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| Earth Resources Observation and Science (EROS) |
LANDSAT Multispectral Scanner (MSS)
Multispectral Scanner (MSS) data from the U.S. Geological Survey's (USGS) EROS Data Center (EDC) provide a historical record of the Earth's land surface from the early 1970's to the early 1990's. Collection of MSS data ceased in October of 1992. The first satellite in the series was originally designated as the Earth Resources Technology Satellite-A (ERTS-A). It used a Nimbus-type platform that was modified to carry sensor systems and data relay equipment. When operational orbit was achieved, it was designated ERTS-1. The satellite continued to function beyond its designed life expectancy of one year and finally ceased to operate on January 6, 1978, more than five years after its launch date. The second in this series of Earth resources satellites (designated ERTS-B) was launched January 22, 1975. It was renamed Landsat 2 by NASA, which also renamed ERTS-1 to Landsat 1. Four additional Landsats were launched in 1978, 1982, 1984, and 1999 (Landsats 3, 4, 5, and 7 (no MSS sensor) respectively). Each successive satellite system had improved sensor and communications capabilities.
Supplemental Information:
NASA was responsible for operating the Landsat series through the early 1980's. In January 1983, operations of the Landsat system were transferred to the National Oceanic and Atmospheric Administration (NOAA). In October 1985, the Landsat system was commercialized. Throughout these changes, the USGS EDC has retained primary responsibility as the Government archive of Landsat data. The Land Remote Sensing Policy Act of 1992 (Public Law 102-555) officially authorized the National Satellite Land Remote Sensing Data Archive and assigned responsibility to the Department of Interior. In addition to its Landsat data management responsibility, the USGS EDC investigates new methods of characterizing and studying changes on the land surface with Landsat data.
Characteristics of the Landsat System
Landsats 1 through 3 operated in a near-polar orbit at an altitude of 920 km with an 18-day repeat coverage cycle. These satellites circled the Earth every 103 minutes, completing 14 orbits a day. Eighteen days and 251 overlapping orbits were required to provide nearly complete coverage of the Earth's surface with 185 km wide image swaths. The amount of swath overlap or sidelap varies from 14 percent at the Equator to a maximum of approximately 85 percent at 81 degrees north or south latitude. Landsat satellites 1 through 3 carried return beam vidicon (RBV) cameras and the MSS sensor. The RBV cameras did not achieve the popularity of the MSS sensor. The MSS sensor scanned the Earth's surface from west to east as the satellite moved in its descending (north-to-south) orbit over the sunlit side of the Earth. Six detectors for each spectral band provided six scan lines on each active scan. The combination of scanning geometry, satellite orbit, and Earth rotation produced the global coverage necessary for studying land surface change. The resolution of the MSS sensor was approximately 80 m with radiometric coverage in four spectral bands from the visible green to the near-infrared (IR) wavelengths. Only the MSS sensor on Landsat 3 had a fifth band in the thermal-IR wavelength.
MSS Scanning Arrangement (86.6 kb)
Landsats 4 and 5 carried both the MSS and the TM sensors; however, routine collection of MSS data was terminated in late 1992. The satellites orbited at an altitude of 705 km and provide a 16-day, 233-orbit cycle with a swath overlap that varies from 7 percent at the Equator to nearly 84 percent at 81 degrees north or south latitude. These satellites were also designed and operated to collect data over a 185 km swath. The MSS sensors flown aboard Landsats 4 and 5 were identical to the ones that were carried on Landsats 1 and 2. The MSS and TM sensors primarily detected reflected radiation from the Earth's surface in the visible and IR wavelengths, but the TM sensor provides more radiometric information than the MSS sensor. The wavelength range for the TM sensor is from the visible (blue), through the mid-IR, into the thermal-IR portion of the electromagnetic spectrum. Sixteen detectors for the visible and mid-IR wavelength bands in the TM sensor provide 16 scan lines on each active scan. Four detectors for the thermal-IR band provide four scan lines on each active scan. The TM sensor has a spatial resolution of 30 meters for the visible, near-IR, and mid-IR wavelengths and a spatial resolution of 120 meters for the thermal-IR band.
The Landsat system provides for global data between 81 degrees north latitude and 81 degrees south latitude.
