AU2003248318B2

AU2003248318B2 - Satellite communication system, receiving earth station and communication satellite switching method

Info

Publication number: AU2003248318B2
Application number: AU2003248318A
Authority: AU
Inventors: Seiya Inoue
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2002-09-30
Filing date: 2003-09-24
Publication date: 2004-12-16
Anticipated expiration: 2023-09-24
Also published as: AU2003248318A1; JP2004128658A; KR20040028585A; KR100554042B1

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): MITSUBISHI DENKI KABUSHIKI KAISHA Invention Title: SATELLITE COMMUNICATION SYSTEM, RECEIVING EARTH STATION AND COMMUNICATION SATELLITE SWITCHING METHOD The following statement is a full description of this invention, including the best method of performing it known to me/us: SATELLITE COMMUNICATION SYSTEM, RECEIVING EARTH STATION AND COMMUNICATION SATELLITE SWITCHING METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a satellite communication system, a receiving earth station and a communication satellite switching method, and particularly to an improvement of a communication satellite switching method in a satellite communication system for performing data transmission between earth stations through a nongeosynchronous satellite.

2. Description of Related Art Satellite communications have been hitherto carried out by using a geosynchronous satellite on the equator, and the satellite remains stationary at all times when it is viewed from earth stations. Therefore, it is unnecessary to switch the satellite being used for communications to another satellite unless an emergency situation such as a satellite trouble or the like occurs. However, various kinds of nongeosynchronous type satellite communication systems have been recently proposed and some of them have been practically used, and in these satellite communication systems, communications are carried out between earth stations by using plural satellites which are moved relatively to the earth stations. Accordingly, when viewed from the earth stations, it is necessary to switch the satellite to be used for communications.

A low earth orbit satellite system (LEO: Low Earth' Orbit), a highly elliptical orbit satellite system (HEO: Highly Elliptical Orbit), etc. are known as representative nongeosynchronous satellites. For example, an iridium H.\Valma\Keep\Specifications\PS0794.FO911.doc 24/09/03 system is known as an example of the former system, and a quasi-zenith satellite system is known as an example of the latter system.

Here, there is considered such a case that satellite switching is carried out in a nongeosynchronous satellite system for transmitting/receiving digital signals between earth stations through a satellite, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, a code division multiple access (CDMA) system, etc. are known as a method of accessing a satellite for satellite communications. In this case, it is assumed that the FDMA system is used as the method of accessing a satellite.

According to the FDMA system, the frequency band owned by a satellite is divided and allocated to each earth station. Each earth station transmits a signal within a frequency band thus allocated. A reception side identifies a transmission station transmitting the signal concerned on the basis of the allocated frequency band in which the signal exists, and picks up a channel to the reception station concerned from the signal. This system has advantages that the access procedure is simple, and the construction of the facilities of each earth station is simplified and the cost can be reduced.

Fig. 11 is a block diagram showing the construction of a conventional satellite communication system. This satellite communication system is a nongeosynchronous satellite system for accessing a satellite in the FDMA system, and Fig. 11 shows a state just before satellite switching is carried out in the satellite communication system.

The satellite communication system comprises a Hs\Valma\Keep\Specifications\P50794.70911.doc 24/09/03 transmitting earth station i, plural communication satellites 2A and 2B and a receiving earth station 3.

Transmission waves from the transmitting earth station 1 can be received by the receiving earth station 3 through any one of the communication satellites 2A, 2B. When the communication satellite 2A is located within antenna beams of the earth stations 1, 3, the communications are carried out through the communication satellite 2A. In this case, the communication satellites 2A, 2B are nongeosynchronous satellites, and thus the orientation of the communication satellite 2A is varied at all times when it is viewed from the earth stations i, 3. Accordingly, it is required to switch the communication satellite being used from the communication satellite 2A to the communication satellite 2B before the communication satellite 2A is moved to the outside of the antenna beams of the earth stations 1, 3. A situation just before the satellite is switched as described above is shown in Fig. 11. Under this situation, the earth stations 1, 3 are allowed to use both the communication satellites 2A, 2B.

When communications are carried out through the communication satellite 2A, an electric wave having a frequency fl is transmitted from an antenna 15A of the transmitting earth station 1 to the communication satellite 2A, and received by a reception antenna 22A of the communication satellite 2A. The wave thus received is converted to an electric wave having a frequency fl' at the communication satellite 2A, and then transmitted from a transmission antenna 25A. The electric wave thus transmitted is received by an antenna 31A of the receiving earth station 3.

