Patent ID: 12234041

DESCRIPTION OF EMBODIMENTS

In embodiments and the drawings, the same or corresponding elements are designated by the same reference numerals. Descriptions of the elements designated by the same reference numerals as the described elements will be omitted or simplified as appropriate.

First Embodiment

A satellite constellation200will be described with reference toFIGS.1to12.

A configuration of a monitoring system100will be described with reference toFIG.1.

The monitoring system100is a system for monitoring a target area of the Earth, and includes the satellite constellation200and a ground facility300.

“Monitoring” may be read as “observing”.

The satellite constellation200is constituted of three or more artificial satellites210.

In the present embodiment, the satellite constellation200is a three-in constellation constituted of three artificial satellites (210A to210C).

However, the satellite constellation200may be constituted of four or more artificial satellites210.

The three artificial satellites (210A to210C) cooperate with each other to monitor the target area of the Earth.

The ground facility300includes a satellite control device310and a satellite communication device320, and controls the satellite constellation200by communicating with each artificial satellite210.

The satellite control device310is a computer that generates each type of command for controlling each artificial satellite (210A to210C), and the satellite control device310includes pieces of hardware such as processing circuitry and an input and output interface. The processing circuitry generates each type of command. An input device and an output device are connected to the input and output interface. The satellite control device310is connected to the satellite communication device320via the input and output interface.

The satellite communication device320communicates with each artificial satellite (210A to210C). Specifically, the satellite communication device320transmits each type of command to each artificial satellite (210A to210C). Further, the satellite communication device320receives monitor data transmitted from each artificial satellite (210A to210C).

A configuration of the artificial satellite210will be described with reference toFIG.2. Each artificial satellite (210A to210B) is constituted as follows.

The artificial satellite210includes a monitoring device211, a monitoring control device212, a communication device213, a propulsion device214, an attitude control device215, and a power supply device216.

The monitoring device211is a device for monitoring the target area of the Earth. For example, the monitoring device211is a visible optical sensor, an infrared optical sensor, or a synthetic aperture radar (SAR). The monitoring device211generates the monitor data. The monitor data is data equivalent to an image showing the target area of the Earth.

The monitoring control device212is a computer that controls the monitoring device211, the propulsion device214, and the attitude control device215, and the monitoring control device212includes the processing circuitry. Specifically, the monitoring control device212controls the monitoring control device212, the propulsion device214, and the attitude control device215according to each type of command transmitted from the ground facility300.

The communication device213is a device that communicates with the ground facility300. Specifically, the communication device213transmits the monitor data to the ground facility300. Further, the communication device213receives each type of command which is transmitted from the ground facility300.

The propulsion device214is a device that provides a propulsive force to the artificial satellite210, and the propulsion device214changes speed of the artificial satellite210. Specifically, the propulsion device214is an electric propulsion machine. For example, the propulsion device214is an ion engine or a Hall thruster.

The attitude control device215is a device for controlling attitude elements such as attitude of the artificial satellite210, angular velocity of the artificial satellite210, and a line-of-sight direction (Line Of Sight) of the monitoring device211. The attitude control device215changes each attitude element in a desired direction.

Alternatively, the attitude control device215maintains each attitude element in the desired direction. The attitude control device215includes an attitude sensor, an actuator, and a controller. The attitude sensor is a gyroscope, an Earth sensor, a sun sensor, a star tracker, a thruster, a magnetic sensor, or the like. The actuator is an attitude control thruster, a momentum wheel, a reaction wheel, a control moment gyro, or the like. The controller controls the actuator according to measurement data of the attitude sensor or each type of command from the ground facility300.

The power supply device216includes a solar cell, a battery, a power control device, and the like, and supplies power to each equipment installed on the artificial satellite210.

The processing circuitry included in each of the satellite control device310and the monitoring control device212will be described.

The processing circuitry may be dedicated hardware or a processor that executes a program stored in a memory.

In the processing circuitry, a part of functions may be realized by dedicated hardware, and the remaining functions may be realized by software or firmware. That is, the processing circuitry can be realized by hardware, software, firmware, or a combination of these.

