Introduction to LANDSAT

The LANDSAT Program is the longest running exercise in the collection of multispectral, digital data of the earth's surface from space. The program has operated continuously since LANDSAT 1 (then the Earth Resources Technology Satellite (ERTS) 1) was launched on July 23, 1972. More than 3 million images from the Multispectral Scanner System (MSS) on LANDSATs 1-5 have been aquired and stored at the National Satellite Land Remote Sensing Data Archive (NSLRSDA) at EROS Data Centre (EDC), Sioux Falls, South Dakota and the LANDSAT international ground stations. The temporal extent of the collection, the characteristics and quality of LANDSAT data, and the ability to collect new data directly comparable to that in the archive, make LANDSAT data a unique resource, one used extensively to address a broad range of issues in earth science, global change science, and making monitoring and assessing land and coastal zone resources.



Landsat Platforms

Five LANDSAT satellites have been successfully launched commencing with LANDSAT 1 in July 1972. All 5 satellites have operated from a repetitive, circular, sun-synchronous, near-polar orbit and on each day-side pass, scan a ground swath 185km wide beneath the satellite. The first three satellites carried the Multispectral Scanner (MSS) as the main imaging instrument with a Return Beam Vidicon (RBV) as a subsiduary. The paths of these satellites were inclined 99 degrees with an 18 day repeat cycle and an equatorial crossing of between 8:50 and 9:30am local time. The last two satellites (LANDSATs 4-5) had the Thematic Mapper (TM) sensor as well as the MSS, are inclined 98 degrees, have a repeat cycle of 16 days and have an equatorial crossing of 9:45am local time. The nominal altitude of the satellites was 920km for LANDSATs 1-3 and 705km for LANDSATs 4-5.

 Orbital Path LANDSATs 4-5                                           Orbital Characteristics of Landsat Satellites

 

The RBV system on LANDSAT 1-2 consisted of three cameras which took simultaneous images of the earth in different spectral bands. The area covered was similar to the MSS images. The images were stored on the RBV photosensitive surfaces from which they were scanned to produce video outputs. The RBV system on LANDSAT  3 consisted of two identical side-by-side instruments which scanned in one spectral band. Each camera produced an image equivalent to a quarter of an MSS scene and the resolution was approximately 40 metres.

LANDSAT 6 was to represent a departure from the earlier satellites with an enhanced Thematic Mapper and no MSS. Unfortunately this satellite, launched in early 1993, was lost on launch without any backup. LANDSAT 5 is currently the only satellite routinely providing Thematic Mapper and MSS data on direct downlink to ground stations. Although the satellite operators are maintaining an optimistic outlook for LANDSAT 5, suggesting that it has sufficient fuel to last the turn of the century, some degradation of the imagery has occurred and this might be expected to worsen.

The next satellite in the LANDSAT series will be LANDSAT 7 and is likely to be a joint project between NASA, NOAA and the USGS. The Department of Defence has apparently pulled out of the LANDSAT Program Management, after funding could not be obtained for a second high resolution sensor on the satellite. The prime instrument on board the satellite will be the Enhanced Thematic Mapper (ETM+). This instrument has a similar 7 bands multispectral capability as the LANDSATs 4-5 but with a 15 metre resolution panchromatic band (0.5 to 0.9 micrometres, visible green to near IR), which is co-registered with the multispectral data. The ETM+ will be capable of collecting 250 day-lit, land mass scenes per day and transmitting these scenes to ground stations via a 150Mbps X-band link. The spacecraft will also have 375GB of solid state memory, sufficient for 100 ETM+ scenes, for delayed transmission.

Landsat 7 is not expected to be launched until 1998.


Data Reception

When within line of sight of a ground receiving station, data from LANDSATs 1-3 were transmitted directly to the station and recorded on magnetic tape. These satellites also had onboard tape recorders so that data from the MSS or RBV could be recorded and transmitted to a ground station when passing over the US. Availability of worldwide MSS data outside the limits of ground reception facilities is relatively good and is available from EROS.

