Satellites

Meteorological satellites provide images and quantitative information about surface features and the lowest 20 km of the atmosphere (WMO, 1996). The contribution of satellite data to hydroclimatology has expanded greatly since the United States launched the Television and Infrared Observation Satellite (TIROS-1) in April 1960 to open the weather satellite era. Early satellites were limited to providing basically cloud images, but by the 1980s reliable measurements of net incoming and outgoing radiation fluxes at the top of the atmosphere became available. More recently, satellites provide information from numerous spectral bands that contribute to understanding of the pre-cipitable water and liquid water content of the atmosphere, winds at cloud level and at the ocean surface, atmospheric temperature and humidity profiles, cloud cover, a cloud's surface temperature, global sea surface temperatures, land surface temperatures, determination of fog, water/ice boundaries, and daily measurements for remote areas where conventional data are lacking. However, it is important to remember that satellite observing systems are viewing upwelling radiances composed of emissions from a range of heights in the atmosphere (WMO, 1996). Consequently, satellite and surface observations should be regarded as complementary and not competing data sources.

Meteorological satellites provide imagery that is digitized data even though the images may look like photographs. All meteorological satellites are equipped with a radiometer. The radiometer produces an image composed of a series of discrete point values in rows and columns called picture elements or pixels. The radiometer measures the intensity of the radiant energy coming from the Earth's surface and the atmosphere in a selected wavelength band identified as a channel. When the radiometer collects a predetermined amount of radiant energy it registers a count, and the number of counts is proportional to the radiation intensity. The area viewed by the radiometer at any given time is known as its footprint, and the total radiation from the footprint is assigned to a pixel located in the footprint's center. Some detail is lost in averaging characteristics within the footprint to produce a single value.

To enlarge the area viewed by the radiometer, a scanning system is employed that physically changes the direction in which the radiometer is pointing. A complete image of a large area of the Earth is constructed by the radiometer when all the pixels in the image have been assigned values. The width of the area scanned by the radiometer defines the swath or the path covered on the Earth's surface for a particular orbit of the satellite.

Satellites can operate in several types of Earth orbit, but the most common orbits are geostationary and polar. A circular orbit is the goal for most

30 North latitude

30 South latitude Fig. 5.1. Sketch of the orbital pattern of a geostationary satellite.

meteorological satellites, but satellites in general do not travel in perfect circles. The exact form of a satellite's orbit is derived from Newton's laws of motion and his law of universal gravitation, which have conceptual roots in Kepler's laws concerning planetary motion. A comprehensive summary of satellite orbits is provided by Kidder (2003).

A geostationary orbit is one in which the satellite is always in the same position with respect to the rotating Earth. The satellite orbits at an elevation of approximately 35 790 km above the Earth because that produces an orbital period equal to the Earth's period of rotation. The satellite appears stationary relative to the Earth because it is orbiting at the same rate and in the same direction as Earth (WMO, 1996), but in reality it is moving through the sky along a path resembling a narrow figure eight (Fig. 5.1). Geostationary satellites were first launched in the 1970s.

A polar-orbiting satellite circles the Earth at near-polar inclination. A true polar orbit has an inclination of 90°, which is the angle between the equatorial plane and the satellite orbital plane. Polar-orbit satellites circle the Earth at an altitude of 700 to 800 km, and the inclination of the satellite can be chosen so it is synchronized with the Sun in what is called a sun-synchronous orbit. This means that the satellite passes over a reference position on Earth at roughly the same local time during each orbital pass. The first weather satellite launched in 1960 was a polar-orbiting satellite.

5.2.1 Geostationary satellites

A network of geostationary satellites provides complete global coverage of all but the extreme north and south polar regions. This global satellite network is coordinated by the Coordination Group for Meteorological Satellites (CGMS) but is actually a collection of different satellites operated by independent agencies around the world. Full global coverage, excluding the polar regions, requires at least five geostationary satellites in orbit at any one time. The CGMS has agreed that operational responsibility for these five geostationary meteorological satellites is nominally the responsibility of four satellite operators. The European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) inherited the Meteorological Satellite (Meteosat) program started by the European Space Agency (ESA). The other three programs are operated by Japan, Russia, and the United States. However, the Geostationary Operational Meteorological Satellite (GOMS), also known as Elektro, deployed by Russia and positioned at 76° E has not produced reliable operational imagery.

