Since the 1960s, the United States has used satellites to observe the earth and its land, oceans, atmosphere, and space environments. Satellites provide a global perspective of the environment and allow observations in areas that may be otherwise unreachable or unsuitable for measurements. Used in combination with ground, sea, and airborne observing systems, satellites have become an indispensable part of measuring and forecasting weather and climate. For example, satellites provide the graphical images used to identify current weather patterns, as well as the data that go into numerical weather prediction models. These models are used to forecast weather 1 to 2 weeks in advance and to issue warnings about severe weather, including the path and intensity of hurricanes. Satellite data are also used to warn infrastructure owners when increased solar activity is expected to affect key assets, including communication satellites or the electric power grid. When collected over time, satellite data can also be used to observe trends and changes in the earth’s climate. For example, these data are used to monitor and project seasonal, annual, and decadal changes in the earth’s temperature, vegetation coverage, and ozone coverage. Environmental satellite programs generally fall into two categories: operational satellites and research and development satellites. Operational environmental satellites contribute to weather and climate predictions on a regular basis, and federal agencies sustain them by launching new satellites as older ones reach the end of their useful lives. Alternatively, research and development satellites are designed to test new technologies or to provide insights into environmental science. While there is not a commitment to sustain the capabilities demonstrated on research and development satellites on subsequent missions, these capabilities can be included on operational satellites if they demonstrate the usefulness of a new measurement or the maturity of new technology. Currently, the United States operates a fleet of operational environmental satellites, as well as multiple research and development satellites. Operational environmental satellites conduct earth observations from space in either a low-earth polar orbit or a geostationary earth orbit. Polar- orbiting satellites circle the earth in an almost north-south orbit within 1,250 miles of the earth, providing global coverage of conditions that affect weather and climate. Each satellite makes about 14 orbits a day. As the earth rotates beneath it, each satellite views the entire earth’s surface twice a day. In contrast, geostationary satellites maintain a fixed position relative to the earth from an orbit of about 22,300 miles in space. Figure 1 describes key characteristics of polar-orbiting and geostationary satellites. The United States currently operates two operational polar-orbiting meteorological satellite systems: the Polar Operational Environmental Satellites (POES) series, which is managed by NOAA, and the Defense Meteorological Satellite Program (DMSP), which is managed by the Air Force. The POES and DMSP programs provide data that are processed to provide graphical weather images and specialized weather products. They also provide the predominant input into numerical weather prediction models, a primary tool for forecasting weather. These satellites also provide data used to monitor environmental phenomena, such as ozone depletion, drought conditions, and energetic particle activity in the near- earth space environment, as well as data sets that are used by researchers to monitor climate change. Currently, one POES and two DMSP satellites are positioned so that they can observe the earth in early morning, midmorning, and early afternoon polar orbits. In addition, a European satellite, called the Meteorological Operational (MetOp) satellite, provides observations in the midmorning orbit. Together, they ensure that, for any region of the earth, the data provided to users are generally no more than 6 hours old. Figure 2 illustrates the current operational polar satellite configuration. NOAA, NASA, and DOD are currently developing the next generation of operational polar-orbiting environmental satellites, called NPOESS. This program was planned to converge the POES and DMSP satellite programs into a single program capable of satisfying both civilian and military requirements for earth and space weather, as well as climate monitoring. As currently defined, NPOESS consists of a series of four satellites, as well as a demonstration satellite called the NPOESS Preparatory Project (NPP). NPP is intended to reduce the risk associated with launching new sensor technologies and to ensure continuity of climate data. The agencies plan to launch NPP in 2011, with the other satellites following at regular intervals to ensure satellite coverage in two orbits through 2026. Due to poor program performance and interagency conflicts over system requirements, the NPOESS program is currently being restructured to allow separate acquisitions by NOAA and DOD. However, it is not yet clear how or when this transition will take place. In addition to the polar satellite program, NOAA also manages an operational geostationary satellite program, called the Geostationary Operational Environmental Satellite (GOES) program. NOAA operates GOES as a two-satellite system that is primarily focused on the United States (see fig. 