Artificial Satellite System and Method for Measuring Distance Between Artificial Satellites

An artificial satellite system according to the present invention includes first and second artificial satellites. An antenna of the first satellite transmits a radio wave (transmission signal) to the second satellite. An antenna of the second satellite receives the transmission signal. A software defined radio of the second satellite generates a signal having the same frequency and phase as the transmission signal has. The antenna of the second satellite transmits a radio wave of a signal generated by the second satellite to the first satellite. The antenna of the first satellite receives the radio wave (reception signal) transmitted by the second satellite. A data processing device of the first satellite determines a relative distance between the first and second satellites by using the difference between a transmission time of the transmission signal and a reception time of the reception signal and the phase difference between the transmission and reception signals.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent Application JP 2023-061559 filed on Apr. 5, 2023, the content of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to an artificial satellite system having a plurality of artificial satellites, and a method for measuring the distance between the artificial satellites.

Constellation which combines multiple artificial satellites to function integrally has been progressing. In order to combine multiple artificial satellites, it is important to measure the position of each artificial satellite and grasp the shape of an artificial satellite group. The measurement of the position of the artificial satellite may be performed using an GNSS (Global Navigation Satellite System) such as a GPS (Global Positioning System), but depending on the position of the artificial satellite, the artificial satellite may not be able to receive signals from the GNSS. It is expected that the utilization range of small artificial satellites will expand in the future. It is becoming necessary to establish a technology of measuring the positions of multiple artificial satellites and the distance between artificial satellites even if the GNSS is not available.

An example of a conventional technology of measuring the positions of multiple artificial satellites (space crafts) has been described in document JP 2015-155897. In the technology described in the document JP 2015-155897, a host ship continuously transmits a wireless frequency signal through three antennas, and a companion ship continuously receives the signals transmitted from the host ship. The companion ship is equipped with a measuring device which measures a propagation time of each reception signal generated from each of the three antennas of the host ship and derives a path difference between a path through which the signal generated from the main antenna propagates and a path through which the signal generated from each of the two sub-antennas propagates. The host ship is equipped with a processing device which determines a relative angular position between the host ship and the companion ship from the result of measurement of the path difference transmitted from the companion ship.

As described above, there has been disclosed in the document JP 2015-155897, the technology of measuring the relative positions and distances of multiple artificial satellites (space crafts) using the propagation time difference (path difference) of the radio waves between the host ship and the companion ship. In the conventional technologies such as the technology described in the document JP 2015-155897, in order to measure mutual distances between multiple artificial satellites, there is a need to synchronize the time between the artificial satellites. In order to perform this time synchronization, a time synchronizing device or system such as an atomic clock is required. For this reason, the conventional technologies are accompanied by a problem that the cost is increased because the device becomes complicated and large, and the processing executed by the device becomes complicated.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a system and method capable of measuring relative distances between a plurality of artificial satellites without time synchronization.

An artificial satellite system of the present invention includes a plurality of artificial satellites including at least a first artificial satellite and a second artificial satellite. Each of the artificial satellites includes a data processing device, a software defined radio, and an antenna. The antenna of the first artificial satellite transmits a radio wave to the second artificial satellite as a transmission signal. The antenna of the second artificial satellite receives the transmission signal. The software defined radio of the second artificial satellite generates a signal having a same frequency and phase as the transmission signal. The antenna of the second artificial satellite transmits a radio wave of the signal generated by the software defined radio of the second artificial satellite to the first artificial satellite. The antenna of the first artificial satellite receives the radio wave transmitted by the second artificial satellite as a reception signal. The data processing device of the first artificial satellite determines a relative distance between the first artificial satellite and the second artificial satellite by using a difference between a transmission time of the transmission signal and a reception time of the reception signal and a phase difference between the transmission signal and the reception signal.

A method for measuring a distance between artificial satellites of the present invention includes the steps of: allowing a first artificial satellite to transmit a radio wave to a second artificial satellite as a transmission signal; allowing the second artificial satellite to receive the transmission signal; allowing the second artificial satellite to generate a signal having a same frequency and phase as the transmission signal; allowing the second artificial satellite to transmit a radio wave of the signal generated by the second artificial satellite to the first artificial satellite; allowing the first artificial satellite to receive the radio wave transmitted by the second artificial satellite as a reception signal; and allowing the first artificial satellite to determine a relative distance between the first artificial satellite and the second artificial satellite by using a difference between a transmission time of the transmission signal and a reception time of the reception signal and a phase difference between the transmission signal and the reception signal.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to provide a system and method capable of measuring relative distances between a plurality of artificial satellites without time synchronization.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method for measuring the distance between an artificial satellite system according to the present invention and an artificial satellite is able to measure relative distances between one artificial satellite and one or more artificial satellites without time synchronization between these artificial satellites. Therefore, it is possible to reduce devices and functions required for time synchronization and reduce the cost of constructing the artificial satellite system.

In the artificial satellite system according to the present invention, a first artificial satellite being a transmitting artificial satellite transmits a radio wave to a second artificial satellite being a receiving artificial satellite. The second artificial satellite transmits a radio wave having the same frequency and phase as the radio wave received from the first artificial satellite to the first artificial satellite. The first artificial satellite determines a relative distance between the first artificial satellite and the second artificial satellite. Further, the first artificial satellite and the second artificial satellite have the same configuration with each other. By being equipped with software defined radios, the first artificial satellite and the second artificial satellite can have both the functions as the first artificial satellite and the second artificial satellite.

The second artificial satellite preferably includes a frequency filter function which allows a radio wave in a frequency band assigned to itself among received radio waves to pass therethrough. The second artificial satellite can transmit a radio wave having the same frequency and phase as the radio wave passed by the frequency filter function to the first artificial satellite. This frequency filter function allows the artificial satellite system according to the present invention to simultaneously determine the relative distances among three or more artificial satellites.

An artificial satellite system according to an embodiment of the present invention and a method for measuring the distance between artificial satellites will hereinafter be described with reference to the drawings. Note that in the drawings used in the present specification, the same or corresponding components are given the same reference numerals, and their repetitive description may be omitted.

