Polar-Orbiting Satellites

Images figure 9 Advanced TIROS - N (ATN) figure 10 One-polar soundings for 00Z map. figure 11 TIROS polar-orbiting environmental satellite. figure 12 Fundamental polar satellite products: AVHRR figure 13 AVHRR channels. figure 14 Two-polar soundings for 00Z map. figure 15 Fundamental polar satellite products: TIROS operational vertical sounder. figure 16 Fundamental polar satellite products: TVOS. figure 17 Atmospheric soundings. figure 18 Summary of fundamental polar-orbiting satellite products. figure 19 Derived polar satellite products. figure 20 Landsat orbit. figure 21 Multispectral Scanner (MSS) sensor. figure 22 Landsat image of Detroit. figure 23 Thematic Mapper (TM) sensor.

A disadvantage of geostationary satellites (figure 9) is that their large distance from the earth's surface compromises their ability to capture fine detailed resolution of the planet surface. Satellites put in low earth orbit, typically 850 km (531 miles) above the surface, can resolve topographic and other environmental features as small as 1 km, and perhaps even much smaller. However, this close proximity to the earth's surface limits their field of view to a swath about 2,500 km (1,560 miles) wide as the satellite orbits the planet. Maximum areal coverage is achieved if the satellite orbits from pole to pole (figure 10). By use of the last equation of the height derivation, with a satellite height above ground of 850 km, we get an orbital period of about 100 minutes. The satellite orbiting from pole to pole crosses the equator going from south to north every 100 minutes, and between these equator crossings the planet will have rotated by 28.8o. This allows the satellite to view a new swath that is adjacent to the west of the swath from the previous pass. By manipulating the height of the satellite, as can be seen from the equation for satellite period, the orbital period can be adjusted to give the desired degree of overlap between adjacent sweeps across the equator. Polar orbiting satellites are sufficiently close to the earth's surface to acquire high resolution data (figure 11). Polar orbiting satellites, like GOES, have vertical sounding units for monitoring temperature and moisture. A second type of instrument on polar orbiting satellites is the radiometer, which measures energy at individual wavelengths of both visible and infrared radiation. The Advanced Very High Resolution Radiometer (AVHRR) (figure 12)senses data in 5 different channels at both 1 km and 4 km resolution. Examples of data from the AVHRR are given in figure 13. Polar orbiting satellites also provide image data for a variety of environmental assessment needs and space environmental monitors similar to those on the GOES satellites. Gathering weather data for use in initializing weather forecasts requires at least daily and hopefully twice daily or more frequent observations. By placing two companion satellites in synchronous low polar orbits, we can acquire data over the entire surface of the earth on a daily (or more frequent) basis (figure 14). A companion pair of polar orbiting satellites can be synchronized (called sun synchronous) so that one passes over the particular location in the morning and the other passes overhead at this location in the early evening. Note that the converging tracks of adjacent orbits give multiple measurements in polar regions. Each satellite takes about 600 vertical profiles of measurements each day. Each profile of measurements includes temperature at 15 levels, precipitable water at 3 levels, total column ozone, cloud cover, and tropopause height (figure 15). An example of data derived from a sounder is given in figure 16. By making such measurements over each of the points in a 3-dimensional volume of atmosphere (figure 17), polar-orbiting satellites provide data for use in computer models used for weather forecasts. A summary of fundamental products of polar orbiting satellites are given in figure 18 and derived satellite products in the following figure (figure 19) . A special NASA polar orbiting satellite, no longer in operation but from which data are still used, is the Landsat satellite (figure 20) which had a multispectral scanning sensor (figure 21). Its very low orbit of 706 km (438 miles) allowed it to get very high resolution images of the earth as is shown for Detroit (figure 22). Another instrument, the thematic mapper (figure 23) operated in both visible and infrared to give detailed land surface images. PREVIOUS: GOES NEXT: International Deployment of Satellites