to the rotating Earth. – The satellite travels around the Earth in the same direction, at an altitude of approximately 22,000 miles (35,800 kilometers). – • Sun-synchronous orbit: An orbit in which a satellite is always in the same position with respect to the rotating Earth at the same time of day. – The satellite travels around the Earth in the same direction, at an altitude of approximately 438 miles (705 kilometers). – Landsat-7 is sun-synchronous, always passing overhead at approximately 10:00 am local time.

Multispectral image: A remote sensing image created using data collected from more than one band in the electro-magnetic spectrum.

• Nadir: the point on the Earth directly below an orbiting satellite.

• Orbital period: The time it takes a satellite to complete one revolution (orbit) around the Earth. – The orbital period of Landsat 7 is about 1.5 hours.

• Panchromatic: Sensitive to all or most of the visible spectrum, between 0.4 and 0.7 micrometers. – Landsat 7 has a panchromatic band.

• Spatial Resolution: A measure of the amount of detail that can be seen in an image; the size of the smallest object recognizable using the detector.

1 • Spectral Resolution: The dimension and number of specific wavelength intervals in the EM spectrum to which a sensor is ‘sensitive’. • Temporal Resolution: How often a given sensor obtains imagery of a particular area (a time reference)

• Radiometric Resolution: The sensitivity of a sensor to differences in recorded radiant flux. Defines the number of just discriminable signal levels. • Scenes: Each Landsat image collected is called a scene. – Each Landsat scene is 115 x 106 miles long. – The globe is divided into 57,784 scenes, and each Landsat scene has about 3 billion bytes of data.

• Three levels of resolution relating to objects discernability – Detection – an object is detected if there is an indication it is different from surroundings – Recognition – an object is recognized if it can be categorized – Identification – Identification made if specific information about object can be determined (i.e., deciduous trees, shopping malls, etc) Remote Sensing • Sensors currently in use • Landsat Thematic Mapper • SPOT Multispectral and Panchromatic Scanner • India Remote Sensing Systems • Canada Radar Satellite • Advanced Very High Radiometric Resolution (AVHRR) • NASA Prototype Sensors – Lewis Hyperspectral Sensor – Clark High Resolution Sensor Electro-Optical Sensors • Electro-Optical sensors use nonfilm detectors to record radiation from a ground scene. • The major types of electro-optical sensors are – The Video Camera – The Vidicon Camera – The Across-Track Scanner – The Along Track Scanner Electro-Optical Sensors Advantages • Can operate in multiple bands of the EM spectrum, within and beyond extents of photographic spectrum • Image data can be transmitted over radio link – a telemetry feature required by satellite systems • In-flight display devices allow for images of ground scenes viewed in near real-time • Systems operating in the thermal IR (TIR) region have day-night capability • The detection process is renewable as detectors can be continuously used • Photographic film serves as both detector and storage medium

2 Digital Conversion • Electro-Optical Sensors are digital systems • Analog-to-digital conversion translates electronic signal to digital numbers (DN) on magnetic tape • DNs are mathematically processed by a computer to correct geometric and radiometric errors (and enhance patterns in original images) • DNs transformed into video image to create visible images – or – transformed into video signals for TV screens – or – transformed to visible light for recording on ordinary photographic film Video Cameras • Video cameras used for remote sensing collect reflected radiation in the 0.4 – 1.1 m range of EM spectrum • A multispectral video system uses an array of black/white video cameras equipped with visible-to- near IR sensitive tubes and filters. • Four-camera array: blue, green ,red and near IR spectral bands collected/isolate – Reflectance data recorded as individual black/white images and color IR composite images.

Vidicon Cameras • Similar to television cameras- covering about same spectral region as video cameras • A latent image is temporarily stored on photoconductive faceplate and scanned by internal electron beam. • Used successfully for imaging Moon and Mars • RBV (Return Beam Vidicon) cameras on first Landsat satellites. Landsat 3 RBV a successful high-resolution mapping device. Across-Track Scanners • Across-track or whiskbroom scanners, developed for military, originally • Two primary types available today: – Multispectral scanner – Thermal IR scanner • Across-track scanners use a rotating or oscillating mirror to scan contiguous series of narrow ground strips at right angles to the flight path – Forward motion of platform means new ground strips covered with successive scan lines

Across-Track Scanners • Two-dimensional record of reflectance and/or emittance information built up along flight path

3 • The Instantaneous Field of View (IFOV) determines how much ground area the scanner “sees” at any given moment – ground area is called “ground resolution” – An object on the ground can only be resolved when its size is equal to or greater than the ground resolution cell size – An object on the ground can only be identified and classified if it is 2 times the ground cell size. Across-Track Scanners • A small IFOV provides spatial detail – but restricts the amount of radiation received by scanner • A large IFOV results in a longer time for the scanner’s mirror to sweep across the ground cell. – A longer dwell time means more radiation enters the detector to create a stronger signal – Large IFOVs are better at detecting small reflectance or emittance variations as the expense of spatial resolution Multispectral Scanners (Across-Track Scanners) • MSS operate simultaneously in the UV, visible, reflected IR and thermal IR regions of EM spectrum. • Spectral channels range from less than 5 to more than 10 – Scanners with >100 channels are hyperspectral • MSS used routinely in both aircraft and spacecraft – Satellite-mounted scanners collect multiple lines of ground data during each sweep – Each detector is responsive to one specific wavelength Thermal IR Scanners (Across-Track Scanners) • Thermal IR scanners operate the same as MSS, but are confined to Thermal IR atmospheric windows of 3 to 5 m and 8 to 14m • Thermal IR detectors are filtered to operate in the 10.5 to 12.5 m due to Ozone absorption in upper atmosphere • Narrow IR bands (in the 8 to 14 m region) have important geologic applications for detection of silicate rocks, carbonate rocks and altered rocks Along-Track Scanners • Along-track, pushbroom, scanner form images without a scanning mirror. • Use linear arrays of very small charged coupled devices (CCD) • A dedicated detector element for each across-track ground resolution cell and a linear array of detectors for each spectral band. A single array may contain as many as 10,000 individual detectors.

