Introduction to Polar Orbiting Systems

We have now reviewed the primary and derived data products from two instruments flown on the GOES satellite platforms, the imager and the sounder.  We will continue to review the imagery from GOES, but now we will turn our attention to remote sensing equipment, and imaging products, from Polar Orbiting Satellites.  

Recall our discussion of satellite orbits, which illustrated that geostationary systems are placed in very high orbits of 35,800km in order to have the same angular velocity as that of the earth.  By contrast, Polar Orbiting satellites are launched into nearly circular sun-synchronous, low-earth orbits, with altitudes that range from 700 to 800km.  Near circular orbits at these elevations have orbital periods ranging from 98-102 minutes.  There are several satellites of this type which we will study, including the operational meteorological sensors of the NOAA Polar Operational Environmental Satellites (POES), and the Defense Meteorological Satellite Program (DMSP).  These platforms produce successive images which overlay eachother from one orbit to the next, which results in global coverage on a daily basis.  

POES Orbits

from: Tutorial on satellite coverages and orbits, prepared by David Johnson of the National Center for Atmospheric Research Mesoscale and Microscale Meteorology (MMM) Division.  

There are also polar orbiting platforms which are designed to produce high resolution imaging of earths geophysical characteristics at the expense of daily global coverage.  These include the US    Landsat  Satellite and the French equivalent known as SPOT.   In an effort to introduce you to some non-meteorological remote sensing applications, we will begin our analysis of polar orbiting platforms by reviewing the imaging capability of  the Landsat Thematic Mapper.  As we proceed with this new system keep in mind the material we have already reviewed on radiative transfer, multi-spectral imaging, and image enhancement.  The primary differences here are in the resolution and application of the reflectivity data from these sensors.  

Landsat: The Thematic Mapper (adapted from the NASA Remote Sensing Tuturial)

Landsats platforms are in  near-polar orbits (inclined ~ 9 degrees to west of longitudinal lines; passing within 8 degrees of the poles) and are sun-synchronous.  Landsats 1-3 make 14 full orbits (each successive one displaced 2875 km [ 1785 miles] to the west) each day (3 over U.S.) and after 252 orbits repeat their previous ground tracks every 18 days; Landsats 4-5, from lower altitude (705 km [437 miles]), after 233 orbits cover the same area again every 16 days. To fully image the entire Earth's land surface (except for polar regions), ~11000 scenes are required.

Two instruments have flow on Landsat platforms, the multispectral imaging senson MSS and a newer version with more channels, called the Thematic Mapper.

• MSS--Multispectral Scanner, a nonphotographic imaging system which utilizes an oscillating mirror and fiber optic sensor array. The mirror sweeps from side to side, transmitting incoming energy to a detector array which sequentially outputs brightness values (signal strengths) for successive pixels, one swath at a time. The forward motion of the sensor platform carries the instrument to a position along its path where an adjacent swath can be imaged. The MSS simultaneously senses radiation using an array of six detectors in each of four spectral bands from 0.5 to 1.1 micrometers.
• TM--Thematic Mapper, a nonphotographic imaging system which utilizes an oscillating mirror and seven arrays of detectors which sense electromagnetic radiation in seven different bands. The thematic mapper sensor is a derivative of the multispectral scanner (MSS) generation of scanners, achieving greater ground resolution, spectral separation, geometric fidelity, and radiometric accuracy.

 For each band detector, the electronic signal from this IFOV results in a single digital value (called its DN or digital number, which for the MSS can range from 0 - 255 [2⁸]). (IFOV is the solid angle through which a detector is sensitive to radiation. In a scanning system this refers to the solid angle subtended by the detector when the scanning motion is stopped. The IFOV is commonly expressed in milliradians.) The DN value is related to the proportionally averaged reflectances from all materials within each IFOV and, since the mix of objects on the ground will constantly change, will vary in DN magnitude from one to the next IFOV. Each IFOV is represented in any b & w image of which it is a part as a tiny point of uniform gray-level tone known as a pixel (a contraction of "picture element") whose brightness is determined by its actual DN value. In a Landsat MSS band image, owing to a sampling rate effect in which there is some overlap between successive 9 microsecond intervals, a pixel has an effective ground-equivalent dimension of 79 by 57 m (259 x 187 ft) but contains the reflectances of the full 79 m² actually viewed .

The continuous stream of pixel values can be used to drive an electronic device that generates a uninterrupted light beam of varying intensity which sweeps systematically over film to produce a b & w photo image in which tone variations are proportional to the DNs in the array. Or, the pixels generated from these sampling intervals can be displayed as an image of each band by feeding their DN values sequentially into an electronic signal array. That is then projected line by line on to a TV monitor (in which the resulting image is an assemblage of light-sensitive spots [also called pixels] of varying brightnesses).