LANDSATs 4-5 did not have onboard tape recorders and relied on direct transmission to ground receiving stations and on transmission via the Tracking and Data Relay Satellite System (TDRSS) to the White Sands ground station in the US. The TDRSS satellites are in geosynchronous orbits and if fully operational (using satellites designated as east and west) allow the acquisition of data for all the earth's surface except for an area within 50 deg north and south and 67 to 82 degrees east. The TDRS-1 (east) satellite operates at 41 degrees west longitude over the Atlantic Ocean off the north-east coast of Brazil. The launch failure of the companion TDRSS satellite during the Challenger explosion in January 1986 limited the availability of MSS and TM from these satellites over Asia and the Pacific region to direct reception. A replacement for this satellite was in place in late 1988 at 171 degrees west longitude slightly south-west of Hawaii. The TDRSS link on Landsat 5 is no longer operational.

Availability of the TDRSS has meant that worldwide MSS and TM from LANDSATs 4-5 outside the limits of direct ground reception faclities is good for the Americas, Africa and Asia west of 67 degrees east. For Asia, east of 82 degrees, availability of data is limited to a few years between 1989 and the early 1990's.


Worldwide Reference System

Data from the LANDSAT satellites is collected in a continuous stream of data along a near vertical path as the satellite moves from north to south. The data is arbitarily divided into nominal scenes which are about 24 second increments of spacecraft time apart, corresponding to a spacing of approximately 160km. This path/row designation is referred to as the LANDSAT Worldwide Reference System (WRS). The rows have been positioned in such a way that Row 60 coincides with the equator. This reference system is different between LANDSATs 1-3 and LANDSATs 4-5 because of the different altitudes of the satellites. This effects the spacing of the paths with LANDSATs 1-3 having 251 paths worldwide and LANDSATs 4-5 having 233. Designation of the rows is similar along each path. A series of WRS path/row maps is available world wide from EOSAT and these normally have the LANDSATs 1-3 WRS on one side and the LANDSATs 4-5 on the reverse. The map sheets are at 10 million scale and 26 are needed for global coverage.

LANDSAT Path designations increase from east to west while SPOT are the reverse.

The standard MSS and TM scenes from each of the LANDSAT satellites have had the same nominal ground coverage of 185km by 170km. Since the LANDSATs 1-3 had 251 paths compared to the 233 of LANDSATs 4-5, there was considerably more overlap between adjacent paths on the earlier satellites. This overlap is shown in the following table.

 

It is possible to view the images stereoscopically in this overlap region and at latitudes greater than 60 degrees there is complete stereoscopic coverage. The quality of the viewing will be dependent on the relief in the overlap area with the angle of the capture varying from zero in the centre of the scene to a maximum of 8 degrees at the extreme edges of the scenes.

 

LANDSAT Ownership

The LANDSAT Program during the 1970's was run by NASA as a high-tech experiment. At the end of 1979, control of the program was passed to the Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and plans were announced for a phased transition to commercialization of the operation of the program. In 1984, the US Congress passed the LANDSAT Remote Sensing Commercialization Act (P.L 98-365) which officially authorised moving the LANDSAT program to the private sector. The tendering process for the commercial operator was won by the Earth Observation Satellite Company (EOSAT) which was jointly owned by General Electric Co. and Hughes Aircraft Co.. The contract included operating LANDSATs 4-5 and designing , developing , launching, and operating the follow-on system.

In October 1992, the Land Remote Sensing Policy Act (P.L. 102-555) was passed effectively reversing the 1984 decision to comercialise the LANDSAT system. This recognised the failure of the commercialization process because the commercial price of data hindering distribution. The Act committed the US to maintain continuity of LANDSAT-type data into the next century and commited funding to NASA and the Department of Defence for the procurement of LANDSAT 7.

In November 1990, EOSAT and NOAA reached an agreement which allowed MSS data older than two years to be sold at the EROS Data Centre (EDC) for the cost reproduction and distribution. This agreement was broadened in 1993 and EOSAT discontinued marketing all MSS data effective Feburary, 1993. This agreement now means that all archived MSS data is available from EDC at relatively low prices.


 
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