The present invention relates to artificial satellites for acquiring an image of the earth such as for meteorological observation, and more particularly relates to an artificial satellite and a meteorological satellite system using the satellite, which can be used to provide a meteorological satellite system having a plurality of geostationary meteorological satellites with a useful complementary function.
Currently, the meteorological satellite system of the world is consisting of five geostationary meteorological satellites and two polar orbit meteorological satellites. In particular, as shown in FIG. 1, the meteorological satellite system is essentially constituted by: a geostationary meteorological satellite, GMS, which is called as "HIMAWARI" of Japan; two geostationary meteorological satellites, GOES-EAST, GOES-WEST, of the U.S.; a geostationary meteorological satellite, METEOSAT, of the European Space Agency; a geostationary meteorological satellite, GOMS, of Russia; and polar orbit meteorological satellites, NOAA and METOP, of the U.S. and the European Space Agency.
The geostationary meteorological satellites refer to those in a geostationary orbit 101, at an altitude of about 35,786 km and a nearly zero inclination angle, and are deployed as one at each of five nearly equidistant positions along the equator. All areas of the earth but polar regions are continuously observable by these geostationary meteorological satellites, images being acquired once in every approximately 30 minutes. In particular, in the case of "HIMAWARI" for example, a spin-scan visible/infrared radiometer is employed as the main observation instrument. The rotation of the satellite on its own axis is used to scan the earth's surface from west to east and displacement of scanning spot along the north and south direction is performed by a motor-driven scan mirror to obtain an image. Specifically due to the fact that a geostationary meteorological satellite is in the geostationary earth orbit, there are such advantages as that a target area can be observed at irregular intervals and that the antenna of an earth station for receiving data from the satellite is required to continuously point to only the same direction.
Further, the polar orbit meteorological satellites are in sun synchronous orbits having altitudes of 833 to 870 km and inclination angles of 98.86 to 98.70 degrees, being deployed as one in each of the two orbits having their orbital planes perpendicular to each other. Frequent observations of the polar regions of the earth can be made by means of these satellites. Especially, since they are in sun synchronous orbits, there is an advantage that images can be obtained of the same solar irradiative conditions, i.e., at the same local time.
Of the existing meteorological satellite system having the above described arrangement, the geostationary meteorological satellites have actually performed for 25 years and the polar orbit satellites for 35 years.
There are problems as will be described below, however, in the currently used meteorological satellite system of the world. First, it is difficult to have standby satellites as a reserve in case of a failure of the five geostationary meteorological satellites. Since a satellite's failure occurs suddenly and unexpectedly, an ordinary approach to be taken as a precaution against such failure is the launching of a standby satellite having the same function for a satellite of which performance should not be interrupted. The problem here is that it is generally difficult to bear the costs for such approach. Although communication satellites are also among the geostationary satellites, their standby satellites are used in applications where an intermission is allowable. In the case of meteorological satellites, however, their standby satellites are of no actual use when the currently used satellites are sound. This is an additional factor for hesitation as to whether or not to have a standby satellite. As a result, only one of the five deployed positions has, without an interruption, been able to provide images of the earth during the 25-year performance period up to the present time. It, too, has not necessarily been free from unstable conditions.
Secondly, there is a problem of noncontinuousness in observation mode among the five geostationary meteorological satellites. In particular, meteorological phenomena in the middle-latitude regions on the earth tend to move from west to east. For this reason, to predict weather conditions of a certain region for a range of several days, it is necessary to grasp the meteorological phenomena of its westward regions. With the observation from a single geostationary meteorological satellite, however, there is a problem that such westward regions are placed outside its observation range. Theoretically in the spirit of this meteorological satellite system, it suffices to use data from a geostationary meteorological satellite which is located west at the next deployment position. However, only two of the five geostationary meteorological satellites use the same earth scan system for acquisition of images. The satellites are of different specifications from each other mainly because the providers of the respective satellites are different. This results in a problem that data received from an adjoining satellite cannot be easily processed. Furthermore, for example due to noncontinuousness of image acquisition time, it has been very difficult to synthesize an image by combining those of different geostationary meteorological satellites when planning an observation of the whole earth.
Thirdly, there is a problem that calibration of data is difficult. While the brightness of images exclusively used in meteorological satellites is determined based on radiant intensity of heat/infrared rays, it is important in meteorological observations to grasp such radiant intensity to an adequate accuracy. In general, however, due to limitation in the weight of an artificial satellite, performance of an accurate data calibration within the satellite alone is not free from difficulties. While this is the reason for trying reciprocity calibrations between the satellites, actual attempts have not been made very often because of the fact that separate data observing the same position at the same time are not readily obtainable between the geostationary meteorological satellites.