Chapter 4 - 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 (EDC-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, SD 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 EDC-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 EDC-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 EDC-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 not 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 EDC-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 images. 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 EDC-DAAC. Although catalogued at EDC, data downlinked to the IGSs must be order from these foreign stations.

 4.2.7 EDC-DAAC

The EDC-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 EDC-DAAC also receives calibration parameter files from the IAS. The EDC-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

NCEP generates weather related products. For Landsat 7, the NCEP will supply cloud cover predict data for use in image scheduling in the MOC

 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. The NISN supports the above institutional facilities.