The Landsat-7 satellite is part of NASA's ESE (Earth Science Enterprise) program, a joint venture of NASA and USGS (United States Geological Survey). The overall mission objective is to extend and improve upon the long-term record of medium-resolution multispectral imagery of the Earth's continental surfaces provided by the earlier Landsat satellites. 1)
Following the loss of Landsat-6 during launch in 1993, Landsat-7 was placed on a fast track for launch in 1998, but was ultimately launched on 15 April 1999 (a one-year delay resulted from having to replace some faulty electronics inside the ETM+ sensor).
The S/C and payload were developed under NASA/GSFC management/procurement responsibility (Landsat Project Scientist: D. Williams). The LS-7 satellite was built by LMMS (Lockheed Martin Missiles and Space) at the facility in Valley Forge, PA. The S/C features the Landsat-6 bus; an onboard recorder in solid state memory (378 Gbit capacity to capture data beyond the range of ground receiving stations, recording rates of 150 Mbit/s, playback with 300 Mbit/s), and a single observation instrument: ETM+. 2) 3)
The Landsat-7 spacecraft is very similar in design to the Landsat-6 satellite. It features three-axis stabilization with a pointing capability of 180 arcsec (3σ) and a pointing knowledge of 45 arcsec (1σ). Attitude control is provided with four reaction wheels and two torque rods, attitude is sensed with a static Earth sensor, 2 magnetometers and gyros. Orbit control and backup momentum unloading is provided through a blow-down monopropellant hydrazine system with a single tank containing 122 kg of hydrazine.
Figure 1: Illustration of the Landsat-7 spacecraft (image credit: NASA)
S/C mass = 2200 kg, dimensions: 4.3 m in length and 2.8 m in diameter, power = 1550 W [about 1000 W average, provided by a silicon cell solar array (4 panels each of size 1.88 m x 2.26 m) and two nickel hydrogen batteries each of 50 Ahr capacity]. S/C design life = 7 years.
Figure 2: Photo of the Landsat-7 spacecraft during pre-launch activities (image credit: NASA)
The onboard processor performs autonomously executed functions for wideband communications, electrical power management, and satellite control. These include attitude control, redundancy management, antenna steering, battery management, solar array pointing maintenance, thermal profile maintenance, and stored command execution.
RF communications: All data communication is CCSDS compliant. An onboard SSR (Solid State Recorder) is used to capture wideband data from the ETM+ (storage capacity of 378 Gbit, equivalent for about 100 scenes or 42 minutes of instrument data).
• S-band (2 omni-directional antennas), 5 W, for TT&C data with real-time telemetry data rates of 1.2 kbit/s and 4.8 kbit/s, and 256 kbit/s of playback data, 2 kbit/s of command data. S-band frequencies of 2106.4 MHz (uplink) and 2287.5 MHz (downlink). The zenith antenna is used for TDRS (Tracking Data and Relay Satellite) communications; the nadir antenna is used for Landsat Ground Network (LGN) communications. Each antenna provides essentially hemispherical coverage.
• X-band (3 steerable antennas), 3.5 W; each antenna transmits data on two channels, with each channel carrying 75 Mbit/s (total of 150 Mbit/s per antenna); up to three separate links are supported. X-band frequencies: 8082.5 MHz, 8212.5 MHz, 8342.5 MHz. The downlink beam width of each antenna is 1.2º.
Ground sites exist at Sioux Falls, South Dakota (Landsat Ground Station, or LGS), Poker Flat, Alaska (Alaska Ground Station, or AGS), Wallops Virginia (WPS), and Svalbard, Norway (SGS). All ground sites are equipped with 11 meter antennas. AGS, SGS, and LGS are capable of receiving both S-band (TT&C) at a downlink rate of 4 kbit/s and X-band (payload) data simultaneously.
Figure 3: Artist's view of the Landsat-7 satellite (image credit: NASA, USGS)
Launch: A launch of Landsat-7 took place on a Delta 2 vehicle from VAFB, CA on April 15, 1999.
