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Section 2 - Observatory Overview

The Landsat 7 satellite was successfully launched from Vandenberg Air Force Base on April 15, 1999. The Delta II launch vehicle left the pad at 11:32 a.m. Pacific Daylight Time and performed flawlessly (Figure 2-1). The spacecraft is a 5000 pound-class satellite designed for a 705 kilometer (km), sun-synchronous, Earth mapping orbit with a 16-day repeat cycle. Its payload is a single nadir-viewing scanning instrument, the Enhanced Thematic Mapper Plus (ETM+). Details about the ETM+ are provided in Section 2.2. 

Launch of rocket containing Landsat 7 satellite 4-15-1999
Figure 2-1. Landsat 7 Launch April 15, 1999

Two radio frequency connections are used on Landsat 7. S-Band is used for commanding, tracking, and housekeeping telemetry operations while X-Band is used for instrument data downlink. A 378 gigabit Solid State Recorder (SSR) can hold 42 minutes of instrument data and 29 hours of housekeeping telemetry concurrently. Power is provided by a single sun-tracking solar array (four 74" by 89.3" panels - 184 square feet) and two 50 amp-hour nickel-hydrogen batteries.

2.1 Concept of Operations

The fundamental Landsat 7 operations concept is to collect, archive, process, and distribute science data in a manner consistent with the operation of previous Landsat missions

2.1.1. Orbit

The orbit of the Landsat 7 satellite is repetitive, circular, Sun-synchronous, and near-polar at a nominal altitude of 705-km (438 miles) at the Equator. The spacecraft crosses the Equator from north to south on a descending orbital node at 10:00 AM ±15 minutes on each pass. Circling the Earth at about 7.5 km/sec, each orbit takes approximately 99 minutes. The spacecraft completes just over 14 orbits per day thus covering the Earth between about 81 degrees north and south latitude (scene center latitude) every 16 days. Figure 2-2 illustrates Landsat 7 orbit characteristics, which is essentially the same as the orbit for Landsat 5 (1984-2013) and Landsat 8 (2013-onward). Landsat 8 has an 8-day temporal offset relative to Landsat 7 to enable a revisit period of a given ground track to be half of an individual satellite’s revisit period.

In addition, as part of EOS, Landsat 7 and Terra were launched and injected into identical 705 km, sun-synchronous orbits in 1999. This same day orbit configuration separates the satellites by about 15 minutes (i.e., equatorial crossing times of 10:00 a.m. ±15 minutes for Landsat 7 and 10:30 a.m. for Terra). A multispectral dataset having both moderate (30 meter) and medium to coarse (250 to 1000 meter) spatial resolution from Terra's Moderate-resolution Imaging Spectroradiometer (MODIS) is thereby acquired on a global basis repetitively and under nearly identical atmospheric, plant physiological, and Earth surface conditions.

Figure 2-2 Illustration of Landsat 7 orbit
Figure 2-2. Illustration of Landsat 7 Orbit

2.1.2 Swath Pattern

Landsat 7 orbits the Earth in a preplanned ground track. The ETM+ sensor onboard the spacecraft obtains data along the ground track at a fixed width or swath as depicted in Figure 2 3. The 16-day Earth repeat coverage cycle for Landsat 7 is known as the swath pattern of the satellite. The paths scheduled for acquisition on any day can be viewed on the Landsat 7 Acquisition Calendar

Figure 2-3 Landsat 7 ETM+ swath separations for single day (left) and a full 16-day cycle (right)
Figure 2-3. Landsat 7 ETM+ Swath Separations for Single Day (left) and a Full 16-Day Cycle (right)

At the Equator, adjacent swaths overlap at the edges by 7.3 percent. Moving from the Equator toward either pole, this sidelap increases because of the fixed 185 km swath width. This mission attribute is quite beneficial for repeat mapping of polar features such as the margin of Antarctica. Table 2 1 shows the amount of sidelap from 0 to 80 degrees latitude in 10-degree increments. Due to the ETM+ swath width, the maximum latitude limit for Landsat 7 coverage is ~82.5°.

Latitude (degrees) Image sidelap percentage
0 7.3
10 8.7
20 12.9
30 19.7
40 29.0
50 40.4
60 53.6
70 68.3
80 83.9

Table 2-1. Image Sidelap of Adjacent Swaths

2.1.3 Worldwide Reference System (WRS)

The standard WRS-2 as defined for Landsat 4 and Landsat 5 was preserved for Landsat 7. WRS-1 was used for Landsat 1-3, which had a higher orbit altitude and different swath pattern. The WRS-2 indexes orbits (paths) and scene centers (rows) into a near-global grid system (for both daytime and nighttime portions of orbits) composed of 233 paths by 248 rows.

The term “row” refers to the latitudinal centerline across a frame of imagery along any given path. As the spacecraft moves along a path, the ETM+ scans the terrain below. During ground processing, the continuous data stream or subinterval of image data is framed into individual scenes that are each 23.92 seconds of spacecraft motion, resulting in 248 rows per complete orbit. The rows have been assigned in such a way that Row 60 coincides with the Equator (descending node). Row 1 of each WRS-2 path starts at 80° 46' N and the numbering increases southward to latitude 81° 51' S (Row 122). Then, beginning with Row 123, the row numbers ascend northward, cross the Equator (Row 184) and continue to latitude 81° 51' N (Row 246). Row 248 is located at latitude 81° 21' N, whereupon the next path begins. Also, there is a coverage hole at both poles where Landsat 7’s inclined orbit does not allow latitudinal coverage above -82º. Figure 2-4 graphically depicts the Landsat path/row scheme.

Successive orbits and spacecraft attitude are controlled to assure minimal variation to either side from the intended ground track. The WRScornerPoints.xls file lists the nominal latitude and longitude for the scene center and four corners of each WRS-2 scene. This includes the ascending rows that are generally not illuminated by the sun. The table uses the notation that positive latitude is north, negative latitude is south, positive longitude is east, and negative longitude is west. The table is sorted by path/row order. WRS-1 and WRS-2 path/row maps are available online in a variety of formats. The path/row number for a specific location is different in WRS-2 than in WRS-1.

The Landsat 7 Acquisition Calendar displays the acquisition dates of specific paths for Landsat 7. 