First Embodiment

A description will be made about an artificial satellite system according to a first embodiment of the present invention, and a method for measuring the distance between artificial satellites. The method for measuring the distance between the artificial satellites according to the present embodiment can be performed by the artificial satellite system according to the present embodiment.

FIGS.1A and1Bare diagrams each showing an example of a plurality of artificial satellites included in the artificial satellite system according to the present embodiment. The artificial satellite system according to the present embodiment includes a plurality of artificial satellites, that is, one transmitting artificial satellite10and one or more receiving artificial satellites20. The transmitting artificial satellite10also has a configuration and function as the receiving artificial satellite20. The receiving artificial satellite20also has a configuration and function as the transmitting artificial satellite10. The transmitting artificial satellite10and the receiving artificial satellite20are, for example, artificial satellites for earth observation, which fly in a geostationary orbit (orbit at an altitude of approximately 36,000 km above the equator).

FIG.1Aillustrates an example in which the artificial satellite system according to the present embodiment is configured of two artificial satellites. The artificial satellite system illustrated inFIG.1Aincludes a single transmitting artificial satellite10and a single receiving artificial satellite20.

For example, the transmitting artificial satellite10transmits a radio wave31for measuring a relative distance to the receiving artificial satellite20to the receiving artificial satellite20. If, after the radio wave31is received, it is a radio wave in a frequency band assigned to the receiving artificial satellite20, the receiving artificial satellite20transmits a radio wave32having the same frequency and phase as the received radio wave31to the transmitting artificial satellite10. Hereinafter, as for the transmitting artificial satellite10and the receiving artificial satellite20, the radio wave to be transmitted is also called a transmission signal, and the radio wave to be received is also called a reception signal.

The transmitting artificial satellite10receives the radio wave32transmitted from the receiving artificial satellite20and determines by calculating the relative distance between the transmitting artificial satellite10and the receiving artificial satellite20from the difference between a transmission time of the radio wave31and a reception time of the radio wave32, the phase difference between the transmission signal (radio wave31) and the reception signal (radio wave32), and in-circuit processing times of the transmitting artificial satellite10and the receiving artificial satellite20. Note that the in-circuit processing times are the times required for signal processing by circuits included in the transmitting artificial satellite10and the receiving artificial satellite20. The in-circuit processing times of the transmitting artificial satellite10and the receiving artificial satellite20can be determined by measuring in advance.

In the artificial satellite system according to the present embodiment, as described above, the receiving artificial satellite20receives the radio wave31transmitted by the transmitting artificial satellite10, the receiving artificial satellite20transmits the radio wave32having the same frequency and phase as the received radio wave31, and the transmitting artificial satellite10receives the radio wave32. That is, the transmitting artificial satellite10receives the radio wave32having the same frequency and phase as the radio wave31transmitted to the receiving artificial satellite20from the receiving artificial satellite20.

The transmitting artificial satellite10and the receiving artificial satellite20have the same configuration with each other and can switch between the operation as a transmitting artificial satellite and the operation as a receiving artificial satellite. That is, the transmitting artificial satellite10can also be operated as the receiving artificial satellite20which receives the transmission signal, and the receiving artificial satellite20can also be operated as the transmitting artificial satellite10which transmits the transmission signal. For example, the transmitting artificial satellite10and the receiving artificial satellite20are configured to be capable of transmission of a transmission signal, reception of a reception signal, reception of a transmission signal, and transmission of a reception signal.

In the artificial satellite system according to the present embodiment, the relative distances can be measured among the multiple artificial satellites without time synchronization in this way. Further, the receiving artificial satellite20transmits the radio wave32having the same frequency and phase as the received radio wave31to the transmitting artificial satellite10in terms of the radio wave in the frequency band assigned to itself. In the artificial satellite system according to the present embodiment, the relative distances among the multiple artificial satellites can be measured simultaneously by changing and assigning the frequency bands processed by the artificial satellites for each artificial satellite.

FIG.1Bshows an example in which the artificial satellite system according to the present embodiment includes four artificial satellites1ato1d.The four artificial satellites1ato1dhave the same configuration with each other and can switch between the operation as the transmitting artificial satellite10and the operation as the receiving artificial satellite20.

For example, the artificial satellite1ais operated as the transmitting artificial satellite10, each of the artificial satellites1bto1dis operated as the receiving artificial satellite20, and the artificial satellite1atransmits a radio wave31and receives a radio wave32. With this, it is possible to measure the relative distance between the artificial satellite1aand each of the artificial satellites1bto1d.Further, for example, the artificial satellite1bis operated as the transmitting artificial satellite10, each of the artificial satellites1aand1cto1dis operated as the receiving artificial satellite20, and the artificial satellite1btransmits a radio wave31and receives a radio wave32. With this, it is possible to measure the relative distance between the artificial satellite1band each of the artificial satellites1aand1cto1d.

As described above, since each of the artificial satellites1ato1dis assigned the frequency band to be processed, the relative distances among the multiple artificial satellites1ato1dcan be measured simultaneously in the artificial satellite system according to the present embodiment.

FIG.2is a block diagram showing an example of a schematic configuration of the artificial satellite system according to the present embodiment. The artificial satellite system4according to the present embodiment includes a transmitting artificial satellite10and a receiving artificial satellite20.

The transmitting artificial satellite10is an artificial satellite which transmits a radio wave31and measures a relative distance to the receiving artificial satellite20. The receiving artificial satellite20is an artificial satellite which returns a radio wave32having the same frequency and phase as the received radio wave31to the transmitting artificial satellite10and whose relative distance is measured by the transmitting artificial satellite10.

The transmitting artificial satellite10includes data processing devices11aand11b, software defined radios12aand12b,and antennas13aand13b.The receiving artificial satellite20includes data processing devices21aand21b,software defined radios22aand22b,and antennas23aand23b.Further, the transmitting artificial satellite10and the receiving artificial satellite20include control devices18and28for controlling the attitudes of the artificial satellites10and20, propulsive mechanisms17and27for driving the artificial satellites10and20, and sensors19and29for grasping the states of the artificial satellites10and20, respectively. For example, any thruster or electric propulsion device or the like can be used for the propulsive mechanisms17and27.

