This application claims the priority of German Patent Document 100 62 951.2, filed Dec. 16, 2001, the disclosure of which is expressly incorporated by reference herein.
The position determination, particularly the orbit determination of geostationary satellites, normally takes place by means of antennas of one or several earth stations. If only one antenna of an earth station is used, the distance of the satellite from the antenna, the elevation angle and the azimuth angle are measured for determining the position. If two or more earth stations are used, respective distance measurements are carried out from the satellite to the antennas.
For determining the position, particularly for determining the orbit of geostationary satellites, navigation satellites may also be used. Particularly the Global Positioning System (GPS) or the equivalent GLONASS System can be used either in addition to the conventional orbit determination methods or as an alternative thereto. Such measures are known, for example, from German Patent Document DE 199 07 235 as well as from S. Averin et al. xe2x80x9cOn Combined Application of GLONASS and GPS Systems in Conditions of Limited Observability of Navigation Satellitesxe2x80x9d, ION GPS 96, Page 187, and on. The alternative use of navigation satellites for the position determination results in a reduction of the required expenditures on the ground and thus in a reduction of costs. The additional use of navigation satellites increases the reliability when determining the position. A position determination by means of navigation satellites can in each case be implemented in a relatively simple manner. It requires only an antenna and a receiver for signals of the navigation satellites on board the satellite.
Navigation satellites, such as GPS satellites or GLONASS satellites fly at an altitude of approximately 20,000 kilometers and radiate their signals toward earth. Geostationary satellites are situated at an altitude of approximately 36,000 km, that is, above the orbits of the navigation satellites. They can therefore receive only signals from navigation satellites which are situated on the opposite side of the earth and whose signals radiate past the earth. This geometric configuration has two effects: First, the distance between the navigation satellite as a transmitter and the geostationary satellite as a receiver is very large, which results in a high free-space attenuation of the signals of the navigation satellites. Second, the signal intensity of the signals of the navigation satellites which radiate past the earth is relatively low because of the normally existing radiation characteristics of the transmitting antennas of the navigation satellites. Both effects result in a low ratio of the signal/noise power density (S/N) at the receiving antenna of geostationary satellite.
Normally, it is suggested to use a receiving antenna having a high directivity for eliminating the problem of the low S/N ratio for receiving navigation signals by geostationary satellites. Although, in principle, this results in an improved SIN following the receiving antenna, such directive antennas are relatively large, heavy and expensive. Mainly the size is problematic because, in the case of a geostationary satellite, the receiving antenna for navigation signals must be mounted on the side oriented toward the earth and a large number of useful-load components and bus components are already mounted on this side. Only small antennas are therefore practical, such as a hemispheric path antenna, which, however are unable to supply the desired improvement of the S/N ratio.
It is therefore an object of the present invention to provide a method of determining the position for geostationary satellites by means of signals of at least one navigation satellite which can still be used effectively at relatively low S/N values.
According to the invention, a method is provided of determining the position of geostationary satellites by means of signals of at least one navigation satellite, which method consists of the following steps:
A determination of data concerning the transit times of the signals from the at least one navigation satellite to the geostationary satellite by a device of the geostationary satellite;
a determination of navigation data of the at least one navigation satellite by a navigation device which is independent of the at least one navigation satellite as well as of the geostationary satellite;
a determination of data concerning the system time of the at least one navigation satellite by a navigation device which is independent of the at least one navigation satellite as well as of the geostationary satellite;
a data exchange between the geostationary satellite and the navigation device for the determination of position data of the geostationary satellite from the transit times, the navigation data as well as the system time of the at least one navigation satellite.