When the communication satellite used for the H, \Vlma\Keep\SpeciEications \P50794 .F0911.doc 24/09/03 communications between the transmitting earth station 1 and the receiving earth station 3 is switched from the communication satellite 2A to the communication satellite 2B, an instantaneous interruption occurs in connection with the switching operation of the communication satellite. In order to shorten the instantaneous interruption time at maximum, it is necessary to simultaneously output both the transmission wave directed to the communication satellite 2A and the transmission wave directed to the communication satellite 2B for some period in the switching operation. In general, the two communication satellites 2A, 2B do not enter the same antenna beam of the transmitting earth station i, and thus the transmitting earth station 1 has two antennas 15A and 15B. Likewise, the receiving earth station 3 also has two antennas 31A and 31B.

Before the satellite switching is carried out, the transmitting earth station 1 transmits an electric wave from the antenna 15B to the communication satellite 2B to be used after the satellite switching. The transmission wave is a signal having a frequency f2 different from the frequency fl, the signal being achieved by superposing the same data as the transmission wave of the frequency fl on an electric wave of the frequency f2. The transmission wave of the frequency f2 is received by the reception antenna 22B of the communication satellite 2B, and then converted to the electric wave of the frequency f2'. The electric wave thus converted is transmitted from the transmission antenna 25B, and received by the antenna 31B of the receiving earth station 3.

If the communication satellite is switched under the state that the same data are transmitted from the transmitting earth station 1 to the receiving earth station H\Valma\Keep\Specificationa\P50794.0911.doc 24/09/03 3 through the two communication satellites 2A and 2B as described above, the instantaneous interruption time can be shortened. The switching of the communication path as described above is generally called as a soft hand-over.

Fig. 12 is a block diagram showing the construction of the transmission earth station 1 of Fig. 11.

The transmitting earth station comprises a modulator 12, two transmitters 13A and 13B and the two transmission antennas 15A and 15B. A transmission data input terminal 11 is a terminal to which transmission digital data are input. The modulator 12 subjects the transmission digital signal to modulation such as QPSK, BPSK or the like to generate a modulated wave in an IF (Intermediate Frequency) band, and outputs the same modulated wave to the transmitters 13A and 13B.

Transmission frequency input terminals 14A, 14B are terminals for indicating frequencies in the RF (Radio Frequency) band of the modulated wave. In the transmitter 13A, an RF signal of the frequency fl is generated on the basis of a transmission frequency setting signal input from the transmission frequency input terminal 14A, subjected to high-frequency amplification and then transmitted from the transmission antenna 15A to the communication satellite 2A.

Likewise, in the transmitter 13B, an RF signal of the frequency f2 is generated on the basis of a transmission frequency setting signal from the transmission frequency input terminal 14B, subjected to high-frequency amplification and then transmitted from the transmission antenna 15B to the communication satellite 2B.

Fig. 13 is a block diagram showing the construction of the communication satellites 2A, 2B of Fig. 11.

The communication satellite 2A comprises a reception H.\Vala\Ceep\Specifications\PSO794. P0911.doc 24/09/03 antenna 22A, a frequency converter 23A, a local oscillator 24A and a transmission antenna 25A. The electric wave of the frequency fl transmitted form the transmitting earth station 1 is received by the reception antenna 22A, and converted to the electric wave of the frequency fl' by the frequency converter 23A. The frequency conversion is carried out on the basis of the oscillation frequency of the local oscillator 24A, and the electric wave after the frequency conversion is transmitted from the transmission antenna 25A to the receiving earth station 3.

Likewise, the communication satellite 2B comprises a reception antenna 22B, a frequency converter 23B, a local oscillator 24B and a transmission antenna 25B. The electric wave of the frequency f2 transmitted from the transmitting earth station 1 is received by the reception antenna 22B, and converted to the electric wave of the frequency f2' by the frequency converter 23B. The frequency conversion is carried out on the basis of the oscillation frequency of the local oscillator 24B, and the electric wave after the frequency conversion is transmitted from the transmission antenna 25B to the receiving earth station 3.

Fig. 14 is a block diagram showing the construction of the receiving earth station 3 of Fig. 11.

The receiving earth station 3 comprises two reception antennas 31A and 31B, two receivers 32A and 32B, two demodulators 34A and 34B, two clock extracting means and 35B, and a switch 36. That is, the receiving earth station 3 comprises the receiving system of two systems and the switch 36 which switches these receiving system.