The Landsat platforms operate from a sun-synchronous, near-polar orbit imaging 185 km (115 miles) ground swaths with a repeat cycle of 16 days for Landsats 4 - 5 (previously 18 days on Landsats 1 through 3). The MSS data were received directly from Landsats 4 and 5 by a network of 16 worldwide ground stations. Also, data were transmitted via a Tracking and Data Relay Satellite (TDRS) to its ground terminal at White Sands, New Mexico, and then relayed via a domestic communications satellite (DOMSAT) to the data processing facility. The MSS digital data were radiometrically corrected and relayed by DOMSAT to the EDC for storage, reproduction into digital and film formats, and distribution to users.
All of the Landsats have been in sun-synchronous orbits with equatorial crossing times ranging from 8:30 a.m. for Landsat 1 to 9 a.m. for Landsat 2 to 9:45 a.m. for Landsat 5.
The TDRS System (TDRSS) satellites are in geosynchronous orbits. This configuration allowed the acquisition of MSS data for nearly all of the Earth's surface except for an area between 50 degrees north and 67 degrees east by 50 degrees south and 82 degrees east. That area may be covered in part by data recorders at the Thailand and India ground stations.
The National Landsat Production System (NLAPS) processing system is currently used by the EDC for processing data. This system replaced the previous EROS Digital Image Processing System (EDIPS) in the mid-1990's. The NLAPS produces systematic (radiometrically and geometrically corrected), precision (map registered), and terrain corrected digital products. User-defined options include variable pixel sizes, image orientations, resampling techniques, horizontal datums, and map projections. Scene shifts and multi-scene products are also available through NLAPS for some acquisition dates (1982 - 1992).
For further information on the NLAPS MSS processing system, refer to:
Since 1972, the Landsat satellites have provided repetitive, synoptic, global coverage of high-resolution multispectral imagery. The characteristics of the MSS and TM bands were selected to maximize each band's capabilities for detecting and monitoring different types of land surface cover characteristics. For example, MSS band 1 can be used to detect green reflectance from healthy vegetation, while MSS band 2 is designed for detecting chlorophyll absorption in vegetation. MSS bands 3 and 4 are ideal for recording near-IR reflectance peaks in healthy green vegetation and for detecting waterland interfaces.
MSS Bands 4, 2, and 1 can be combined to make false-color composite images, where band 4 controls the amount of red, band 2 the amount of green, and band 1 the amount of blue in the composite. This band combination makes vegetation appear as shades of red with brighter reds indicating more vigorously growing vegetation. Soils with sparse or no vegetation will range from white (sand) to green or brown, depending on moisture and organic matter content. Water bodies appear blue. Deep, clear water appears dark blue to black in color, while sediment-laden or shallow waters appear lighter in color. Urban areas appear blue-gray in color. Clouds and snow appear as bright white, and they are usually distinguishable from each other by the shadows associated with the clouds.
Landsats 1 through 5 provided similar coverage. However, the higher altitude of Landsats 1 through 3 resulted in a different swathing pattern with the IFOV being 56 meters in the cross-track direction by 79 meters in the along-track direction (183.7 feet by 259.2 feet, respectively).
MSS scenes from Landsats 4 and 5 have an instantaneous field of view (IFOV) of 68 meters in the cross-track direction by 82 meters in the along-track direction (223 by 272.3 feet, respectively). To understand this concept, consider a ground scene composed of a single 82- by 82-m area. The scan monitor sensor ensures that the cross-track optical scan is 185 km at nominal altitude regardless of mirror scan nonlinearity or other perturbations of mirror velocity. Cross-track image scan velocity is nominally 6.82 meters per microsecond. After 9.958 microseconds, the 82- by 82-m image has moved 67.9 meters. The sample taken at this instant represents 15 meters of previous information and 68 meters of new information. Therefore, the effective IFOV of the MSS detector in the cross-track direction must be considered to be 68 meters which corresponds to a nominal ground area of 68 meters, by 82 meters at the satellite nadir point. Using the effective IFOV in area calculation eliminates the overlap in area between adjacent pixels.