Each of the data processing devices11a,11b,21a,and21bis a calculation device and has a function of calculating the relative distance between the transmitting artificial satellite10and the receiving artificial satellite20using the difference between a transmission time of a transmission signal (radio wave31) and a reception time of a reception signal (radio wave32), and the phase difference between the transmission signal and the reception signal. Further, each of the data processing devices11a,11b,21a,and21bhas a function of controlling each of the software defined radios12a,12b,22a,and22b.The data processing devices11aand11bincluded in the transmitting artificial satellite10may be configured of one data processing device, or they may be configured of data processing devices different from each other. The data processing devices21aand21bincluded in the receiving artificial satellite20may be configured of one data processing device, or they may be configured of data processing devices different from each other.

The software defined radios12aand22a,and12band22brespectively have a function of generating the signal of a radio wave to be transmitted and a function of processing a received radio wave.

Each of the software defined radios12aand22ahas a function of according to a command from each of the data processing devices11aand21a,generating an analog signal corresponding to the command. For example, the software defined radio22aincluded in the receiving artificial satellite20generates a radio wave (analog signal) having the same frequency and phase as the radio wave received from the transmitting artificial satellite10by the receiving artificial satellite20. When the software defined radio22ahas a frequency filter function to be described later, the software defined radio22agenerates a radio wave having the same frequency and phase as the signal passed by the frequency filter function.

The software defined radios12band22beach have a function of processing the received radio wave, e.g., a function of converting the analog signal received by each of the transmitting artificial satellite10and the receiving artificial satellite20into a digital signal. In the transmitting artificial satellite10, the software defined radio12aand the software defined radio12bmay be configured of one software defined radio, or they may be configured of software defined radios different from each other. In the receiving artificial satellite20, the software defined radio22aand the software defined radio22bmay be configured of one software defined radio, or they may be configured of software defined radios different from each other.

The antennas13aand23ahave a function for transmitting analog signals generated by the software defined radios12aand22aas radio waves. Each of the antennas13band23bhas a function for receiving a radio wave. In each of the transmitting artificial satellite10and the receiving artificial satellite20, the antenna to transmit the radio wave and the antenna to receive the radio wave may be configured of one antenna, or they may be configured of antennas different from each other. Further, the transmitting artificial satellite10and the receiving artificial satellite20may each have a plurality of antennas13band23bwhich receive the radio waves.

In the following, the data processing devices11aand11bmay also be represented as a data processing device11, the software defined radios12aand12bmay also be represented as a software defined radio12, the antennas13aand13bmay also be represented as an antenna13, the data processing devices21aand21bmay also be represented as a data processing device21, the software defined radios22aand22bmay also be represented as a software defined radio22, and the antennas23aand23bmay also be represented as an antenna23, respectively.

According to the command from the data processing device11, the control devices18and28are capable of controlling the propulsive mechanisms17and27to drive the artificial satellites10and20.

FIG.3is a diagram showing in more detail an example of basic configurations of the transmitting artificial satellite10and the receiving artificial satellite20. As described above, the transmitting artificial satellite10and the receiving artificial satellite20has the same configuration with each other. Therefore, hereinafter, the transmitting artificial satellite10will be described as a representative, and the basic configuration of the transmitting artificial satellite10is shown inFIG.3. Further, inFIG.3, for the sake of simplification of the drawing, the data processing devices11aand11band the software defined radios12aand12bare shown as one data processing device11and one software defined radio12, respectively.

As a configuration for transmitting a radio wave, the software defined radio12includes a D/A converter121a,a filter122a,an amplifier123a,a mixer124a,an amplifier125a,and a filter126a.Also, as a configuration for receiving a radio wave, the software defined radio12includes a filter126b,an amplifier125b,a mixer124b,an amplifier123b,a filter122b,and an A/D converter121b.Further, the software defined radio12has a local oscillator127. The local oscillator127is a device which generates a signal for modulation (frequency conversion) of an analog signal.

A description will first be made about the configuration for transmitting the radio wave in terms of the transmitting artificial satellite10.

The D/A converter121a(digital-analog converter121a) performs D/A conversion (digital-analog conversion) according to a command from the data processing device11to generate an analog signal. The filter122aeliminates high frequency noise caused by the D/A conversion from the generated analog signal. The amplifier123aamplifies the amplitude of the generated analog signal. The mixer124amodulates the generated analog signal. That is, the mixer124aconverts the frequency of the analog signal by multiplying the generated analog signal by the signal from the local oscillator127. The amplifier125aamplifies the amplitude of the modulated analog signal. The filter126aeliminates a noise component from the modulated analog signal. The antenna13aconverts the modulated analog signal (electric signal) into a radio wave.

Note that the software defined radio12may include one or more of the filter122a, the filter126a,the amplifier123a,and the amplifier125a,or may not include any of these.

A description will next be made about the configuration for receiving the radio wave in terms of the transmitting artificial satellite10.

The antenna13bconverts the received radio wave into an electric analog signal. The filter126beliminates a noise component from the analog signal. The amplifier125bamplifies the amplitude of the analog signal. The mixer124bmodulates the analog signal. That is, the mixer124bconverts the frequency of the analog signal by multiplying the analog signal by the signal from the local oscillator127. The amplifier123bamplifies the amplitude of the modulated analog signal. The filter122beliminates noise or an unnecessary frequency band caused by the frequency conversion from the modulated analog signal. The A/D converter121b(analog-digital converter121b) performs A/D conversion (analog-digital conversion) to convert the modulated analog signal into a digital signal.

Note that the software defined radio12may include one or more of the filter126b, the filter122b,the amplifier125b,and the amplifier123b,or may not include any of these.

In the present embodiment, the transmitting artificial satellite10uses the same local oscillator127when transmitting and receiving the radio waves. That is, the software defined radio12performs frequency conversion processing in generating a transmission signal and frequency conversion processing for a reception signal. When the error in initial phase occurs in the local oscillator127, phase errors occur when the mixers124aand124beach covert the frequency of the analog signal. In the present embodiment, since the same local oscillator127is used when transmitting and receiving the radio waves, it is possible to cancel out the phase errors each other, and it is possible to prevent the error in the initial phase due to the local oscillator127from occurring.