Orbit: Sun-synchronous polar orbit (AM orbit), altitude = 705 km, inclination = 98.2º, period = 99 minutes, repeat coverage = 16 days, the nominal descending equator crossing time is at 10:00 to 10:15 hours.
As of early 2001 Landsat-7 is flying in the so-called “morning constellation,” also referred to as morning train with EO-1 (a few minutes apart), SAC-C and Terra. The objective is to compare coincident imagery from the ETM+ and ALI instruments. The “paired scene” images are used to evaluate the performance of ALI. In fact, the EO-1 and SAC-C spacecraft joined the constellation already on Nov. 21, 2000. The overall objective is to obtain synergistic effects for data interpretation and analysis.
Figure 4: Original morning constellation configuration (image credit: NASA)
Sensor complement: (ETM+)
ETM+ (Enhanced Thematic Mapper Plus):
ETM+ was built by Raytheon SBRS (Santa Barbara Remote Sensing), Goleta, CA. ETM+ is an 8-band whiskbroom scanning radiometer consisting of:
• A primary mirror that sweeps side-to-side (cross-track) to produce forward and revers image scans, and
• A scan line corrector (SLC) mirror assembly that sweeps forward-to-aft to compensate for the forward motion of the spacecraft during integration time. The motion of these mirrors deviates from an ideal line profile, introducing along- and cross-track geometric distortions that require compensation.
The principal functional differences between the ETM and the former TM series are the addition of a 15 m resolution panchromatic band and two 8-bit “gain” ranges. The ETM+ adds a 60 m resolution thermal band, replacing the 120 m band on ETM/TM (band No. 6). Design life = 7 years. 5)
Figure 5: ETM+ block diagram (image credit: SBRS)
Figure 6: Schematic of the ETM+ optics subsystem (image credit: NASA)
The Scan Mirror Assembly (SMA) provides the cross-track scanning motion to develop the 185 km long scene swath. The SMA consists of a flat mirror supported by flex pivots on each side (which have compensators to equalize pivot reaction torque), a torquer, a scan angle monitor (SAM), 2 leaf spring bumpers and scan mirror electronics (SME). The bi-directional SMA sweeps the detector's line of sight in west-to-east and east-to-west directions in cross-track direction, while the spacecraft's orbital path provides the north-south motion.
The ETM+ scanner contains 2 focal planes that collect, filter, and detect the scene radiation in a swath, 185 km wide. The primary focal plane consists of optical filters, detectors, and preamplifiers for 5 of the 8 ETM+ spectral bands (bands 1-4, 8). The second focal plane is the cold focal plane which includes the optical filters, infrared detectors, and input stages for ETM+ spectral bands 5,6, and 7. The temperature of the cold focal plane is maintained at 91 K using a radiative cooler. The detector line arrays (16 for VNIR bands, 32 for PAN, and 8 detectors for TIR) of the whiskbroom scanner are oriented in the along-track direction. This arrangement provides a parallel coverage of 480 m along-track in one scan sweep (cross-track direction). The wide along-track coverage permits sufficient integration time for all cells in each scan sweep.
Table 1: Landsat-7 ETM+ bandwidth specifications
The ETM+ also includes a number of radiometric enhancements to achieve an absolute radiometric uncertainty of <5% (bands 1-4). Two new calibration devices were added: FAC (Full Aperture Calibrator), and PAC (Partial Aperture Calibrator). ETM+ uses three independent onboard calibration systems (plus preflight calibration), representing a significant step forward in absolute radiometric calibration accuracy. 6) 7)
• A full-aperture solar diffuser (FASC) on the inner surface of the aperture door that illuminates the focal planes with diffusely reflected solar energy when commanded into position
• A partial-aperture solar reflector (PASC) that illuminates the focal planes with attenuated solar energy, once per orbit
• Internal calibrator (IC). Calibration lamps that project calibrated energy onto the focal planes via the main calibration shutter, once per scan, during the scan mirror turnaround.