In addition to the four operators recognized by the CGMS, at least two other nations have satellite programs. An experimental geostationary meteorological satellite (Fengyun-2) launched by China is positioned at 105° E, but it has not achieved operational status. The multipurpose Indian National Satellite (INSAT-2) deployed by India and positioned at 93.5° E has meteorological capabilities but is reserved mainly for national use.

The three programs that have successfully established operational satellites for CGMS use a variety of instrument configurations on the spacecraft. The European region served by EUMETSAT has a Meteosat satellite positioned on the Greenwich Meridian and the equator above the Gulf of Guinea (Fig. 5.2). Meteosat provides imagery in one visible and eleven IR channels, including bands for ozone and carbon dioxide. A new satellite with advanced application features called Meteosat Second Generation (MSG) became operational in January 2004 and was renamed Meteosat-8. The Japanese Geostationary Meteorological Satellite (GMS), also known as Himawari or sunflower, is in a geostationary orbit at 140° E. The GMS has one scanning channel in the visible spectrum and three in the IR. In May 2003, GMS-5 handed over its observation operation to a United States satellite moved from storage to 155° E. This became necessary when GMS-5 exceeded its life expectancy and two satellite launches to replace GMS-5 failed.

The United States maintains and operates two Geostationary Operational Environmental Satellites (GOES) known as GOES-East and GOES-West. GOES satellites carry multispectral instruments with flexible scan modes for observing the atmosphere and inferring atmospheric vertical temperature and moisture structure. GOES-East is positioned at 75° W and the equator where it views North and South America and most of the Atlantic Ocean. GOES-West is positioned at 135° W and the equator, which permits it to view North America and the Pacific Ocean basin (Fig. 5.3). The two satellites together provide day and night imagery of the Earth extending from 20° W to 165° E.

http://weather.msfc.nasa.gov/GOES/.)"/>
Fig. 5.2. Visible image from Meteosat-7 for 2 March 2007 at 1200 UTC. (Image courtesy of NOAA from their website at http://weather.msfc.nasa.gov/GOES/.)

Operation of environmental satellites in the United States is the responsibility of the National Oceanic and Atmospheric Administration. NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) operates the satellites and manages the processing and distribution of data and images these satellites produce daily. The prime customer is NOAA's National Weather Service, which uses satellite data to create forecasts and weather advisories. Satellite information is also shared with various Federal agencies, other countries, and the private sector.

Most geostationary satellites are geosynchronous because the Earth rotates at a rate slightly less than once every 24 hours. The satellite is allowed to drift within a predetermined area before corrections are made by on-board thrusters. The satellite's drift arises from anomalies in the Earth's gravitational field. The corrections maintain the satellite in a fixed position in the sky relative to the Earth's surface. Each satellite has a lifetime of about 5 years.

The scanning system of a geostationary satellite allows about 42° of the Earth's surface to be viewed from a single satellite. The geostationary satellite

http://www.goes.noaa.gov/goesfull.html.)"/>
Fig. 5.3. GOES-West visible image for 19 April 2006 at 2100 UTC. (Image courtesy of NOAA and the National Environmental Satellite, Data, and Information Service from their website at http://www.goes.noaa.gov/goesfull.html.)

can monitor developments in the field of view continuously and in almost realtime. However, a geostationary satellite's altitude of 36 000 km above the Earth's surface means the imagery resolution is lower than with polar-orbiting satellites deployed at lower altitudes. Also, geostationary satellites are limited to approximately 60 degrees of latitude at a fixed point over the Earth, and they provide distorted images of polar regions with poor spatial resolution. Even with these limitations, geostationary satellites provide continuous monitoring necessary for intensive data analysis. These satellites circle the Earth in a geosynchronous orbit that is high enough to allow the satellites a full-disk view of the Earth.

5.2.2 GOES instruments

Scanning and transmission systems on geostationary satellites vary with the country of origin, but they are broadly similar. In general, the basic instrumentation on these satellites measures Earth-emitted and reflected radiation. These measurements are then used to derive atmospheric temperature, winds, moisture, and cloud cover. Some recent satellites are equipped with instruments capable of measuring a broader range of conditions.

