Satellite Remote Sensing

Remote sensing provides spatial coverage by measurement of reflected and emitted electromagnetic radiation, across a wide range of wavebands, from the earth's surface and surrounding atmosphere. The improvement in technical tools of meteorological observation, during the last twenty years, has created a favourable substratum for research and monitoring in many applications of sciences of great economic relevance, such as agriculture and forestry. Each waveband provides different information about the atmosphere and land surface: surface temperature, clouds, solar radiation, processes of photosynthesis and evaporation, which can affect the reflected and emitted radiation, detected by satellites. The challenge for research therefore is to develop new systems extracting this information from remotely sensed data, giving to the final users, near-real-time information.

Over the last two decades, the development of space technology has led to a substantial increase in satellite earth observation systems. Simultaneously, the Information and Communication Technology (ICT) revolution has rendered increasingly effective the processing of data for specific uses and their instantaneous distribution on the World Wide Web (WWW).

The meteorological community and associated environmental disciplines such as climatology including global change, hydrology and oceanography all over the world are now able to take advantage of a wealth of observational data, products and services flowing from specially equipped and highly sophisticated environmental observation satellites. An environmental observation satellite is an artificial Earth satellite providing data on the Earth system and a Meteorological satellite is a type of environmental satellite providing meteorological observations. Several factors make environmental satellite data unique compared with data from other sources, and it is worthy to note a few of the most important:

• Because of its high vantage point and broad field of view, an environmental satellite can provide a regular supply of data from those areas of the globe yielding very few conventional observations;

• The atmosphere is broadly scanned from satellite altitude and enables large-scale environmental features to be seen in a single view;

• The ability of certain satellites to view a major portion of the atmosphere continually from space makes them particularly well suited for the monitoring and warning of short-lived meteorological phenomena; and

• The advanced communication systems developed as an integral part of the satellite technology permit the rapid transmission of data from the satellite, or their relay from automatic stations on earth and in the atmosphere, to operational users.

These factors are incorporated in the design of meteorological satellites to provide data, products and services through three major functions:

• Remote sensing of spectral radiation which can be converted into meteorological measurements such as cloud cover, cloud motion vectors, surface temperature, vertical profiles of atmospheric temperature, humidity and atmospheric constituents such as ozone, snow and ice cover, ozone and various radiation measurements;

• Collection of data from in situ sensors on remote fixed or mobile platforms located on the earth's surface or in the atmosphere; and

• Direct broadcast to provide cloud-cover images and other meteorological information to users through a user-operated direct readout station.

The first views of earth from space were not obtained from satellites but from converted military rockets in the early 1950s. It was not until 1 April 1960 that the first operational meteorological satellite, TIROS-I, was launched by the USA and began to transmit basic, but very useful, cloud imagery. This satellite was such an effective proof of concept that by 1966 the USA had launched a long line of operational polar satellites and its first geostationary meteorological satellite. In 1969 the USSR launched the first of a series of polar satellites. In 1977 geostationary meteorological satellites were also launched and operated by Japan and by the European Space Agency (ESA). Thus, within 18 years of the first practical demonstration by TIROS-I, a fully operational meteorological satellite system (Fig. 3) was in place, giving routine data coverage of most of the planet. This rapid evolution of a very expensive new system was unprecedented and indicates the enormous value of these satellites to meteorology and society. Some four decades after the first earth images, new systems are still being designed and implemented, illustrating the continued and dynamic interest in this unique source of environmental data.

Figure 3: Nominal configuration of the space-based sub-system of the Global Observing System in 1978.

By the year 2000, WMO Members contributing to the space-based subsystem of the Global Observing System had grown. There were two major constellations in the space-based Global Observing System (GOS) (Fig. 4). One constellation was the various geostationary satellites, which operated in an equatorial belt and provided a continuous view of the weather from roughly 70°N to 70°S. The second constellation in the current space-based GOS comprised the polar-orbiting satellites operated by the Russian Federation, the USA and the People's Republic of China. The METEOR-3 series has been operated by the Russian Federation since 1991.

Figure 4: Nominal configuration of the space-based sub-system of the Global Observing System in 2000.