From the initiation of the LS-7 system, greater attention was paid toward the long-term characterization and calibration of the data than for earlier LS missions. In particular, an IAS (Image Assessment System) was incorporated into the ground processing system. The objective of IAS is to characterize and calibrate the instrument data over the life of the mission. 8) 9) 10) 11) 12)
Figure 7: Cutaway illustration of the ETM+ instrument (image credit: SBRS)
Table 2: Some parameters of the ETM+ instrument
Cross calibration was performed between ETM+ and ALI [(Advanced Land Imager) on the EO-1 (Earth Observing-1) mission] image pairs using two approaches. One approach was based on image statistics of large common areas between the image pairs. The other approach was based on vicarious calibration that compares the measured radiance obtained from the sensor to the predicted at-sensor radiance using the surface measurements propagated to the sensor via radiative transfer code. 13) 14) 15)
Figure 8: Detector projection at the prime focal plane (image credit: NASA)
Table 3: Overview of Landsat series imaging instrument parameters
Operational events and status regarding the sensor ETM+ :
The SLC (Scan Line Corrector) on the ETM+ instrument failed on May 31, 2003. The SLC function is to compensate for the forward motion of the satellite during data acquisition. As a consequence of this operational anomaly, individual image scans overlap and also leave large physical gaps near the edge of each picture. Only portions of the image near the center are left completely unfettered and valid. Overall, about 30 percent of the total image is missing in each downlinked picture.
Spacecraft controllers immediately suspended normal LS-7 operations and limited activity to just spacecraft housekeeping and operations related to the anomaly investigation and recovery effort. However, subsequent efforts to recover the SLC were not successful and the problem appears to be permanent. Without an operating SLC, the ETM+ line of sight now traces a zig-zag pattern along the satellite ground track (Figure 10). The resulting gaps in coverage range from none at the center of the scan to 14 pixels at the extreme edges of the scan.
As of Sept. 16, 2003, LS-7 has resumed its normal Long Term Acquisition Plan (LTAP) scheduling of approximately 250 scenes per day, and all data will now be acquired in SLC-off mode. ETM+ is still capable of acquiring useful image data with the SLC turned off, particularly within the central portion of any given scene.
As of Oct. 2003, the USGS/NASA Landsat team is trying to develop means to compensate for the SLC malfunction, with image processing methods and acquisition strategies to exploit the remaining observation capability of the LS-7 system. The team is refining gap-filling techniques that merge data from multiple acquisitions. They are also developing modifications to the LS-7 acquisition scheme to acquire two or more clear scenes as near in time as possible to facilitate this gap-filling process. These merged images potentially resolve most, if not all, of the missing data problems.
Figure 9: ETM+ Scan Line Correction (image credit: NASA)
Figure 10: ETM+ scan coverage with and without the operating Scan Line Corrector (image credit: USGS)
The USGS reinitiated routine collection of ETM+ data with the SLC turned off on July 14, 2003 and began distributing this data in late October 2003. Initially, the USGS offered data products with a fixed maximum interpolation of two pixels in their fully processed data products. Beginning in mid-Feb. 2004, the maximum amount of interpolation became user selectable. Starting in May 2004, USGS began providing the first in a series of data products to help make the SLC-off data more usable. 16) 17) 18)
Figure 11: Impact of the ETM+ SLC anomaly (image credit: USDA FAS Agricultural Applications Seminar 2006) 19)
Figure 12: Landsat-7 image of the Lena Delta in northern Russia observed on July 7, 2000 (image credit: USGS)
Legend to Figure 12: The Lena river of 4,400 km in length, is one of the largest rivers in the world. The Lena Delta Reserve is the most extensive protected wilderness area in Russia. It is an important refuge and breeding grounds for many species of Siberian wildlife. 20)
• January 2014: Landsat-7 continues its science mission and based on fuel estimates will continue to do so until at least mid-2016. Landsat-7, while its imagery is slightly degraded due to the scan-line corrector failure, continues to provide global coverage and an 8-repeat cycle for the Landsat Mission when combined with Landsat-8. 21)
• The LS-7 spacecraft is operational in 2013 with the known degradations of the ETM+ instrument, collecting global data. 22)
- Launched on April 15, 1999 with a 5 year design life, the Landsat-7 mission just began its 14th year on-orbit.