Further, since the transmitting artificial satellite10and the receiving artificial satellite20have the same configuration with each other, it is easy to mutually switch the transmitting artificial satellite10and the receiving artificial satellite20. Therefore, the relative distances between one artificial satellite and one or more artificial satellites can be easily measured even from any artificial satellite.

FIG.4is a diagram showing an example of sequence control when measuring the relative distance between the transmitting artificial satellite10and the receiving artificial satellite20by mutually switching the transmitting artificial satellite10and the receiving artificial satellite20. Two artificial satellites2aand2bare shown inFIG.4.

UsingFIG.4, a description will be made about a case where the artificial satellite2aserves as the transmitting artificial satellite10and the artificial satellite2bserves as the receiving artificial satellite20, and a case where the artificial satellite2bserves as the transmitting artificial satellite10and the artificial satellite2aserves as the receiving artificial satellite20. First, the artificial satellite2aserving as the transmitting artificial satellite10measures the relative distance to the artificial satellite2bserving as the receiving artificial satellite20. Next, the artificial satellite2bserving as the transmitting artificial satellite10measures the relative distance to the artificial satellite2aserving as the receiving artificial satellite20.

First, a ground station3on the ground transmits a measurement-start command c101to the artificial satellite2awhich serves as the transmitting artificial satellite10. Then, the artificial satellite2atransmits a radio wave31to the artificial satellite2bwhich serves as the receiving artificial satellite20. Further, the artificial satellite2areceives a radio wave32from the artificial satellite2bto determine the relative distance to the artificial satellite2band transmits a signal indicating that the measurement of the relative distance is completed to the ground station3.

Next, the ground station3transmits a measurement-start command c101to the artificial satellite2bwhich serves as the transmitting artificial satellite10. Then the artificial satellite2btransmits a radio wave31to the artificial satellite2awhich serves as the receiving artificial satellite20. Further, the artificial satellite2breceives a radio wave32from the artificial satellite2ato determine the relative distance to the artificial satellite2aand transmits a signal indicating that the measurement of the relative distance is completed to the ground station3.

Note that mutual switching between the transmitting artificial satellite10and the receiving artificial satellite20may be performed by transmitting the measurement-start command c101by the ground station3as in the example shown inFIG.4, or it may be performed by any other method.

FIG.5is a diagram showing an example of sequence control of the artificial satellite system according to the present embodiment. A description will be made about the configuration of the artificial satellite system according to the present embodiment with reference toFIG.5.

Hereinafter, for the purpose of simplifying the description, a description will be made about an example in which the artificial satellite system includes one transmitting artificial satellite10and two receiving artificial satellites20(receiving artificial satellite20aand receiving artificial satellite20b). However, the artificial satellite system according to the present embodiment is not limited to this example and can include an arbitrary number of transmitting artificial satellites10and receiving artificial satellites20.

InFIG.5, processing50to measure the relative distance to the receiving artificial satellite20aby the transmitting artificial satellite10and processing51to measure the relative distance to the receiving artificial satellite20bby the transmitting artificial satellite10are executed in parallel.

The data processing device11of the transmitting artificial satellite10generates a digital signal for transmitting a transmission signal (i.e., radio wave31).

The D/A converter121aof the software defined radio12of the transmitting artificial satellite10generates an analog signal expressed by an equation (1), for example according to a command from the data processing device11. Note that the analog signal of the equation (1) is a common signal as an example, and the transmitting artificial satellite10may generate any other signal, for example, a signal having a certain frequency bandwidth.

In the equation (1), j indicates an imaginary unit, f1and f2indicate the frequencies of analog signals, ΔtDA1indicates a processing time associated with digital-analog conversion, ϕDA11indicates a phase difference caused due to a processing time difference in the signal of the frequency f1, and ϕDA12indicates a phase difference caused due to a processing time difference in the signal of the frequency f2. The frequency f1is included in a frequency band assigned to the receiving artificial satellite20a.The frequency f2is included in a frequency band assigned to the receiving artificial satellite20b.

The processing time ΔtDA1is a value which depends on the processing performance of the D/A converter121a.The processing time becomes a constant value for a constant signal amount unless the influence of noise is considered. Therefore, the processing time ΔtDA1is a value which can be determined by measuring it in advance as the in-circuit processing time. The phase difference ϕDA11and the phase difference ϕDA12can also be determined from the in-circuit processing time measured in advance.

The mixer124aof the software defined radio12converts the frequency of an analog signal and generates a transmission signal (i.e., radio wave31) of the transmitting artificial satellite10. The frequency-converted analog signal is represented by an equation (2).

In the equation (2), ϕ1indicates an error in the initial phase of the local oscillator127of the transmitting artificial satellite10, and fL.O.indicates the frequency of the signal generated by the local oscillator127.

The antenna13of the transmitting artificial satellite10transmits the transmission signal to the receiving artificial satellite20aand the receiving artificial satellite20b.

The antennas23of the receiving artificial satellite20aand the receiving artificial satellite20breceive the transmission signals from the transmitting artificial satellite10. The signals received by the receiving artificial satellite20aand the receiving artificial satellite20bare represented by equations (3) and (4) respectively.

In the equation (3), phase differences ϕla1and ϕla2are phase differences which occur in the process of radio waves propagating from the transmitting artificial satellite10to the receiving artificial satellite20a.In the equation (4), phase differences ϕlb1and ϕlb1are phase differences which occur in the process of radio waves propagating from the transmitting artificial satellite10to the receiving artificial satellite20b.

Both the signals represented by the equations (3) and (4) are represented by the sum of a signal of a frequency (f1+fL.O.) and a signal of a frequency (f2+fL.O.) and include a signal processed by the receiving artificial satellite20aand a signal processed by the receiving artificial satellite20b.

The phase differences included in the equations (3) and (4) depend on the distance between the artificial satellites and the frequency of the transmission signal, and are respectively represented by equations (5) and (6) using the speed of light c.

In the equation (5), lais the distance between the transmitting artificial satellite10and the receiving artificial satellite20a.In the equation (6), lbis the distance between the transmitting artificial satellite10and the receiving artificial satellite20b.

The mixers124bof the software defined radios22in the receiving artificial satellites20aand20bconvert the frequencies of the signals received by the receiving artificial satellites20aand20b.The signals (baseband signals) generated by converting the frequencies by the mixers124bof the receiving artificial satellites20aand20bare represented by equations (7) and (8) respectively.