4 • Each array is located in the focal plane of the instrument’s imaging optics so that an entire ground strip in the across-track direction is focused onto the detector elements at the same time w/o mechanical scanning Along-Track Scanner Advantages • Improved spectral resolution due to longer dwell time on each ground resolution cell

• Greater reliability and longer operating life due to elimination of moving parts

• High geometric accuracy because of fixed geometry of detector arrays

• Light weight and lower power requirements

Electro-Optical Sensor – Earth Observation Satellites • Landsat Program – Landsat – “Land Satellite” – 1st international satellite program designed specifically for collecting a view of the Earth’s surface – Started by NASA and Department of Interior – images would be acquired on a worldwide basis with users from any country gaining access to collected data – First known as “The Earth Resources Technology Satellite Program (ERTS) until 1975 – Launched July 23, 1972 – operated until 1978 (long after expected one-year expiration date!) Landsat Program • Landsat 2 launched January 22, 1975 • Landsat 3, 4, and 5 launched in 1978, 1982 and 1984, respectively • January 1983 – operation of Landsat system transferred to NOAA (National Oceanic and Atmospheric Administration) • 1985 – Landsat system commercialized and Earth Observation Satellite Company (Space Imaging, now) assumed responsibility Landsat Systems Characteristics • Landsats 4 and 5 carry both MSS and TM sensors - however, routine collection of MSS data terminated in 1992 • Satellites orbit at an altitude of 705 km, with a 16-day cycle. • Satellites designed and operate to collect data over a 185 km swath. • MSS and TM sensors detect reflected radiation from Earth’s surface in visible and near IR wavelengths.

5 Acquisition of Data - Landsat • TM data is received directly from Landsat 5 to a network of 15 worldwide ground stations • Data is recorded on high density magnetic tapes and sent to Space Imaging’s Image Data Processing Center in Maryland • Earlier, Landsat 4 and 5 TM data was sent to TDRS (Tracking and Data Relay Satellites) then relayed to Space Imaging’s data processing facility. – Failure of K-band transmission on Landsat 4 and 5 stopped this transmission Sample Landsat TM image along Missouri River Landsat 1,2, and 3 Swathing Pattern

Landsat 4 and 5 Swathing Pattern

6 Current Landsat TM Ground Receiving Stations

7 Landsat Characteristics • The wavelength range for the TM sensors is from visible through mid-IR into the Thermal-IR section of EM spectrum • All five Landsats have been in Sun-synchronous orbits, providing global coverage between 81 degrees north latitude and 81 degrees south latitude Landsat TM image of San Francisco Bay

8 Domestic Satellite Relay Across-Track Scanner Mirror System

• Landsat 7 getting ready for Launch

9 Landsat Schematics

10 SPOT –(Systeme Pour l’Observation de la Terre) • Began in France in 1978 – a long-term commercial system • Carries two identical HRV pushbroom scanners (High Resolution Visible) operating in one of two modes: – Panchromatic – a single visible band (0.51 – 0.73m) with a spatial resolution of 10 m x 10 m – Multispectral – three images produced: one for each band – 0.50-0.59m (green), 0.61-0.68m (red) and 0.79-0.89m (near IR) with a spatial resolution of 20 m x 20 m SPOT Satellite Characteristics

11 • A sun-synchronous orbit at an altitude of 832 km • Capability to view same area from two widely separated locations enables full scene stereo images to be produced (Figure 3.19) • SPOT 2 launched January 21, 1990 with SPOT 1 and SPOT 2 orbiting concurrently in identical orbits 1800 apart • SPOT 4 launched mid 1990s with increased spectral capabilities

Sample Spectral Signatures for SPOT

GIS: Land Use and Land Cover • Land use is a description of man’s relationship with the land. • Land cover is the natural or man-made composition that cover the Earth’s surface at a certain location • Major emphasis of classification system research is to incorporate information derived from remotely sensed data • System devised by USGS represents a national classification scheme being used in operational mapping programs, and GIS Features of USGS System • The USGS system is hierarchical and incorporates features of several existing classification systems amenable to remotely sensed data • Criteria: – Identification of land use and land cover should an accuracy of 85% or greater – Different interpreters should be able to repeat someone else’s identification

12 – Categorization should permit vegetation and other land cover types to describe present activities USGS Classification System • Comparison should be possible with future land use data • Minimum areas and Image Resolution • The minimum area that can be classified as to land use and land cover depends on: – The scale and resolution of the original sensor image or data source – The scale of data compilation image interpretation – The final scale of the land use information or map USGS Land Use Classification • Analysis of area development means to be able to identify and map land use and land cover change over time – These changes are indicators of rural, urban and industrial growth in an area – GIS must handle both positional data and attribute data as databases may contain many variables including soil, rock types, land ownership, land use/land cover, population demographics, image data, etc. – Planning organizations need vast amounts of accurate and timely information on physical resources and related socioeconomic factors to help guide management and planning decisions A State-of-the-Art GIS capability • Accept data inputs in one or more formats • Store and maintain information with necessary spatial relationship • Manipulate data (search, retrieve, compute) • Develop models and ‘what if’ scenarios • Present data as output in various ways • Five technical elements of GIS: – Encoding (data structure – Raster or Vector) – Data Input – Data Management – Data Manipulation/Analysis – Output