The electric waves from the communication satellites 2A, 2B are received by the reception antennas 31A, 31B, and input to the receivers 32A, 32B. The receivers 32A, 32B H\Valma\Keep\Speciications\P50794 .F911.doc 24/09/03 subject the respective received waves to low-noise amplification, and then subjected to the frequency conversion to the IF band. At this time, a reception frequency setting signal for indicating a reception frequency is input to the reception frequency input terminals 33A, 33B so that the frequencies.(IF frequencies) after the conversion at the receivers 32A, 32B are equal to each other.

The demodulators 34A, 34B demodulate the IF signals output from the receivers 32A, 32B to digital data, respectively. At this time, clock signals are extracted from the reception signals by the clock extracting means 35B, respectively. The clock signal is determined on the basis of the variation point of the reception data, and demodulation is carried out on the basis of the extracted clock signal in each of the demodulators 34A, 34B.

The switch 36 serves as switching means for outputting any one of the digital data sequences output from the demodulators 34A, 34B to the reception data output terminal 37. That is, any one of the communication satellite 2A and the communication satellite 2B is selected by the switch 36. The instantaneous interruption time can be shortened by performing the switching operation of the switch 36 during a period for which both the digital data sequences are received.

When the communication satellite being used is switched from the communication satellite 2A to the communication satellite 2B, the switch 36 is switched from the communication satellite 2A side to the communication satellite 2B side during the period for which both the data from the communication satellites 2A and 2B can be received. After the switching operation has been completed, Ht\Valma\Keep\Specifications\P5O794.F0911.doc 24/09/03 t 9 O the transmitting earth station 1 stops the transmission to ci the communication satellite 2A, and carries out signal 0 z transmission therefrom to the receiving earth station 3 by i using only the communication satellite 2B.

ci The signal transmission path length from the 00transmitting earth station 1 to the receiving earth station 3 is different between the communication satellite 00 2A and the communication satellite 2B. Therefore, although the received digital data of the two systems o 10 output from the demodulator 34A and the demodulator 34B are the same digital data sequence, a predetermined delay time difference occurs between the two digital data sequences.

As described above, in the conventional satellite communication system which makes an access in the FDMA system using nongeosynchronous satellites, when the satellite switching is carried out, the data reception is carried out through different communication paths and the two digital sequences having the delay time difference are switched. Accordingly, data missing or data duplication occurs in the received digital data sequence before and after the switching operation. When the data missing occurs, instantaneous interruption of data and frame synchronization step-out are induced. When the data duplication occurs, the frame synchronization step-out is induced. That is, there is a problem that in any case the received data are discontinuous and thus this system is unsuitable for the digital data communications.

SUMMARY OF THE INVENTION According to one aspect of the present invention, there is provided a satellite communication network for performing data transmission from a transmitting earth Ht\stella\Keep\Speci\NTB\P50794 .70911.amended pages.doc 24/11/04 o station to a receiving earth station through a first ci communication satellite before satellite switching and 0 Z through a second communication satellite after satellite Iswitching; the transmitting earth station, first ci communication satellite and receiving earth station OO forming a first system; the transmitting earth station, second communication satellite and receiving station OO forming a second system; characterized in that the i receiving earth station comprises: time marker extracting means for extracting a time marker from each reception data of the two systems by the receiving earth station; a buffer memory in which reception data are written on the basis of the time marker thus extracted; time marker delaying means for delaying any one of the time markers of the two systems; read-out means for reading out the reception data of the two systems from the buffer memory on the basis of the time marker thus delayed; and switching means for switching the reception data of the two systems thus read out and outputting the reception data thus switched.

According to a further aspect of the present invention, there is provided a receiving earth station for receiving data from a transmitting earth station through a first communication satellite before satellite switching and through a second communication satellite after satellite switching; the transmitting earth station, first communication satellite and receiving earth station forming a first system; the transmitting earth station, second communication satellite and receiving station forming a second system; the receiving earth station comprising: H:\.tella\Keep\Speci\NTB\P50794.F0911.amended pages.doc 24/11/04

II

O time marker extracting means for extracting a time ci marker from each reception data of the two systems at the 0 z receiving earth station; IC) buffer memories in which the reception data are ci written on the basis of the time markers thus extracted; time marker delaying means for delaying any one of

OO

the time markers of the two systems; 00 read-out means for reading out the reception data of C' the two systems from the buffer memories on the basis of o 10 the time marker thus delayed; and Ci switching means for switching the reception data of the two systems thus read out and outputting the reception data thus switched.

