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An Overview of Meteorological Satellites
Ann Mazuk, John Haas, W. John Hussey, Leslie Belsma, and John Bohlson
Accurate weather forecasts have a direct bearing on military operations and commercial enterprises. Aerospace has played an integral role in helping to develop and enhance U.S. meteorological satellite systems.
In late summer 2004, a series of powerful hurricanes swept through the southeastern United States. Homes and businesses were demolished, roads and bridges were washed away, and coastal structures were carried out to sea. Fortunately, local authorities were notified early enough to begin evacuations and emergency preparations. These early warnings were made possible by the fleet of weather satellites circling the globe.
The ability to track potentially devastating storms is just one benefit of these weather satellites; other applications range from detection of burning forests to identifying the frozen boundaries between snow and ice. Weather satellites help the agricultural sector determine the best time to plant and harvest crops and help the transportation sector chart optimal air and sea routes. They play a role in monitoring Earth's climate change and have proved useful in identifying global transport of air pollutants. The military uses information from weather satellites in planning combat missions and assessing conditions within theaters of operation. Such information can influence decisions on a practical level—such as whether to load electro-optical precision guided munitions, which require good weather, or other weapons that do not.
Most weather satellites have both infrared and visible radiometers on board. The visible sensors observe reflected light in the 0.4 to 1.1 micron region of the electromagnetic spectrum, the same as the human eye. The infrared sensors observe emitted radiation in the 3 to 14 micron region and work by detecting differences in temperature. Weather satellites also carry special radiometers, either microwave or infrared sounders, which provide profiles of temperature and moisture at different levels in the atmosphere as well as information on Earth's surface.
To get a complete picture of emerging and evolving weather patterns, two types of satellites are needed. Geostationary satellites, which maintain a constant view of one complete hemisphere, provide a more immediate assessment of existing weather conditions. Polar-orbiting satellites, circling the globe at a much lower altitude, provide global coverage needed to support longer forecasts and weather models. In the past, organizations such as the Department of Defense and the National Oceanic and Atmospheric Administration (NOAA) maintained separate polar systems. In the near future, the needs of these agencies from polar orbit will be met by a single constellation. All of these missions are controlled from a NOAA facility.
POES
Artist's rendering of a NOAA satellite in orbit, part of the Polar-orbiting Operational Environmental Satellite (POES) system. |
Today's polar-orbiting weather satellites trace their roots to the Television Infrared Observation Satellite (TIROS), launched in 1960. Four generations of TIROS (also known as NOAA) satellites were eventually deployed. The latest, NOAA N, will serve as the next NOAA Polar-orbiting Operational Environmental Satellite (POES) system. The constellation includes two primary satellites and several backup units in sun-synchronous polar orbit (850-kilometer altitude). The two primary spacecraft circle the globe every 102 minutes; one is in a morning orbit (crossing the equator from north to south at 7:30 a.m. local time), and the other is in an afternoon orbit (crossing the equator from south to north at 1:40 p.m.). The satellites are generally known by their NOAA designation—alphabetical before launch (e.g., NOAA J) and numerical on orbit (e.g., NOAA 14).
These satellites are equipped with high-resolution radiometers, infrared sounders, and advanced microwave sounders. They also include a system to collect data from moving platforms (e.g., buoys) and a space environment monitor. Data from these sensors contribute to global climate models by providing global atmospheric temperature and humidity profiles from Earth's surface to the upper stratosphere. The data are also used to determine ocean surface temperature, total atmospheric ozone levels, precipitable water, and cloud height and coverage (see sidebar, The Polar Orbit).
Aerospace support to the POES program started in 1997, during its operational convergence with the Defense Meteorological Satellite Program (DMSP) at NOAA's Suitland, Maryland, facility. Aerospace was instrumental in preparing the ground system for the launch and operation of NOAA N and N' and also developed the ground-system specification upgrades for these two spacecraft. Aerospace also provided in-depth support for the NOAA N Microwave Humidity Sounder instrument and interface. In the course of this work, Aerospace developed command procedures, developed a database, validated instrument data, and conducted simulations. The Microwave Humidity Sounder provides vertical water vapor profiles from Earth's surface to about 12 kilometers for use in global forecast models. Aerospace also supported NOAA 15–17 simulations, launch, checkout, and anomaly resolution.