Figure 2-4 Illustration of WRS-2 path/row scheme
Figure 2-4. Illustration of WRS-2 path/row Scheme

2.1.4 Scheduling

Planning and scheduling all satellite activities takes place in three categories: long-term planning, short-term planning, and daily scheduling

2.1.4.1 Long-Term Planning

The prime goal of the Landsat 7 mission is to refresh the global land surface imagery archive. Because the Landsat 7 mission's orbit profile is a repetitive 16-day cycle, the Long Term Acquisition Plan (LTAP, see Section 4) was designed years before the Landsat 7 satellite launch. This provided ample time for coordination with the science community, program management, international resources, and project elements. The LTAP also addresses the needs of land cover classes such as glaciers, reefs, tropical forests, and land ice. The Mission Operations Center (MOC) scheduling subsystem was built around the long-term plan and uses it to generate daily schedules of both instrument and SSR activities.

2.1.4.2 Short-Term Planning

The objectives of short-term planning are to schedule communication contacts for telemetry, tracking, and commanding services on the Landsat Ground Network (LGN). The system is designed to include special requests into the scheduling process and to generate daily reports summarizing the disposition of imaging requests and time-ordered scheduled ETM+ imaging events for the latest 37-hour period.

2.1.4.3 Daily Scheduling

On a daily basis, the MOC scheduling system generates a set of imaging, SSR activities, and X-Band downlink services based on a number of criteria including global refresh requirements, request priority, SSR or other resource availability, and cloud cover predictions. To assist in the scheduling process, the MOC receives planning aids from the flight dynamics subsystem within the MOC, cloud cover predicts from the National Centers for Environmental Prediction (NCEP), and assessed cloud cover feedback on newly archived imagery from the EROS Landsat Processing System (LPS).

2.1.5 Tracking and Spacecraft Control

The two-channel Radio Frequency (RF) communications system on Landsat 7 provides S-band (narrowband) telemetry for housekeeping data and tracking ability. The S-Band communications are conducted through two omni S-band antennas located on opposite sides of the spacecraft (nadir and zenith pointing). The zenith antenna is used for Tracking Data and Relay Satellite System (TDRSS) communications; the nadir antenna is used for LGN communications. Each antenna provides essentially hemispherical coverage.

Telemetry data are generated and recorded at all times and contain all of the information required to monitor and assess the health of the satellite, verify day-to-day operations, and assist in anomaly resolution. The MOC receives tracking telemetry on a daily basis that shows the position and velocity of the spacecraft. Tracking data are used to compute the actual spacecraft position and velocity for the last 61 hours and generate predicted orbit state vectors for the next 72 hours. The orbit state vectors must provide an attitude accuracy of 375 meters at 40 hours and must be uplinked daily in order to maintain the satellite within mission parameters.

The predicted orbit state vectors are compared against the old orbit state vectors in the same way the flight software makes this comparison on the satellite after the receipt of the new orbit state vectors load. Once validated, the new state vectors are uplinked to the satellite and activated. On-board software interpolates this new data to generate the positional information used for on-board navigation and contained in the Payload Correction Data (PCD).

2.1.6 Orbit Maneuvers

During the Landsat 7 mission, the MOC Operators perform three types of orbit maneuvers to ensure the safety of the satellite, as well as maintain that the satellite's orbit remains within the Landsat 7 mission parameters. The three types of orbit maneuvers are:

  • In-plane maintenance
  • Inclination maintenance
  • Collision avoidance or risk mitigation

The in-plane maintenance maneuver, also called a drag make-up or delta-velocity maneuver, maintains the semi-major axis within an acceptable tolerance of the mission orbit semi-major axis. The semi-major axis is biased high and permitted to decay over time. The bias applied to the orbit varies with the amount of environmental drag, which is a function of solar activity. These maneuvers are performed every few weeks during a period of high environmental drag, and every few months during a period of low drag.

The inclination maintenance maneuver involves a yaw slew by approximately +/- 90 degrees. The operators perform the inclination maneuver to keep the satellite’s descending node within a 30-minute box (09:45 a.m.to 10:15 a.m.). This type of maneuver was performed each fall until 2012, when it was performed in spring after an in-plane maneuver designed for a high-drag environment instead encountered low-drag and sent the spacecraft rapidly drifting out of its control box. The schedule returned to the fall the following year. In 2014 and 2015, smaller inclination maneuvers were performed in both the spring and fall, to conserve fuel and position the satellite for a long end-of-mission drift downward from the 10:15 a.m.extent of the box.

Collision avoidance or risk mitigation maneuvers are executed if the risk of a collision or close approach to space debris or another constellation is judged significantly high, based on inputs from the space debris monitoring agencies and the committee that monitors the constellations. Space debris is actively tracked and a system has been initiated in recent years to give satellite operators notice of close approaches with debris. Additionally, the morning and afternoon Earth-observing constellations have grown over the years and occasionally there is a close approach between constellations.

The demand for orbit maneuvers is determined by the MOC flight dynamics subsystem, which uses tracking data from the Space Network (SN) and LGN stations as well as housekeeping telemetry data to monitor the orbit and decide when the orbit has been perturbed sufficiently to require an orbit adjust. The resulting maneuver is reviewed by the USGS Flight System Manager (FSM) and Flight Operations Team (FOT) Engineers.

Once the maneuver is approved, MOC Operators generate a set of satellite commands which are then reviewed by the FOT Engineers. The FOT coordinates the orbit maneuver times and calculates post-maneuver position and velocity. On the day of the maneuver, the FSM makes the final decision to perform the maneuver and the FOT sends the command load to the satellite.

Following the maneuver, the MOC conducts TDRSS contacts for calculating the new orbit and evaluates the performance of the satellite components used during the orbit adjust. This information is used to calculate the remaining propellant on-board the satellite.

After many of the inclination maneuvers, imaging was performed while the ETM+ focal planes slowly returned to their nominal operating temperatures. The resulting engineering data was used to enhance the instrument calibration curves by extending them into the non-nominal operating temperature ranges.

