The Landsat 7 mission

Abstract

The Landsat Earth observation approach introduced in 1972 created a new way of monitoring land cover and land use globally. The Landsat 7 mission, successfully launched on April 15, 1999, continues those observations and demonstrates significant progress in precise numerical radiometry, spectral differentiation, and seasonally repetitive monitoring. Substantial improvements in calibration procedures, both prior to launch and during normal operations, have also been made to ensure long-term stability in the acquired spectral radiometry. Landsat 7 data acquisitions are being driven by a long-term data acquisition plan that was designed to ensure that substantially cloud-free, seasonal coverage would be recorded and archived in the US for all land areas of the globe. NASA competitively selected a Landsat Science Team, consisting of representatives from US universities and government agencies, to exploit the Landsat 7 record for global change research. This team is addressing the technical and analytical means to process and analyze the core of this observation record, and for the first time in the history of the Landsat mission, the technical and operational aspects of the mission are being driven by the goals of the US science community. The expected outcome of these efforts is a rapid improvement in understanding the Earth system, as well as conceptual knowledge that will underpin significant advancements in the application of this technology for commercial, operational, educational, and research purposes. Pathways to achieve effective Landsat continuity in the early decades of the 21st century are also being given careful attention, and there is no question that the lessons learned from the Landsat 7 mission will strongly influence these next-generation sensor systems.

Introduction

The launch of Landsat 7 on April 15, 1999 from Vandenberg Air Force Base, California, marked a significant advance in Landsat's more than quarter century mission to monitor the Earth's land and shallow sea areas. Begun in 1972, Landsat observatories pioneered the use of space platforms for systematic collection of land images (Short, Lowan, Freden, & Finch, 1976). These measurements produced a virtual revolution in earth science research, revealing the importance of remotely sensed images for monitoring the patterns and processes that define the Earth's land areas. In fact, the science and technology introduced by the continuing Landsat mission have served as a primary stimulus for current interest in Earth System Science and global change investigations, in particular the role of land conditions and dynamics in the Earth system.

The tremendous success of Landsat 7 has become clear during the last year. As of June 2000, over 75,000 images had been transmitted to the US archive at the US Geological Survey (USGS) EROS Data Center (EDC), with nearly 40% of this total having less than 10% cloud cover. In addition, the quality of the imagery, in terms of geodetic navigation, radiometric calibration, and signal-to-noise characteristics, has been superb, and represents a major advance over previous Landsat missions. Taken together, these early results suggest that Landsat 7 has emerged as a cornerstone for the US remote sensing program.

The success of Landsat 7 belies the troubled and confused history of US Landsat operations during the last three decades Goward, 1989, Marshall, 1989a, Marshall, 1989b, National Research Council, 1995. Much of the confusion centers on unrealized assumptions concerning possible commercial development of Earth imaging as well as misunderstandings concerning the information content and thus the scientific and applications potential of a Landsat-type observatory. From 1972 to 1982, the Landsat mission was sustained as a NASA experimental activity, directed toward demonstrating the potential of these observations in studying the Earth's land areas Freden & Gorden, 1983, Sheffner, 1994, Short et al., 1976, Williams et al., 1984. In 1982, a transition of the program to the commercial sector was initiated. The EOSAT Corporation took over the Landsat program, under the direction of the National Oceanic and Atmospheric Administration (NOAA), in an effort to develop the commercial potential of the mission. This business experiment ceased in 1992 as a result of a congressional act, the “1992 Land Remote Sensing Act” (Sheffner, 1994). Based on this act, the Landsat program, beginning with Landsat 7, returned to Federal Government management. Initially, the Department of Defense, NASA, and NOAA shared management responsibility for Landsat 7. Over the last 5 years this management structure has changed with the mission now being shared between NASA and the USGS.

The return of Landsat operations to government management reflected not simply concerns with the success of the commercial efforts. During this time period a new emphasis on global change science placed Landsat-type observations in a central role for evaluating the status and potential changes of Earth's land areas Bretherton, 1988, IGBP, 1992, National Research Council, 1993. Both NASA and USGS are committed to exploiting Landsat 7 as a basic component of their contributions to the US Global Change Research Program (Committee on Earth and Environmental Sciences, 1990).

Landsat-type observations fill an important niche between the highly repetitive but coarse spatial resolution observations from the NOAA AVHRR, NASA EOS MODIS and French VEGETATION instruments and the ultra-high spatial resolution, local observatories such as the IKONOS instrument operated by the Space Imaging Corporation. Landsat provides systematic global coverage at a frequency sufficient to capture seasonal variations and at a spatial resolution where land cover dynamics, under the influence of natural processes and human activities, are clearly evident. If indeed we are to link global change to local environmental conditions, then Landsat-type observations will remain a fundamental requirement.

With Earth System Science clearly in mind, NASA incorporated Landsat 7 into its Earth Observation System (EOS) plans in 1994 and established a Landsat Project Science Office at the NASA Goddard Space Flight Center (GSFC). During 1995, the agency solicited proposals to form a Landsat Science Team to oversee science and application interests in the mission. This team was selected and funded in 1996. These activities reflected NASA's renewed interest in Landsat as a fundamental component of their earth science goals.

In keeping with this global change science interest, several aspects of the Landsat 7 mission were given particular attention, to ensure that this observatory would strongly support its new EOS role, as well as to better serve the broader applications community. Specific items given attention by the NASA Landsat Project Science Office included instrument characterization and calibration, development of an Image Assessment System, and an automated Long-Term Acquisition Plan (LTAP) to ensure that a US archive of calibrated, global observations is compiled on an annual basis (Goward et al., 1999). The Landsat Science Team has periodically reviewed these activities in their capacity as representatives of the scientific user community. This is the first time in the history of the Landsat program when such a focussed interchange between the NASA managers and the US science community has been undertaken, a process that has laid the groundwork for this revolutionary Landsat 7 era.

Landsat 7 capability to acquire high-quality global imagery has stimulated the science community to find new ways of processing and analyzing these data. Various members of the Landsat Science Team are pursuing research activities that are directed toward exploiting large volumes of the observations. To achieve this goal they have proposed advanced computational approaches for automated preprocessing, data storage, and analysis procedures. This research draws upon the 28-year heritage of Landsat research as well as novel concepts that have resulted from preparations for the NASA Terra mission. The team supports an initiative to develop a large-capacity computational facility to analyze a significant proportion of the Landsat 7 observations. Such a facility should be configured to handle at least 10,000 Landsat scenes per year, with the objective of answering key land science questions, such as the role of global forest dynamics in the Earth's carbon balance. A prototype of such a facility, the Research Environment for Advanced Landsat Monitoring (REALM) has been implemented at the University of Maryland to demonstrate the technical feasibility of achieving this critical mission goal (Masek, Shock, & Goward, 2000).

Recent efforts by NASA to plan future earth science missions have revealed strong support for continuation of the Landsat mission. Among the several missions that were proposed under NASA's 1998 Request for Information on future activities (see NASA report “Report of the Workshop on NASA Earth Science Enterprise Post-2002 Missions”), a Landsat-type follow-on mission received high priority from the earth science community. Anticipating such interest, NASA recently launched the Earth Observing-1 mission, which is dedicated to testing advanced sensors and related hardware for possible deployment in future Landsat missions. In addition, active consideration for a Landsat-type follow-on for deployment in the 2005 time period is now underway. The pathways to achieve effective Landsat continuity in the early decades of the 21st century are being given careful attention. The lessons learned from the Landsat 7 mission will strongly influence these next-generation sensor systems.