Artist's rendition of a GOES (Geostationary Operational Environmental Satellite) spacecraft. |
GOES
NOAA's geostationary weather satellites trace their roots to NASA's Applications Technology Satellite (ATS), launched in December 1966. ATS incorporated a spin-scan camera: The motion of the satellite spinning about its axis, which was parallel to Earth's axis, generated the east-west scan motion, while a stepping motor provided the north-south scan motion. This innovative concept enabled meteorologists to track severe storms and cloud motions and derive wind speed and direction at cloud altitude.
NOAA eventually assumed operational responsibility for the ATS system, using it as a model for its subsequent Geostationary Operational Environmental Satellite (GOES) system. Today, the GOES system consists of two geostationary spacecraft located at 75 degrees and 135 degrees west longitude. They provide constant coverage of the contiguous 48 states, the southern part of Alaska, Hawaii, and adjacent ocean areas through visible and infrared sensors. The version now in development, GOES-R, will scan Earth nearly six times faster than the current system and provide about 60 times the amount of data. The first launch is planned for 2012.
Aerospace support to the GOES program started in 1993, when Aerospace researchers began simulations for the first of the three-axis stabilized spacecraft, GOES I, which eventually flew as GOES 8 (the naming convention for GOES is similar to that of TIROS—alphabetical before launch, numerical after). In this effort, Aerospace was able to apply specialized expertise gained through years of experience with three-axis-stabilized meteorological spacecraft. Aerospace supported all GOES I–M simulations, launches, and checkout. Aerospace was an integral part of the GOES N–P development process, including integration, test, and simulations. Aerospace developed GOES N fault-protection command procedures for safely changing spacecraft modes, using the spacecraft contractor's guidelines to develop actual spacecraft commands and procedures. Aerospace was also instrumental in testing the new GOES N–P ground system, including the archive subsystem.
DMSP
DMSP is the military counterpart to POES. The DMSP spacecraft are in polar sun-synchronous orbit at about the same orbital period and altitude as the POES spacecraft. The sensors aboard each system differ, and unlike POES data, DMSP data are encrypted. DMSP data are continuously transmitted for real-time use within its path and also stored onboard and relayed to central weather agencies around the world.
DMSP began as an all "blue suit" Air Force program with surge Aerospace support, starting with the recovery of a tumbling F-1, the first Block 5D-1 satellite, launched in 1976. Since then, Aerospace has provided increasing technical support to all phases of the program. The impact of Aerospace's general systems engineering and integration expertise was the dramatic improvement in satellite lifetime starting with F-6, the first Block 5D-2 series of satellites, and enhanced reliability of the primary cloud-imaging sensor, the Operational Line Scan.
DMSP satellite F-12 caught this view of blowing dust in southwest Afghanistan in August 2003. (Air Force Weather Agency) |
The Operational Line Scan sensor produces cloud imagery globally at smooth resolution (2.77 kilometers) and, for areas of special interest, at fine resolution (0.55 kilometer). The DMSP cloud-cover imagery is used to find, track, and locate targets during combat and to assess mission impact after engagement. Aerospace is directly involved in developing applications that exploit DMSP and other weather satellite data to support tactical operations. In 1999, Aerospace developed a prototype system to convert DMSP and POES imagery to high-resolution quantitative assessments of percent cloud cover, type, and height used in combat mission planning in Bosnia. Aerospace continues to develop this system and has recently added NASA Earth Observing System (EOS) Moderate-resolution Imaging Spectroradiometer (MODIS) data for the first operational use of MODIS cloud products by the Air Force Weather Agency.
In addition to cloud cover, many other atmospheric phenomena such as aerosols, smoke, and haze can affect a weapon's ability to detect and acquire a target. The Operational Line Scan sensor's visible channel can detect smoke and dust storms and identify snow and ice fields. The system has provided vivid images of dust storms blanketing the Iraqi desert and has helped pinpoint oil fires.
Combat forces rely on surface winds and ocean data for planning amphibious landings and soil moisture data for assessing ground maneuverability of armored divisions. Both atmospheric moisture and sudden increases in temperature with height (inversions) can affect high-precision targeting of artillery gunfire and ballistic missiles. It can also affect aircraft performance. DMSP carries a suite of microwave sensors for imaging, temperature sounding, and moisture sounding. On the most recent DMSP satellite, launched in the fall of 2003, these three sensors were combined into one unit. The microwave imager is used to specify wind speed over the oceans; it can detect snow cover and, for some snow conditions, provide estimates of snow depth. It can detect and distinguish sea ice from its surrounding waters and determine surface types over land, classifying bare soil and identifying vegetation (see sidebar, Microwave Data).
