Patent Description:
An orbit of an artificial satellite is affected by an elliptic effect of Earth (an Earth flattening effect), asymmetry with respect to an equatorial surface of the Earth, and high-order components compared to calculating the Earth as a mass point without a volume. For this reason, an effect such as an orbit surface circling with respect to an inertial space, or the like is generated.

An artificial satellite (geostationary orbit satellite) that circulates on a geostationary orbit flies about <NUM>,<NUM> kilometers above the equator and circulates around the Earth in about a day while synchronizing with a rotation of the Earth. For this reason, the geostationary orbit satellite appears to be stationary in the sky when viewed from a specific point on a ground. That is, it is possible to always monitor the specific point by a monitoring means installed on the geostationary orbit satellite.

Since geostationary orbit satellite flies at about <NUM>,<NUM> kilometers above the equator, monitoring by the geostationary orbit satellite is to be monitoring from a long distance. Further, when monitoring a mid-latitude (for example, near <NUM> degrees north latitude), monitoring by the geostationary orbit satellite is to be monitoring by viewing from an angle. For this reason, it is difficult to make the monitoring by the geostationary orbit satellite into one with high resolution.

The geostationary orbit satellite has traditionally been put into the geostationary orbit by chemical propulsion. For this reason, it is necessary to install a large amount of propellant on the geostationary orbit satellite, and it has been difficult to install a photographing means having a large aperture and a long focal length on the geostationary orbit satellite.

The chemical propulsion or the electric propulsion is utilized as a propulsion means for an artificial satellite.

For example, in an artificial satellite called a super-low altitude demonstrator, the electric propulsion is utilized. The super-low altitude demonstrator maintains an orbit altitude (about <NUM> kilometers) where atmospheric resistance cannot be ignored, by accelerating by the electric propulsion, and obtains an effect of monitoring with high resolution.

The super-low altitude demonstrator does not have synchronization with the rotation of the Earth like the geostationary orbit satellite. For this reason, it is impossible to always monitor the specific point by monitoring with the super-low altitude demonstrator.

Patent Literature <NUM> discloses a system for observing an observation goal area in a short time, after making an observation plan, by using a plurality of observation satellite groups.

The system disclosed in Patent Literature <NUM> requires a large number of observation satellites. Further, it is difficult to observe the observation goal area by making the large number of observation satellites cooperate with each other.

<CIT> discloses an elliptical satellite system which emulates the characteristics of geosynchronous satellites.

<CIT> discloses an elliptical orbit satellite, system, and deployment with controllable coverage characteristics.

<CIT> discloses an earth observation method and system, an observation satellite, an operating ground system and a program for the same.

<CIT> discloses a method for controlling the attitude guidance of a satellite, a satellite, pluralities of satellites, and an associated computer program.

<CIT> discloses a photodetector and an optical sensor mounted on an artificial satellite.

The present invention aims to facilitate observation of a target area by making a smaller number of artificial satellites cooperate with each other.

A satellite constellation according to the present invention as recited in claims <NUM> and <NUM>.

According to the present invention, observation of a target area by making three artificial satellites cooperate with each other is facilitated.

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.

A satellite constellation <NUM> will be described with reference to <FIG>.

A configuration of a monitoring system <NUM> will be described with reference to <FIG>.

The monitoring system <NUM> is a system for monitoring a target area of the Earth, and includes the satellite constellation <NUM> and a ground facility <NUM>.

"Monitoring" may be read as "observing".

The satellite constellation <NUM> is constituted of three or more artificial satellites <NUM>.

In the present embodiment, the satellite constellation <NUM> is a three-in constellation constituted of three artificial satellites (210A to 210C).

However, the satellite constellation <NUM> may be constituted of four or more artificial satellites <NUM>.

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

The ground facility <NUM> includes a satellite control device <NUM> and a satellite communication device <NUM>, and controls the satellite constellation <NUM> by communicating with each artificial satellite <NUM>.

The satellite control device <NUM> is a computer that generates each type of command for controlling each artificial satellite (210A to 210C), and the satellite control device <NUM> includes 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 device <NUM> is connected to the satellite communication device <NUM> via the input and output interface.

The satellite communication device <NUM> communicates with each artificial satellite (210A to 210C). Specifically, the satellite communication device <NUM> transmits each type of command to each artificial satellite (210A to 210C). Further, the satellite communication device <NUM> receives monitor data transmitted from each artificial satellite (210A to 210C).

A configuration of the artificial satellite <NUM> will be described with reference to <FIG>. Each artificial satellite (210A to 210B) is constituted as follows.

The artificial satellite <NUM> includes a monitoring device <NUM>, a monitoring control device <NUM>, a communication device <NUM>, a propulsion device <NUM>, an attitude control device <NUM>, and a power supply device <NUM>.