- Robust global acquisitions are continuing, collecting nearly 400 scenes a day.
- Fuel-based end-of-life is 2017.
- Successfully completed the underflight with LDCM, collecting all images that LDCM collected for a three day period.
Figure 13: These images show a portion of the Great Salt Lake, Utah as seen by LS-7 (left) and LS-8 (LDCM) satellites (right); both images were acquired on March 29, 2013 (image credit: USGS, Ref. 22)
Legend to Figure 13: On March 29-30, 2013, the LDCM was in position under the Landsat 7 satellite. This provided opportunities for near-coincident data collection from both satellites. The images below show a portion of the Great Salt Lake in Utah, and the Dolan Springs, Arizona area, the latter of which is used in Landsat calibration activities. 23)
Legend to Figure 14: This image, released on Dec. 21, 2012, shows the largest outlet glacier on Greenland’s east coast, discharging ice into the surrounding oceans. In this image one can see hundreds of icebergs speckling the water. A recent study based on satellite observations revealed that over the past 20 years the ice melting in Greenland and Antarctica has contributed about 11 mm to the global sea-level rise. This image clearly shows the glacier’s calving front, where ice breaks away. Over the years, satellite images have shown that this front has retreated – an indication that the glacier is getting smaller over time.
• On July 23, 2012, the Landsat program marked its 40th anniversary, representing the longest continuous global record of Earth observations from space. Through four decades, Landsat satellites have taken specialized measurements of Earth's continents and surrounding coastal regions, enabling people to study many aspects of our planet and to evaluate the dynamic changes caused by both natural processes and human practices. The long record of Landsat spectral information is a historical archive unmatched in quality, detail, coverage, and length. 25)
The data-rich USGS archive built from the Landsat satellites since 1972, with more than three million images, represents the surface of Earth over a 40-year period, a story of our physical world unparalleled in the history of science.
• The LS-7 spacecraft is operational in 2012 with the known degradations of the ETM+ instrument. 26)
Legend to Figure 15: The 3 Landsat images were captured by the Landsat-1, -4 and -7 satellites. Visible is the Lisan Peninsula (bottom center) that forms a land bridge through the Dead Sea. Deep waters are dark blue, while pale blue shows salt ponds and shallow waters to the south. The pale pink and sand-colored regions are desert lands. Denser vegetation appears bright red. - The expansion of massive salt evaporation projects on the Dead Sea are clearly visible in this time series of images taken by Landsat satellites operated by NASA and the USGS (U.S. Geological Survey). The USGS preserves a 40-year archive of Landsat images that is freely available data over the Internet.
• The LS-7 spacecraft is operational in 2011 (11 years of on-orbit operations). 28)
• LS-7 is operational in 2010 providing image data, however with the known degradations of the ETM+ instrument. Expendable fuel, necessary for stabilizing the orbit and angle of the satellite, will run out in 2012. 29) 30) 31)
Figure 16: Overview of LS-7 component failures/recoveries over its life until Jan. 2010 (image credit: USGS, Ref. 30)
• The Landsat 7 ETM+ instrument is currently operating in SAM (Scan Angle Monitor) mode to control the motion of the scan mirror during imaging. Over time, wear of the scan mirror assembly will cause the instrument to lose the ability to synchronize the calibration shutter with the scan mirror. - Current projections show this to occur between March 2007 and January 2008. As a result, changes to operations and software are necessary to switch the instrument to an alternate mode, known as “bumper mode.” The Landsat-5 Thematic Mapper underwent a similar change in 2002. 32)
• In preparation for this event, on March 3rd and 19th, 2006, the Landsat-7 flight operations team successfully tested bumper mode operation over several geometric calibration sites. Analysis of the preliminary data show the movement of one of the antennas impacts image acquisition, but this error can be corrected by the ground processing system. Additional in-depth research is underway, but this successful bumper mode test is a positive indicator for the continuation of the Landsat-7 mission.