According to a further aspect of the present invention, there is provided a communication satellite switching method for performing data transmission from a transmitting earth station to a receiving earth station through a first communication satellite before satellite switching and through a second communication satellite after satellite switching; the transmitting earth station, first communication satellite and receiving earth station forming a first system; the transmitting earth station, second communication satellite and receiving station forming a second system; the method comprising: a time marker extracting step of extracting a time marker from each reception data of the two systems; a buffer writing step of writing reception data into a buffer memory on the basis of the time marker thus extracted; a time marker delaying step of delaying any one of the time markers of the two systems; H,\Stella\Kep\Spei\NTB\P50794.FQ911.aended pages.doc 24/11/04 0 0 ci

I

oo C0 00 en 00 0 0 0 uN 12 a buffer read-out step of reading out the reception data of the two systems from the buffer memory on the basis of the time marker thus delayed; and a switching step of switching the reception data of the two systems thus read out and outputting the reception data thus switched.

BRIEF DESCRIPTION OF THE DRAWINGS 10 Fig. 1 is a diagram showing the construction of a satellite communication system according to a first embodiment of the present invention; Fig. 2 is a block diagram showing a construction of a transmitting earth station 4 of Fig. 1; Fig. 3 is a block diagram showing a construction of a receiving earth station 5 of Fig. 1; Fig. 4 is a timing chart showing an example of the operation of buffer memories 51A and 51B; Fig. 5 is a diagram showing an example of the relationship between communication satellites 2A and 2B and earth stations 4, Fig. 6 is a timing chart showing an example of H. \stella\Keep\Speci\NTB\P5Q794.FO911.amended pages.doc 24/11/04 reception signals in the receiving earth station 5 of Fig.

Fig. 7 is a diagram showing the construction of the satellite communication system according to a second embodiment of the present invention; Fig. 8 is a block diagram showing an example of a construction of a receiving earth station 6 of Fig. 7; Fig. 9 is a diagram showing an example of the construction of the satellite communication system according to a third embodiment of the present invention; Fig. 10 is a block diagram showing an example of a construction of a receiving earth station 7 of Fig. 9; Fig. 11 is a block diagram showing the construction of a conventional satellite communication system; Fig. 12 is a block diagram showing the construction of a transmitting earth station 1 of Fig. 11; Fig. 13 is a block diagram showing the construction of communication satellites 2A and 2B of Fig. 11; and Fig. 14 is a block diagram showing the construction of the receiving earth station 3 of Fig. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments according to the present invention will be described hereunder with reference to the accompanying drawings.

[First Embodiment] Fig. 1 is a schematic diagram showing an example of the construction of a satellite communication system according to a first embodiment of the present invention, and shows a state just before satellite switching is carried out in the system concerned. The satellite H\Valma\Keep\Specifications\P0794 .F0911.doc 24/09/03 communication system according to this embodiment is a nongeosynchronous satellite system for accessing a satellite in the FDMA system, and it comprises a transmitting earth station 4, plural communication satellites 2A and 2B and a receiving earth station Comparing the satellite communication system of Fig.

1 with the conventional satellite communication system (Fig. 11), the constructions of the transmitting earth station 4 and the receiving earth station 5 are different therebetween. A time marker is inserted in a transmission signal at the transmitting earth station 4, and a time marker contained in a reception signal is extracted at the receiving earth station 5, whereby the communication satellites 2A, 2B are switched to each other without inducing any instantaneous interruption of reception data.

Fig. 2 is a block diagram showing an example of the construction of the transmitting earth station 4 of Fig. 1.

The transmitting earth station 4 is constructed by equipping time marker inserting means 40 to the conventional transmitting earth station 1, and a time marker comprising a predetermined pattern data is periodically inserted in transmission digital data input from the input terminal 11. The transmission digital data having the time marker inserted therein is modulated by the modulator 12, and the same modulated signal is subjected to frequency conversion and high-frequency amplification in the transmitters 13A and 13B, and then transmitted from the antennas 15A and 15B as in the case of the conventional transmitting earth station i.

Fig. 3 is a block diagrams showing an example of a construction of the receiving earth station 5 of Fig. 1.