Satellite sensors observe the atmosphere with electromagnetic radiation either passively or actively. Most operational systems on meteorological satellites are passive sensors receiving scattered, reflected, or emitted radiation from the atmosphere or the Earth's surface. Sensors used on GOES satellites are representative of the most widely used sensor systems on geostationary satellites (WMO, 1996). Therefore, a brief description of GOES instruments is provided to illustrate the general nature of these instruments.

The present GOES satellites have a sensor array that includes both imager and sounder instruments. An imager is a radiometer designed to sense radiant and solar reflected energy from sampled areas of the Earth. It is capable of calculating cloud cover and surface temperature remotely from space. The surface temperature can be at the Earth's surface, a cloud's surface, the ocean's surface, or any other surface whose temperature is required. A sounder is a radiometer designed to provide data from which atmospheric temperature and moisture profiles, surface and cloud-top temperatures, pressure, and ozone distribution can be deduced by mathematical analysis. The atmospheric profiles produced by the sounder are similar to those achieved using radiosondes. However, the sounder system is capable of producing profiles for many more locations than is possible with radiosondes.

The GOES imager (Fig. 5.4) has a five-band multispectral capability to detect different wavelengths of energy in narrow wavelength bands called channels. This allows the imager to identify visible light, emitted longwave radiation, and other radiation wavelengths. The GOES imager has five channels that monitor

Louver sun shield

Radiant Radiant cooler cooler

Optical port assem

Louvei

Radiant Radiant cooler cooler

Optical port assem

Louvei

mirror

Telescope secondary Telescope primary mirror mirror

Fig. 5.4. GOES Imager diagram. (Drawing courtesy of NASA and the Goddard Space Flight Center from their website at http://goespoes.gsfc.nasa.gov/.)

mirror

Telescope secondary Telescope primary mirror mirror

Fig. 5.4. GOES Imager diagram. (Drawing courtesy of NASA and the Goddard Space Flight Center from their website at http://goespoes.gsfc.nasa.gov/.)

Fig. 5.5. GOES Imager visible image of North America for 2 March 2007 at 2015 UTC. Clouds cover most of Canada and the northern United States, and a band of clouds parallels the Atlantic coastline and extends southward into the Gulf of Mexico. (Image courtesy of NASA and the Global Hydrology and Climate Center from their website at http://weather.msfc.nasa.gov/GOES/.)

Fig. 5.5. GOES Imager visible image of North America for 2 March 2007 at 2015 UTC. Clouds cover most of Canada and the northern United States, and a band of clouds parallels the Atlantic coastline and extends southward into the Gulf of Mexico. (Image courtesy of NASA and the Global Hydrology and Climate Center from their website at http://weather.msfc.nasa.gov/GOES/.)

radiation at the specific wavelengths. Visible and IR images are acquired independently and continuously with a flexible scan system. The greatest advantage of having both visible and IR capability is that monitoring continues both day and night. With the imager's multispectral design and sensitivity it can detect temperature fluctuations, variation in low-level moisture, and track hurricanes from their inception as tropical storms. The imager provides views every 15 minutes with a spatial resolution of 1 km for visible images (Fig. 5.5) and 4 km for IR data.

The GOES imager visible channel centered on the wavelength of 0.65 mm supplies daylight images of clouds, haze, severe storms, snowcover, and volcanic activity. The near IR channel with a central wavelength of 3.9 mm reveals ground fog, fires, volcanoes, sea surface temperatures, and permits discrimination between water clouds and snow or ice crystal clouds. An IR channel centered at 6.7 mm provides a window on upper-level water vapor and is used for

Radiant Radiant cooler patch cooler Scan assembly

Radiant Radiant cooler patch cooler Scan assembly

wheel primary secondary assembly mirror mirror

Fig. 5.6. GOES Sounder diagram. (Drawing courtesy of NASA and the Goddard Space Flight Center from their website at http://goespoes.gsfc.nasa.gov/.)

wheel primary secondary assembly mirror mirror

Fig. 5.6. GOES Sounder diagram. (Drawing courtesy of NASA and the Goddard Space Flight Center from their website at http://goespoes.gsfc.nasa.gov/.)

identifying upper-level moisture sources, the presence of atmospheric humidity, mid-level moisture content and advection, and tracking of mid-level atmospheric motions. Information on jet stream features, surface temperatures, and the location of heavy rainfall is provided by a longwave IR channel centered at 10.7 mm. A second longwave IR channel centered at 12.0 mm supplies images used to observe daily temperature changes and cold cloud tops, the detection of airborne dust and volcanic ash, and identification of low-level moisture. GOES-12 launched in July 2003 replaced the 12.0 mm channel with a 13.3 mm centered channel to provide improved data for estimating cloud amount and cloud heights.