Empirically, the different partial processes which are conventionally used for the position determination by means of navigation satellites, make different demands on the SIN ratio of the received signals. Thus, a transit time measurement or the measuring of fractional parts of the transit time can still be effectively carried out at a lower S/N ratio than the determination of the remaining navigation data and of data concerning the system time, which are conventionally transmitted within the scope of the signals emitted by the navigation satellites. Within the scope of the present invention, it is now provided that only those partial processes are still to be carried out within the geostationary satellite for which a low SIN ratio of the received signals of at least one navigation satellite is sufficient, thus particularly only the determination of data concerning the transit times of the signals. The other partial processes are carried out by means of external navigation devices which may be situated, for example, on the earth surface or in a suitable position relative to the navigation satellite on an orbit around the earth and for which an access to navigation data and data concerning the system time of the at least one navigation satellite is more easily possible than for the geostationary satellite. In particular, this applies to navigation devices which are situated at locations at which the signals of the navigation satellites can be received with a better S/N ratio than at the location of the geostationary satellite or to navigation devices which can access navigation data and data concerning the system time of the at least one navigation satellite by means of a different type of data access.
On the basis of a data exchange between the geostationary satellite and the navigation device, a position determination can then take place for the geostationary satellite. As a result of this reduction of the part of the data which has to be determined by the geostationary satellite itself, to the pure determination of data concerning the transit time of the signals, a position determination can also still take place effectively at a low S/N ratio.
In a first further development of the invention, it is provided that a reception of signals of the at least one navigation satellite takes place by a navigation device which has at least one navigation receiver, and a determination of navigation data and data concerning the system time of the at least one navigation satellite takes place by the navigation device from the received signals. Such a method can be used for navigation devices which are situated at locations at which the signals of the navigation satellites can be received with a better S/N ratio than at the location of the geostationary satellite or for navigation devices. These locations may be on earth or in an orbit around the earth.
As an alternative, it may be provided that, by means of the navigation device, an extraction takes place of navigation data and data concerning the system time of the at least one navigation satellite in a data bank. Here, the fact is therefore that navigation data and data concerning the system time of the at least one navigation satellite is accessed by a different type of data access.
Each of the two above-mentioned further developments has the result that the knowledge of navigation data and data concerning the system time of the at least one navigation satellite are present in the navigation device, which can now be used for the position determination of the geostationary satellite. This position determination can take place either within the geostationary satellite or it can also take place within the navigation device.
Thus, it can either be provided that a transmission of the navigation data and of the data concerning the system time to the geostationary satellite takes place by means of a transmitting device which is connected with the navigation device in the sense of a data exchange, and a determination of position data of the geostationary satellite takes place on the basis of the transit time data, the navigation data as well as the data concerning the system time within the geostationary satellite.
However, as an alternative, it may also be provided that a transmission of the data concerning the transit time to the navigation device takes place by means of a transmitting device of the geostationary satellite; a determination of position data of the geostationary satellite takes place on the basis of transit time data, of the navigation data as well as the data concerning the system time within the navigation device; and a transmission of the determined position data takes place at the geostationary satellites by means of a transmitting device which is connected with the navigation device in the sense of a data exchange.
For a clear determination of the transit time data, it mayxe2x80x94depending on the type and method of operation of the at least one navigation satellite and the signals transmitted by the latterxe2x80x94be necessary that a clear determination of the transit time data takes place while taking into account an estimated position of the geostationary satellite. For this purpose, for example, special estimating devices, such as Kalman filters, should therefore be provided inside the satellite, or an external estimation of the position can take place, for example, in an earth station, and the corresponding data can then be transmitted to the satellite.
The determination of the transit times can take place by any suitable method known from the prior art. Thus, it may, for example, be provided that the data concerning the transit times are determined by means of a delay lock loop (DLL).
According to the system, it may be helpful or even necessary that a transmission of data concerning the system time takes place from the navigation device to the geostationary satellite, and a determination of the transit time data takes place while taking into account the data concerning the system time. Thus, in such cases, a corresponding data exchange can or must also be provided between the navigation device and the satellite.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
A special embodiment of the present invention will be explained in the following by means of FIGS. 1 and 2.