Dedicated hardware is, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC, an FPGA, or a combination of these.

ASIC is an abbreviation for Application Specific Integrated Circuit.

FPGA is an abbreviation for Field Programmable Gate Array.

A pointing function of the artificial satellite210will be described.

The artificial satellite210includes a pointing function for directing a monitoring direction to the target area.

For example, the artificial satellite210includes a reaction wheel. The reaction wheel is a device for controlling the attitude of the artificial satellite210. Body pointing is realized by controlling the attitude of the artificial satellite210by the reaction wheel.

For example, the monitoring device211includes a pointing mechanism. The pointing mechanism is a mechanism for changing the line-of-sight direction of the monitoring device211. For example, a drive mirror or the like is used as the pointing mechanism.

A monitoring function of the monitoring device211will be described.

The monitoring device211has a resolution variable function and an autofocus function.

The resolution variable function is a function for changing resolution of the monitor data.

The autofocus function is a function for setting a focal point to the monitoring target.

The satellite constellation200will be described with reference toFIGS.3to7.

FIG.3illustrates the satellite constellation200when viewed in a normal line direction of an orbit surface103.

FIG.4illustrates the satellite constellation200when viewed from the orbit surface103. For example,FIG.4illustrates the satellite constellation200when viewed from the sky above the equator.

FIGS.5,6and7illustrate a long axis of an elliptical orbit of each artificial satellite (210A to210C) circling centering on Earth101along the orbit surface103. The orbit surface103is a surface along which the elliptical orbit of each artificial satellite (210A to210C) is arranged.

Each artificial satellite (210A-210C) circulates on a sun-synchronous elliptical orbit. Each elliptical orbit has a high eccentricity and an orbit inclination angle. That is, the orbit of each artificial satellite (210A to210C) is a sun-synchronous orbit, also an inclined orbit, and also the elliptical orbit.

Monitoring of the Northern Hemisphere during a daytime is maintained by the three artificial satellites (210A-210C).

The elliptical orbit of each artificial satellite (210A to210C) is a non-frozen orbit.

That is, the elliptical orbit of each artificial satellite (210A to210C) is not a frozen orbit, and the long axis of each elliptical orbit circles centering on the Earth101inside the orbit surface103over time.

The three artificial satellites (210A to210C) alternately monitor the target area of the Earth101from a perigee, an apogee, or a midpoint. The midpoint is a point located between the perigee and the apogee.

At the perigee, it is possible to monitor with high resolution but for a short time.

At the apogee, it is possible to monitor for a long time but with low resolution.

Each long axis of three elliptical orbits is inclined at equal intervals of about 120° in a circumferential direction of the orbit surface. An azimuth direction is equivalent to a longitude direction, that is, an east-west direction.

The long axis of each elliptical orbit circles based on the Earth101, but a relative relationship between the three elliptical orbits is maintained.

The normal line direction of the orbit surface103is maintained.

For this reason, a sun incident angle is maintained relative to each artificial satellite (210A to210C).

At 12:00 pm, a phase of each artificial satellite (210A-210C) is not correlated with the latitude of the target area of the Earth101. A phase of the artificial satellite210is equivalent to a position of the artificial satellite210on the orbit of the artificial satellite210.

One of the three artificial satellites (210A to210C) can monitor the target area of the Earth101.

Therefore, it is possible to monitor the target area almost continuously.

When the artificial satellite210passes in the sky above the target area of the Earth101on an apogee side of the elliptical orbit, the artificial satellite210monitors the target area of the Earth101for a long time but with low resolution.

When the artificial satellite210passes in the sky above the target area of the Earth101on a perigee side of the elliptical orbit, the artificial satellite210monitors the target area of the Earth101with high resolution but for a short time.

A specific example of the elliptical orbit of each artificial satellite (210A to210C) is as follows. However, the following values are approximate values.

An altitude of the circular orbit, which is a basis of the elliptical orbit, is 5,100 kilometers.

An eccentricity of the elliptical orbit is 0.418.

An orbit inclination angle is 122 degrees.

An apogee altitude is 9,898 kilometers.