The Operational Line Scan sensor is the only operational weather sensor sensitive enough in the visible spectrum to view clouds by moonlight; thus, it can provide low-light (nighttime) visible imagery. The nighttime infrared imagery can help mission planners visualize the cloud cover and weather conditions over areas of interest and determine which sites are optimal to target. Nighttime imagery also plays a role in battlefield damage assessment; comparing images before and after the mission can indicate whether a target has been hit.
Earth's city lights as seen by the DMSP Operational Line Scan sensor. The civil community has found ways to exploit this nighttime visible imagery. By collecting data of city lights, the entire globe can be mapped to create a database of stable lights over a period of about six months. This data can be useful in detecting fires and lightning, quantifying power outages, and estimating population densities and economic development. (NASA) |
During the recent conflicts in Bosnia, Afghanistan, and Iraq, DMSP data collected over combat theaters were used daily in the planning of air, sea, and ground operations. The weather affects military planning at every level and timescale. By observing developing weather systems in the polar regions and over the oceans, weather satellites have vastly improved the capability to forecast many days in advance in support of strategic planning. At the other end of the spectrum, the instant data downlinks allow satellite data over the area of interest to be swiftly integrated in aerial mission planning, even for the 30-minute targeting time line.
Weather forecasts (typically cloud cover and visibility) also influence weapon selection. Electro-optical, infrared, and laser weapons use sensors to guide them to the target and require a clear line of sight. Pilots use weather forecasts produced prior to takeoff to predict the probability of target acquisition, and, consequently, target prioritization and weapon selection.
NPOESS
In 1994, President Clinton signed a directive merging DMSP and POES into a single system, the National Polar-orbiting Operational Environmental Satellite System (NPOESS). The evolution from the current POES and DMSP programs will take place over the next five to nine years, with full operational capability in 2013.
Artist's rendition of the NPOESS spacecraft. (Northrop Grumman) |
The operational concept for NPOESS consists of a constellation of spacecraft flying at an altitude of 828 kilometers in three sun-synchronous orbital planes (crossing the equator from south to north at 1:30, 5:30, and 9:30 p.m. local time). The satellites will carry visible, infrared, and microwave imagers and sounders, with special emphasis on atmospheric temperature and moisture sounding.
The 13 different instrument payloads on NPOESS will observe significantly more phenomena than their predecessors. In fact, NPOESS is expected to deliver about 8 terabytes of data per day, more than the current POES and DMSP systems combined. Each NPOESS spacecraft generates data at a rate of 20.0 megabits per second. Thus, the entire volume of data generated by the civil POES system in its 40 years of operations would be generated in less than 12 days by NPOESS. This increase in data volume will increase demands on the front-end processors and data-assimilation systems used to initialize and update global and regional numerical weather prediction models. To minimize delays, NPOESS stored mission data will be relayed to national weather processing centers located in the continental United States within 28 minutes from observation.
NPOESS spacecraft are being designed for precise orbit control to maintain altitude, nodal crossing times, and repeat ground tracks (repeat cycles of approximately 17 days). Because the requirements for data refresh are different for many of the 55 environmental parameters monitored, not all instrument payloads will fly in each orbit. In addition, certain orbital characteristics, as well as considerations of instrument field of view, have determined the payload configurations for each orbit. In addition, as a result of discussions between the United States and the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), sounding data acquired from the Infrared Atmospheric Sounding Interferometer and the Microwave Humidity Sounder onboard the European Metop satellite (also to be in polar orbit, crossing the equator from north to south at 9:30 a.m.) will augment NPOESS data.
NPOESS environmental data records (EDRs), listed by sensor. |
Final design, prototype, and fabrication of the sensor suites and algorithms necessary to support NPOESS has begun, with delivery of the first flight units scheduled for 2005 to support the NPOESS Preparatory Project, a risk-reduction mission for NPOESS and a data-continuity mission for NASA. Aerospace has been involved since the program's inception, providing technical guidance on requirements definitions and developing the requisite source-selection documentation for each sensor and system. Aerospace now serves as the technical lead for the majority of the developmental sensors as well as the algorithms and ground system. In fact, Aerospace participation extends to every level of the project, from broad program management to individual task groups.
Conclusion
The sensors aboard POES, GOES, and DMSP provide information about surface winds over broad areas of the oceans, cloud water content, rain rate, water-vapor content, and land surface temperature. As these systems merge and evolve, Aerospace will work to ensure that practical requirements for atmospheric, oceanic, terrestrial, climatic, and solar-geophysical data will guide the development of visible, infrared, and microwave imagers and sounders that will achieve greater accuracy and timeliness of observations.
Acknowledgement
The authors thank Rich Pastore for his assistance in preparing this article.
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