The receiving earth station 5 is constructed by equipping H\Valma\Keep\Specifications\PS0794 .F911.doc 24/09/03 the conventional receiving earth station 3 with two time marker extracting means 50A and 50B, two buffer memories 51A and 51B and time marker delaying means 56.

The time marker extracting means 50A, 50B extracts a time marker from demodulated data of the demodulator 34A, 34B to detect the position of the timer marker inserted at the transmission side in the demodulated data. The timer marker extracted by the timer marker extracting means is input to the buffer memory 51a, 51B, respectively.

The timer marker extracted by any one timer marker extracting means is input to the timer marker delaying means 56 to be delayed by a predetermined delay time, and then input to both the buffer memories 51A and 51B. Fig. 3 shows a case where the time marker is input from the time marker extracting means 50A to the time marker delaying means 56.

The buffer memory 51A serves as storage means for temporarily storing demodulated data of the demodulator 34A, and it has not only input and output terminals for the demodulated data, but also a writing clock input terminal 52A, a reading clock input terminal 53A, a writing time marker input terminal 54A and a reading time marker input terminal The clock signal extracted by the clock extracting means 35A is input to each of the writing clock input terminal 52A and the reading clock input terminal 53A. The time marker extracted by the time marker extracting means is input to the writing time marker input terminal 54A, and the time marker delayed by the timer marker delaying means 56 is input to the reading time marker input terminal The data writing into the buffer memory 51A is H.\Vlma\Keep\Specification\P50794 .F0911.doc 24/09/03 carried out on the basis of the clock signal extracted by the clock extracting means 35A and the time marker extracted by the time marker extracting means SOA. That is, the demodulated data which are sectioned on the basis of the time marker are written at the signal reception time.

The data read-out from the buffer memory 51A is carried out on the basis of the clock signal extracted by the clock extracting means 35A and the time marker delayed by the time marker delaying means 56. After a predetermined delay time elapses, the demodulated data sectioned on the basis of the time marker are successively read out.

Like the buffer memory 51A, the buffer memory 51B serves as storage means for temporarily storing the demodulated data of the demodulator 34B, and it has not only input and output terminals for the demodulated data, but also a writing clock input terminal 52B, a reading clock input terminal 53B, a writing time marker input terminal 54B and a reading time marker input terminal The same type of signal as the buffer memory 51A is input to the writing input terminal of the buffer memory 51B. That is, the clock signal extracted by the clock extracting means 35B is input to the writing clock input terminal 52B, and the time marker extracted by the time marker extracting means 50B is input to the writing time marker input terminal 54B.

Furthermore, the common signal to the buffer memory 51A is input to the reading input terminal of the buffer memory 51B. That is, the clock signal extracted by the clock extracting means 35A is input to the reading clock input terminal 53B, and the time marker delayed by the time marker delaying means 56 is input to the reading time marker input terminal H.\Valma\Keep\Specification\P50794.F0911.doc 24/09/03 That is, the data are sectioned on the basis of the time marker and written into the buffer memory 51B at the data reception time. The data writing is carried out in non-synchronism with the buffer memory 51A. On the other hand, the data reading from the buffer memory 51B is carried out in synchronism with the buffer memory 51B.

Therefore, when a delay time difference occurs due to a communication path difference, a time difference also occurs in the writing operation of the reception data between the buffer memories 51A and 51B. However, no time difference occurs in the reading operation of the reception data between the buffer memories 51A and 51B. Accordingly, no instantaneous interruption occurs even when the data sequence is switched to another one by the switch 36.

Fig. 4 is a timing chart showing an example of the operation of the buffer memories 51A and 51B. The top side of Fig. 4 shows writing data into the buffer memory 51A, the second top side of Fig. 4 shows writing data into the buffer memory 51B, the third top side of Fig.

4 shows reading data from the buffer memory 51A, and the bottom side of Fig. 4 shows reading data from the buffer memory 51B.

In Fig. 4, it is assumed that the communication path via the communication satellite 2A is longer than the communication path via the communication satellite 2B. In this case, the data to be written into the buffer memory 51A are written later than the writing data to be written into the buffer memory 51B. However, the reception data are read out from the buffer memories 51A and 51B on the basis of the time markers which are sectioned and written every time marker and delayed, whereby the reception data can be read out from the buffer memories 51A and 51B in H \Valma\Keep\Specifications\P50794. F0911 .doc 24/09/03 synchronism.