A perigee altitude is 302 kilometers.

The relationship between the altitude and the latitude of the elliptical orbit of each artificial satellite (210A to210C) will be described with reference toFIG.8.

A dotted line represents the elliptical orbit of the artificial satellite210A. An alternate long and short dash line represents the elliptical orbit of the artificial satellite210B. A broken line represents the elliptical orbit of the artificial satellite210C.

“Ha” is the perigee altitude, and “Hc” is an all-time-perigee usage altitude. The all-time-perigee usage altitude is an altitude at which the target area can be monitored from the perigee side by at least one of the three artificial satellites (210A to210C).

“Hb” is the apogee altitude, and “Hd” is the always-apogee usage altitude. The always-apogee usage altitude is an altitude at which the target area can be monitored from the apogee side by at least one of the three artificial satellites (210A to210C).

Each usage altitude (Hd, Hc) is equivalent to an altitude of an intersection of two elliptical orbits in a graph ofFIG.8.

A length of time that the artificial satellite210stays in the sky above the target area is referred to as staying time of the artificial satellite210.

On the apogee side, the staying time of each artificial satellite (210A to210C) is long, and a field of view of each artificial satellite (210A to210C) is wide.

The satellite control device310sets each of the usage altitude Hb and a viewing angle of the monitoring device211so that the target area fits within the field of view while each artificial satellite (210A to210C) flies at an altitude higher than the usage altitude Hd. Consequently, always monitoring the target area is possible.

Since a passage time of each artificial satellite (210A to210C) is short on the perigee side, monitoring from the perigee side does not have all-time characteristics. However, no matter what latitude the target area is at, at least one of the artificial satellites (210A to210C) can monitor the target area from an altitude lower than the usage altitude Hc.

The satellite control device310sets resolution of the monitoring device211so that a desired resolution can be achieved at the usage altitude Hc. Consequently, monitoring the target area with high resolution is possible.

Adjustments of a satellite altitude and the orbit inclination angle will be described with reference toFIGS.9and10.

The long axis of the elliptical orbit of the artificial satellite210is correlated with the satellite altitude. For this reason, by finely adjusting the altitude of each artificial satellite (210A to210C), a relative angle of the elliptical orbit when viewed from the normal line direction of the orbit surface can be maintained. A condition of the satellite altitude for maintaining the relative angle of the elliptical orbit is referred to as an “altitude condition”.

Sun-synchronization of the elliptical orbit is realized by a correlation between the satellite altitude and the orbit inclination angle. For this reason, by finely adjusting the orbit inclination angle of each artificial satellite (210A to210C), the sun-synchronization of the elliptical orbit can be maintained. A condition of the orbit inclination angle for maintaining the sun-synchronization is referred to as an “inclination angle condition”.

Therefore, by realizing both the altitude condition and the inclination angle condition, it is possible to run the satellite constellation200while the relative angle of the elliptical orbit is maintained and while the sun-synchronization of the elliptical orbit is maintained.

The satellite control device310generates a command for adjusting the altitude of each artificial satellite (210A to210C). Further, the satellite control device310generates a command for adjusting the orbit inclination angle of each artificial satellite (210A to210C). Then, the satellite communication device320transmits these commands to each artificial satellite (210A to210C).

For each artificial satellite (210A to210C), the monitoring control device212adjusts the satellite altitude and the orbit inclination angle according to these commands. Specifically, the monitoring control device212controls the propulsion device214according to these commands. The propulsion device214can adjust the satellite altitude and the orbit inclination angle by changing satellite speed.

InFIG.9, a black circle depicted inside the Earth101represents the North Pole.

When flight speed of the artificial satellite210accelerates, the altitude of the artificial satellite210rises. Then, when the altitude of the artificial satellite210rises, ground speed of the artificial satellite210decelerates.

When the flight speed of the artificial satellite210decelerates, the altitude of the artificial satellite210lowers. Then, when the altitude of the artificial satellite210lowers, the ground speed of the artificial satellite210accelerates.