2.2 Enhanced Thematic Mapper Plus (ETM+)

Landsat 7's sensor - the ETM+ (see Figure 2-5) - was built by Santa Barbara Remote Sensing (SBRS). The sensor is a derivative of the Thematic Mapper (TM) engineered for Landsat 4 and Landsat 5, but is more closely related to the ETM that was lost during the Landsat 6 launch failure. The primary performance-related enhancements of the ETM+ over the TM are the addition of two gain ranges, the panchromatic band (Band 8, originally added for ETM), the improved spatial resolution for the thermal band (Band 6), and the addition of two solar calibrators.

Figure 2-5 Landsat 7 ETM+ sensor and major components
Figure 2-5. Cut-Away View of ETM+ Sensor and its Major Components

Reflected energy from the Earth's surface energy passes through a number of major ETM+ subsystems, depicted schematically in Figure 2-6, before the solid-state detectors at the focal plane can collect it. The bidirectional Scan Mirror Assembly (SMA) sweeps the detector's line of sight in approximately west-to-east and east-to-west directions across track, while the spacecraft's orbital path provides the general north-south motion. As originally designed, a Ritchey-Chretien telescope focuses the energy onto a pair of motion compensation mirrors (i.e. Scan Line Corrector (SLC)) where it is redirected to the focal planes. The SLC is required due to the compound effect of along-track orbital motion and cross-track scanning, which would lead to significant overlap and underlap in ground coverage between successive scans, unless this is compensated for in the sensor design (see Section 2.2.3.3).

Figure 2-6 Illustration of ETM's optical path from target area to detectors on the focal plane
Figure 2-6. Illustration of  the ETM's Optical Path from Target Area to Detectors on the Focal Plane

The aligned energy encounters the Prime Focal Plane (PFP), where the silicon detectors for Bands 1-4 and Band 8 are located. A portion of the scene energy is redirected from the PFP by the relay optics to the Cold Focal Plane (CFP) where the detectors for Bands 5, 6, and 7 are located. The temperature of the CFP is maintained at a predetermined setting of 91 Kelvin (K) using a radiative cooler. The spectral filters for the bands are located directly in front of the detectors.

2.2.1 Detector Geometry

The relative position of all detectors from both focal planes with respect to their actual ground projection geometry is illustrated in Figure 2-7. The even-numbered detectors are arranged in a row normal to the scan direction while the odd-numbered detectors are arranged in a parallel row, off exactly one Instantaneous Field of View (IFOV) in the along-scan direction. This arrangement provides for a contiguous bank of 32, 16, and 8 detectors for Band 8, Bands 1-5 and 7, and Band 6 respectively. The detector arrays are swept left to right (forward) and right to left (reverse) by the scan mirror. With each sweep or scan, an additional 480 meters (32, 16, and 8 data lines at a time) of along-track image data is added to the acquired subinterval.

Figure 2-7 Detector projection at ETM+ focal planes
Figure 2-7. Detector Projection at the ETM+ Focal Planes

2.2.1.1 Band Offsets

During a scan, the actual ground observed by each band's detectors is not identical due to the horizontal spacing of detector rows within and between bands. Referring to the ground projection illustration in Figure 2-7, note the spacing between bands as measured in 30 meter or 42.5 microradian IFOVs. Taken individually, these numbers represent a band's unique leading edge preamble that occurs before coincident data are collected by a band's forward or reverse focal plane neighbor. Taken cumulatively, these numbers represent the first order zero fill offsets that LPS uses during Level-0R processing to achieve image registration of the raw, uncorrected data. Other factors such as detector offsets within a band and sample timing must be considered to calculate registration offsets accurately.

2.2.1.2 Detector Offsets

Band 8 detector rows are separated by two 42.5µ IFOVs, which translates to four 15 meter samples. The Band 8 odd and even detectors are sampled simultaneously, twice per minor frame (i.e., one sample). The registration offsets for the odd and even detectors therefore always differ by four samples for both forward and reverse scans.

The detector rows within Bands 1-5 and Band 7 are separated by two and a half 42.5µ IFOVs. This design works because the multiplexer samples the even detectors one half of an IFOV later than the odd detectors within a minor frame of data. The delay effectively separates the odd and even detectors an integral multiple of IFOV's apart in sampling space. A two IFOV odd-to-even detector spacing is realized on forward scans while three IFOV spacing occurs on reverse scans. The registration offsets for forward and reverse scans always differ by these amounts.

The Band 6 odd and even detectors are separated by five 42.5µ IFOVs which translates to 2.5 Band 6 samples. The odd and evens are sampled, however, in alternating minor frames, which separate the odd and even detectors, an integral multiple of IFOVs. A two Band 6 IFOV odd-to-even detector spacing is realized on forward scans while a three Band 6 IFOV spacing occurs on reverse scans. The registration offsets for forward and reverse scans always differ by these amounts.

2.2.1.3 Registration Offsets

Over the years, different Ground System Engineers have characterized Landsat sensor focal plane offsets in different ways that resulted in negative and positive offsets depending upon the forward and reverse scan directions and origin of the image grid. For ETM+, all shifts are declared as positive from column 1 in the 0R image buffers. These 8-bit buffers are 3300, 6600, and 13,200 elements in size for the 60-, 30-, and 15-meter bands, respectively.

2.2.2 ETM+ Sample Timing

The ETM+ data stream is composed of a continuous succession of major frames. A major frame contains the data for an entire period of one complete scan of the ETM+ scan mirror. A major frame is partitioned into minor frames—a specific pattern for organizing groups of ETM+ data words. This pattern is based on the architecture of the Landsat 7 Auxiliary Electronics Module (AEM) that samples, digitizes, and groups analog video signals from the ETM+ scanner to form scene data. The minor frame data structure is 85 words (8 bits) in length consisting of 16 separate groups of 5 words, 4 words from Band 6 along with a one-word spare.

The AEM generates the line sync code at the beginning of each new scan line and inserted into the minor frame zero position, which is also called Scan Line Start (SLS). The time code pattern preempts all video data except Band 6 even though its data for the first minor frame is invalid. The six minor frames immediately following the SLS minor frame describe the spacecraft time code as illustrated in Figure 2-8. The Band 6 data alternate between the odd and even detectors for each successive minor frame and are synchronized to odd pixel data for the first minor frame. The first valid Band 6 data are from the even detectors and occur in minor frame two.

