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 10, 2017: Cambodia has one of the fastest rates of forest loss in the world. In broad swaths of the country, densely forested landscapes—even those in protected areas—have been clear-cut over the past decade. Much of the forest has been cleared for rubber plantations and timber. 21)
- Scientists from the University of Maryland and the World Resources Institute's Global Forest Watch have been using Landsat satellite data to track the rate of forest loss on a global scale. Though other countries have lost more acres in recent years, Cambodia stands out for how rapidly its forests are being cleared.
- Between 2001 and 2014, the annual forest loss rate in Cambodia increased by 14.4 %. Put another way, the country lost a total of 1.44 million hectares—or 14, 400 km2—of forest. Other countries with accelerating rates of forest loss include Sierra Leone (12.6 %), Madagascar (8.3%), Uruguay (8.1%), and Paraguay (7.7%).
- The transformation of Cambodia's landscape has been profound, as revealed by this pair of Landsat images. The image of Figure 13, captured by Landsat 7's ETM+ (Enhanced Thematic Mapper Plus) on December 31, 2000, shows intact forest near the border of the Kampong Thom and Kampong Cham provinces. On October 30, 2015, OLI (Operational Land Imager) on Landsat 8 captured the image of Figure 14, in which much of the forest has been replaced by a grid-like pattern of roads and fields and by large-scale rubber plantations. Clear-cutting has also chewed away at the edges of densely forested areas (dark green) and replaced them with exposed soil, croplands, and mixed forests (brown and light green).
- Researchers working with Landsat data and other economic datasets have demonstrated that changes in global rubber prices and a surge of land-concession deals have played key roles in accelerating Cambodia's rate of deforestation. Concession lands are leased by the Cambodian government to domestic and foreign investors for agriculture, timber production, and other uses. Researchers found that the rate of forest loss within concession lands was anywhere from 29 to 105% higher than in comparable lands outside the concessions.
- Work by Matthew Hansen and his University of Maryland Global Land Analysis and Discovery (GLAD) lab has played a key role in revealing the scope of deforestation. In 2013, the group published their first global map of forest change. The map of Figure 15, based on Hansen's work, depicts the extent of forest loss throughout Cambodia between 2000 and 2014. Much of it has occurred in the past five years.
- In conjunction with the World Resources Institute, GLAD has developed a new weekly alert system: deforestation is detected by satellites with each new Landsat image, and users can subscribe for email updates. The freely available alert system is already operating for Congo, Uganda, Indonesia, Peru, and Brazil. The researchers hope to have the system operating for Cambodia and the rest of the tropics in 2017.
Figure 13: Landsat-7 ETM+ image of Cambodia, acquired on December 31, 2000 (image credit: NASA Earth Observatory, image by Joshua Stevens, using Landsat data from the USGS)
Figure 14: Landsat-8 OLI image of the same region in Cambodia, acquired on October 30, 2015 (image credit: NASA Earth Observatory, image by Joshua Stevens, using Landsat data from the USGS)
Figure 15: This map, based on Hansen's work, depicts the extent of forest loss throughout Cambodia between 2000 and 2014 (image credit: NASA Earth Observatory)
• November-December 2016: Landsat-7's duty cycle has been raised to 105%, resulting in ~15 more scene acquisitions per day. On average, Landsat-7 is now acquiring around 470 scenes per day. The USGS is committed to continuity by extending Landsat-7's operational life until the launch of Landsat-9 late in 2020. Once retired, plans are being prepared to use Landsat-7 to test satellite-refueling technology via the NASA Restore-L mission. 22)
- In May 2016, NASA officially moved forward with plans to execute the ambitious, technology-rich Restore-L mission, an endeavor to launch a robotic spacecraft in 2020 to refuel a live satellite. The mission – the first of its kind in LEO (Low Earth Orbit), will demonstrate that a carefully curated suite of satellite-servicing technologies are fully operational. The current candidate client for this venture is Landsat-7, a government-owned satellite in LEO. 23)
- Beyond refueling, the Restore-L mission also carries another, weighty objective: to test other crosscutting technologies that have applications for several critical upcoming NASA missions. As the Restore-L servicer rendezvous which, grasps, refuels, and relocates a client spacecraft, NASA will be checking important items off of its technology checklist that puts humans closer to Mars exploration.
• September 21, 2016: Today the Landsat project celebrates the 50th anniversary of Secretary of the Interior Stewart Udall's 1966 announcement of "Project EROS (Earth Resources Observation Satellites)". Udall's vision paved the way for what we today know as Landsat, and gave the world the confidence to create satellite systems to monitor our planet with a new perspective. 24)
- Secretary Udall's vision to create "a program aimed at gathering facts about the natural resources of the Earth from earth-orbiting satellites" was an idealistic goal at the time, but on July 23, 1972, the first Earth Resources Technology Satellite (ERTS) was launched from Vandenberg Air Force Base in California. In 1975, it was renamed Landsat 1. Since then, six more Landsat satellites have followed, collectively capturing millions of images of Earth, and creating an impressive archive that has been available at no charge since 2008.