As illustrated inFIG.10, if the propulsion device214generates thrust in a direction orthogonal to the orbit surface at a point (equinox) where the artificial satellite210crosses in the sky above the equator, the orbit inclination angle can be finely adjusted effectively.

With rotation of the Earth101, the target area moves independently of the orbit surface of each artificial satellite (210A to210C). Further, each artificial satellite (210A to210C) flies on the elliptical orbit regardless of movement of the target area.

For this reason, even if both the altitude condition and the inclination angle condition are satisfied, the three artificial satellites (210A to210C) are not necessarily able to cooperate with each other in a timely manner to constantly monitor the target area.

By accelerating or decelerating each artificial satellite (210A to210C) in the orbit surface, the three artificial satellites (210A to210C) can cooperate with each other in a timely manner to constantly monitor the target area.

Then, the satellite control device310generates a command for accelerating or decelerating each artificial satellite (210A to210C) in the orbit surface. Then, the satellite communication device320transmits the generated command to each artificial satellite (210A to210C).

After that, the satellite control device310generates the commands for adjusting the satellite altitude and the orbit inclination angle of each artificial satellite (210A to210C). Then, the satellite communication device320transmits the generated commands to each artificial satellite (210A to210C).

Consequently, optimally adjusting a monitoring condition in a short term and maintaining the relative relationship of the elliptical orbit of each artificial satellite (210A to210C) in a long term are possible.

A position of the target area and a position of each artificial satellite (210A to210C) can be managed by using a common coordinate system. Then, by using the common coordinate system, each artificial satellite (210A to210C) can be controlled according to the position of the target area.

A specific example of the common coordinate system is an Earth fixed coordinate system. The Earth fixed coordinate system is a coordinate system adopted by the Quasi-Zenith Positioning Satellite of Japan and the GPS of the United States.

GPS is an abbreviation for Global Positioning System.

The satellite control device310can calculate an optimum pointing condition for orienting the direction to the target area in consideration of a satellite attitude condition in an inertial space.

The satellite control device310generates a command indicating the optimum pointing condition for each artificial satellite (210A to210C). Then, the satellite communication device320transmits the generated command to each artificial satellite (210A to210C).

The monitoring control device212controls the pointing function of the artificial satellite210according to the command from the ground facility300.

The monitoring control device212may control the attitude control device215, or control the pointing mechanism of the monitoring device211.

For monitoring the target area, it is effective to shorten a relative distance between the target area and each artificial satellite (210A to210C). Further, it is effective to image under a condition in which a solar altitude is high, that is, to image under a condition in which there is bright.

Hence, the satellite control device310generates a command for adjusting a flight position of each artificial satellite (210A to210C). Then, the satellite communication device320transmits the generated command to each artificial satellite (210A to210C).

After that, the satellite control device310generates commands for adjusting the satellite altitude and the orbit inclination angle of each artificial satellite (210A to210C). Then, the satellite communication device320transmits the generated commands to each artificial satellite (210A to210C).

Consequently, optimally adjusting the monitoring condition in a short term and maintaining the relative relationship of the elliptical orbit of each artificial satellite (210A to210C) in a long term are possible.

The sun-synchronous orbit will be described with reference toFIG.11. A black circle depicted in the Earth101represents the North Pole. A line depicted through a center of the Earth101represents the equator.

The sun-synchronous orbit is an orbit in which the sun incident angle is maintained. That is, when the orbit of the artificial satellite210is the sun-synchronous orbit, the sun incident angle relative to the orbit surface of the artificial satellite210does not change throughout a year.

A sun-synchronous circular orbit will be described with reference toFIG.12.

The sun-synchronous circular orbit is a sun-synchronous orbit, also a circular orbit, and also an inclined orbit.

In the sun-synchronous circular orbit, an adoptable range of the orbit altitude is from about 500 kilometers to about 5,700 kilometers. At about 500 kilometers or lower, since effect of atmospheric resistance cannot be ignored, the sun-synchronization cannot be maintained. At about 5,700 kilometers or higher, since the circling of the orbit surface due to effect of an Earth ellipsoid reaches a limit, the sun-synchronization cannot be maintained.