Scene data transmission for the other bands starts at the minor frame boundary immediately following the time code and continues until the start of the next end-of-line pattern code, which is mechanically / optically triggered. For reference, 6313 minor frames of scene data are nominally transmitted during any given scan cycle. The digitized scene data can be organized into either of two minor frame scene data formats as depicted in Figure 2-8 and Figure 2-9. Bands 1-5 are allocated to Format 1 while Band 7 and Band 8 are allocated to Format 2. The two Band 6 data streams allocated to Format 1 and Format 2 are obtained with low gain and high gain settings, respectively.

Figure 2-8 Illustration of ETM+ sample timing, format 1
Figure 2-8. Illustration of ETM+ Sample Timing, Format 1

Figure 2-9 Illustration of ETM+ sample timing, format 2
Figure 2-9. Illustration of ETM+ Sample Timing, Format 2

2.2.3 ETM+ Subsystems

2.2.3.1 Scan Mirror Assembly (SMA)

The SMA provides the cross-track scanning motion to develop the 185 km-wide scene swath. The SMA consists of a flat mirror supported by flex pivots on each side, a torquer, Scan Angle Monitor (SAM), two leaf spring bumpers, and Scan Mirror Electronics (SME). The motion of the mirror in each direction is stopped by the bumper and is boosted by precision torque pulses during the turnaround period. The SME microprocessor as determined from the SAM mirror angle pulses determine the amount of torque applied. SAM mirror angle pulses are used by the multiplexer to synchronize the detected scene data. There are two redundant sets of SMEs, SME 1 and an identical backup package, SME 2. Additionally, both SMEs have a primary SAM mode of operation and a backup Bumper Mode of operation. Table 2-2 lists the SMA characteristics.

Parameter Specifications
Swath width at 0 degrees North 185 km
Swath width at 40 degrees North 187 km
Scan length at 0 degrees North 480 m
Scan length at 40 degrees North 484 m
Active scan amplitude 7.695 degrees
Scan period 142.925 milliseconds
Scan frequency 6.997 Hz
Active scan time 60.743 milliseconds
Turnaround time 10.719 milliseconds
Obejct plane scan rate 4.42191 rad/sec
Mirror scan rate 2.21095 rad/sec
30-m IFOV dwell time 9.611 microseconds
Scan line length
(excludes turn-around)
6320 IFOVs
Inertia <10.83 kg-cm-(sec2)
Clear aperature 53.467 cm x 41.275 cm

Table 2-2. Scan Mirror Assembly Characteristics

Current Performance

Regular monitoring has revealed the ETM+ scan period is increasing with time due to growth in turnaround time, probably caused by bumper wear. However, the active scan time is showing increased variability, especially in the forward scans. The rate of growth of turnaround time, however, appears to be stable. The impact to ETM+ data is that scan gaps will gradually increase with time. Also, the scan mirror could, theoretically, lose synchronization with the calibration flag if scan time gets too long. This would effectively end the Landsat 7 mission. However, this mirror behavior is similar to what was observed in Landsat 5 TM mission’s lifetime and has proven to be manageable over the Landsat 7 mission.

Physical wear of the mirror bumpers led to growth in time to synchronize the scan mirror and calibration flag at instrument start-up. With the rate of wear rapidly increasing, the instrument warm-up time was being increased to compensate for this while the EROS product generation systems were updated to handle bumper mode processing. On April 1, 2007, the ETM+ was transitioned from the SAM mode to Bumper mode. This also impacts the scan mirror deviation correction.

The Gimbaled X-band Antenna (GXA), when maneuvered in an across-track slew, sometimes disturbs the ETM+ scan mirror. The resulting impact to ETM+ data is a wider than normal (see Figure 2-10) variation in scan line length. Most extreme examples exceed the maximum allowable scan length leading, which leads to dropped scans. This phenomenon was not observed on earlier missions because pointable X-Band antennae are a new component on Landsat 7.

Figure 2-10 ETM+ GXA data anomaly caused by X-band antenna across-track slew maneuver
Figure 2-10. ETM+ GXA Data Anomaly Caused by an X-Band Antenna Across-Track Slew Maneuver

The scan mirror may lose synchronization or, in extreme cases, restart during imaging. Such an occurrence is correlated with regions of high electron flux (see Figure 2 11) at 705 km orbital altitude particularly in Polar Regions and within the South Atlantic anomaly. The correlation was confirmed by a July 2000 solar flare which resulted in 14 anomalies in a single day. The impact to ETM+ data is dropped scans and calibration flag incursions (see Figure 2-11) into the Earth imaging area. The scan mirror controller sees an "extra" timing pulse and thus loses synchronization with the mirror calibration flag. This phenomenon may also have occurred on Landsat 5 and impacted TM data.

Figure 2-11 Modeled regions of high electrol flux at 705 km (left) and GXA anomaly in regions of high electron flux (right)
Figure 2-11. Modeled Regions of High Electrol Flux at 705 km (left) and GXA Anomaly in Regions of High Electron Flux (right)

2.2.3.2 Telescope

The telescope is a Ritchey-Chretien configuration with a primary and secondary mirror and baffles. Both the tube-like central baffle and the outer housing have a series of annular baffles for stray light control. The telescope structure is constructed using a graphite-epoxy laminate, which has a very low coefficient of thermal expansion and thus eliminates problems due to thermal expansion. However, the graphite-epoxy laminate is hygroscopic and can change dimensions due to moisture absorption. Table 2-3 summarizes the telescope's characteristics. 

Parameter Specifications
Primary mirror clear aperature outer diameter 40.64 cm
Primary mirror clear aperature inner diameter 16.66 cm
Telescope effective clear aperture 1020 cm2
Effective focal length 243.8 cm
f/# 6.0
Mirror material Ultra-Low Expansion (ULE) glass
Mirror coating Enhanced silver

Table 2-3. ETM+ Telescope Characteristics

2.2.3.3 Scan Line Corrector (SLC)

The SLC is an electro-optical mechanism composed of two parallel mirrors set at an angle on a shaft. The SLC is positioned behind the primary optics and compensates for the along-track motion of the spacecraft occurring during an active SMA cross-track scan. As a result, a rectilinear scan pattern is produced using the SLC instead of the zigzag pattern that would be produced without it (see Figure 2-12). Table 2-4 lists the SLC characteristics.