• July 2016: Soil salinity is one of the most important problems affecting many areas of the world. Saline soils present in agricultural areas reduce the annual yields of most crops. This research deals with the soil salinity mapping of the Seyhan plate of Adana district in Turkey from the years 2009 to 2010, using remote sensing technology. In the analysis, multitemporal data acquired from Landsat-7 ETM+ satellite in four different dates (19 April 2009, 12 October 2009, 21 March 2010, 31 October 2010) are used. 25)
- The main objectives of this study are: (i) to understand the spectral reflectance characteristics of saline soil in the Seyhan plate, (ii) to explore the potential of Landsat imagery to detect and map the soil salinity over the study area, (iii) to analyze the correlation between field measurements and Landsat imagery, and (iv) to produce the soil salinity map according to high, moderate and low saline content.
- Study area: Çukurova is a district in south central of Turkey covering the provinces of Adana and Mersin (Figure 16). It is located in the coordinates of 37º02'52'' North latitudes and 35º17'54'' East longitudes. The total area of the Çukurova is about 38,000 km2, Turkey's biggest delta plain with a large stretch of flat and fertile land, which is among the most agriculturally productive areas of the world. The climate is relatively mild and humid in the winter months and the alluvial soils make the area highly suitable for agriculture. The Akarsu irrigation basin and the Seyhan plate are located in the Çukurova plain.
- In this study, the Landsat-7 ETM+ satellite images with 30 m resolution were used. The images were georectified to a UTM (Universal Transverse Mercator) coordinate system, using WGS (World Geodetic System) 1984 datum, assigned to north UTM zone 36 and Path 175 Row 34, 35. The most compatible and close dates were selected according to the dates of field work not to have any problems like seasonal changes. The dates of satellite images used are given in the Table 4.
Table 4: The dates of field measurements and Landsat-7 acquisitions
- Ground truth measurements: Fieldwork performed by Cukurova University, Remote Sensing and GIS group in May 2009, October 2009, October 2010 and March 2010, were used in the analysis. In these periods, different numbers of soil samples and soil EC (Electrical Conductivity ) were collected by using the EM-38 device. As a total, 688, 269,153, and 27 samples were collected in the periods of 12-Oct-2009, 31-Oct-2010, 19- Apr-2009 and 21-Mar-2010, respectively. The dataset consists of four Landsat images belonging to the years 2009 and 2010 winter and summer cropping seasons; hence it is possible to evaluate the soil salinity conditions in both seasonal and annual periods.
- As a first step, preprocessing of Landsat images is applied. Several salinity indices such as NDSI (Normalized Difference Salinity Index), BI (Brightness Index) and SI (Salinity Index) are used besides some vegetation indices such as NDVI (Normalized Difference Vegetation Index), RVI (Ratio Vegetation Index), SAVI (Soil Adjusted Vegetation Index) and EVI (Enhanced Vegetation Index) for the soil salinity mapping of the study area.
Figure 16: The map and satellite image of the study area (image credit: Istanbul Technical University)
Results (Ref. 25):
- As a first approach, simple linear regression technique was applied to each individual bands and low correlation (r2: - 30.89% to 20.02%) was observed. As a second approach, the multiple linear regression was applied to all bands of satellite images. Among all different band combinations tested, the correlation of 18 sampled points of March 21, 2010 with EC measurements, showed the highest correlation (78.40%) due to near simultaneous acquisition of the satellite data (3 days).
- After the correlation analysis, the satellite data showing the highest correlation (March 21, 2010) were chosen to map the soil salinity in the area As observed, the highest saline soils in the study area are taking place in the region covering reeds due to the presence of high amount of salt in the lake. Besides, the other parts of the Seyhan plate are being affected by medium to low soil salinity and the salt is mostly accumulated in the lower part of the study area.
- As a final step, the percentages of the salt-affected fields in the study area were evaluated. taking into two major fields into account, it was seen that bare soil fields are rather more influenced by salinization than the wheat fields due to having direct interaction.
The following list is a summary of the main conclusions, drawn from this study:
- The compatibility between the satellite data and the field data; the more simultaneous satellite data and field measurements are used, the better a correlation can be observed.
- Radiometric quality of Landsat-7 ETM+data: the missed lines can make it difficult to get the exact value of a pixel or visually affect the image interpretation. Although the scan line error of Landsat-7 ETM+ can be corrected using the available remote sensing software, the original values are modified.