Here, in the case where the time markers are inserted in transmission data, if the delay time difference occurring through the communication satellites 2A, 2B is not more than a half of the time marker period, the time markers extracted from the respective reception signals from the communication satellites 2A, 2B can be associated with each other. Therefore, it is favorable that the period of the time markers is set to be twice or more as high as the maximum delay time difference through the communication satellites 2A, 2B before and after the predetermined switching.

Furthermore, by setting any one time marker to be a half or more of the time marker period in the time marker delaying means 56, the reception data can be synchronously read out in all cases where the synchronization can be performed on the basis of the time marker concerned.

Fig. 5 is a diagram showing an example of the relationship between the communication satellite 2A, 2B and the earth station 4, 5, and an example of the communication path length based on each of the communication satellites 2A, 2B is shown. In this case, in order to simplify the description, there is considered such a case that the transmitting earth station 4 and the receiving earth station 5 are located at the same position on the surface of the earth, and the communication satellite 2A, 2B goes around a circular orbit passing through the zenith of the earth station 4, It is assumed that the satellite switching is carried out from the communication satellite 2A to the communication satellite 2B when the elevation angle (EL) of the communication satellite 2A viewed from the earth H.\Valma\Keep\Specifications\P50794.FO911.doc 24/09/03 station 4, 5 is equal to 20 degrees and the elevation angle of the communication satellite 2B is equal to 10 degrees, and also the altitude of each of the communication satellites 2A, 2B is equal to 1000km. At this time, the distance from the earth station 4, 5 to the communication satellite 2A is equal to 2123.7 km, and the distance from the earth station 4, 5 to the communication satellite 2B is equal to 2762.2 km.

If the communication path length difference in the earth station 4, 5 and the communication satellite 2A, 2B can be neglected, the communication path length difference based on the communication satellites 2A, 2B is determined by the distances from the earth stations 4, 5 to the communication satellites 2A, 2B. Therefore, the delay time difference caused by the path length difference between the path extending from the transmitting earth station 4 via the communication satellite 2A to the receiving earth station 5, and the path extending from the transmitting earth station 4 via the communication satellite 2B to the receiving earth station 5 is determined according to the following equation on the assumption that the velocity of light is equal to 300000km/sec.

((2762.2 2123.7)/300000} x 2 4.25 (msec) Fig. 6 is a timing chart showing an example of the reception signal in the receiving earth station 5 of Fig.

The top side of Fig. 6 shows reception data from the communication satellite 2A, and the bottom side of Fig. 6 shows reception data from the communication satellite 2B. When data transmission of 64kbps (bit per second) is carried out, the delay difference corresponding to 64kbps x 4.25msec 272 bits occurs between the communication satellite 2A and the communication satellite H\Valma\Keep\Specifications\P0794 .F911.dec 24/09/03 2B.

It is assumed that it is unknown which the communication satellite 2A or 2B provides a longer communication path length. In this case, if the time marker is not inserted at a period of 272 bits x 2 544 bits or more at the transmitting earth station 4, the timer markers received from the communication satellites 2A, 2B cannot be associated with each other at the receiving earth station and thus the reception data received from these satellites cannot be synchronized with each other.

Accordingly, if the insertion period of the time marker is twice or more as long as the maximum delay time difference caused by the communication satellites 2A, 2B, the reception data therefrom can be synchronized with each other at all times.

The satellite communication system according to this embodiment stores the reception data of the two systems into the buffer memories 51A, 51B respectively, delays the time marker extracted from any one reception data and reads out the data from both the buffer memories 51A,l 51B on the basis of the delayed time marker. Therefore, the data sequences from both the buffer memories 51A, 51B can be output in synchronism with each other.

Particularly, data read-out is carried out from both the buffer memories 51A, 51B on the basis of the clock signal extracted from any one reception data, and thus the data sequences which are perfectly synchronized with each other even in clock level can be output from both the buffer memories 51A, 51B. With this construction, when the satellite switching is carried out, neither instantaneous interruption nor synchronization step-out occurs even when two data sequences being received are switched to each H.\Valma\Keep\Specification\P50794.F0911.doc 24/09/03 other by the switch 36.

[Second Embodiment] Fig. 7 is a diagram showing an example of the construction of a satellite communication system according to a second embodiment of the present invention. This satellite communication system comprises a transmitting earth station 1, two communication satellites 2A and 2B, and a receiving earth station 6. As compared with the satellite communication system (of the first embodiment) of Fig. 1, this embodiment is different from the first embodiment in that a conventional transmitting earth station 1 is used as the transmission side.