In the equation (7), ϕ2ais an error in the initial phase of the local oscillator127of the receiving artificial satellite20a.In the equation (8), ϕ2bis an error in the initial phase of the local oscillator127of the receiving artificial satellite20b.

As described above, each of the receiving artificial satellites20aand20bis assigned with the frequency band of the signal (radio wave) to be processed. Each of the software defined radios22of the receiving artificial satellites20aand20bhas the frequency filter function which allows only the signal (radio wave) in the frequency band assigned to each of the receiving artificial satellites20aand20bto pass through. That is, using the frequency filter function of the software defined radio22, each of the receiving artificial satellites20aand20bis capable of processing only the signal in the frequency band assigned to itself, of the received signals and transmitting the signal having the same frequency and phase as the signal passed through by the frequency filter function to the first artificial satellite10.

Here, the frequency filter function of the software defined radio22will be described.

FIG.6Ais a diagram for describing the frequency filter function. An example of the distribution of strength versus frequency of the signal is shown inFIG.6A.

As shown in the left diagram ofFIG.6A, for example, the frequency band of the signal received by the receiving artificial satellite20ais assumed to be f1lto f2u. Further, the frequency band assigned to the receiving artificial satellite20ais assumed to be f1lto fc. Then, the receiving artificial satellite20auses the filter function s101to allow only the signals in the range of the frequency band of f1lto fcamong the received signals to pass therethrough as shown in the right diagram ofFIG.6A.

Assuming that the frequency band assigned to the receiving artificial satellite20bis fcto f2u, and the receiving artificial satellite20breceives the signals in the frequency band of f1lto f2u, the receiving artificial satellite20buses the filter function to allow only the signals in the range of the frequency band of fcto f2uamong the received signals to pass therethrough.

Further, the frequency filter function in each of the receiving artificial satellites20aand20bmay extract the signals to be passed by fixing the frequency band to be filtered and adjusting the frequency of the signal generated by the local oscillator127. That is, the frequency filter function can be achieved by changing the frequency to be frequency-converted (frequency to be shifted), without changing the frequency band to be filtered according to the frequency band assigned to each of the receiving artificial satellites20aand20b.

FIG.6Bis a diagram for describing the frequency filter function of adjusting the frequency of the signal generated by the local oscillator127. An example of the distribution of strength versus frequency of the signal is illustrated inFIG.6B.FIG.6Billustrates such that in a strength distribution in an upper stage and a strength distribution in a lower stage, the magnitudes of the corresponding frequencies become equal to each other. In the example illustrated inFIG.6B, to make the description easier to understand, the receiving artificial satellite20aand the receiving artificial satellite20bare configured to allow only the signals in the range of the same frequency band of f1lto fcwith each other to pass therethrough.

Assume that the frequency band to be filtered is fixed to f1lto fcin the receiving artificial satellite20a.In the receiving artificial satellite20a,as illustrated in the left and central drawings in the upper stage ofFIG.6B, a frequency conversion function s102shifts the frequency by the same amount (frequency fL.O.of the signal generated by the local oscillator127) as that in the transmitting artificial satellite10. On the other hand, in the receiving artificial satellite20b,as illustrated in the left and central drawings in the lower stage ofFIG.6B, a frequency conversion function s102shifts the frequency by a frequency f′L.O.(=fL.O.+fc−f1l). Thereafter, as illustrated in the central and right drawings in the upper stage ofFIG.6Band in the central and right drawings in the lower stage ofFIG.6B, a filter function s101is used to allow only the signals in the range of the frequency band of f1lto fcto pass through the receiving artificial satellite20aand the receiving artificial satellite20b.

Referring back to the description of the configuration of the artificial satellite system with reference toFIG.5.

As already described, the frequencies of the signals received by the receiving artificial satellites20aand20bare converted by the mixers124bof the software defined radios22of the receiving artificial satellites20aand20bto generate the baseband signals (equations (7) and (8)).

The signals transmitted to the transmitting artificial satellite10from the receiving artificial satellites20aand20bare represented by equations (9) and (10) respectively.

In the equation (9), ϕAD2aand ϕDA2aare respectively phase differences which occur accompanying processing times for analog-to-digital conversion and digital-to-analog conversion of the receiving artificial satellite20a.In the equation (10), ϕAD2band ϕDA2bare respectively phase differences which occur accompanying processing times for analog-to-digital conversion and digital-to-analog conversion of the receiving artificial satellite20b.These processing times depend on the performance of the A/D converter121band the D/A converter121aand become constant values for a constant signal amount unless the influence of noise is taken into consideration. Therefore, these processing times are values which can be determined by measuring the same in advance as the in-circuit processing times. The above phase differences can be determined from the in-circuit processing times measured in advance.

The receiving artificial satellites20aand20brespectively receive the signals represented by the equations (3) and (4) from the transmitting artificial satellite10and transmit the radio waves each having the same frequency and phase as the signals represented by the equations (3) and (4) to the transmitting artificial satellite10. The signals represented by the equations (9) and (10) are signals that the receiving artificial satellites20aand20btransmit to the transmitting artificial satellite10, and they can be considered to have the same frequency and phase as the signals represented by the equations (3) and (4).

However, the equations (9) and (10) indicate the signals passed by the receiving artificial satellites20aand20bthrough their frequency filter functions. Therefore, the signal represented by the equation (9) is a signal of a frequency (f1+fL.O.), and the signal represented by the equation (10) is a signal of a frequency (f2+fL.O.). Further, even if the signal received by each of the receiving artificial satellites20aand20band the signal transmitted therefrom are intended to make the phases the same value, the phase difference associated with the analog-to-digital conversion and digital-to-analog conversion actually occurs. Therefore, the equations (9) and (10) indicate the signals each having the phase having considered the phase difference (ϕAD2a+ϕDA2ain the equation (9), and ϕAD2b+ϕDA2bin the equation (10)).

The signal received by the transmitting artificial satellite10is represented by an equation (11).

The mixer124bof the software defined radio12of the transmitting artificial satellite10converts the frequency of the signal represented by the equation (11) to obtain a signal represented by an equation (12).