- Spatial resolution of Landsat-7 satellite 30 m data: the higher resolution makes the sampling easier in the image data.
- Spectral resolution of Landsat-7 satellite data: Hyperspectral sensors can be a better solution, since they capture a large amount of narrow bands.
As a continuation of this study, it is planned to test the other multispectral/hyperspectral sensor data in the same area to evaluate the effectiveness of the different spectral/spatial resolutions on the salinity mapping in the future.
• In November 2015, Landsat-7 still acquires geometrically and radiometrically accurate data worldwide, and methods have been established that allow users to fill the data gaps. 26)
- Landsat data support a vast range of applications in areas such as global change research, agriculture, forestry, geology, land cover mapping, resource management, water, and coastal studies. Specific environmental monitoring activities such as deforestation research, volcanic flow studies, and understanding the effects of natural disasters all benefit from the availability of Landsat data. In recent years, Landsat data have also been used to track oil spills and to monitor mine waste pollution. Table 5 lists Landsat bands and describes the use of each band to help users determine the best bands to use in data analysis.
Table 5: The bands of each Landsat satellite and descriptions of how each band is best used
Figure 17: Timeline and history of the Landsat Missions, which started in 1972 (image credit: USGS, Ref. 26)
• October 12, 2015: The delta plain of the Mississippi River is disappearing. The lobe-shaped arc of coastal land from the Chandeleur Islands in eastern Louisiana to the Sabine River loses a football field's worth of land every hour. Put another way, the delta has shrunk by nearly 5,000 km2 over the past 80 years. That's as if most of Delaware had sunk into the sea. 27)
- Though land losses are widely distributed across the 300 km wide coastal plain of Louisiana, Atchafalaya Bay stands as a notable exception. In a swampy area south of Morgan City, new land is forming at the mouths of the Wax Lake Outlet and the Atchafalaya River. Wax Lake Outlet is an artificial channel that diverts some of the river's flow into the bay about 16 km west of where the main river empties.
- This series of false-color satellite images chronicles the growth of the two deltas between 1984 and 2015. All of the images were acquired by instruments on Landsat satellites: the Thematic Mapper on Landsat 5, the Enhanced Thematic Mapper Plus on Landsat 7, and the Operational Land Imager on Landsat 8. A combination of shortwave infrared, near infrared, and green light was used to accentuate differences between land and water. Water appears dark blue; vegetation is green; bare ground is pink. All of the images were acquired in autumn, when river discharge tends to be low.
Figure 18: The Atchafalaya River Delta and Wax Lake Outlet, acquired by Landsat-5 on Nov. 4, 1984 (image credit: NASA)
Figure 19: The Atchafalaya River Delta and Wax Lake Outlet, acquired by Landsat-7 on Oct. 12, 2015 (image credit: NASA)
- Both deltas are being built by sediment carried by the Atchafalaya River. The Atchafalaya is a distributary of the Mississippi River, connecting to the "Big Muddy" in south central Louisiana near Simmesport. Studies of the geologic history of the meandering Mississippi have shown that—if left to nature—most of the river's water would eventually flow down the Atchafalaya. But the Old River Control Structure, built in the 1960s by the U.S. Army Corps of Engineers, ensures that only 30 percent of the Mississippi flows into the Atchafalaya River, while the rest of the keeps moving toward Baton Rouge and New Orleans.
- Even the Atchafalaya's flow has been sub-divided. In 1941, the Army Corps opened the Wax Lake Outlet, a dredged channel designed to reduce the severity of floods in Morgan City. About 40 percent of the Atchafalaya's discharge gets channeled through the Wax Lake Outlet.
- Even with the reduced flow, the Atchafalaya carries enough sediment to build land. While geologists first noticed mud deposits building up in Atchafalaya Bay in the 1950s, new land first rose above the water line in 1973 after a severe flood. Since then, both deltas have grown considerably. According to one estimate by scientists from Louisiana State University (LSU), the Atchafalaya and Wax Lake Outlet deltas have combined to grow by 2.8 km2 per year.
- The rate of growth has varied considerably, mainly due to the timing of major floods and hurricanes. Floods transport large volumes of extra sediment to Atchafalaya Bay, while hurricanes redistribute sediment within the bay and transport it offshore into deeper waters. Hurricanes also destroy coastal vegetation that would otherwise protect land from erosion.
- The two deltas added a combined 34 km2 of land between 1989 and 1995, for instance, but lost 2 km2 between 1999–2004, according to the LSU team. The land loss coincided with a period when hurricanes Allison, Isidore, and Lili battered Atchafalaya Bay and there were no major floods to replenish sediments.