Figure 2-12 ETM+ scan line corrector (SLC) effect
Figure 2-12. ETM+ Scan Line Corrector (SLC) Effect

 

Parameter Specification
Scan frequency 13.99 Hz
Scan period 71.462 ms
Scan rate in object space 9.610 µrad/sec
SLC rotation rate 576.6 µrad/sec
SLC linear scan angle 35.02 µrad
Linear scan angle in object space 583.7 µrad
Mirror separation 4.064 cm
Linear image displacement amplitude 0.14224 cm
Linear image displacement rate 23.4188 cm/sec
Mirror material nickel-plated beryllium
Mirror coating enhanced silver

Table 2-4. Scan Line Corrector (SLC) Design Parameters

Current Performance
On May 31, 2003, the SLC failed at approximately 21:45 UTC. Subsequent efforts to recover the SLC were not successful, and the problem is permanent for the mission. Without an operating SLC, the ETM+ line of sight now traces a zig-zag pattern along the satellite ground track (see Figure 2-12) resulting in wedge shaped scan-to-scan gaps (and alternating overlap areas), which increase in magnitude away from nadir.

As a result, the imaged areas are duplicated, with a width that increases toward the edge of the scene. When the Level-1 data are processed, the duplicated areas are removed, leaving data gaps. An estimated 22 percent of any given scene is lost due to the SLC failure. The maximum width of the data gaps along the edge of the image would be equivalent to one full scan line, or approximately 390 to 450 meters and the location of the missing scan lines vary from scene to scene. Figure 2-13 shows the scan gaps in a Landsat 7 SLC-off image.

Figure 2-13. The top image shows a subset of a SLC-on scene. The middle, a scene from the same area after the SLC failed. The bottom image shows the middle image, after the SLC gaps were largely filled by interpolation.
Figure 2-13. Portions of Landsat 7 image SLC-on (top), SLC-off (middle), and Gap-fill (bottom)

Despite the SLC failure, Landsat 7 ETM+ is still capable of acquiring useful image data with the SLC turned off (SLC-off), particularly within the central portion of any given scene (see Figure 2-13). Various interpolation and compositing techniques were investigated to expand the coverage of useful data. An interpolation example can be seen in Figure 2-13, which suggests but does not fully detail the impact on image quality. In Figure 2-13, the top image shows a subset of an SLC-on scene. The middle image shows a scene from the same area after the SLC failed. The bottom image shows the middle image with the SLC-off gaps largely filled by interpolation.

Besides interpolating across these unresolved areas, gap-filled products can be produced by using two methods: Phase One, which used a full Landsat 7 image (pre-2003) to fill the gaps of the SLC-off scene (i.e., using data from the top panel in Figure 2-13 to fill in the gaps in the middle panel); and Phase Two, which incorporated more than two SLC-off scenes together to create a final product:

Phase One: SLC-off to SLC-on - 6/1/2004 to 11/18/2004
Phase Two: SLC-off to SLC-off - 11/18/2004 to 2008 when the Open Data Policy was established. 

To better characterize the SLC-off scenes in support of Phase Two, a gap phase statistic was added to the scene metadata to specify the location of the gap pattern relative to the scene center. This statistic is used to find another SLC-off acquisition of the same WRS-2 scene that could be used to fill in the gaps of the first image. Additionally, the browse product was modified to show the gap size, in pixels, across the width of the image. This provides the user with an idea of the gap size in their area of interest.

In part due to these efforts and the undisturbed central portion of each scene, as well as other enhancements to image processing of ETM+ data by ground systems, the USGS decided to continue the Landsat 7 mission, acquiring ETM+ image data in "SLC-off" mode since July 14, 2003.

2.2.3.4 Prime Focal Plane (PFP)

The PFP assembly consists of three major subassemblies; the Prime Focal Plane Array (PFPA) and two preamplifier stacks. The PFPA is located at the focal plane (surface) of the telescope. Table 2-5 lists its characteristics. The PFPA is a monolithic silicon focal plane made up of five detector arrays: Band 1 through Band 4 and Band 8 (panchromatic). The arrays for Bands 1 through Band 4 contain 16 detectors divided into odd-even rows. The array for the Band 8 contains 32 detectors also divided into odd-even rows. The system focus is optimized for Band 8, which has the highest spatial resolution. The preamplifiers are mounted on the PFP assembly, and consist of two stacks of flat hybrid modules. On top of each stack is a cylindrical, black radiative cooling tower to help dissipate the heat from the preamplifiers.

Parameter Bands 1-4 Bands 8
Number of detector 16 per band 32
Detector size 0.0103632 x 0.0103632 cm 0.0051816 x 0.0044958 cm
Detector area 10.741914 x 10ˉ5 cm2 2.50064016 x 10ˉ5 cm2
IFOV size 42.5 µrad 21.25 µrad x 18.5 µrad
Center to center spacing along track 0.0103632 cm 0.0051816 cm
Center to center spacing between rows 0.025908 cm 0.0207264 cm

Table 2-5. Prime Focal Plan Assembly Design Parameters

2.2.3.5 Relay Optics

The ETM+'s Relay Optics consist of a graphite-epoxy structure containing a folding mirror and a spherical mirror, which are used to relay the imaged scene from the PFPA to the Band 5, 6, and 7 detectors on the Cold Focal Plane Array (CFPA). Table 2-6 lists the characteristics of the Relay Optics. The Relay Optics have a magnification of 0.5. This magnification is used because of the reduced physical size of the Band 6 detectors.

Parameter Specifications
Folding mirror clear aperture outer diameter 7.9756 cm
Folding mirror clear aperture inner diameter 1.36398 cm
Sperical mirror clear aperture diameter 14.06652 cm
Magnification 0.5
f/# 3.0
Mirror material ULE glass
Mirror coating Aluminum, SiO overcoat

Table 2-6. Relay Optics Design Parameters

2.2.3.6 Cold Focal Plane (CFP)

The CFP assembly is mounted on the cold stage of the Radiative Cooler (RC), operates at a nominal temperature of 91.4 K, and can be controlled to one of three set points (91.4 K, 95 K, or 105 K) by a heater on the back of the substrate. The higher temperatures are backups in case the RC efficiency degrades. Table 2-7 lists the characteristics of the CFP assembly. The CFP assembly contains the detector arrays for Bands 5, 6, and 7. Each band is a separate array. The Band 5 and Band 7 arrays contain 16 detector elements. The nominal spatial resolution of Band 5 and Band 7 is the same as Band 1 through Band 4. The Band 6 array is fabricated from mercury cadmium telluride. This photoconductive array shows a significant decrease in responsivity from 90 K to 105 K. The Band 6 array contains eight detector elements.