In the first embodiment of the present invention, the timer marker is periodically inserted at the transmitting earth station 1. The time marker may have a fixed pattern having a constant period which is twice or more as long as the propagation delay time difference between the communication paths via the communication satellites 2A, 2B. Therefore, when there is any pattern which is not specially inserted at the transmitting earth station, but satisfies the above condition, this pattern may be used as the time marker. Particularly, by using a periodic pattern contained in transmission digital data in advance, it is unnecessary to insert a special time marker at the transmitting earth station 1.

Fig. 8 is a block diagram showing an example of the construction of the receiving earth station 6 of Fig. 7.

In place of the time marker extracting means 50A, 50B at the receiving earth station 5 of Fig. 3, frame synchronized pattern extracting means 60A, 60B are equipped, and in place of the time marker delaying means 56, frame synchronized pattern delaying means 66 is equipped.

H\Valma\Keep\Specifications\P50794 .F911.doc 24/09/03 When a transmission digital data sequence is sectioned into data frames comprising plural data, a frame synchronizing pattern is normally inserted at the head of each frame. The frame synchronizing pattern may be used as the time marker.

The frame synchronizing pattern extracting means extracts a frame synchronizing pattern from demodulated data of the demodulator 34A, 34B to detect the position of the frame synchronizing patter in the demodulated data. The frame synchronizing pattern extracted by the frame synchronizing pattern extracting means 60A, 60B is input to the buffer memory 51A, 51B, respectively.

The frame synchronizing pattern extracted by any one frame synchronizing pattern extracting means 60A, 60B is input to the frame synchronizing pattern delaying means 66, delayed by a predetermined delay time and then input to both the buffer memories 51A and 51B. Fig. 8 shows a case where the output of the frame synchronizing pattern extracting means 60A is input to the frame synchronizing pattern delaying means 66.

When the transmission digital data is sectioned into multi-frames comprising plural frames in the data sequence, a multi-frame synchronizing pattern to be inserted in a transmission digital data sequence every multi-frame may be used in place of the frame synchronizing pattern.

Particularly, when the period of the frame synchronizing pattern is shorter than the delay time difference between the communication paths, it is favorable to use the multiframe synchronizing pattern.

In this embodiment, the frame synchronizing pattern or the multi-frame synchronizing pattern contained in the transmission digital data is extracted in the receiving H \Valma\Keep\Specifications\P50794 .F0911.doc 24/09/03 earth station without inserting any time marker at the transmitting earth station, and the received data sequences of the two systems are synchronized with each other.

Therefore, the facilities of the transmitting earth station can be simplified, and the cost can be reduced.

In this embodiment, the frame synchronizing pattern and the multi-frame synchronizing pattern are used as the time marker. However, the pattern of this invention is not limited to the patterns as described above insofar as it is periodically inserted. That is, various patterns may be used as the time marker in accordance with the pattern period and the delay time difference.

[Third Embodiment] Fig. 9 is a diagram showing an example of the construction of the satellite communication system according to a third embodiment of the present invention.

The satellite communication system of this embodiment comprises a transmitting earth station 4, two communication satellites 2A and 2B, and a receiving earth station 7. The construction of the receiving earth station 7 is different from that of the satellite communication system of Fig. 1 (the first embodiment).

Fig. 10 is a block diagram showing an example of the construction of the receiving earth station 7 of Fig. 9.

This embodiment is different from the receiving earth station 5 of Fig. 3 in that clock generating means 70 is equipped. The clock generating means 70 serves to generate a clock signal independent of the reception signal, and a reference clock of the receiving earth station may be used.

The clock signal from the clock generating means 70 is input to the reading clock input terminals 53A, 53B of the buffer memories 51A, 51B, and reception data are read out H\Valma\Keep\Specifications\P50794.F0911.doc 24/09/03 from the buffer memories 51A, 51B in synchronism with the clock signal concerned.

In this case, the same effect as the first embodiment can be achieved. In addition, although it is ordinarily required to equip the earth station to a dropper buffer or plesiochronous buffer separately in order to keep the clock synchronization relationship with the earth system.

