Landsat 7 takes pictures of the earth
and sends the data to the ground stations around the
world.
In 1992, the U.S. Congress authorized
the procurement, launch and operation of a new Landsat
satellite. This new system, Landsat 7, was launched
in April, 1999. It is latest in a series of earth observation
satellites dating back to 1972. The thirty year record
of data acquired by the Landsat satellites constitutes
the longest continuous record of the earth's continental
surfaces. Preservation of the existing record and continuation
of the Landsat capability were identified in the law
as critical to land surface monitoring and global change
research. Landsat 7 has a unique and essential role
in the realm of earth observing satellites in orbit.
No other system matches Landsat's combination of synoptic
coverage, high spatial resolution, spectral range and
radiometric calibration. In addition, the Landsat Project
is committed to providing Landsat digital data to the
user community in greater quantities, more quickly and
at lower cost than at any previous time in the history
of the Project. The earth observing instrument on Landsat
7, the Enhanced Thematic Mapper Plus (ETM+), replicates
the capabilities of the highly successful Thematic Mapper
instruments on Landsat 4 and 5*. The ETM+ also includes
new features that make it a more versatile and efficient
instrument for global change studies, land cover monitoring
and assessment, and large area mapping than its design
forebears. The primary new features on Landsat 7 are:
- a panchromatic band with 15m spatial resolution
- on board, full aperture, 5% absolute radiometric calibration
- a thermal IR channel with 60m spatial resolution
The instrument is supported by a ground network that
receives ETM+ data via X-band direct downlink only
at a data rate of 150 Mbps. The primary receiving
station at the US Geological Survey's (USGS) EROS
Data Center (EDC) in Sioux Falls, South Dakota.
Substantially cloud- free, land and coastal scenes
will be acquired by EDC through real-time downlink,
and by playback from an on-board, solid state, recording
device. The capacities of the satellite, instrument
and ground system are sufficient to allow for continuous
acquisition of all substantially cloud free scenes
at the primary receiving station. In addition, a world-wide
network of receiving stations is able to receive real-time,
direct downlink of image data via X-band. Each station
is able to receive data only for that part of the
ETM+ ground track where the satellite is in sight
of the receiving station.
The Landsat 7 systeminsures continuity of Thematic
Mapper type data into the next centuries. These data
are made available to all users through EDC at the
cost of fulfilling user requests. Browse data (a lower
resolution image for determining image location, quality
and information content) and metadata (descriptive
information on the image) will be available, on-line,
to users within 24 hours of acquisition of the image
by the primary ground station. EDC will process all
Landsat 7 data received to "Level
0R" ( i.e. corrected for scan direction
and band alignment but without radiometric or geometric
correction) and archive the data in that format. A
systematically corrected product (Level
1G) will be generated and distributed to users
on request. The user hasthe option of performing further
processing on the data on user-operated digital processing
equipment or by a commercial, value added firm.
The Landsat 7 spacecraft, built by Lockheed Martin, Valley Forge,
Pennsylvania. The ETM+ instrument is a product of Hughes Santa Barbara Remote
Sensing. Construction of both is managed through contracts between the
manufacturers and the NASA Goddard Space Flight Center, Greenbelt, Maryland.
The Landsat Project, as defined by Congress in 1992
and amended by Presidential Decision Directive/NSTC-3
in May, 1994, is managed cooperatively by the National
Aeronautics and Space Administration (NASA), the National
Oceanic and Atmospheric Administration (NOAA), and
the USGS. Responsibility for construction of the spacecraft
and instrument lied with NASA. The Landsat Project
is part of the NASA's global change initiative - the
Earth Observing System, administered by the NASA Office
of Mission to Planet Earth. Landsat 7 will be operated
by NOAA. Data processing, archiving and distribution
will be performed by USGS with direction from NOAA.
These functions are executed in coordination with
the EDC Distributed Active Archive Center (LP
DAAC) of NASA's Earth Observing System Data
and Information System (EOSDIS) at EDC.
*The ETM+ is similar to the ETM instrument on Landsat 6. Landsat 6 was
launched in October, 1993, but failed to obtain orbit.
An orbit is completed approximately
every 99 minutes allowing the spacecraft to achieve
over 14 orbits a day. Therefore, total coverage of the
Earth occurs in 16 days. The descending orbit from north
to south crosses the equator between 10:00 and 10:15
(local time) on each pass.
Data acquisition requests will not be available to the
public due to the robust nature of the Landsat 7 Long Term
Acquisition Plan (LTAP). The LTAP was derived from
recommendations put forth by the scientific community. The
purpose of the plan is to optimize the use of the Enhanced
Thematic Mapper+ (ETM+) to create a global archive of the
processes on the planet's land surfaces. The LTAP takes
into account the calibration plan, planned satellite
maneuvers, and international imaging needs.
The Mission Operation Center (MOC)
schedules a 48-hour set of activities based on a number of
criteria, including global refresh requirements, request
priority, solid state recorder or other resource
availability, and cloud cover predictions. After the
Flight Operations Team (FOT) has iterated through the
scheduling function and resolved all conflicts, the MOC
will pass the 48-hour schedule to the load generation
function for final compilation. To view the 24 hour
expected acquisition schedule, please go to: http://ls7pm3.gsfc.nasa.gov/scenes/Default.html
For additional info on the LTAP,
please go to: http://ltpwww.gsfc.nasa.gov/IAS/htmls/igs.html