Parameter Bands 5-7 Band 6
Number of detectors 16 per band 8
Detector size 0.004826 cm x 0.0051816 cm 0.0103632 cm x 0.0103632 cm
Detector area 10.741914 c 10-5 cm2 2.50064016 x 10-5 cm2
IFOV size 42.5 µrad x 39.4 µrad 42.5 µrad x 85.0 µrad
Center to center spacing along track 0.0051816 cm 0.0103632 cm
Center to center spacing between rows 0.012954 cm 0.025908 cm

Table 2-7. Cold Focal Plane Design Parameters

2.2.3.7 Radiative Cooler (RC)

The RC cools the CFP by radiating internal heat out to cold space. It has a cold stage, an intermediate stage, a radiation shield, and a combination Earth shield and cooler door. Temperature-controlled outgas heaters (controlled to 318 K) are mounted on both the cold and intermediate stages to provide temporary heating of the cold surfaces should on-orbit contamination occur. The cold stage outgas heater also serves as a backup for the CFP heater. The flat rectangular corners of the RC structure that extends beyond the main circular cross section serve as radiation elements to dissipate heat from the Bands 5, 6, and 7 preamplifiers that are inside. Table 2-8 shows the RC's characteristics.

Parameter Specification
Horizontal field of view 160°
Vertical field of view 114°
Intermediate stage radiator area 660 cm2
Cold stage radiator area 435 cm2
Nomincal intermediate stage temperature 134 K
Cold stage temperature 91.4 K
Cold staget minimum temperature 82 K
Outgas temperature - both temperature - both stages 318 K
Cold stage backup temperatures 95 K, 105 K

Table 2-8. Radiative Cooler Design Parameters

2.2.3.8 Spectral Filters

The nominal wavelength location of the ETM+ spectral bands and the nominal ETM+ IFOV size and associated ground resolution, for a 705-km satellite altitude, are shown in Table 2-9. The ETM+ spectral bandwidth is determined by the overall combination of all optical elements, the spectral filters, and the detector response. The spectral filters, located immediately in front of each detector array, are the dominant items that establish the optical bandpass for each spectral band. The prime focal plane assembly has a filter housing that contains filters for Bands 1 through 4 and the panchromatic band, Band 8. The cold focal plane assembly has a filter housing that contains filters for Bands 5, 6, and 7.

Spectral Band Wavelength width (µm) IFOV Size (µm) Resolution (m)
1 0.450 to 0.515 ± 0.005 42.5 x 42.5 ±4.3 30
2 0.525 to 0.605 ± 0.005 42.5 x 42.5 ±4.3 30
3 0.630 to 0.690 ± 0.005 42.5 x 42.5 ±4.3 30
4 0.775 to 0.900 ± 0.005 42.5 x 42.5 ±4.3 30
5 1.550 to 1.750 ± 0.010 39.4 x 42.5 ±4.3 30
6 10.40 to 12.50 ± 0.100 85.0 x 85.0 ±9.0 60
7 2.080 to 2.350 ± 0.020 39.4 x 42.5 ±4.3 30
8 0.520 to 0.900 ± 0.010 18.5 x 21.3 ±4.3 15
Detector wavelength limits have the same ± uncertainty. 
Resolution is the nominal ground spatial resolution. 

Table 2-9. ETM+ Spectral Bands, IFOV Size, and Ground Resolution

2.3 Spacecraft Overview

The Landsat 7 platform provides a variety of elements necessary for mission success, including power, orbit and attitude control, telemetry and communications, and data storage and transmission capabilities for the ETM+ sensor.

Figure 2-14 Landsat 7 satellite as viewed from sun-facing side
Figure 2-14. Landsat 7 satellite as viewed from sun-facing side

The satellite attitude control uses precision mode, which is a combination of stellar and inertial guidance sensors to maintain the spacecraft platform within 0.015 degrees of Earth pointing. Narrowband communications include processing of real-time and stored commands, processing and authentication of command messages, and transmission of telemetry data, which is the collection of housekeeping and satellite processor reports.

Wideband communications for payload data transmission to the ground incorporates four X-Band transmitters, switchable to three steerable antennas. Also, an on-board SSR is used to store imagery when out of view of ground stations, for subsequent transmission to the ground when the satellite is back within a station’s view.

The on-board 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. Figure 2-15 shows the current (ca. 2017) status of the individual Landsat 7 satellite subsystems

Figure 2-15 Status of Landsat spacecraft and subsystems (ca.2017)
Figure 2-15. Status of Landsat Spacecraft and Subsystems (ca.2017)

2.4 Sold State Recorder

The Solid State Recorder (SSR) is used to capture wideband science data from the ETM+ and narrowband housekeeping telemetry data from satellite subsystems. The SSR records and plays back data in numbered logical blocks, which are used by the MOC in commanding the recorder.

The SSR accepts two inputs from the ETM+ at 75 Mbps. The SSR was designed to hold up to 42 minutes (approximately 105 scenes) of data at 150 Mbps. Memory board shutdowns over the years, likely due to single event upsets, have reduced the capacity to approximately 84 scenes. The boards are believed recoverable, should Program Management deem it necessary to make the attempt. For now, the reduced capacity is not adversely impacting the LTAP.

The SSR records ETM+ Channel Access Data Units (CADU) as two bitstreams, each at a nominal rate of 75 Mbps. CADUs are recorded in the same order as received from the ETM+. Partial CADUs may be recorded if the ETM+ collection interval extends beyond the commanded SSR record interval, if the ETM+ is turned off before the end of the SSR data recording area is achieved, or as a result of a ground command to disable wideband recording.