• The Landsat-7 spacecraft and its payload reached the end of its five-year design life on April 15, 2004.
Status of LS-7 spacecraft gyros:
The LS-7 project de-powered one of its gyros on May 5, 2004, due to indications of anomalous behavior. The spacecraft has three two-degrees-of-freedom gyros and needs two at any time to maintain attitude control. A risk assessment reported a 40 percent likelihood of another gyro failure by July 2005. A team was assembled to modify the software on board the spacecraft to operate in what is being termed Virtual Gyro (V-Gyro) mode. In this mode, if another gyro fails, the attitude control system would use the remaining gyro, along with existing onboard instrumentation and new control logic, to maintain attitude control.
As of February 1, 2006, the Landsat-7 team developed and uploaded flight software that can act like a ”virtual” gyro -- ready to use if another gyro fails. The enhanced capability was designed, developed, tested, and implemented with no interference to ongoing Landsat-7 operations.
LS-7 ground segment and data handling policy:
Landsat-7 data is received and distributed to the user community by USGS (capturing and processing 250 Landsat scenes per day and delivering at least 100 of the scenes to users each day).NASA/GSFC performed on-orbit mission operations until Oct. 1, 2000; after that responsibility for flight operations and LS-7 management was transferred to the USGS/EDC (flight operations, maintenance, and management of all ground data reception, processing, archiving, product generation, and distribution). The operating philosophy changed to the effect that ETM+ data covering the global continental surfaces are being archived in the USA. The ETM+ archive is continually being updated as data become available. This data policy differs from the past (Landsat-4 and -5), where data was only acquired from the S/C on the basis of customer requests. The new archiving policy will substantially increase the amount of data available to the user community. 33) 34) 35)
The Landsat-7 ground system design includes an Image Assessment System (IAS) to provide users with ancillary information needed to generate useful, radiometrically calibrated and geometrically corrected ETM+ digital imagery. Another aspect of the new data handling policy is that ETM+ data will be distributed from the archive in an essentially raw form. Users are responsible for the task of preprocessing their imagery (i.e. radiometric and geometric corrections). The price tag for Landsat-7 imagery is substantially lower than for the commercial products of Landsat-4 and -5.
• In April 2008, the USGS announced to open the Landsat-7 archive providing free access to the entire user community. Previously acquired imagery from Landsat 1 through Landsat 5, is also now available for download at no charge using the same standard processing format. The release schedule is shown in Table 4. 36)
As of early January 2009, over 225,000 scences were downloaded since Oct. 1, 2008. Newly acquired Landsat 7 ETM+ SLC-off and Landsat 5 TM images with less than 40 percent cloud cover are automatically processed and made available for immediate download. Imagery with greater than 40 percent cloud cover can be processed upon request. Once the requested scenes are processed, an email notification is sent to the customer with instructions for downloading. These scenes will then become accessible to all users. 37)
Table 4: Schedule of Landsat archival data availability via Internet
Figure 17: Schematic illustration of LS-7 space segment and ground segment (image credit: NASA)
The direct downlink service to a global network of existing Landsat ground stations (in X-band via each of three directional antennas) is maintained. All real-time image data (within view of a licensed ground station) are directly downlinked via the three X-band links. TDRSS may be used for TT&C data relays in S-band (backup). The prime (US) ground receiving station for the Landsat-7 archive is located at the EROS Data Center (EDC) in Sioux Falls, South Dakota. A second (US) reception facility near Fairbanks, AK, acquires coverage of Alaska and international coverage using the onboard recorder. An additional receiving station in Svalbard/Spitzbergen (Norway) provides backup reception. All data received at either Fairbanks or Svalbard is being shipped to EDC for archiving and distribution.
Figure 18: Active Landsat ground stations (image credit: USGS) 38)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.