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Landsat 7 Ground System
4.1 Overview

The Landsat 7 ground system consists of both Landsat 7 unique components as well as institutional services. The unique components include the Mission Operations Center (MOC), Landsat Ground Station (LGS), Landsat Processing System (LPS), the Image Assessment System (IAS), the Level 1 Product Generation System (LPGS), the EROS Data Center Distributed Active Archive Center (LP-DAAC), and the international ground stations (IGS).

The institutional support systems consist of the Landsat Ground Network (LGN), Space Network (SN), The National Centers for Environmental Prediction (NCEP), The Flight Dynamics Facility (FDF), and the NASA Integrated Support Network (NISN).

The ground system context diagram illustrates both unique and institutional components and their data flow relationships end-to-end. A brief description of each is presented below.

4.2 Unique Ground System Components

4.2.1 MOC

The MOC, located at Goddard Space Flight Center (GSFC) in Greenbelt, MD 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, and to command and control the Landsat 7 space vehicle, monitor its health and status, analyze the performance of the space vehicle, and maintain flight software, 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 are provided by the Flight Dynamics Facility. The MOC is staffed by the Flight Operations Team (FOT) which is comprised of console analysts, mission planners, subsystem engineers, and supervisor/managers.

In addition to the ground sites, the Tracking Data and Relay Satellites (TDRS), operated by the NASA's Space Network are utilized. Together these sites provide the ability to downlink real-time and stored housekeeping data (S-band), and to command the spacecraft. In addition, tracking services and spacecraft clock maintenance capabilities are provided. All network lines are provided by the NASCOM division of NASA.

4.2.2 Landsat Ground Stations

The LGS, located at EDC, in Sioux Falls, SD is a receive site for the wideband X-Band downlinks of payload data from the space vehicle. In addition to LGS, the Alaska Ground Station (AGS) and the Svalbard Norway Ground Station (SGS) receive payload data downlinks. Payload data downlinked to SGS and AGS is captured on tape and shipped to LGS which serves as a front-end processor. The LGS also supports S-Band command telemetry operations, as well as tracking. The LGS is a Landsat 7 unique component of the Landsat Ground Network (LGN); the other LGN components are institutional services.

The LGS acquires ETM+ wideband data directly from the Landsat 7 spacecraft by way of two 150 megabit-per-second (Mbps) X-band return links, each at a different frequency. LGS separates each X-band data into two 75-Mbps channels ( I and Q), and transmits the acquired wideband data over four 75-Mbps LGS output channels to the LPS where they are recorded.

4.2.3 Landsat Processing System

The LPS, located at EDC, in Sioux Falls, South Dakota records all wideband data, at real-time rates, into its wideband data stores. A complete data set is represented by an I-Q channel pair. One channel holds bands 1 through 6, and the other holds bands 7 and 8 and a second gain form of 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 payload correction data (PCD). Channel accumulations represented by bands 1 through 6 and 6 through 8 become formats 1 and 2, respectively. PCD and MSCD are generated twice, once for each format. Their contents should be identical.

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 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 and to generate browse. Cloud cover scores are generated on a scene-by-scene and quadrant-by-quadrant basis. Metadata are generated for the entire subinterval and on a scene-by-scene basis. The image data, PCD, MSCD, calibration data, and metadata are structured into HDF-EOS for each format and sent to the LP-DAAC for long term archival in subinterval form. The two formats of data are united when a Landsat 7 0R product is ordered. The browse is sent to the LP-DAAC separately for use as an online aid to ordering.

4.2.4 Level 1 Product Generation System

The LPGS, located at EDC, generates Level 1 products in response to user requests received from the LP-DAAC. Radiometric and geometric processing is performed by LPGS on Level 0R data to create Level 1 products. Users can order either 1R (radiometrically corrected only) or 1G (radiometrically and geometrically corrected) products. The 1G products are classified as systematic meaning the class of corrections applied are derived from spacecraft data only. Enhanced geometric accuracy is possible with the application of ground control and terrain models but these are not used by LPGS.