During playback, the two 75 Mbps bitstreams are read out of memory and sent to the broadband switching unit. A second pair of 75 Mbps bitstreams can also be played back for a total aggregate rate of 300 Mbps. The bitstreams include the CADUs generated by the ETM+. Record intervals, each corresponding to an ETM+ collection interval, which consists of one or more Landsat scenes, may be subdivided for playback if more than one scene is collected. In this case, each resulting subinterval is defined such that data in the vicinity of each subinterval boundary are included (redundantly) with both subintervals. Each subinterval includes all of the CADU data required to process the subinterval as a separate ETM+ collection. As a result, individual subintervals may contain partial CADUs. The SSR contains error detection and a correction capability to preserve the integrity of the stored wideband and narrowband data. Reed-Solomon encoding is performed on record while Reed-Solomon decoding is performed on playback to recover data from possible dynamic  Random Access Memory (RAM) list errors.

Narrowband data are captured by the SSR from the telemetry data formatter. The SSR accepts two input rates of 1.216 Kilobits per Second (Kbps) or 4.864 Kbps and plays back stored telemetry data at 256 Kbps to the S-Band transponder. The SSR is capable of either recording or playing back wideband data (but not both simultaneously) while simultaneously recording and/or playing back narrowband telemetry data. S-Band telemetry data is stored separately from wideband image data downlinked by the X-Band transponder and can be recorded when the wideband capability is shut down due to anomalies or power concerns.

2.5 Ground System

2.5.1 Overview

The Landsat 7 ground system consists of both Landsat 7 unique components as well as institutional services. The Landsat 7 unique components include the MOC, LGN, LPS, LPGS and IAS (both explained further in Section 2.5), the Landsat Archive Manager (LAM), and a number of IGS. 

The institutional support systems consist of the SN, the EOS Near-Earth Network (NEN), NCEP, the Land Processes Distributed Active Archive Center (LP DAAC) at EROS (used from 1999–2004), and the NASA Integrated Support Network (NISN).

The ground system context diagram in Figure 2-16 illustrates both the unique and institutional components and their data flow relationships from the satellite to the end-users of the data.

Figure 2-16 Landsat 7 End-to-end data flow
Figure 2-16. Landsat 7 end-to-end data flow

2.5.2 Unique Landsat Ground System Components

2.5.2.1 Mission Operations Center (MOC)

The MOC, located at NASA Goddard Space Flight Center (GSFC), is the focal point for all space vehicle operations. The MOC provides the facilities, hardware, software, procedures, and personnel required to accomplish Landsat 7 planning and scheduling, to command and control the Landsat 7 space vehicle, to monitor its health and status, to analyze the performance of the space vehicle, and to maintain flight and MOC ground software. The MOC also detects, investigates, and resolves spacecraft anomalies. Flight Dynamics functions (such as maneuver planning, planning aid generation, and orbit determination) have been incorporated into the MOC systems. The MOC is staffed by the FOT, which is comprised of Console Analysts, Mission Planners, Flight Dynamics Engineers, Subsystem Engineers, and Supervisors / Managers.

2.5.2.2 Landsat Ground Network (LGN)

The Landsat 7 Ground Network includes four stations:

  • LGS – Landsat Ground Station, Sioux Falls, South Dakota, USA
  • SGS – Svalbard Ground Station, Svalbard, Norway
  • ASN – Australian Ground Station, Alice Springs, Australia
  • NPA – North Pole Ground Station (NPA3 and NPA4), North Pole, Alaska, USA

These ground stations are receiving sites for the X-Band downlinks of ETM+ science data from the satellite. They downlink both real-time and playback ETM+ wideband data directly from the Landsat 7 spacecraft by way of one or two 150 Mbps X-Band return links, each at a different frequency. The SGS, ASN, and NPA stations do not do any further processing but forward the downlinked data to the LGS via high-speed communication lines. The LGS separates each X-Band data into two 75 Mbps channels (I and Q), and transmits the acquired wideband data over 75 Mbps LGS output channels to the LPS where they are recorded and made available for further processing.

The LGS, NPA3/4, and SGS stations conduct S-Band communications with the satellite, sending command and data loads and receiving real-time and playback telemetry data. These stations also support spacecraft tracking. The ASN is capable of receiving S-Band downlinks from the satellite, but is not currently used for command and data uploads or for tracking.

In addition to the ground sites, Landsat 7 uses the Tracking and Data Relay Satellite System (TDRSS), operated by the SN headquartered at NASA GSFC. TDRSS enables S-band downlink of real-time and stored housekeeping data and uplink of command data. These are used for Landsat 7 real-time command and telemetry monitoring during on-orbit operations on both a scheduled basis as well as for possible emergency operations on a call-up basis. Data rates are limited by the absence of a high-gain antenna on Landsat 7; therefore, housekeeping recorder dumps are not supported. TDRSS is also used for tracking data collection for generation of Landsat 7 spacecraft orbital state vectors for use in precision attitude control. The NISN at GSFC provides all network lines, voice communications, ground communication interfaces among the control centers and ground stations.

2.5.2.3 International Ground Stations (IGS)

The IGS are satellite data receiving stations located around the world. They provide data reception, processing, and distribution services for their user community. They receive Landsat 7 payload data via X-Band direct downlink. The coverage areas for the IGS depict the Earth's land areas that are regularly imaged. The X-Band direct downlink data includes the PCD required for image processing.

Earlier in the Landsat 7 mission, the IGS used an online tool to submit downlink requests for the land scenes within their station view, assigning relative priorities to the scenes to aid in scheduling. Due to duty cycle considerations and the large number of participating IGSs, it was not a given that all IGS downlink requests would be scheduled; the priorities were used in the decision process for deciding which scenes to reject, if necessary. Use of the tool was discontinued in 2011, when it was determined that the low number of participating stations at that time and the amount of available duty cycle made it possible to provide full coverage scheduling for the IGS.

The IGS receives schedule and orbital element data from the MOC for their scheduled requests. After receipt of the downlink data, the IGS provides a copy of the data to EROS, where it is incorporated into the Landsat archive catalog.

Although catalogued at EROS, data downlinked to the IGS originally had to be ordered directly from these foreign stations. In 2010, the USGS began the Landsat Global Archive Consolidation (LGAC) effort to acquire as many copies as possible of these early Landsat data. To date (ca. 2017), more than 5 million Landsat scenes from all satellites in the series, 70 percent of them unique, have been added to the EROS Landsat archive. See Figure 2-17 and the IGS Network web page for a map and a current list of Landsat 7 ground stations.