FIG.12illustrates attribute values of the circular orbit at which the number of circulations of the artificial satellite210in a day is an integer. Each attribute value is an approximate value. That is, each numerical value indicated inFIG.12is an approximate number including a rounding error.

The attribute values of the circular orbit illustrated inFIG.12are examples of the attribute values for realizing the sun-synchronization. The number of circulations of the artificial satellite210in a day is not necessary an integer, and there exist many attribute values of the circular orbit for satisfying the sun-synchronization.

The eccentricity of the circular orbit is zero, and when the eccentricity is changed, the circular orbit becomes the elliptical orbit.

The sun-synchronous elliptical orbit largely depends on an orbit long axis length. Specifically, twice a radius of the sun-synchronous circular orbit is a rough standard for the orbit long axis length.

The radius of the sun-synchronous circular orbit, that is, a distance from the center of the Earth to the sun-synchronous circular orbit, is calculated by adding the radius of the Earth to an altitude-from-ground-surface of the circular orbit. The radius of the Earth is about 6,378 kilometers.

The radius of the sun-synchronous circular orbit required for making the artificial satellite210circulate seven rounds in a day is about 11,522 kilometers. This radius is calculated by adding the radius (about 6,378 kilometers) of the Earth to the altitude-from-ground-surface (about 5,144 kilometers) of the circular orbit. Therefore, the long axis length of the sun-synchronous elliptical orbit required for making the artificial satellite210circulate seven rounds in a day is about 23,000 kilometers. This long axis length is calculated by doubling the radius (about 11,522 kilometers) of the sun-synchronous circular orbit.

The long axis length of the elliptical orbit depends on the apogee altitude and the perigee altitude. A ratio of the apogee altitude and the perigee altitude can be anything. For example, when the apogee altitude is about 10,000 kilometers and the perigee altitude is about 300 kilometers, the long axis length of the elliptical orbit is the above length (about 23,000 kilometers).

That is, by changing the eccentricity and finely adjusting parameters such as the apogee altitude and the perigee altitude, it is possible to find the sun-synchronous elliptical orbit required for making the artificial satellite210circulate seven rounds in a day.

The sun-synchronous elliptical orbit for seven rounds has the orbit inclination angle of about 140 degrees. That is, the apogee and the perigee are located in the sky at a latitude of plus or minus 40 degrees (=180-140). For this reason, this elliptical orbit is suitable for monitoring Japan located at a latitude of about 40 degrees.

Similarly, for the number of circulations other than the seven rounds, the sun-synchronous elliptical orbit can be obtained.

That is, by changing the eccentricity and finely adjusting the parameters such as the apogee altitude and the perigee altitude, it is possible to find the sun-synchronous elliptical orbit for any number of circulations.

Characteristics of First Embodiment

Main characteristics of the first embodiment will be described.

The satellite constellation200includes three or more artificial satellites210that monitor the target area of the Earth.

Each of the three or more artificial satellites210circulates on the elliptical orbit having the sun-synchronization and the orbit inclination angle.

The long axis of each elliptical orbit is tilted at equal intervals of about 120° along a circumferential direction of the orbit surface. That is, the long axis of each elliptical orbit forms an equal angle with each long axis of two adjacent elliptical orbits on the orbit surface.

The ground facility300includes the satellite control device310and the satellite communication device320, and controls the satellite constellation200.

The satellite control device310generates adjustment commands for each artificial satellite210of the satellite constellation200. The adjustment commands are commands for adjusting the altitude of the artificial satellite210and the orbit inclination angle of the elliptical orbit of the artificial satellite210.

The satellite communication device320transmits the generated adjustment commands to the artificial satellite210for each artificial satellite210of the satellite constellation200.

For each artificial satellite210of the satellite constellation200, the altitude of the artificial satellite210and the orbit inclination angle of the elliptical orbit of the artificial satellite210are adjusted according to the adjustment commands. Consequently, the sun-synchronization of the elliptical orbit of each artificial satellite210is maintained, and also, relative angles between the long axis of the elliptical orbit of each artificial satellite210and the long axes of the elliptical orbits of the other artificial satellites210are maintained.