A more sophistical multispectral imaging sensor, named the Thematic Mapper (TM) was added to Landsats 4 (1982), 5 (1984), and 6 (the latter failed to attain orbit during launch and thus has never returned data). These flew on a redesigned, more advanced platform. Although similar in operational modes to the MSS (which flew on Landsats 1-3 and was also part of the 4 and 5 payload, to maintain continuity), the TM consists of 7 bands that have these characteristics:

The six reflectance bands obtain their effective resolution at a nominal orbital altitude of 705 km (437 miles) through an IFOV of 0.043 mrad; the IFOV for the thermal channel is 0.172 mrad.  These sensors, like the broad-band visible channel on the GOES sounder, measure surface reflectance, but in much narrower spectral bands.  

Band 1 is superior to MSS 4 in detecting some features in water; it also allows quasi- natural color composites to be put together. Band 5 is sensitive to variations in water content, both in leafy vegetation and as soil moisture; it also distinguishes between clouds (appearing dark) and bright snow (light). This band also responds to variations in ferric iron (Fe₂ O₃) content in rocks and soils, with materials containing this substance showing higher reflectances as its percentage increases. Band 7 likewise reacts to moisture contents and is especially suited to detecting hydrous minerals (such as clays or certain alteration products) in geologic settings. Band 6 can distinguish a radiant temperature difference of ~ 0.6 degrees C and is helpful in discriminating rock types whose thermal properties permit varying extents of heating and consequent differences in near surface temperatures; it often can pick out changes in ground temperatures due to moisture variation and can single out vegetation due to its evaporative cooling effect. The higher resolution achieved in the reflective bands is a significant aid in picking out features and classes whose minimum dimension is usually on the order of 30 m (100 ft) . Thus, houses and smaller buildings, which were unresolvable in MSS images, can often be discerned.

THE NEW LANDSAT 7 Satellite is scheduled for launch in two weeks:  April 15, 1999.

Background Material from USGS Earthshots

The following material is mirrored here from the USGS website called Earthshots: Satellite Images of Environmental Change.  This provides an electronic tutorial on Landsat images and their use in illustrating environmental changes through viewing changes in time of the same remotely sensed scene.  This website includes a very useful introduction to surface based remote sensing, how multispectral images are derived from the MSS and TM instruments on Landsat, and how the images are displayed and interpretted.  Before moving on to a very specific image interpretation example, we will review the following material:

Articles available from the USGS Earthshots website:

• History of remote sensing
• Landsat
• EMR represented by RGB
• Beyond looking at pictures
  

Link to very cool  Landsat image of Charlottesville  prepared by the Global Environmental Change Program, a research group within the Department of Environmental Sciences program here at UVA.

On-line Image Processing Tutorial

Link to Thematic Mapper Tutorial of Morro Bay, California (from NASA Remote Sensing Tutorial)

Best MSS Bands for Identifying Surface Features
Item	Category	Best Bands	Salient Characteristics
a.	Clear Water	7	Black tone in black and white and color.
b.	Silty Water	4,7	Dark in 7; bluish in color.
c.	Nonforested Coastal Wetlands	7	Dark gray tone between black water and light gray land; blocky pinks, reds, blues, blacks.
d.	Deciduous Forests	5,7	Very dark tone in 5, light in 7; dark red.
e.	Coniferous Forest	5,7	Mottled medium to dark gray in 7, very dark in 5; brownish-red and subdued tone in color,
f.	Defoliated Forest	5,7	Lighter tone in 5, darker in 7 and grayish to brownish-red in color, relative to normal vegetation.
g.	Mixed Forest	4,7	Combination of blotchy gray tones; mottled pinks, reds, and brownish-red.
h.	Grasslands (in growth)	5,7	Light tone in black and white; pinkish-red.
i.	Croplands and Pasture	5,7	Medium gray in 5, light in 7, pinkish to moderate red in color depending on growth stage.
j.	Moist Ground	7	Irregular darker gray tones (broad);darker colors.
k.	Soils-bare Rock-Fallow Fields	4,5,7	Depends on surface composition and extent of vegetative cover. If barren or exposed, may be brighter in 4 and 5 than in 7, Red soils and red rock in shades of yellow; gray soil and rock dark bluish; rock outcrops associated with large land forms and structure.
1.	Faults and Fractures	5,7	Linear (straight to curved), often discontinuous; interrupts topography; sometimes vegetated.
m.	Sand and Beaches	4,5	Bright in all bands; white, bluish, to light buff.
n.	Stripped Land-Pits and Quarries	4,5	Similar to beaches ­ usually not near large water bodies; often mottled, depending on reclamation.
o.	Urban Areas: Commercial Industrial	5,7	Usually light toned in 5, dark in 7, mottled bluish-gray with whitish and reddish specks.
p.	Urban Areas: Residential	5,7	Mottled gray, with street patterns visible; pinkish to reddish.
q.	Transportation	5,7	Linear patterns, dirt and concrete roads light, in 5; asphalt dark in 7.

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