Figure 2-17 Historical International Ground Stations (above) and (below) Active Landsat Ground Stations, circa 2017
Figure 2-17. (Top) Historical International Ground Stations and (Bottom) Active Landsat Ground Stations, circa 2018

2.5.2.4 Landsat Processing System (LPS)

The LPS records all wideband data, at real-time rates, into its wideband data stores. An I-Q channel pair represents a complete data set. One channel holds Bands 1 through Band 6, and the other channel holds Band 7 and Band 8 and the second gain setting from Band 6. The LPS retrieves and processes each channel of raw wideband data, at lower than real-time rates, into separate accumulations of Earth image data, calibration data, Mirror Scan Correction Data (MSCD), and PCD. Channel accumulations represented by Band 1 through Band 6 and Band 6 through Band 8 become Format 1 and Format 2, respectively. PCD and MSCD are generated twice, once for each format and their contents should be identical.

The LPS spatially reformats Earth imagery and calibration data into Level-0R data. This involves shifting pixels by integer amounts to account for the alternating forward-reverse scanning pattern of the ETM+ sensor, the odd-even detector arrangement within each band, and the detector offsets inherent to the focal plane array engineering design. All LPS Level-0R corrections are reversible; the pixel shift parameters used are documented in the IAS CPF. 

During LPS processing, Format 1 bands are duplicated, radiometrically-corrected, and used to assess cloud cover content through the Automatic Cloud Cover Assessment algorithm (ACCA) (see Section 5.6.10) and to generate a browse image. Cloud cover scores are generated on a scene and quadrant basis. Metadata are generated for the entire subinterval and on a scene basis. The image data, PCD, MSCD, calibration data, and metadata are structured into Hierarchical Data Format - Earth Observing System (HDF-EOS) for each format and sent to the LAM for long term archival in subinterval form. The two formats of data are united when a Landsat 7 Level-0R product is ordered. The browse image and a subset of the metadata are used to provide online aids to ordering.

2.5.2.5  Landsat Product Generation System (LPGS)

The LPGS at EROS uses multiple geometric algorithms to create Level-1 products from the raw Level-0R data. The LPGS is composed of multiple subsystems that each perform a unique function. See Section 5 for a full list of the LPGS Subsystems along with descriptions of the purpose and functions of each.

2.5.2.6 Image Assessment System (IAS)

The IAS is responsible for the off-line assessment of image quality to ensure compliance with the radiometric and geometric requirements of the spacecraft and the ETM+ sensor throughout the life of the Landsat 7 mission. Section 5 provides a full description of the IAS.

2.5.2.7 Landsat Archive Manager (LAM)

The LAM  at EROS was originally implemented as a backup to the LP-DAAC, a part of the Earth Observing System Data and Information System (EOSDIS). In 2004, it became the primary system providing data archival and distribution functions for Landsat 7. In addition to the Level-0R data received from LPS, the LAM also receives and utilizes the CPF from the IAS.

2.5.3 NASA Institutional Ground System Components

2.5.3.1 Space Network (SN)

Landsat 7 uses the TDRSS, operated by the Space Network (SN) headquartered at NASA GSFC. The SN, which includes the TDRSS and the ground terminals at the White Sands Complex, provides space-to-space and space-to-ground data relay services. TDRSS enables S-Band downlink of real-time and stored housekeeping data and uplink of command data. These are used for Landsat 7 real-time command and telemetry monitoring during on-orbit operations on both a scheduled basis as well as for possible emergency operations on a call-up basis. Data rates are limited by the absence of a high-gain antenna on Landsat 7; therefore, housekeeping recorder dumps are not supported. TDRSS is also used for tracking data collection for the generation of Landsat 7 spacecraft orbital state vectors for use in precision attitude control.

2.5.3.2    EOS Near-Earth Network (NEN)

The EOS NEN Ground Stations include the following:

  • NP3/4 – North Pole Ground Station, North Pole, AK; S-Band and X-Band
  • SGS – Svalbard Ground Station, Svalbard, Norway; S-Band and X-Band
  • MGS – McMurdo Ground Station, South Pole; S-Band

Figure 2-18 depicts the Svalbard Ground Station (SGS) in Norway. Since the launch of Landsat 7, the Alaska station has been at times located at Gilmore Creek, Poker Flat, and now at North Pole, Alaska.

All three stations provide S-Band services including both real-time and playback housekeeping telemetry support, command capabilities, and two-way Doppler tracking. Additionally, the SGS and both NP3 and NP4 record Landsat 7 X-Band downlinks; all stations forward recorded data to the LGS over high speed communication lines.

Figure 2-18 Svalbard Ground Station (SGS) located on Platåberget, Spitsbergen Island, near Longyearbyen in the Svalbard Archipelago.
Figure 2-18. Svalbard Ground Station (SGS) located on Platåberget, Spitsbergen Island, near Longyearbyen in the Svalbard Archipelago.

2.5.3.3    NASA Integrated Support Network (NISN)

NISN is a global system of communications transmission switching and terminal facilities that provide NASA with wide area communications services. The NISN at GSFC provides all network lines, voice communications, and ground communication interfaces among the control centers and ground stations. NISN was implemented in 2003 to serve the needs of NASA's users for the transmission of digital data, voice, and video information in the most cost effective manner possible. This single integrated network project replaced multiple independent special purpose networks that had served individual NASA customers for several decades. NISN supports all the institutional facilities mentioned previously and provides communications between the MOC at GSFC and the LGS at EROS.

2.5.3.4    Flight Dynamics Facility (FDF)

The FDF, an institutional support element located at GSFC, provided pre-launch planning and analysis including star catalog generation and orbit injection maneuver planning; post-launch, the FDF provided orbit and attitude determination, planning and scheduling aid generations, as well as maneuver planning. Within the first year after launch, most FDF activities and responsibilities were migrated to workstations in the MOC used by the Mission Planners for orbit determination, attitude determination, ephemeris data generation, maneuver planning support, and generation of planning and scheduling aids (including in-view predictions for IGS, SN, and LGN). Today, the FDF institutional facility retains responsibility for star catalog maintenance, local oscillator frequency reporting, and SN tracking data preprocessing. They also participate in conjunction analysis and collision avoidance monitoring across all active GSFC Earth-observing missions.


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