Background information and status of Landsat satellites.
| Satellite | Launched | Decommissioned | Sensors |
|---|---|---|---|
| Landsat 1 | July 23, 1972 | January 6, 1978 | MSS and RBV |
| Landsat 2 | January 22, 1975 | February 25, 1982 | MSS and RBV |
| Landsat 3 | March 5, 1978 | March 31, 1983 | MSS and RBV |
| Landsat 4 | July 16, 1982 | June 15, 2001 | TM and MSS *** |
| Landsat 5 | March 1, 1984 | ** | TM and MSS *** |
** currently operational
*** MSS data acquisition suspended in 1992
The MSS sensors were line scanning devices observing the Earth at a right angle to the orbital track. The cross-track scanning was accomplished by an oscillating mirror; six lines were scanned simultaneously in each of the four spectral bands for each mirror sweep. The forward motion of the satellite provided the along-track scan line progression. All five Landsats have carried the MSS sensor which responds to Earth-reflected sunlight in four spectral bands. Landsat 3 carried an MSS sensor with an additional band, designated band 8, that responded to thermal (heat) infrared radiation.
The radiometric range of bands for the MSS sensor is shown below:
| Landsats 1-3 | Landsats 4-5 | Wavelength (micrometers) |
|---|---|---|
| Band 4 | Band 1 | 0.5 - 0.6 |
| Band 5 | Band 2 | 0.6 - 0.7 |
| Band 6 | Band 3 | 0.7 - 0.8 |
| Band 7 | Band 4 | 0.8 - 1.1 |
| Band 8 | 10.4 - 12.6 |
Micrometers and their relationship to the electromagnetic spectrum are explained in the glossary.
Line-Start Anomaly (Landsat 3)
Beginning in the spring of 1979, the MSS sensor flown aboard Landsat 3 began to experience a problem with proper synchronization of scan lines. NASA performed a series of tests during the period of April 8-24, 1979, in an attempt to correct the situation. However, in the fall of 1979, the problem began occurring with increasing frequency. After that period, line-start anomalies affected 20 percent to 40 percent of all Landsat 3 MSS images acquired.
The MSS sensor was equipped with a hardware feature that generated a line-start pulse to activate the MSS detectors at the beginning of each mirror sweep. The problem occurred when this start pulse was either absent or of an abnormally low amplitude. Because the detectors were not activated at the appropriate time, no data were acquired during the initial part of the scan. A second pulse did activate the detectors when the mirror was about 30 percent of the way across a scan, but the early (western) portion of the intended scan area was lost by that time.
Because there are six detectors per band, multiples of the six scan lines were involved. The number of scan lines affected in a given frame was random and depended on how many line-start pulses were missed. As many as 40 percent of the scenes are affected, and the number of affected scans varies from 10 percent to 90 percent of the scans in the image.
Before the occurrence of this problem, all scan lines were left-justified during routine processing at NASA; that is, they were aligned at the left side of the image. For every line-start pulse missed, a set of six scan lines was, thus, aligned with those that had normal start pulses. This caused that set of scan lines to shift on the ground about 48 km (29.8 miles) to the left of where they should have been.
After the first appearance of the line-start anomaly, NASA contracted to have the ground hardware modified to alleviate this problem. Basically, the hardware properly aligns the good portions of the affected scan lines with respect to the other scan lines so that the ground features line up correctly. Unfortunately, the 30 percent of the data that was missed in each scan line is lost; missing areas are represented in the image as blanks. The corrected images are technically more than 70 percent usable, meaning that much of the image area is unaffected, but their appearance is aesthetically poor. All quality code references to line-start anomaly scenes refer only to the unaffected areas.
The resolution for the MSS sensor is shown below:
| Landsats 1-3 | Landsats 4-5 | Meters |
|---|---|---|
| Band 4 | Band 1 | 79/82* |
| Band 5 | Band 2 | 79/82 |
| Band 6 | Band 3 | 79/82 |
| Band 7 | Band 4 | 79/82 |
| Band 8** | 237 |
* The nominal altitude was 920 km for Landsats 1, 2, and 3. Nominal altitude for Landsats 4 and 5 was 705 km. The resolutions, dependent on altitude, are approximately 79 and 82 meters respectively.
** Landsat 3 only.
Application and Related Data Sets
Landsat data have been used by government, commercial, industrial, civilian, and educational communities in the U.S. and worldwide. These data are being used to support a wide range of applications in areas such as global change research, agriculture, forestry, geology, resources management, geography, mapping, water quality, and oceanography. Landsat data have potential applications for monitoring the conditions of the Earth's land surface. The images can be used to map anthropogenic and natural changes on the Earth over periods of several months to two decades. The types of changes that can be identified include agricultural development, deforestation, natural disasters, urbanization, and the development and degradation of water resources. The MSS archive retains over 630,000 scenes with a data volume of 20 terabytes.
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Landsat Orbit 29.4 kb)
TDRS Coverage Gap (8.7 kb)