The GOES sounder (Fig. 5.6) is a 19-channel discrete-filter radiometer covering the spectral range from the visible wavelengths to 15 mm. The sounder utilizes four sets of detectors to collect and identify variations within the Earth's atmosphere using a scanning broad infrared spectrum. It operates independently of the imager and makes simultaneous observations using a flexible scanning system similar to the one used by the imager. The sounder is equipped with a search and rescue transponder, and it serves as a space weather monitor.

The sounder's multi-element detector array permits it to simultaneously sample four separate fields or atmospheric columns. A rotating filter wheel brings spectral filters into the optical path of the detector array and provides the IR channel definition. It measures one visible channel and emitted radiation in 18 thermal IR bands divided into three detector groups that support several derived products (Fig. 5.7). The spatial resolution of sounder data is 1 km for the visible channel and 2 km for the IR channels.

The three-axis, body-stabilized spacecraft design of GOES enables the imager and sounder to constantly view a specified area of the Earth and frequently image clouds, monitor Earth's surface temperature and water vapor fields, and

Fig. 5.7. GOES Sounder derived convective available potential energy (CAPE) for North America on 2 March 2007 at 1700 UTC. This energy relates to the energy available for thunderstorm development. (Image courtesy of NOAA and the National Environmental Satellite Data and Information Service from their website at http://www.orbit.nesdis.noaa.gov/smcd/opbd/goes/soundings/.)

Fig. 5.7. GOES Sounder derived convective available potential energy (CAPE) for North America on 2 March 2007 at 1700 UTC. This energy relates to the energy available for thunderstorm development. (Image courtesy of NOAA and the National Environmental Satellite Data and Information Service from their website at http://www.orbit.nesdis.noaa.gov/smcd/opbd/goes/soundings/.)

sound the atmosphere of its vertical thermal and vapor structures. In addition, GOES satellites have flexible scanning that allows small-area imaging as well as simultaneous and independent imaging and sounding. These two features allow continuous data gathering from both instruments. While the imager collects digitized data, the sounder profiles temperature and moisture levels down through the atmosphere. Together the imager and sounder provide a stream ofinformation on the movement ofwinds and moisture and temperature in the clouds. The viewing capability of current GOES satellites provides imagery useful beyond the previous north/south limits of geostationary satellites. This capability permits tracking of icebergs and monitoring snow and ice cover up to the Arctic and Antarctic circles.

5.2.3 Polar-orbiting satellites

A polar-orbiting satellite provides an observational platform for the entire Earth's surface. Polar-orbiting meteorological satellites complement the data provided by geostationary satellites. The orbit of a polar-orbiting satellite crosses close to both poles, and it follows an almost north-south direction. These satellites are inserted at altitudes of 800 to 1000 km, which imparts a relatively high speed to the satellite. Polar-orbiting satellites complete about 14 orbits per day with an orbital period of approximately 100 minutes. This results in the satellite passing over the same region on the Earth's surface twice a day at 12 hour intervals. The lower altitude of polar-orbiting satellites makes possible a finer ground resolution image than geostationary satellites, and it produces a ground swath of about 3300 km.

Numerous meteorological, commercial geophysical remote sensing (e.g. Landsat and the French Systeme Probatoire d'Observation de la Terre or SPOT), and communications satellites are in polar orbits. Polar-orbiting meteorological satellites have been deployed by China, Russia, and the United States. The Polar Operational Environmental Satellite (POES) Program is a cooperative effort between NASA and NOAA, the United Kingdom (UK), and France. In the United States, POES includes spacecraft developed and launched for the NOAA Polar-orbiting Operational Environmental Satellites (NPOES) program and the Defense Meteorological Satellite (DMS) program. Real-time data transmissions from the DMS satellites are encrypted, but DMS data are sent daily to the National Geophysical Data Center and the Solar Terrestrial Physics Division for archiving and can be available for civil use.

