Abstract:
A satellite provides a time signal that is adjusted for position of a receiver on the surface of the Earth. A receiver determines appropriate local time from the time signal, and can adjust a local clock accordingly, the satellite time signal in the form of a rotating beam containing angular information to determine the position of the satellite transmitter for proper synchronization. A fixed receiver can also measure the radiation angle of the transmitted beam.

Description:
This Application is based on and claims benefit of German Application No. 197 42 100.8 filed Sep. 24, 1997, to which a claim of priority is hereby made. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a process for transmitting a time signal from a satellite as well as process for receiving a time signal at a ground based receiver. 
     2. Description of the Related Art 
     A terrestrial time signal transmitter for example the DCF-77 transmitter of the Federal Institute of Physical Engineering at Frankfurt um Main, transmits its time signal in the long wave frequency band in order to facilitate long range transmission. However, despite the high transmitting power, a range of only 1,200 to 2,000 kilometres results. In addition this time signal is designed only for one national time and furthermore uses a special transmitter frequency) and intrinsic encoding, so that in an area in foreign countries the receiver has to be suitable for several different time signals or else it is no longer capable of reading the signal. In an area at sea far removed from the coast, reception is in general no longer possible. Setting the time with the aid of satellite positioning systems (GPS) is certainly possible, however in this country they lack the supplementary information such as daylight saving time, leap second and so on, so that an involved semi-manual adjustment is necessary in order to maintain the actual local time. 
     So-called multiple radio clocks are also known, which make it possible to receive or exploit time signals in different countries. However, it is necessary in this connection for the clock to be adjusted by the user so that the time of the place in which it is located is known. These multiple radio clocks are, however, not able to function in all countries. 
     A process for determining a position of a receiver is known from U.S. Pat. No. 5,408,444. In order to be able to set the correct time in this receiver, its position has to be determined using at least three satellites of the GPS satellite system. If the position is established, the time adjustment is carried out using a correction value for this position, the said correction value being filed in a data bank of the receiver. 
     Likewise from U.S. Pat. No. 5,574,670, it is known how to fix the position, by means of the GPS system, of a receiver situated on the ground. In addition, provision is made in the receiver situated on the ground to redirect the antenna to the respective orbit. 
     From DF 43 13 945 A1, several satellites are combined together to form a satellite system. For the position determination of the receiver, which is to receive the time signal, merely the doppler curve over time is used. However, position determinations of this type are very imprecise. 
     The purpose of the invention is to specify a process for transmitting and receiving a time signal, in which process simply a transmitter for determining the position of the receiver has to be provided in order to be able to set the actual local time. 
     SUMMARY OF THE INVENTION 
     The process of the invention provides that, to achieve global reception of the time signal, the transmission occurs from an aerospace vehicle moving relative to a point on the earth&#39;s surface. The time signal is transmitted according to the invention with a particular frequency or several frequencies by the aerospace vehicle, which moves around an orbit with a high inclination. Further, provision is made that the signal rotates in the form of a beam at the transmitter in a predeterminable orbit and the rotating transmission beam contains angular information which is used to determine the direction of the transmitter. If the time signal transmits with one frequency, the distance between transmitter and receiver can be determined using the doppler curve. If several frequencies are used, the distance between transmitter and receiver can be determined by the propagation time scatter. In addition, because the angular information is acquired by the receiver from the rotating beam, the position of the receiver is determined in order to be capable of ascertaining whether the receiver is the left or right of a ground track of the aerospace vehicle. Consequently, an accurate position fixing of the receiver is possible, so that the actual local time can be set in the receiver. For determining the position of the receiver the radiated signal is therefore not radiated downwards uniformly, but rotates by means of a rotating beam. This rotation can be produced either by mechanically-driven antennas or by suitable electronic means. The rotating beam is altered in a suitable fashion as a function of the radiated angular position so that the instantaneous radiation angle can be determined from the received signal. This can be carried out for example by an auxiliary frequency, so that each angular position, or that is to say each range of angular positions between 0° and 360°, has a defined auxiliary frequency. An angle of 90° or 270° at the time of the greatest convergence then defines the side of the flyby. 
