Meteorological satellites

(aerospace engineering) Earth-orbiting spacecraft carrying a variety of instruments for measuring visible and invisible radiations from the earth and its atmosphere.

Satellites dedicated to the observation of meteorological phenomena and atmospheric or surface properties used for weather forecasting. Operational meteorological satellites provide routine observations of weather conditions as well as an ever expanding range of environmental properties, such as aerosol, dust and ash clouds from volcanic eruptions, ozone, and land vegetation cover. For this reason, they are known in the United States as operational environmental satellites. See also Meteorology; Satellite (spacecraft); Satellite meteorology.

Optical imaging sensors

The first recognized application of orbital observation was the visual exploitation of cloud images associated with weather systems. Recent instruments, such as the Advanced Very High Resolution Radiometer (AVHRR) on NOAA satellites, use a variety of quantitative applications, such as remote sensing of sea surface temperature, monitoring changes in land vegetation, and discriminating between different kinds of clouds. There is a pervasive trend to increase the number of spectral bands in imaging sensors, from 5 channels in the current AVHRR to 36 channels in the experimental Moderate-resolution Imaging Spectroradiometer (MODIS) developed by NASA. These channels sample the full spectrum of backscattered solar radiation in the visible, near-infrared, and longwave infrared, and a good part of the emitted terrestrial radiation spectrum (thermal infrared). This multiplicity of spectral bands allows the detection of a wide variety of features, from aerosols and smoke in the atmosphere to chlorophyll in the ocean. See also Cloud; Remote sensing; Weather.

Except for observing polar regions, or providing meteorological support to operations in remote locations worldwide, the ideal platforms for cloud imaging are those in geosynchronous equatorial orbit, also known as geostationary orbit, at the precise altitude (35,900 km) where the orbital period matches the period of rotation of the Earth, so that the satellite appears to hover over a fixed location at the Equator. The international system of four to six geostationary meteorological satellites provides uninterrupted visibility of the global tropics and midlatitudes (up to 60° north and south at the satellite longitude) with the ability to monitor fast-developing weather systems that often are the most dangerous. The sharpness of cloud images (1-km picture elements in the visible), as well as the ability to scan the same scene repeatedly at time intervals as short as 5 minutes, allow for tracking the apparent motion of clouds, deducing wind velocity, and instantaneously assessing the strength of developing storms, a valuable capability in warm climate regions. See also Earth rotation and orbital motion; Tropical meteorology.

Imaging microwave radiometers

Also interesting is the detection of diverse atmospheric properties and surface features using multifrequency microwave radiometers with small antenna beams. Water molecule absorption of microwave radiation emitted by the ocean provides an accurate estimation of total precipitable water in the atmospheric column. Microwave radiation emitted by the relatively homogeneous moist atmosphere below is scattered in a recognizable way by waterdrops and ice particles in rain clouds, thus providing an indirect means to estimate precipitation rates. Microwave radiation contrast discriminates ice floes from open ocean water, and wet from dry soil. Microwave radiometry enables diagnostics of sea state and wind strength over the surface of the ocean, or the sea surface temperature. The principal design constraint of imaging microwave radiometers is the diffraction limit of the sensor—large apertures are desirable, but bulky antennas are a problem because mechanical scanning is needed to preserve radiometric accuracy. In order to achieve reasonably small footprints, microwave sensors are currently deployed in low Earth orbit. See also Microwave; Precipitation (meteorology); Radiometry.

Sounding sensors

The retrieval of temperature profile and water vapor information from spectral data is a difficult and not a fully determined mathematical problem. The solutions are highly sensitive to spectral resolution and small errors in radiometric measurements. The latest Atmospheric Infra-Red Sounder (AIRS) instrument developed by NASA is expected to yield temperature profiles as accurate as balloon measurements, 1°C within each successive 1-km-thick layer of the lower atmosphere. See also Hydrometeorology; Infrared radiation.

Atmospheric sounders operate in the thermal infrared, using the absorption bands of carbon dioxide molecules (3.7–4.9 μm and 13–15 μm), and in the microwave spectrum, using the 54-GHz absorption band of oxygen. Emitted radiation is much weaker and atmospheric sounders correspondingly less sensitive in the microwave region. However, nonprecipitating clouds are largely transparent to such relatively long wavelengths, thus allowing all-weather albeit less accurate observations.

Measurements of temperature and moisture are used mainly to update numerical weather prediction computations that forecast the circulation of the global atmosphere several days in advance. For this quantitative application, a delay of a few hours is immaterial but homogeneous global coverage is essential. Thus, atmospheric sounders are principally deployed on Sun-synchronous polar orbits. The parameters of these circular low Earth orbits are selected from a discrete set of altitudes (800–1000 km) and inclinations (retrograde quasi-polar) that allow the orbital plane to drift by about 1° of longitude per day and match the change in Sun-Earth direction. Thus, a Sun-synchronous satellite crosses the Equator at (nearly) the same local time on every successive orbit.

Active sensors

Orbital systems are now powerful enough to probe the atmospheric medium or the surface with beams of electromagnetic radiation generated in space. The first operational sensor of this kind was a coarse radar or scatterometer that measured microwave radiation backscattered by the ocean surface. Backscatter is sensitive to surface roughness and thus provides a measurement of vector wind speed over the ocean (as well as a coarse all-weather mapping of sea ice).

Various radar altimeters have been used to map the changing topography of the ocean surface (principally to reconstruct the oceanic circulation from measured altitude gradients). Higher-frequency experimental radar and lidar systems are being tested to profile the distribution and optical properties of aerosol and cloud ice particles and waterdrops. The first demonstration of a space-borne precipitation radar in being conducted with the United States-Japan Tropical Rain Measuring Mission (TRMM) launched in 1997. Rain rate can be deduced from the three-dimensional distribution of ice particles and water in rain clouds, as observed by the TRMM satellite. See also Lidar; Meteorological radar; Radar meteorology.

Yet another promising technology will determine wind velocity in clear air from direct measurements of the frequency shift (Doppler effect) of multiple laser pulses backscattered by aerosol and other diffusive particles. Global wind measurements will provide an invaluable enhancement of the worldwide meteorological observing network, especially at low latitudes where the wind field cannot be deduced from atmospheric pressure. See also Doppler radar.