POES spacecraft have meteorological and geophysical importance because of their high-resolution global coverage and well-calibrated channels. Similar benefits are realized from the ESA European Remote Sensing (ERS) satellites equipped with both radiometers and synthetic aperture radar (SAR). The Canadian satellite RADARSAT provides SAR images for both scientific and commercial applications, and the research emphasis is on polar regions. The NASA-centered international Earth Observing System (EOS) includes the Terra and Aqua satellites launched in 1999 and 2002, respectively. These satellites are intended to monitor the health of the planet with Terra emphasizing land and Aqua emphasizing the many forms of water (Parkinson, 2003).

Most polar-orbiting satellites operate in a sun-synchronous orbit in which the orbit is tilted slightly towards the northwest and does not actually go over the poles. A sun-synchronous polar-orbiting satellite passes over a reference position on Earth at roughly the same local time during each orbital pass and is identified by the time it crosses the equator. An early morning satellite will make its ascending pass over the equator in the early morning, independent of Earth's east to west rotation. With each subsequent pass, the satellite will cross the equator southbound about 11 degrees westward due to the Earth's rotation. The time the satellite passes over the equator is usually between mid-morning and mid-afternoon on the sunlight side of the orbit. The actual orbital track is due to a combination of the orbital plane of the satellite coupled with the rotation of the Earth beneath the satellite. The orbital plane of a sun-synchronous orbit must also rotate approximately one degree each day to keep pace with the Earth's surface. In this way, it images its swath at about the same sun time during each pass, so that lighting remains roughly uniform. The repeated coverage by polar-orbiting satellites enables regular data collection at consistent times as well as long-term comparisons. Sun-synchronism produces time-constant illumination conditions of the observed surfaces, except for seasonal variations in sunlight duration.

http://goespoes.gsfc.nasa.gov/poes/.)"/>
Fig. 5.8. Sketch of a POES satellite. (Drawing provided by NASA and the Goddard Space Flight Center from their website at http://goespoes.gsfc.nasa.gov/poes/.)

NOAA class satellites and Russian Meteor class satellites have orbits that cross very close to the poles on each revolution of the Earth. NOAA has two polar-orbiting satellites in the Advanced Television Infrared Observation Satellite (TIROS-N or ATN) series (Fig. 5.8). The orbits are circular and sun synchronous. The morning orbit crosses the equator at 7:30 a.m. local time and has an altitude of 830 km. The afternoon orbit crosses the equator at 1:40 p.m. local time and has an altitude of 870 km. The circular orbit permits uniform data acquisition by the satellite and efficient control of the satellite by the NOAA Command and Data Acquisition (CDA) stations located near Fairbanks, Alaska, and Wallops Island, Virginia. Operating as a pair, these satellites ensure that data for any region of the Earth are no more than six hours old.

5.2.4 Polar-orbiting satellite instrumentation

Instrumentation on NOAA's polar-orbiting satellites serves as a representative case due to the accessibility of the data from these instruments. The TIROS-N series satellites are three-axis-stabilized spacecraft that carry seven scientific instruments and two for Search and Rescue. The primary instrument is an Advanced Very High Resolution Radiometer (AVHRR), which is a five-channel multispectral scanner covering visible to thermal IR wavelengths in the range from 0.58 to 12.5 mm. One channel observes in the visible band, one in

Fig. 5.9. NOAA-18 Advanced Very High Resolution Radiometer (AVHRR) 4 km resolution composite image of snowcover across Canada and the United States on 15 February 2007. (Image courtesy of NOAA and the National Environmental Satellite Data and Information Service from their website at http://www.orbit.nesdis.noaa.gov/smcd/emb/snow/.)

Fig. 5.9. NOAA-18 Advanced Very High Resolution Radiometer (AVHRR) 4 km resolution composite image of snowcover across Canada and the United States on 15 February 2007. (Image courtesy of NOAA and the National Environmental Satellite Data and Information Service from their website at http://www.orbit.nesdis.noaa.gov/smcd/emb/snow/.)

the near-IR, and three in the thermal-IR portion of the spectrum. The AVHRR scans the Earth's surface from side to side perpendicular to the satellite's ground track. Each scan covers an area about 2 km high and 3000 km wide.