The satellite control device310generates a control command for controlling the propulsion device of the artificial satellite210for each artificial satellite210of the satellite constellation200. The control command is a command for adjusting the position of the artificial satellite210on the elliptical orbit of the artificial satellite210.

The satellite communication device320transmits the generated control command to the artificial satellite210for each artificial satellite210of the satellite constellation200.

For each artificial satellite210of the satellite constellation200, the position of the artificial satellite210on the elliptical orbit of the artificial satellite210is adjusted according to the control command. Consequently, each artificial satellite210cooperate with the other artificial satellites210to constantly monitor the target area.

For each artificial satellite210of the satellite constellation200, the adjustment commands are executed after the control command is executed.

Further, the satellite control device310generates for each artificial satellite210of the satellite constellation200, the control command for controlling the propulsion device of the artificial satellite210. The control command is a command for adjusting the speed of the artificial satellite210.

The satellite communication device320transmits the generated control command to the artificial satellite210for each artificial satellite210of the satellite constellation200.

For each artificial satellite210of the satellite constellation200, the speed of the artificial satellite210is adjusted according to the control command. Consequently, a relative position of the artificial satellite210towards the target area of the Earth is adjusted during a target time range assigned to the artificial satellite210.

For each artificial satellite210of the satellite constellation200, the adjustment commands are executed after the control command is executed.

The target time range is a time range in which the target area is monitored.

Each artificial satellite210of the satellite constellation200has the pointing function for changing the monitoring direction.

The satellite control device310generates a pointing command for each artificial satellite210of the satellite constellation200. The pointing command is a command for controlling the pointing function of the artificial satellite210.

The satellite communication device320transmits the generated pointing command to the artificial satellite210for each artificial satellite210of the satellite constellation200.

For each artificial satellite210of the satellite constellation200, the pointing function of the artificial satellite210is controlled according to the pointing command. Consequently, the monitoring direction of the artificial satellite210is directed to the target area of the Earth during the target time range assigned to the artificial satellite210.

Each artificial satellite210has the pointing function for changing the monitoring direction.

Each artificial satellite210includes the monitoring control device212.

The monitoring control device212directs the monitoring direction to the target area of the Earth by controlling the pointing function.

Each artificial satellite210includes the monitoring device211and the monitoring control device212.

The monitoring device211has the resolution variable function.

The monitoring control device212adjusts the resolution of the monitoring device211by controlling the resolution variable function of the monitoring device211.

Each artificial satellite210includes the monitoring device211and the monitoring control device212.

The monitoring device211has an autofocus function.

The monitoring control device212sets a focal point of the monitoring device211to the target area by controlling the autofocus function of the monitoring device211.

Each artificial satellite210includes the communication device213that communicates with the ground facility300.

The communication device213has a dynamic range corresponding to change in the relative distance between the ground facility300and the artificial satellite210.

Effect of First Embodiment

Since the three artificial satellites (210A to210C) alternately stay in the vicinity of the apogee for a long time, it is possible to always monitor the target area. Further, since the three artificial satellites (210A to210C) alternately pass in the vicinity of the perigee, it is possible to observe the target area with high resolution.

Second Embodiment

For a mode in which each long axis of three elliptical orbits of the satellite constellation200is tilted at equal intervals from each other in an elevation direction, mainly matters different from the first embodiment will be described with reference toFIGS.13to20.

The elevation direction is equivalent to a latitude direction, that is, a north-south direction.

As described in the first embodiment, it is possible to find the sun-synchronous elliptical orbit for any number of circulations.

A sun-synchronous elliptical orbit for 14 rounds/day can be found by finely adjusting each of the apogee altitude at 1,500 kilometers and the perigee altitude at 300 kilometers.

The elliptical orbit for 14 rounds/day has the orbit inclination angle of about 98 degrees. For this reason, this elliptical orbit is suitable for monitoring the target area located at a latitude of plus or minus 82 degrees (=180-98).

FIGS.13and14illustrate a sun-synchronous elliptical polar orbit.