The ability of geostationary satellites to provide a continuous view of weather systems make them invaluable in following the motion, development, and decay of such phenomena. Even such short-term events such as severe thunderstorms, with a life-time of only a few hours, can be successfully recognized in their early stages and appropriate warnings of the time and area of their maximum impact can be expeditiously provided to the general public. For this reason, its warning capability has been the primary justification for the geostationary spacecraft. Since 71 per cent of the Earth's surface is water and even the land areas have many regions which are sparsely inhabited, the polar-orbiting satellite system provides the data needed to compensate the deficiencies in conventional observing networks. Flying in a near-polar orbit, the spacecraft is able to acquire data from all parts of the globe in the course of a series of successive revolutions. For these reasons the polar-orbiting satellites are principally used to obtain: (a) daily global cloud cover; and (b) accurate quantitative measurements of surface temperature and of the vertical variation of temperature and water vapour in the atmosphere. There is a distinct advantage in receiving global data acquired by a single set of observing sensors. Together, the polar-orbiting and geostationary satellites constitute a truly global meteorological satellite network.

Satellite data provide better coverage in time and in area extent than any alternative. Most polar satellite instruments observe the entire planet once or twice in a 24-hour period. Each geostationary satellite's instruments cover about í4 of the planet almost continuously and there are now six geostationary satellites providing a combined coverage of almost 75%. Satellites cover the world's oceans (about 70% of the planet), its deserts, forests, polar regions, and other sparsely inhabited places. Surface winds over the oceans from satellites are comparable to ship observations; ocean heights can be determined to a few centimetres; and temperatures in any part of the atmosphere anywhere in the world are suitable for computer models. It is important to make maximum use of this information to monitor our environment. Access to these satellite data and products is only the beginning. In addition, the ability to interpret, combine, and make maximum use of this information must be an integral element of national management in developed and developing countries.

The thrust of the current generation of environmental satellites is aimed primarily at characterizing the kinematics and dynamics of the atmospheric circulation. The existing network of environmental satellites, forming part of the GOS of the World Weather Watch produces real-time weather information on a regular basis. This is acquired several times a day through direct broadcast from the meteorological satellites by more than 1,300 stations located in 125 countries.

The ground segment of the space-based component of the GOS should provide for the reception of signals and DCP data from operational satellites and/or the processing, formatting and display of meaningful environmental observation information, with a view to further distributing it in a convenient form to local users, or over the GTS, as required. This capability is normally accomplished through receiving and processing stations of varying complexity, sophistication and cost.

In addition to their current satellite programmes in polar and geostationary orbits, satellite operators in the USA (NOAA) and Europe (EUMETSAT) have agreed to launch a series of joint polar-orbiting satellites (METOP) in 2005. These satellites will complement the existing global array of geostationary satellites that form part of the Global Observing System of the World Meteorological Organization. This Initial Joint Polar System (IJPS) represents a major cooperation programme between the USA and Europe in the field of space activities. Europe has invested 2 billion Euros in a low earth orbit satellite system, which will be available operationally from 2006 to 2020.

The data provided by these satellites will enable development of operational services in improved temperature and moisture sounding for numerical weather prediction (NWP), tropospheric/stratospheric interactions, imagery of clouds and land/ocean surfaces, air-sea interactions, ozone and other trace gases mapping and monitoring, and direct broadcast support to nowcasting. Advanced weather prediction models are needed to assimilate satellite information at the highest possible spatial and spectral resolutions. It imposes new requirements on the precision and spectral resolution of soundings in order to improve the quality of weather forecasts. Satellite information is already used by fishery-fleets on an operational basis. Wind and the resulting surface stress is the major force for oceanic motions. Ocean circulation forecasts require the knowledge of an accurate wind field. Wind measurements from space play an increasing role in monitoring of climate change and variability. The chemical composition of the troposphere is changing on all spatial scales. Increases in trace gases with long atmospheric residence times can affect the climate and chemical equilibrium of the Earth/ Atmosphere system. Among these trace gases are methane, nitrogen dioxide, and ozone. The chemical and dynamic state of the stratosphere influence the troposphere by exchange processes through the tropopause. Continuous monitoring of ozone and of (the main) trace gases in the troposphere and the stratosphere is an essential input to the understanding of the related atmospheric chemistry processes.

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