AVHRR data are transmitted to the ground via a High Resolution Picture Transmission (HRPT) broadcast that contains AVHRR data in three formats. The HRPT provides unprocessed real-time, digital imagery at 1.1 km resolution. The onboard processor records full resolution Local Area Coverage (LAC) data at 1.1 km resolution for subsequent transmission during a station overpass. Global Area Coverage (GAC) data are produced by sampling and averaging onboard the satellite the 1.1 km AVHRR measurements and portraying them as 4 km data (Gutman et al., 2000). An archive of the AVHRR data is provided by NOAA's National Environmental Satellite, Data, and Information Service (Gutman et al., 2000; Jacobowitz et al., 2003).

A second data transmission consists of only image data from two of the AVHRR channels, called Automatic Picture Transmission (APT). The information transmitted in the APT format provides users with imagery at 4 km resolution (Fig. 5.9). For users who want to establish their own direct-readout receiving station, low-resolution imagery data in the APT service can be received with inexpensive equipment, while the highest-resolution data transmitted in the HRPT service utilizes a more complex receiver.

Since 1998, the NOAA polar-orbiting satellites instrumentation has included the Advanced TIROS Operational Vertical Sounder (ATOVS). This instrument suite includes a 20-channel High Resolution Infrared Sounder (HIRS), a 15-channel Advanced Microwave Sounding Unit-A (AMSU-A), and a 5-channel Advanced Microwave Sounding Unit-B (AMSU-B). HIRS provides atmospheric sounding data in cloud-free regions. AMSU-A is designed for measuring vertical temperature profiles by selecting channels sensitive to emissions from different depths into the atmosphere. AMSU-B is designed for measuring vertical atmospheric humidity profiles in a similar manner. These sensors scan the entire Earth's surface over a 24hour period. The ATOVS data are archived with the earlier TIROS Operational Vertical Sounder (TOVS) data at the Goddard Distributed Active Archive Center.

Instruments on the Terra and Aqua satellites complement the data acquired on earlier satellites, extend the record of these data, and improve the spatial resolution of some data. Aqua carries six instruments that take multiple readings across a wide span of the electromagnetic spectrum. Three instruments work together to form Aqua's sounding suite, which is composed of the Atmospheric Infrared Sounder (AIRS), the AMSU, and the Humidity Sounder for Brazil (HSB). AIRS is a 2382-channel high-spectral-resolution sounder whose primary purpose is to obtain atmospheric temperature and humidity profiles from the surface up to 40 km. The AMSU and HSB are similar to instruments on NOAA satellites, and they provide complementary measurements to the AIRS that are important under overcast sky conditions. The Advanced Microwave Scanning Radiometer for EOS (AMSR-E) provided by Japan is a 12-channel conically scanning passive-microwave radiometer measuring vertically and horizontally polarized radiation at frequencies from 6.9 to 89 GHz. The inclusion on AMSR-E of several channels measuring at wavelengths where there is little atmospheric interference permits the instrument to obtain surface data even in the presence of substantial cloud cover. This feature and the passive-microwave measurements mean that the AMSR-E has an all-weather and day-or-night capability for surface variables. The Moderate Resolution Imaging Spectroradiometer (MODIS) and the Clouds and the Earth's Radiant Energy System (CERES) instruments on Aqua are similar to instruments on Terra. MODIS is a 36-channel cross-track scanning radiometer utilizing wavelengths between 0.4 and 14.5 mm. MODIS has the finest spatial resolution for data from any of the Aqua instruments. CERES is a 3-channel broadband scanning radiometer that measures both the shortwave and longwave components of Earth's radiation balance. Two CERES instruments on Aqua operating in fixed and rotating scanning modes provide energy flux measurements that produce highly accurate radiation balance measurements (Parkinson, 2003).

Was this article helpful?

0 0
Waste Management And Control

Waste Management And Control

Get All The Support And Guidance You Need To Be A Success At Understanding Waste Management. This Book Is One Of The Most Valuable Resources In The World When It Comes To The Truth about Environment, Waste and Landfills.

Get My Free Ebook


Post a comment