- As is typical of deltas in southern Louisiana, the Atchafalaya and Wax Lake Outlet deltas have grown southward into the Gulf of Mexico. While the newer, outer lobes are periodically submerged by the sea and are still free of vegetation, various types of plants—including reeds and willows—colonize the higher elevations of the sandbars. Vegetation is critical to maintaining new land because roots stabilize sediment and prevent erosion.
- The Atchafalaya delta has grown at a faster rate than its neighbor—about 1.6 km2 per year, versus 1.2 km2 per year for the Wax Lake delta. The difference is due to regular dredging and channel widening on the lower Atchafalaya, which delivers extra sediment to its delta. In the sequence of images, the emergence of new dredged islands—which appear pink—is particularly noticeable between 1993 and 1994. Due to the lack of dredging, Wax Lake delta is more natural in character, with a more symmetric, lobate shape.
- So why are the deltas in Atchafalaya Bay growing while the rest of Louisiana's coastline is retreating? The key reason is that the Atchafalaya delivers sediment to the coast at a pace that allows it to settle into shallow water and to maintain marshes. In contrast, an extensive series of levees keep the Lower Mississippi's water flowing in a narrow channel that whisks water and sediment past natural floodplains. Instead of building new land along the mouth of the Mississippi, the controlled river sends jets of sediment-rich water directly into the relatively deep waters of the Gulf of Mexico and toward the edge of the continental shelf.
- With sea level rising rapidly due to natural geological processes, climate change, and human activities, predictions for the future of the Mississippi Delta are grim. Even the land gains in Atchafalaya Bay will do little to offset the losses elsewhere, according to geologists. Most scientists expect the Mississippi Delta Plain to lose roughly 5,000 km2 of land over the next 50 years.
- "We are looking carefully at the Wax Lake and Atchafalaya deltas as models for building new land and preserving some of our coastal marshlands," said Harry Roberts, director of the Coastal Studies Institute at LSU. "If we start diverting significant portions of the water and sediment from the main channel of the Mississippi River into adjacent wetlands, lakes, and bays—as happens now in Atchafalaya Bay—we'll be taking an important first step toward saving a significant part of Louisiana's coastal plain."
Figure 20: Satellite view of the Louisiana coastline with the studied areas of the Atchafalaya River Delta and Wax Lake Outlet, circled in red. The Mississippi River is the long meandering blue line, farthest right (east), image credit: USGS, NASA)
• On April 15, 2015, Landsat-7 was on orbit for 16 years, well past its 5-year design life. A number of spacecraft components are being monitored (e.g., attitude control system, remote telemetry command box, power control unit) but overall, Landsat-7 is acquiring more imagery than at any point in its history (collecting about 475 scenes/day, ~22% of pixels are missing per scene (faulty scan-line corrector). 28) 29)
- There was considerable discussion on the end-of-mission for Landsat-7. Fuel depletion is expected by late 2018, but the imaging life can be extended with reduced orbit maintenance. Preliminary analysis shows that if Landsat-7 is authorized to continue imaging outside of its nominal 10:00 AM equator crossing time window, the mission could be extended until early 2020. At that point, however, the local solar time would be 9:15 AM. The LST (Landsat Science Team) encouraged continued imaging until 2020. As Landsat 5 orbit decayed to an approximate 9:15 AM orbit in the late 1990s, there is precedent for imaging outside the nominal crossing time.
• Landsat-7 is operational in 2015 with the known degradations of the ETM+ instrument, collecting global data.
• On April 2014, the Landsat-7 satellite has now been observing Earth from outer space for 15 years. 30)
• Landsat-7 status on March 3, 2014: The spacecraft continues its science mission. 31)
- Currently, the Landsat-7 archive is providing 1,708,353 scenes of ETM+ data which corresponds to ~ 1,586 TB Raw and L0Ra Data (average scene size of 487 MB). 32)
Figure 21: Landsat-7 spacecraft status in 2014 (image credit: USGS)
• 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. 33)
• The LS-7 spacecraft is operational in 2013 with the known degradations of the ETM+ instrument, collecting global data. 34)
- 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 22: 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. 34)
Legend to Figure 22: 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. 35)
Legend to Figure 23: 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. 37)
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. 38)
Legend to Figure 24: 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). 40)
• 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. 41) 42) 43)
Figure 25: Overview of LS-7 component failures/recoveries over its life until Jan. 2010 (image credit: USGS, Ref. 42)
• 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. 44)
• 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. 45) 46) 47)
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 6. 48)
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. 49)
Table 6: Schedule of Landsat archival data availability via Internet
Figure 26: 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 27: Active Landsat ground stations (image credit: USGS) 50)
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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 (firstname.lastname@example.org).