A number of user-selectable options exist for configuring a 1G product. These include band selection, map projection, grid cell size, resampling methodology, rotation, product size, and output format. These details can be found in the chapter on Data Products .

4.2.5 Image Assessment System

The IAS, located at EDC, is responsible for the off-line assessment of image quality to ensure compliance with the radiometric and geometric requirements of the spacecraft and ETM+ sensor throughout the life of the Landsat-7 mission.

In addition to its assessment functions, the IAS is responsible for the radiometric and geometric calibration of the Landsat 7 satellite and ETM+. The IAS periodically performs radiometric and geometric calibration and updates the CPF. This file is stamped with applicability dates and sent to the LP-DAAC (EDC) for storage and eventual bundling with outbound Level 0R products. The CPF also is sent to international ground stations via the MOC. The CPF supplies the radiometric and geometric correction parameters required during Level 1 processing to create superior products of uniform consistency across the Landsat 7 system. Operational activities occur at EDC while less frequent assessments and calibration certification are the responsibility of the Landsat-7 Project Science Office at the Goddard Space Flight Center (GSFC) in Greenbelt, Maryland.

4.2.6 International Ground Stations

The IGSs are satellite data receiving stations located around the world. They provide data receive, processing, and distribution services for their user community. They receive Landsat 7 payload data via X-Band direct downlink. The acquisition circles for the IGSs depict the Earth's land areas that are regularly imaged. The X-Band direct downlink data includes the PCD required for image processing. The IGSs submit downlink requests to the MOC and receive schedule and orbital element data from the MOC. In addition, the IGSs return metadata for their station holdings to the LP-DAAC. Although catalogued at EDC, data downlinked to the IGSs must be order from these foreign stations.

4.2.7 LP-DAAC

The LP-DAAC, located at Sioux Falls, SD is part of the Earth Observing System (EOS) Data and Information System (EOSDIS). It provides information management, user interface, and data archival and distribution functions for a variety of data types including Landsat 7. In addition to the Level 0R data received from LPS, the LP-DAAC also receives calibration parameter files from the IAS. The LP-DAAC performs billing and accounting functions and serves as the repository for user-oriented documentation.

4.3 Institutional Ground System Components

4.3.1 Landsat Ground Network
LGN consists of multiple communications sites which provide S-Band and X-Band communication support to the Landsat 7 mission. The LGN institutional services include AGS, the Wallops Island, VA (WPS) ground station, and the SGS in Norway. The AGS, WPS, and SGS are managed by the Wallops Facility.

4.3.2 Space Network
The SN, which includes the Tracking and Data Relay Satellites (TDRSs) and the ground terminals at the White Sands Complex, provides space-to-space and space-to-ground data relay services. These are used for Landsat 7 real-time command and telemetry monitoring during on-orbit operations on a scheduled basis and possible emergency operations on a call-up basis. The SN collects Landsat 7 space vehicle tracking data for FDF processing. The SN is managed by GSFC.

4.3.3 National Centers for Environmental Prediction

The National Centers for Environmental Prediction (NCEP) provide timely, accurate and continually improving worldwide forecast guidance products. NCEP, a critical part of the National Oceanic and Atmospheric Aministration's National Weather Service, is the starting point for nearly all weather forecasts in the United States.

NCEP generates weather related products. For Landsat 7, the NCEP supplies cloud cover predict data to the MOC for image scheduling.

4.3.4 Flight Dynamics Facility
FDF, an institutional support element located at GSFC, provides workstations in the MOC which are used by the Flight Operations Team (FOT) for orbit determination, attitude determination, ephemeris data generation, maneuver planning support, and generation of planning and scheduling aids (including in-view predictions for IGSs, SN, and the LGN). The FDF institutional facility retains responsibility for star catalog maintenance, local oscillator frequency reporting, and SN tracking data preprocessing.