     A receiver independently determines its own geographic position on the earth from the received signals of the time signal transmitter and fixes the actual local time from that, without user intervention being necessary. A normal satellite cannot be considered for such a time signal because either the altitude is too high because of the required life and consequently the required incoming-signal levels are not obtained or the inclination of the orbit is too low, so that the entire surface of the earth cannot be radiated. With a low-flying satellite or space station (at an altitude of for example 200 km to 400 km) with a high orbit inclination, it is possible however to cover the earth&#39;s surface within the region of ±70 to 80 degrees of latitude. With a high orbit inclination, the entire earth&#39;s surface is overflown in the course of time by the satellite or space station. 
     By means of a special antenna geometry of the device according to the invention, the scanned area of the earth&#39;s surface can be expanded in width so that only the polar regions cannot be provided for 
     Terrestrial radio clocks are normally synchronised only once a day, in order to save the battery. This normally takes place at night because the changeover between daylight saving time and winter time also occurs at that time. With a space-supported radio clock, this is not so readily feasible since the transmitter must stay in the reception area for the given time. That is why the time signal transmitter transmits other supplementary data on the time of the next overflight for a particular area in addition to the basic time information, so that the receiver already knows in advance the contact time of reception. On first switching-on the clock or on losing the contact times the receiver switches on again only briefly in order to ascertain whether the time signal can be received. A quiet period is then inserted which is shorter than one reception time window, so that a possible contact cannot be missed. As soon as the first reception contact has been established, the clock goes over to the normal switching-on cycle. 
     The reception area for a particular point on the ground of the transmitter can extend over several time zones. That is why the receiver must determine, how far the instantaneous point on the ground, for which the transmitted data was calculated, is removed from its own geographic position. Two alternatives are provided for this: 
     1. During an overflight by the satellite on space station relatively close to the receiver, the doppler shift in the received frequency caused by the high velocity of the transmitter is so large that the time of the overflight, and therefore the distance, can be determined from the sudden change in frequency and from the form of the frequency jump. 
     2. During a relatively far distant flyby of the transmitter, the propagation time scatter of different frequencies (and therefore the dependency of the wave motion velocity of propagation on the wavelength or frequency) while passing through the earth&#39;s ionosphere is exploited. The electrically-conducting upper atmospheric layers (ionosphere) impede the propagation radio waves depending on the frequency of the transmitted signal of varying strength. This causes the simultaneously radiated signals of various frequencies to arrive at the receiver at different times. If the electrical conductivity of the ionosphere is known, the distance of the transmitter from the receiver can be determined from this time shift. The current characteristics for the ionosphere can be determined by ground station, or the time signal transmitter itself continually measures the ionosphere, by analysing the echo of a test signal. 
     In a further advantageous design of the invention, the earth&#39;s surface is subdivided into numbered zones for saving in memory and computer requirements inside the receiver. The transmitter then transmits a number of the current zone and the previously mentioned supplementary information in addition to the time signal. These data are stored in the receiver. The transmitter therefore can also predict orbit corrections and time change-overs and communicate these to the receiver. By the division of the earth into suitable zones, which do not have to be identical to the international time/zones, the receiver is therefore capable of calculating the actual time in which the receiver is situated, by simple offset- addition or subtraction of the transmitted time information. 
     The transmitter transmits the actual times and the supplementary information continuously and in an iterative manner. So that the receiver does not have to wait the full period for an already started data packet before the transmission of a complete packet can be started, easily recognised synchronisation signals are embedded in the data stream, so that the analysis can be started in the middle of a packet as well. This minimises the time for which the receiver has to be activated and therefore decreases the electrical current consumption of the clock. 
     In accordance with international regulations, transmitters on satellites or space stations are not permitted to exceed a certain transmitter power (power flux density), so that other systems are not interfered with. In order to meet this boundary condition, the so-called spread spectrum technique is used in the process according to the invention, actually so that separate encoding and modulation can be carried out. The transmitter signal is then shifted periodically by a given frequency shift in the transmitter frequency (sweeping). This sweeping and all other changes in the transmitter signal occur synchronously and phase-locked to the time standards on board the time signal transmitter, so that the received time can be determined from the instantaneous sweep frequency and the sweep phase position with a resolution into the microseconds range. 