The data processing device11of the transmitting artificial satellite10measures the signal represented by the equation (12) as a signal represented by an equation (13).

Here, ϕAD11and ϕAD12are phase differences each of which occurs accompanying the processing time for the analog-to-digital conversion of the transmitting artificial satellite10. The processing time depends on the performance of the A/D converter121band becomes a constant value for a constant signal amount unless the influence of noise is taken into consideration. Therefore, the processing time can be determined by measuring the same in advance as the in-circuit processing time. The phase differences ϕAD11and ϕAD12can also be determined from the in-circuit processing time measured in advance.

It is understood that as shown in the equation (13), the signal received by the transmitting artificial satellite10has information about a phase difference ϕla1corresponding to the distance between the transmitting artificial satellite10and the receiving artificial satellite20aand a phase difference ϕlb2corresponding to the distance between the transmitting artificial satellite10and the receiving artificial satellite20b.

The data processing device11of the transmitting artificial satellite10performs frequency analysis of a transmission signal and a reception signal. The data processing device11calculates the approximate value of the relative distance between the transmitting artificial satellite10and each of the receiving artificial satellites20aand20bfrom the difference between the transmission time of the transmission signal (radio wave31) and the reception time of the reception signal (radio wave32) according to an equation (14).

In the equation (14), lais the relative distance between the transmitting artificial satellite10and the receiving artificial satellite20a,and lbis the relative distance between the transmitting artificial satellite10and the receiving artificial satellite20b.Further, ΔTaindicates the time difference between the time (transmission time) at which the transmitting artificial satellite10transmits the signal, and the time (reception time) at which the transmitting artificial satellite10receives the signal transmitted by the receiving artificial satellite20a.ΔTbindicates the time difference between the time at which the transmitting artificial satellite10transmits the signal and the time at which the transmitting artificial satellite10receives the signal transmitted by the receiving artificial satellite20b.

The data processing device11calculates the cross-power spectrum between the transmission signal and the reception signal and calculates a phase difference ϕ of each frequency component. The data processing device11corrects the calculated phase difference ϕ as in an equation (15) using the phase difference determined by measurement in advance, which is caused by the processing of the analog-to-digital conversion and the digital-to-analog conversion of the transmitting artificial satellite10.

In the equation (15), ϕcais the corrected phase difference for the transmitting artificial satellite10and the receiving artificial satellite20a,and99cbis the corrected phase difference for the transmitting artificial satellite10and the receiving artificial satellite20b.

The data processing device11corrects the approximate value (equation (14)) of the relative distance between the transmitting artificial satellite10and each of the receiving artificial satellites20aand20bas in an equation (16) using the phase difference corrected like the equation (15), to thereby determine a relative distance labetween the transmitting artificial satellite10and the receiving artificial satellite20aand a relative distance1bbetween the transmitting artificial satellite10and the receiving artificial satellite20b.

In the equation (16), quotient represents the quotient of division.

FIG.7is a diagram showing an example of a flowchart of a method for measuring the distance between the artificial satellites, according to the present embodiment. The flowchart shown inFIG.7is executed by the transmitting artificial satellite10which measures the relative distance to the receiving artificial satellite20.

An example of a procedure for the method for measuring the distance between the artificial satellites will be described usingFIG.7. The method for measuring the distance between the artificial satellites, according to the present embodiment may be performed by other procedures and methods without being not limited to the procedure and method shown in the flowchart ofFIG.7.

In processing step s103, an operator or the data processing device11of the transmitting artificial satellite10determines an assumed distance for the relative distance between the artificial satellites desired to be measured. In the present embodiment, the assumed distance is an approximate distance assumed as the relative distance between the transmitting artificial satellite10and the receiving artificial satellite20. The assumed distance may be determined by, for example, operation conditions of the artificial satellite or may be determined from conditions for placing each artificial satellite into the orbit.

In processing step s104, the operator or the data processing device11determines the frequency of a radio wave31transmitted by the transmitting artificial satellite10. This frequency is preferably determined based on the assumed distance determined in processing step s103. For example, it is desirable to determine the frequency of the radio wave31so that the radio wave31has a wavelength longer than the assumed distance.

In processing step s105, the operator or the data processing device11determines the gain of each of the amplifiers123a,123b,125a,and125bof the transmitting artificial satellite10and the receiving artificial satellite20. The gain is preferably determined based on the assumed distance determined in processing step s103. For example, since the radio wave is reduced in strength due to its propagation, it is desirable that the gain is larger as the assumed distance determined in processing step s103is longer.

In processing step s106, the transmitting artificial satellite10generates a transmission signal (signal of radio wave31) by the data processing device11and transmits the radio wave31through the antenna13a.

In processing step s107, the data processing device11performs processing of fast Fourier Transform (FFT) to the transmission signal. With the processing of fast Fourier Transform, the data processing device11can perform frequency analysis of the signal (transmission signal) of the transmitted radio wave31.

In processing step s108, the data processing device11determines whether or not the radio wave32(reception signal) from the receiving artificial satellite20have been received, that is, whether the reception signal is present or absent. When it is determined that the reception signal is present, processing in processing step s109is executed. Note that the transmitting artificial satellite10is able to receive the radio waves32from the multiple receiving artificial satellites20.

In processing step s109, the data processing device11performs processing of fast Fourier Transform (FFT) to the reception signal. With the processing of fast Fourier Transform, the data processing device11performs frequency analysis of the signal (reception signal) of the received radio wave32, and can determine whether the reception signal is a signal transmitted from any artificial satellite.

In processing step s110, the data processing device11calculates the cross-power spectrum between the reception signal and the transmission signal. The data processing device11calculates the cross-power spectrum by multiplying those obtained by making a result of the fast Fourier Transform processing of the transmission signal conjugated with a result of the fast Fourier Transform processing of the reception signal.

In processing step s111, the data processing device11determines the receiving artificial satellite20which measures the relative distance. The data processing device11can determine, for example, a predetermined receiving artificial satellite20or a receiving artificial satellite20commanded from outside among the multiple receiving artificial satellites20as the receiving artificial satellite20which measures the relative distance.