FIG.13illustrates an elliptical polar orbit when viewed from the sky above the North Pole. A black circle depicted in the Earth101represents the North Pole. In the following diagrams, the black circle depicted in the Earth101represents the North Pole.

FIG.14illustrates the elliptical polar orbit when viewed from the sky above the equator. A line depicted in the Earth101represents the equator. In the following diagrams, the line depicted in the Earth101represents the equator.

In the sun-synchronous elliptical polar orbit, the northernmost end of the orbit surface crosses directly below the Sun102at 12:00 μm.

A specific example of the sun-synchronous elliptical polar orbit is an elliptical orbit whose LST on the orbit surface is 12:00 pm. LST is an abbreviation for Local Sun Time.

The elliptical orbit for 14 rounds/day is an orbit similar to a so-called polar orbit since the elliptical orbit for 14 rounds/day has the orbit inclination angle of about 98 degrees. That is, the elliptical orbit for 14 rounds/day is similar to the elliptical polar orbits illustrated inFIGS.13and14.

First Examples

FIGS.15and16illustrate the satellite constellations200in which each artificial satellite (210A to210C) circulates on the elliptical orbit with LST 12:00.

The elliptical orbit with LST 12:00 is an elliptical orbit whose LST on the orbit surface is 12:00 pm (seeFIG.15).

The satellite constellation200is constituted of the three artificial satellites (210A to210C), and each artificial satellite (210A to210C) circulates on the elliptical orbit. Each long axis of the three elliptical orbits is tilted evenly by 120 degrees from each other in an elevation direction (latitude direction, north-south direction) (seeFIG.16). Consequently, the artificial satellite210that monitors from a perigee side and the artificial satellite210that monitors from an apogee side alternately fly in the sky above the target area of the Earth101.

Each artificial satellite (210A to210C) circulates one round in about 100 minutes. That is, each artificial satellite (210A to210C) revisits in the sky above the target area once every about 100 minutes. For this reason, each artificial satellite (210A to210C) may be able to monitor the target area a plurality of times during a sunshine time range.

In the elliptical orbit with LST 12:00, 12:00 pm is basically an optimum monitoring time.

Before and after 12:00 pm, there are monitoring opportunities at around 10:20 and around 13:40. However, since the long axis of the elliptical orbit circles, the target area is viewed from an angle in the monitoring opportunities before and after 12:00 pm, and the monitoring condition is deteriorated.

For this reason, it is effective to shift the time range in which the three artificial satellites (210A to210C) fly in the sky above the target area, by changing the LSTs of the three elliptical orbits.

Second Examples

FIGS.17and18illustrate the satellite constellation200in which each artificial satellite (210A to210C) circulates on the elliptical orbits whose LST times are different from each other.

The artificial satellite210A circulates on the elliptical orbit with LST 12:00.

The artificial satellite210B circulates on an elliptical orbit with LST 13:30. The elliptical orbit with LST 13:30 is an elliptical orbit whose LST on the orbit surface is 13:30.

The artificial satellite210C circulates on an elliptical orbit with LST 10:30. The elliptical orbit with LST 10:30 is an elliptical orbit whose LST on the orbit surface is 10:30.

By making the long axis of the elliptical orbit with LST 12:00, circle in an azimuth direction (longitude direction, east-west direction) by plus or minus 22.5 degrees, the elliptical orbit with LST 10:30 and the elliptical orbit with LST 13:30 are formed (SeeFIG.17).

Each long axis of the three elliptical orbits is evenly tilted by 120 degrees from each other in an elevation direction (latitude direction, north-south direction) (seeFIG.18).

Consequently, the artificial satellite210which monitors from the perigee side and the artificial satellite210which monitors from the apogee side alternately fly directly above the target area of the Earth101.

For this reason, when monitoring opportunities before and after each LST are included, it is possible to intermittently monitor the target area approximately between 9:00 and 15:00.

Since the ground speed of each artificial satellite (210A to210C) is fast, constantly monitoring for a long time is not possible. However, since the number of circulations in a day by each artificial satellite (210A to210C) is large, it is possible to have an opportunity to monitor the target area about 9 to 12 times by the three artificial satellites (210A to210C).