4.3.5 NASA Integrated Support Network
NISN is a global system of communications transmission switching and terminal facilities that provide NASA with long-haul communications services. NISN was implemented to serve the needs of all of NASA's users for the transmission of digital data, voice, and video information in the most cost effective manner possible. The single integrated network project is replacing the independent special purpose networks that have served individual customers for decades. The NISN supports the above institutional facilities.

3.1 Sensor Overview

Landsat 7's sensor - the Enhanced Thematic Mapper Plus (ETM+) - was built by SBRS. The sensor is a derivative of the Thematic Mapper (TM) engineered for Landsats 4 and 5, but is more closely related to the Enhanced Thematic Mapper (ETM) lost during the Landsat 6 failure. The primary performance related changes of the ETM+ over the TM's are the addtion of the panchromatic band and two gain ranges (added for Landsat 6), the improved spatial resolution for the thermal band, and the addition of two solar calibrators.

The ETM+ design provides for a nadir-viewing, eight-band multispectral scanning radiometer capable of providing high-resolution image information of the Earth's surface when operated from Landsat 7, a 3 axis stabilized spacecraft located in a near polar, sun-synchronous and circular orbit at a 705 km nominal altitude, with an orbit inclination of 98.2 degrees. The ETM+ is designed to collect, filter and detect radiation from the Earth in a swath 185 km wide as it passes overhead and provides the necessary cross-track scanning motion while the spacecraft orbital motion provides an along-track scan.

3.2 ETM+ Design

ETM+ Optical Path
Figure 3.2 ETM+ Optical Path.
Scene energy passes through a number of major ETM+ subsystems, depicted in Figure 3.2, before it is collected by the solidstate detectors at the focal plane. The bidirectional scan mirror assembly (SMA) sweeps the detector's line of sight in west-to-east and east-to-west directions across track, while the spacecraft's orbital path provides the north-south motion. A Ritchey-Chretien telescope focuses the energy onto a pair of motion compensation mirrors (i.e. scan line corrector) where it is redirected to the focal planes. The scan line corrector is required due to the compound effect of along-track orbital motion and cross-track scanning which leads to significant overlap and underlap in ground coverage between successive scans.

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

3.2.1 ETM+ Detector Geometry

Figure 3.4 illustrates the relative position of all the detectors from both focal planes with respect to their actual ground projection geometry. 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 IFOV in the along scan direction. This arrangment 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 which covers a ground swath approximately 185 kilometers wide. 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.

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. Again referring to the ground projection illustration in figure 3.3 one should note the spacing between bands as measured in 30 meter 42.5 microradian IFOVs. Taken individually, these numbers represent a band's unique leading edge preamble that occurs before coincident data is 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 0R processing to achieve image registration at the level 0 level. Other factors such as detector offsets within a band and sample timing must be considered to calculate registration offsets accurately.

Detector Projection at the Prime Focal Plane
Figure 3.4 Detector Projection at the Prime Focal Plane

Detector Offsets
Band 8 detectors rows are separated by 2 42.5 &microrad IFOVs which translates to 4 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 will therefore always differ by four samples for both forward and reverse scans.

The detector rows within bands 1-5 and 7 are separated by 2.5 42.5 IFOVs. This seemingly curious design makes sense because the multiplexer samples the even detectors .5 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 2 IFOV odd-to-even detector spacing is realized on forward scans while a 3 IFOV spacing occurs on reverse scans. The registration offsets for forward and reverse scans will always differ by these amounts.

The band 6 odd and even detectors are separated by 5 42.5 &microrad IFOVs which translates to 2.5 band 6 samples. The odd and evens are sampled, however, in alternating minor frames which separates the odd and even detectors an integral multiple of IFOVs. A 2 band 6 IFOV odd-to-even detector spacing is realized on forward scans while a 3 band 6 IFOV spacing occurs on reverse scans. The registration offsets for forward and reverse scans will always differ by these amounts.

3.2.2 Registration Offsets

Over the years, different ground system engineers have characterized Landsat 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 Landsat 7 we have declared all shifts as postive 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.

Source: http://landsathandbook.gsfc.nasa.gov