In processing step s112, the data processing device11determines the phase difference between the transmission signal (radio wave31) and the reception signal (radio wave32) from the cross-power spectrum calculated in processing step s110in terms of the radio waves in the frequency band assigned to the receiving artificial satellite20determined in processing step s111. As a result, the transmitting artificial satellite10is capable of acquiring the phase difference between the radio wave32received from the receiving artificial satellite20that measures the relative distance and the transmitted radio wave31.

In processing step s113, the data processing device11subtracts the correction value (value obtained by converting the in-circuit processing time of each of the transmitting artificial satellite10and the receiving artificial satellite20into the phase) from the phase difference obtained in processing step s112, to thereby correct the phase difference. With the correction of the phase difference, the data processing device11can correct the processing time difference generated in the transmitting artificial satellite10and the receiving artificial satellite20. Note that the data processing device11may update the correction value during operation by installing in the inside thereof a feedback circuit (for example, see a fourth embodiment).

In processing step s114, the data processing device11uses the corrected phase difference and the difference between the transmission time of the transmission signal (radio wave31) and the reception time of the reception signal (radio wave32) to calculate the relative distance between the transmitting artificial satellite10and the receiving artificial satellite20.

The artificial satellite system and the method for measuring the distance between the artificial satellites according to the present embodiment include the configurations described above and can measure the relative distances among the multiple artificial satellites without time synchronization. Further, the artificial satellite system and the method for measuring the distance between the artificial satellites according to the present embodiment can simultaneously measure the relative distances between one artificial satellite and multiple artificial satellites.

Second Embodiment

A description will be made about an artificial satellite system and a method for measuring the distance between artificial satellites, according to a second embodiment of the present invention. Regarding the present embodiment, a description will hereinafter be made mainly about points different from the first embodiment.

In the present embodiment, a transmitting artificial satellite10adjusts speed and position of the artificial satellite using a frequency shift (Doppler shift) of a reception signal (radio wave32) for a transmission signal (radio wave31), which is caused by the Doppler effect due to a relative speed between the transmitting artificial satellite10and a receiving artificial satellite20. In the present embodiment, the speed of the artificial satellite is adjusted, thereby to make it possible to keep the relative distance between the artificial satellites and make the relative position between the artificial satellites constant.

FIG.8is a diagram showing an example of a flowchart of sequence control to control the speed of the artificial satellite. The flowchart shown inFIG.8is executed by the transmitting artificial satellite10. Processing step s103to processing step s109inFIG.8are the same as those in the first embodiment.

In processing step s115, the data processing device11of the transmitting artificial satellite10compares the results of frequency analysis by fast Fourier Transform (FFT) processing of the transmission signal and the reception signal to determine whether there is a frequency shift of the reception signal for the transmission signal. When the frequency shift is present, the data processing device11determines a frequency shift amount and executes processing in processing step s116.

In processing step s116, the data processing device11calculates a relative speed v of the receiving artificial satellite20to the transmitting artificial satellite10from the frequency shift amount determined in processing step s115using an equation (17).

In the equation (17), ft1and ft2each represent the frequency of the signal (transmission signal) which is transmitted by the transmitting artificial satellite10, and fr1and fr2each represent the frequency of the signal (reception signal) which is received by the transmitting artificial satellite10.

In processing step s117, the data processing device11adjusts the speed and attitude of the transmitting artificial satellite10so that the relative speed v determined in processing step s116becomes small. The data processing device11is capable of adjusting the speed and attitude of the transmitting artificial satellite10using the propulsive mechanism17(FIG.2) such as the thruster or the electric propulsion device. The data processing device11is capable of adjusting the position of the transmitting artificial satellite10by adjusting the speed and attitude of the transmitting artificial satellite10.

In the present embodiment, the speed of the artificial satellite can be adjusted to keep the relative distance and position between the artificial satellites constant, and the shape of the distribution of multiple artificial satellites is maintained.

Third Embodiment

A description will be made about an artificial satellite system and a method for measuring the distance between artificial satellites, according to a third embodiment of the present invention. Regarding the present embodiment, a description will hereinafter be made mainly about points different from the first embodiment.

In the present embodiment, the relative position (relative distance) between the transmitting artificial satellite10and the ground station3is measured to thereby measure the absolute position of the artificial satellite based on the position of the ground station3.

FIG.9is a diagram showing an example of a configuration for measuring the absolute position of the artificial satellite in the present embodiment. The transmitting artificial satellite10includes a plurality of antennas13(FIG.2) each of which receives a reception signal (radio wave32), and measures the relative distances to a plurality of receiving artificial satellites20by using the data processing device11. The ground station3is installed on the ground. The transmitting artificial satellite10is capable of receiving radio waves transmitted from the ground station3by the multiple antennas13and measuring the relative distances to the ground station3by using the data processing device11.

The data processing device11of the transmitting artificial satellite10determines the relative positions between the transmitting artificial satellite10and the multiple receiving artificial satellites20in accordance with the principle of triangulation using the relative distances between the transmitting artificial satellite10and the multiple receiving artificial satellites20. Further, the data processing device11determines the relative position between the transmitting artificial satellite10and the ground station3in accordance with the principle of triangulation using the plural relative distances between the transmitting artificial satellite10and the ground station3, which have been obtained by the plural antennas13.

The ground station3is installed on the ground and is capable of measuring the absolute position. The data processing device11of the transmitting artificial satellite10can acquire the absolute position of the ground station3and determine the absolute position of the transmitting artificial satellite10from the relative position between the transmitting artificial satellite10and the ground station3, on the basis of the absolute position. Then, the data processing device11is capable of determining the absolute position of the receiving artificial satellite20from the absolute position of the transmitting artificial satellite10and the relative position between the transmitting artificial satellite10and each receiving artificial satellite20.

In the present embodiment, the absolute positions of the transmitting artificial satellite10and the multiple receiving artificial satellites20can be measured in the manner described above.

Fourth Embodiment

A description will be made about an artificial satellite system and a method for measuring a distance between artificial satellites, according to a fourth embodiment of the present invention. Regarding the present embodiment, a description will hereinafter be made mainly about points different from the first embodiment.

In the present embodiment, software defined radios12and22of a transmitting artificial satellite10and a receiving artificial satellite20each include a feedback circuit and correct an error (ϕ1, ϕ2a, ϕ2b) in the initial phase of the local oscillator127using a referenced signal.