However, the buffer memories 51A, 51B can be made to serve as the above buffer, so that the device can be simplified and the cost can be reduced.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

H.\Valma\Keep\Specification\P50794.POS11.doc 24/09/03

Claims

2. The satellite communication network according to claim i, wherein the transmitting earth station is equipped with time marker inserting means for periodically inserting a time marker to transmission data.
3. The satellite communication network according to claim 1, wherein the time marker is inserted in the transmission data at a period that is twice or more as Ht\stella\Keep\Speci\NTB\P50794.P0911.amended pages.doc 24/11/04 r 26 o long as a propagation delay time difference in the ci communication paths of the two systems. O z 4. The satellite communication network according to in claim 1, wherein the time marker extracting means extracts ci a frame synchronizing pattern or multi-frame synchronizing O0 pattern in reception data as the time marker. A receiving earth station for receiving data from OO a transmitting earth station through a first communication satellite before satellite switching and through a second communication satellite after satellite switching; the transmitting earth station, first communication satellite and receiving earth station forming a first system; the transmitting earth station, second communication satellite and receiving station forming a second system; the receiving earth station comprising: time marker extracting means for extracting a time marker from each reception data of the two systems at the receiving earth station; buffer memories in which the reception data are written on the basis of the time markers thus extracted; time marker delaying means for delaying any one of the time markers of the two systems; read-out means for reading out the reception data of the two systems from the buffer memories on the basis of the time marker thus delayed; and switching means for switching the reception data of the two systems thus read out and outputting the reception data thus switched.
6. The receiving earth station according to claim wherein the time marker extracting means extracts a frame synchronizing pattern or multi-frame synchronizing pattern in the reception data as the time marker. H:\etella\Keep\Speci\NTB\P50794,FOS911,amended pagesdoc 24/11/04 27 o 7. The receiving earth station according to claim ci wherein the time marker delaying means delays the timer 0 Z marker by a half or more of an insertion period of the tfl time marker. ci
8. The receiving earth station according to claim 00 further comprising clock extracting means for extracting a clock signal from each of the reception data of the two 00 systems, wherein the reception data are written into the buffer memories on the basis of the clock signals extracted from the reception data, and the reception data of the two systems are read out from the buffer memories on the basis of the clock signal extracted from any one reception data.
9. The receiving earth station according to claim further comprising clock extracting means of the two systems for extracting clock signals from the reception data of the two systems and clock generating means for generating a clock signal, wherein the reception data are written into the buffer memories on the basis of the clock signals thus extracted, and the reception data of the two systems are read out from the buffer memories on the basis of the clock signal from the clock generating means. A communication satellite switching method for performing data transmission from a transmitting earth station to a receiving earth station through a first communication satellite before satellite switching and through a second communication satellite after satellite switching; the transmitting earth station, first communication satellite and receiving earth station forming a first system; the transmitting earth station, second communication satellite and receiving station forming a second system; the method comprising: H;\stella\Keep\Speci\NTB\P50794.P0911 amended pages.doc 24/11/04 28 O a time marker extracting step of extracting a time ci marker from each reception data of the two systems; 0 z a buffer writing step of writing reception data into V) a buffer memory on the basis of the time marker thus ci extracted; a time marker delaying step of delaying any one of 0 the time markers of the two systems; 00 a buffer read-out step of reading out the reception Ci data of the two systems from the buffer memory on the o 10 basis of the time marker thus delayed; and C a switching step of switching the reception data of the two systems thus read out and outputting the reception data thus switched.
11. The communication satellite switching method according to claim 10, further comprising a step of a time marker inserting step of inserting the time marker to the transmission data.
12. The communication satellite switching method according to claim 10, wherein in the time marker insertion step, the time marker to be inserted into the transmission data has a period which is twice or more as long as the propagation time difference in transmission paths of the two systems.
13. A network as claimed in any.one of claims 1 to 4, and substantially as herein described with reference to the accompanying drawings.
14. A receiving earth station as claimed in any one of claims 5 to 9, and substantially as herein described with reference to the accompanying drawings. H:\stella\Keep\Speci\NTB\P50794.FD91iamended pages-doc 24/11/04 S29 0 O 15. A method as claimed in any one of claims 10 to 12, and substantially as herein described with reference O Z to the accompanying drawings. iA Dated this 24th day of November 2004 OO MITSUBISHI DENKI KABUSHIKI KAISHA By their Patent Attorneys 00 GRIFFITH HACK C Fellows Institute of Patent and Cr o 10 Trade Mark Attorneys of Australia ci H \stella\Keep\Speci\NTB\P50794.F0911.amended pages.doc 24/11/04