Third Examples

FIGS.19and20illustrate the satellite constellation200including the artificial satellites (210A to210C) each of which circulates on the elliptical orbits whose LST times are different from each other.

The artificial satellite210A circulates on the elliptical orbit with LST 12:00.

The artificial satellite210B circulates on an elliptical orbit with LST 15:00. The elliptical orbit with LST 15:00 is an elliptical orbit whose LST on the orbit surface is 15:00.

The artificial satellite210C circulates on an elliptical orbit with LST 9:00. The elliptical orbit with LST 9:00 is an elliptical orbit whose LST on the orbit surface is 9:00.

By making the long axis of the elliptical orbit with LST 12:00, circle in an azimuth direction (longitude direction, east-west direction) by plus or minus 45 degrees, the elliptical orbit with LST 9:00 and the elliptical orbit with LST 15:00 are formed (seeFIG.19).

Each long axis of the three elliptical orbits is evenly tilted by 120 degrees from each other in an elevation direction (latitude direction, north-south direction) (seeFIG.20).

Consequently, the artificial satellite210which monitors from the perigee side and the artificial satellite210which monitors from the apogee side alternately fly directly above the target area of the Earth101.

For this reason, when the monitoring opportunities before and after each LST are included, it is possible to intermittently monitor the target area approximately between 7:30 to 16:30.

By the three artificial satellites (210A to210C), it is possible to monitor with high resolution during all time ranges in a daytime but intermittently.

Characteristics of Second Embodiment

Main characteristics of the second embodiment will be described.

The satellite constellation200includes three or more artificial satellites210which monitor the target area of the Earth.

At least one of the three or more artificial satellites210circulates on the orbit on which the northernmost end of the orbit surface crosses directly below the Sun at 12:00 pm.

The orbit of each of the three or more artificial satellites210is the elliptical orbit having the sun-synchronization and the orbit inclination angle.

The long axis of each elliptical orbit forms an equal angle with each long axis of two adjacent elliptical orbits in the longitude direction.

At least one of the three or more artificial satellites210circulates on the orbit whose local sun time on the orbit surface is 12:00 pm.

The satellite constellation200includes three artificial satellites (210A to210C).

One artificial satellite210A circulates on a first elliptical orbit. The northernmost end of the orbit surface of the first elliptical orbit crosses directly below the Sun at 12:00 pm.

One (210B) of two artificial satellites circulates on a second elliptical orbit. The long axis of the second elliptical orbit forms a defined angle with the long axis of the first elliptical orbit on a plus side of the latitude direction.

The other one (210C) of the two artificial satellites circulates on a third elliptical orbit. The long axis of the third elliptical orbit forms a defined angle with the long axis of the first elliptical orbit on a minus side of the latitude direction.

The defined angle is an angle of 45 degrees or smaller.

Effect of Second Embodiment

Even if the elliptical orbit of the artificial satellite210A is an orbit similar to a so-called polar orbit, the same effect as that in the first embodiment can be obtained.

Third Embodiment

As to running of the satellite constellation200, mainly matters different from the first embodiment and the second embodiment will be described.

In order to verify feasibility of the satellite constellation200, one artificial satellite210is manufactured (developed), and one manufactured (developed) artificial satellite210is put into an orbit. The ground facility300controls the one artificial satellite210.

Then, after the feasibility of the satellite constellation200is verified, the satellite constellation200by the three artificial satellites210is run. The ground facility300controls the three artificial satellites210.

During preparation of the satellite constellation200, one or two artificial satellites210may be prepared in advance. The ground facility300controls the one or two artificial satellites210.

Supplement to Embodiments

The embodiments are examples of preferred modes, and are not intended to limit the technical scope of the present invention. The embodiments may be implemented partially or may be implemented being combined with other modes.

REFERENCE SIGNS LIST

100: monitoring system,101: Earth,102: sun,103: orbit surface,200: satellite constellation,210: artificial satellite,211: monitoring device,212: monitoring control device,213: communication device,214: propulsion device,215: attitude control device,216: power supply device,300: ground facility,310: satellite control device,320: satellite communication device.