3). These satellites are uniquely positioned to provide broad, continuously updated coverage of atmospheric and surface conditions on the earth, as well as the space environment surrounding the earth. For example, geostationary satellites observe the development of hazardous weather events, such as hurricanes and severe thunderstorms, and track their movement and intensity to help reduce or avoid major losses of property and life. In addition, the geostationary satellites track space weather variables such as solar X-ray fluctuations and high-energy particles that are used in identifying emerging solar storms. NOAA is currently developing the next generation geostationary series, called GOES-R. GOES-R is expected to provide satellite data products to users more quickly and to provide better clarity and precision than prior geostationary satellites. It is expected to be a two-satellite system, launching in 2015 and 2017, and is considered critical to the United States’ ability to maintain the continuity of data required for weather forecasting through 2028. In addition to operational polar and geostationary satellites, the United States operates research satellites to better understand scientific earth processes and to develop new technologies. Since the early 1990s, NASA has launched 18 research satellites under its Earth Observing System, and plans to launch 6 more by 2013. These satellites continue to provide global and seasonal earth system measurements, which have provided a better understanding of human impacts on the earth, as well as improved disaster prediction and mitigation technologies. They are used both by NASA’s research communities and by other agencies, including the U.S. Department of Agriculture, for operational and decision-making purposes. NASA is now planning the next generation of research satellites, called its Earth Systematic Missions program. This program consists of three series of satellites to advance understanding of the climate system and climate change. In addition to its earth observation activities, NASA has been working to understand and measure solar activity in the space environment. For example, the observations of solar winds from its Advanced Composition Explorer mission and solar X-ray images from its Solar and Heliospheric Observatory mission are used for both solar research and space weather forecasting. DOD also develops environmental research satellites in support of its mission when a need is identified. For example, the Navy and others developed the WindSat program to demonstrate new capabilities for measuring the ocean surface wind vectors from space and to demonstrate an instrument that was originally planned for the NPOESS mission. In addition, DOD’s Communication/Navigation Outage Forecasting System satellite is expected to develop a capability for detecting and forecasting space weather events that lead to disruptions in communication signals in high-frequency radios and Global Positioning System (GPS) satellites. Environmental satellites gather a broad range of data that are transformed into a variety of products. Satellite sensors observe different bands of radiation wavelengths, called channels, which are used for remotely determining information about the earth’s environment. When first received, satellite data are considered raw data. To make them usable, NOAA, NASA, and DOD operate data processing centers that format the data so that they are time-sequenced and include earth location and calibration information. After formatting, these data are called raw data records. The data centers further process the raw data records into channel-specific data sets, called sensor data records and temperature data records. These data records are then used to derive weather and climate products called environmental data records and climate data records. Environmental data records generally support near-term weather observations and include a wide range of atmospheric products detailing cloud coverage, temperature, humidity, and ozone distribution; land surface products showing snow cover, vegetation, and land use; ocean products depicting sea surface temperatures, sea ice, and wave height; and characterizations of the space environment. Combinations of these data records (raw, sensor, temperature, and environmental data records) are also used to derive more sophisticated products, including the forecasts that result from weather prediction modeling. In contrast, climate data records identify longer term variations in the climate and include observations of the land, ocean, and atmosphere. While environmental and climate data products use much of the same data, the two user communities’ needs differ. In order to deliver timely weather forecasts and warnings, meteorologists require the rapid delivery of environmental data. Alternatively, scientists involved in climate monitoring, prediction, and research require accurate, precise, and consistent data over long periods of time. Figure 4 is a simplified depiction of the various stages of environmental satellite data processing, and figure 5 depicts examples of two different weather products. Figure 6 depicts an example of a climate data record. One subset of satellite-provided environmental weather information is climate data. Satellite-provided climate data are used in combination with ground and ocean observing systems to understand seasonal, annual, and decadal variations in the climate.