FIG.10is a block diagram showing an example of a schematic configuration of the software defined radio12in the present embodiment. The software defined radio12of the transmitting artificial satellite10includes feedback circuits401aand401b.Although not shown in the drawing, the software defined radio22of the receiving artificial satellite20also include similar feedback circuits. A description will hereinafter be made about the transmitting artificial satellite10.

In the present embodiment, a local oscillator127aused at the time of signal transmission, and a local oscillator127bused at the time of signal reception are assumed to be different from each other in the software defined radio12. The local oscillator127ais used to convert the frequency of an analog signal when transmitting a signal by the antenna13a.The local oscillator127bis used to convert the frequency of an analog signal received by the antenna13b.

The feedback circuit401aincludes a D/A converter402aand a mixer403a.The feedback circuit401bincludes an A/D converter402band a mixer403b.

The data processing device11transmits a digital signal (reference signal R) to the feedback circuit401a.The reference signal R is a reference signal for measuring and correcting the phase difference between the local oscillators127aand127b.Any signal can be used for the reference signal R.

The feedback circuit401aconverts the digital signal (reference signal R) received from the data processing device11into an analog signal by the D/A converter402a.Then, the feedback circuit401aperforms frequency conversion on the analog signal using the signal from the local oscillator127ain the mixer403aand transmits the reference signal R subjected to the frequency conversion to the feedback circuit401b.

The feedback circuit401breceives the reference signal R from the feedback circuit401aand performs frequency conversion on the reference signal R using the signal from the local oscillator127bin the mixer403b.Then, the feedback circuit401bconverts the reference signal R into a digital signal by the A/D converter402b.

The data processing device11receives the reference signal R from the feedback circuit401b.Then, the data processing device11determines the phase difference between the digital signal transmitted to the feedback circuit401aand the digital signal received from the feedback circuit401b,that is, the difference between the phase of the transmitted reference signal R and the phase of the received reference signal R. This phase difference corresponds to the phase difference between the two local oscillators127aand127b,and it becomes the factor of an error when measuring the relative distance between the artificial satellites.

Therefore, the data processing device11determines the phase difference using the reference signal R and subtracts the phase difference (the phase difference between the transmitted reference signal R and the received reference signal R) from the phase obtained from a reception signal S which is received by the data processing device11, to thereby correct the phase difference between the local oscillators127aand127b.

In the present embodiment, when transmitting the signal and receiving the signal, the same local oscillator are not used and different local oscillators are used, it is possible to correct the phase difference between the two local oscillators by including the feedback circuits401aand401bto use the reference signal R.

Fifth Embodiment

A description will be made about an artificial satellite system and a method for measuring the distance between artificial satellites, according to a fifth embodiment of the present invention. Regarding the present embodiment, a description will hereinafter be made mainly about points different from the first embodiment.

In the present embodiment, the shape (shape of distribution of multiple artificial satellites) of an artificial satellite group configured of a plurality of artificial satellites is estimated.

FIG.11is a diagram showing an example of an artificial satellite group comprised of a plurality of artificial satellites. To make the description easier to understand,FIG.11shows an artificial satellite group comprised of three artificial satellites1a,1b,and1cas an example. However, the artificial satellite group can have two or four or more artificial satellites.

Each of the artificial satellites1a,1b,and1chas the same configuration as the transmitting artificial satellite10(that is, it has the same configuration as the receiving artificial satellite20) and can switch between the operation as the transmitting artificial satellite10and the operation as the receiving artificial satellite20. In the present embodiment, first, the artificial satellite1ais used as a reference, and the artificial satellite1ameasures the resistance distance to each of the artificial satellites1band1c.Next, the artificial satellite1cis used as a reference, and the artificial satellite1cmeasures the relative distance to the artificial satellite1b.

From these measurement results, the positions of the artificial satellites1a,1b,and1care represented by an equation (18) in an orthogonal coordinate system arbitrarily determined in advance.

In the equation (18), (xa, ya, za) represents the position of the artificial satellite1a,(xb, yb, zb) represents the position of the artificial satellite1b,and (xc, yc, zc) represents the position of the artificial satellite1c.Further, Rabrepresents the relative distance between the artificial satellite1aand the artificial satellite1b,Racrepresents the relative distance between the artificial satellite1aand the artificial satellite1c,and Rcbrepresents the relative distance between the artificial satellite1cand the artificial satellite1b.

Next, assuming that in order to estimate the shape of the artificial satellite group, the artificial satellite1ais used as a reference point, and the line connecting the artificial satellite1aand the artificial satellite1cis a reference line k103, the positions of the artificial satellite1aand the artificial satellite1care represented by an equation (19).

Using the equation (18) and the equation (19), it can be seen that the position of the artificial satellite1bfollows an equation (20).

It can be seen from the equation (20) that the artificial satellite1bis positioned on a circle k102of a radius R. Note thatFIG.11is drawn as a circle (ellipse) seen from the diagonal direction of the circle k102.

Further, it can be seen that the shape of the artificial satellite group comprised of the artificial satellites1a,1b,and1cis a specific triangle whose lengths of three sides are relative distances Rab, Rac, and Rcb.

In the way described above, the data processing devices11of the artificial satellites1a,1b,and1cdetermine the mutual relative distances Rab, Rac, and Rcbin the artificial satellites1a,1b,and1cand determine the positions of the artificial satellites1a,1b,and1cfrom these relative distances, thereby making it possible to estimate the shape of the artificial satellite group.

Incidentally, when the arrival angle of the reception signal (radio wave32) and the like are also measured by increasing the number of antennas for the artificial satellites1ato1c,etc., the data processing device11can determine the shape of the artificial satellite group uniquely.

It should be noted that the present invention is not limited to the foregoing embodiments, and the foregoing embodiments may be variously modified. The foregoing embodiments have been described in detail, for example, in order to facilitate the understanding of the present invention. The present invention is not limited to embodiments including all the above-described elements. Some elements of an embodiment may be replaced by the elements of another embodiment. Further, elements of an embodiment may be added to another embodiment. Furthermore, some elements of each embodiment may be deleted, subjected to the addition of other elements, or replaced by other elements.

REFERENCE SIGNS LIST