Metsat Instrumentation; AVHRR¶
Contents
The prime sensor used now for many years on U.S. metsats is AVHRR, whose characteristics are given. Several other satellites that have proved valuable in weather monitoring are listed with brief narratives.
Metsat Instrumentation; AVHRR
The chief tool for conducting meteorological observations is still any sensor that images in the Visible spectrum. The target of interest is primarily clouds: their types, distribution, extent, rates and directions of movements, and potential for rainfall. Clouds and other components of weather systems-the atmosphere, and the oceans-are also well-suited to sensing in various regions of the Infrared. Water and gases have distinct absorption bands in these regions as well as characteristic emissions. Generally, when we mention an infrared sensor regarding metsats, we refer to the thermal-infrared region of the spectrum, particularly in the interval from 10-14 µm but also the 3-5 µm region. In those segments of the EM spectrum clouds are cooler than the land and usually the water as well. If imaged conventionally, clouds would appear dark, contrary to their visible-region appearance. The image is customarily inverted (photographically when printed, or simply by numerical transformation of the digital data to make low values large and high values smaller, prior to printing or displaying) to produce images with clouds as whitist features in the scene.
For nearly 20 years now, the workhorse imaging sensor on many metsats has been the Advanced Very High Resolution Radiometer (AVHRR ). The AVHRR has five channels, whose characteristics are:
Band Number |
Wavelengths (µm) |
Main Uses |
1 |
0.58-0.68 |
Day clouds; snow; ice |
2 |
0.725-1.10 |
Surface water; snow; ice |
3 |
3.55-3.93 |
Fires; clouds at night |
4 |
10.30-11.30 |
Day/nite cloud & surface temperatures and mapping |
5 |
11.50-12.50 |
Same as 4; water vapor |
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AVHRR has flown on NOAA’s polar-orbiting satellites, starting with TIROS-N, and is still very much in use today (GOES series). The ground resolution at nadir for this instrument, for a field of view of 1.4 milliradians (producing a swath width of 2,400 km [1,491 mi]), when flown at 830 km (516 mi) is 1,100 m (3,608 ft).
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In general, metsats do not require the higher resolutions of land-imaging satellites, because their principal targets are large assemblages of clouds. The highest resolution system, at 600 m (1968 ft) are sensors onboard DMSP (Defense Meteorological Satellite Program) spacecraft. The tradeoff is to couple the sensor(s) to an optical telescope with a large field of view (FOV) that creates a wide swathwidth or full disk coverage, depending on orbital parameters. We can join successive swaths to produce timely mosaics that cover large regions or most of the Earth over a 24-hour cycle. Therein lies a big difference between Metsats and Landsats, in that the former seek clouds, while the latter can’t see through them, unless they happen to be the targets of interest. Landsat, SPOT and other satellites produce excellent subregional cloud pictures suited to mesoscale analysis.
Other imaging sensors in active use on U.S. metsats (we detail several later in the section), together with descriptions of some obsolete sensors include the two-channel Operational Line Scanner (OLS) on DMSP satellites, the 6-channel Coastal Zone Color Scanner (CZCS) on Nimbus-7, and Total Ozone Mapping Spectrometer (TOMS) on several recent satellites. We don’t review sensors on foreign satellites except for brief mention of pertinent characteristics when we introduce their images.
The Heat Capacity Mapping Mission (HCMM; page 9-8) provided useful visible and thermal imagery of clouds. The Earth Radiation Budget Satellite (ERBS) measures incident and reflected, and longwave radiation to determine the energy budget related to solar insolation and outgoing re-radiation (with a terrestrial target). Active radar and passive microwave sensors also operate on some metsats. They can be particularly effective in detecting and tracking several forms of precipitation, such as rain droplets. Radars on Seasat and the SIR series (flown on Shuttles) produce images specifically for oceanographic studies. Nimbus 5 and 6 carried the Electrically Scanning Microwave Radiometer (ESMR) and Nimbus 7 included the Scanning Multichannel Microwave Radiometer (SMMR). The Special Sensor Microwave Imager (SSM/I ) operates on DMSP satellites. Japanese, Canadian, and European satellites also mount radar or microwave systems, which look at water and land targets.