Landsat, a remote sensing Earth Resources Technology satellite, has been acquiring repetitive coverage of the Earth since 1972 when Landsat 1 was launched. Since then, four others have been in operation. Landsat 1, 2, and 3 flew in a circular orbit 913 kilometers (570 miles) above the Earth's surface and circled the Earth every 103 minutes, or about 14 times a day. Landsat 4 and 5 fly about 705 kilometers (440 miles) above the Earth and circle every 98 minutes.
Landsat 4 and 5 are still operating. Landsat 6 was launched in 1993 but did not achieve orbit and was lost. Landsat 7 is in planning stages. For more information on Landsat 7 status, see the Landsat Program Home Page.
Operational Dates: Landsat 1: July 1972 - January 1978 Landsat 2: January 1975 - February 1982 Landsat 3: March 1978 - March 1983 Landsat 4: July 1982 - present Landsat 5: March 1984 - present Sensors: Multispectral Scanner (MSS): Landsats 1,2,3,4,5 Return Beam Vidicon (RBV): Landsats 1,2,3 Thematic Mapper (TM): Landsats 4,5
Multispectral Scanner
141k GIF - 42k JPEG © photo #95-1705
Landsat 1, 2 and 3 carried two remote sensor systems: a Return Beam Vidicon (RBV) or
television system and a Multispectral Scanner (MSS). Landsats 4 and
5 carried the MSS sensor and an advanced MSS called a Thematic Mapper which records in
seven spectral bands and obtains high resolution images (30m/pixel) of selected areas. The
MSS pictured here is on display in the National Air and Space Museum.
MSS: Band Wavelength (um) Resolution 4 0.5-0.6 (green) 80 m 5 0.6-0.7 (red) 80 m 6 0.7-0.8 (near IR) 80 m 7 0.8-1.1 (near IR) 80 m 8 10.4-12.6 (thermal) 240 m --Landsat 3 only
The LANDSAT satellite orbits the Earth and obtains continuous imagery (one image after another) of the ground below along its orbital path. Each image is also referred to as a "scene". Landsat circles the Earth in a nearly polar orbit (circling the Earth from pole to pole). As the Earth rotates below it, Landsat can cover almost the entire globe with successive strips of imagery. Landsat covers a ground track between 81° North and 81° South latitude.
A portion of the
Landsat 3 orbit path is shown in this map. Each red dot on the map shows an approximate
center point where a scene was acquired. This map is used to determine which image
contains an area of interest. The Path is the path of travel of the LANDSAT
satellite and the Row is a line (east-west) connecting successive paths as they
line up next to each other.
Scenes tend to overlap, but a particular feature or city will not necessarily be directly in the center point of a scene. Each "Landsat of the U.S.A." scene covers an area about 185 km on a side.
Satellite images are not like photographs on film. Satellite sensors pick up light or radiation reflected from the Earth's surface and record the data digitally.
Satellite sensors can be designed to see wavelengths of light (radiation) that human eyes cannot. Typically, a sensor will collect data in a number of different wavelengths or bands. For instance, the LANDSAT MSS sensor records in 4 or 5 different bands. Two bands are in the range of what the human eye can see, but the remaining 3 bands are in wavelengths beyond human vision (2 bands in the near infrared and one band in the thermal infrared range).
When the data from a LANDSAT sensor is sent to Earth via radio telemetry, each band has its own set of data. In other words, for a sensor that records data in 5 bands, there are 5 separate sets of data or "images" for each scene. A scientist receives these data and performs image processing on a computer to interpret the scene. Any color scheme can be assigned to each band. Often the color is chosen to highlight certain features. Once each band is processed, the separate images for each band can be combined (layered like a sandwich) to create a complete or composite image. Because the image may not reflect the real colors of the Earth a human eye would see, it is called a false color image.
The utility of satellite imagery is in how scientists choose to process an image. Scientists can distinguish certain features from others based on their spectral characteristics or how strongly a feature shows up in one wavelength of light compared to another. For instance, vegetation reflects strongly in the infrared wavelengths. Healthy vegetation reflects differently than unhealthy vegetation. Sensors that can record radiation in the infrared wavelengths obtain data that can be used to distinguish between healthy and unhealthy vegetation. This is only one example, but the applications are endless. Environmental monitoring, ecological research, and urban planning are just a few broad disciplines that utilize satellite imagery.
LINK: LANDSAT Program Home Page.
Landsat Images of the U.S.A.