     For adjustment of the time on board the time signal transmitter, on the one hand control signals from a ground or control station are used, on the other hand the time signal transmitter itself can decode the time signals of national time transmitters during overflight in order to synchronise itself with them. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is now described in more detail using a design example, with reference to the drawings which show:. 
     in FIG. 1 a diagrammatic illustration of a space-supported global time signal system, and 
     in FIG. 2 a diagrammatic representation of the world, and 
     in FIG. 3 a typical reception area on each, and 
     in FIG. 4 a graph of a doppler shift. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a space supported global time signal system  1 , which is used for distributing an almost globally-received time signal  14  in order to produce an automatic adjustment of clocks to the prevailing local time in which the clock is situated. The time signal system  1  has an aerospace vehicle  2  in the form of a satellite  3 , a receiver unit  4 , a time signal generator  5  and a ground station  10 . 2  together with an antenna  10 . 1 . 
     The satellite has a time signal transmitter  6  which serves to distribute or send out the time signal  14  as well as other supplementary information. The time signal  14  is indicated symbolically in the representation in FIG. 1 by a semicircular wave train and therefore no conclusion can be drawn on the actual propagation direction of the time signal  14  and the supplementary information. The device required for operation of the satellite  3 , for example power supply, or flight control, are not provided with reference markings for reason of clarity. 
     The receiver unit  4 , which is situated on the ground  7 , has a time signal receiver  8  and a clock  9 . The clock  9 , which preferably also can be designated as a wristwatch, and the time signal receiver  8  are connected to each other by a connecting line so that synchronisation information can be transmitted from the time signal receiver  8  to the clock  9 . 
     The time signal generator  5  is used to produce a time base by means of an atomic clerk for example, The time signal generator  5  is connected to the ground station  10 . 2 , also described as a control station. The ground station together with its antenna  10 . 1  is used to transmit a signal, which is indicated by an arrow  15  in FIG.  1  and is used for synchronizing the on board time of the satellite  3 . 
     The orbit of the satellite  3  is indicated in FIG. 1 by an arrow  13 . An additional arrow  16  marks a signal flow direction of the time signal  14  from the time signal transmitter  6  to the time signal receiver  8 . 
     FIG. 2 shows in diagrammatic representation the earth  7  which is divided into several segments of zones  17 . Two adjacent zones  17  are separated from one another by a zone border  18 , which runs parallel to the meridans of longitude or to the parallels of latitude, so that the zones  17  are quasi quadratic or rectangular in shape. The zones  17  can be selected as far as possible so that they correspond roughly with the existing time zones on each  7 ; however this is only approximately possible, since there are few straight time zone boundaries in the world. In FIG. 2 the zones  17  are only drawn diagrammatically and therefore no conclusion can be drawn on its actual size in practice the size of the zone  17  can be dimensioned so that it is smaller than the reception area. The satellite  3  together with its orbit  19  is drawn only diagramatically to complete the picture. The correct flight path, or that is to say the correct orbit,  19  can be inferred from FIG. 3, which is described in more detail below. 
     In a developed view of the earth, FIG. 3 shows the reception area  20  of the satellite  3  on the earth  7 . A high inclination, or that is to say a large inclination of the orbit of the satellite  3  produces an orbit  19  which has a sinusoidal form. Several passes of the satellite  3  around the earth therefore results in extensive coverage of an almost global reception area  20 . In FIG. 3 the reception area  20  of the satellite  3  is drawn so that a reception code  21  projected onto the earth  7  is instantaneously situated over Europe. In FIG. 3 it can be easily recognised that the reception cone  21  projected onto the earth  7 , the said cone being formed by the development of the earth  7  elliptically in the illustration, includes the whole of Europe and thus sweeps over several real existing time zones. 
     FIG. 4 shows a graph  22  with an exemplary frequency curve  25  of a doppler shift, as received from the viewpoint of the time signal receiver  8 . Time is laid off on the abscissa  23  and frequency on the ordinate  24  on the graph  22  of FIG. 4. A dashed vertical line  26  marks an overfly time t 0  at which the time signal receiver  8  is at the minimum distance from the time signal transmitter  6 . The area to the left of the dashed line  26  indicates the approach of the time signal transmitter  6  to the time signal receiver  8  and corresponds to the area to the right of the line  26 , the area in which the time signal transmitter  6  is going away from the time signal receiver  8 . The larger the velocity component of the time signal transmitter  6  towards the time signal receiver  8 , the closer the satellite  3  is flying by the time signal receiver  8 , and the more marked (i.e. the larger) the frequency shift within the bounds of the overfly time t 0 . Consequently, the time signal receiver  8  can determine from the frequency response curve  25  the distance to the time signal transmitter  6  frequency. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.