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

The monitoring control device <NUM> is a computer that controls the monitoring device <NUM>, the propulsion device <NUM>, and the attitude control device <NUM>, and the monitoring control device <NUM> includes the processing circuitry. Specifically, the monitoring control device <NUM> controls the monitoring control device <NUM>, the propulsion device <NUM>, and the attitude control device <NUM> according to each type of command transmitted from the ground facility <NUM>.

The communication device <NUM> is a device that communicates with the ground facility <NUM>. Specifically, the communication device <NUM> transmits the monitor data to the ground facility <NUM>. Further, the communication device <NUM> receives each type of command which is transmitted from the ground facility <NUM>.

The propulsion device <NUM> is a device that provides a propulsive force to the artificial satellite <NUM>, and the propulsion device <NUM> changes speed of the artificial satellite <NUM>. Specifically, the propulsion device <NUM> is an electric propulsion machine. For example, the propulsion device <NUM> is an ion engine or a Hall thruster.

The attitude control device <NUM> is a device for controlling attitude elements such as attitude of the artificial satellite <NUM>, angular velocity of the artificial satellite <NUM>, and a line-of-sight direction (Line Of Sight) of the monitoring device <NUM>. The attitude control device <NUM> changes each attitude element in a desired direction. Alternatively, the attitude control device <NUM> maintains each attitude element in the desired direction. The attitude control device <NUM> includes 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 facility <NUM>.

The power supply device <NUM> includes a solar cell, a battery, a power control device, and the like, and supplies power to each equipment installed on the artificial satellite <NUM>.

The processing circuitry included in each of the satellite control device <NUM> and the monitoring control device <NUM> will 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 satellite <NUM> will be described.

The artificial satellite <NUM> includes a pointing function for directing a monitoring direction to the target area.

For example, the artificial satellite <NUM> includes a reaction wheel. The reaction wheel is a device for controlling the attitude of the artificial satellite <NUM>. Body pointing is realized by controlling the attitude of the artificial satellite <NUM> by the reaction wheel.

For example, the monitoring device <NUM> includes a pointing mechanism. The pointing mechanism is a mechanism for changing the line-of-sight direction of the monitoring device <NUM>. For example, a drive mirror or the like is used as the pointing mechanism.

A monitoring function of the monitoring device <NUM> will be described.

The monitoring device <NUM> has 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 constellation <NUM> will be described with reference to <FIG>.

<FIG> illustrates the satellite constellation <NUM> when viewed in a normal line direction of an orbit surface <NUM>.

<FIG> illustrates the satellite constellation <NUM> when viewed from the orbit surface <NUM>. For example, <FIG> illustrates the satellite constellation <NUM> when viewed from the sky above the equator.

<FIG>, <FIG> and <FIG> illustrate a long axis of an elliptical orbit of each artificial satellite (210A to 210C) circling centering on Earth <NUM> along the orbit surface <NUM>.

The orbit surface <NUM> is a surface along which the elliptical orbit of each artificial satellite (210A to 210C) 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 to 210C) 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 to 210C) is a non-frozen orbit.

That is, the elliptical orbit of each artificial satellite (210A to 210C) is not a frozen orbit, and the long axis of each elliptical orbit circles centering on the Earth <NUM> inside the orbit surface <NUM> over time.

The three artificial satellites (210A to 210C) alternately monitor the target area of the Earth <NUM> from 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 <NUM> ° 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 Earth <NUM>, but a relative relationship between the three elliptical orbits is maintained.

The normal line direction of the orbit surface <NUM> is maintained.

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

At <NUM>:<NUM> pm, a phase of each artificial satellite (210A-210C) is not correlated with the latitude of the target area of the Earth <NUM>. A phase of the artificial satellite <NUM> is equivalent to a position of the artificial satellite <NUM> on the orbit of the artificial satellite <NUM>.

One of the three artificial satellites (210A to 210C) can monitor the target area of the Earth <NUM>.

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

When the artificial satellite <NUM> passes in the sky above the target area of the Earth <NUM> on an apogee side of the elliptical orbit, the artificial satellite <NUM> monitors the target area of the Earth <NUM> for a long time but with low resolution.

When the artificial satellite <NUM> passes in the sky above the target area of the Earth <NUM> on a perigee side of the elliptical orbit, the artificial satellite <NUM> monitors the target area of the Earth <NUM> with high resolution but for a short time.

A specific example of the elliptical orbit of each artificial satellite (210A to 210C) 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 <NUM>,<NUM> kilometers.

An eccentricity of the elliptical orbit is <NUM>.

An orbit inclination angle is <NUM> degrees.

An apogee altitude is <NUM>,<NUM> kilometers.

The relationship between the altitude and the latitude of the elliptical orbit of each artificial satellite (210A to 210C) will be described with reference to <FIG>.

A dotted line represents the elliptical orbit of the artificial satellite 210A. An alternate long and short dash line represents the elliptical orbit of the artificial satellite 210B. A broken line represents the elliptical orbit of the artificial satellite 210C.

"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 to 210C).

"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 to 210C).

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

A length of time that the artificial satellite <NUM> stays in the sky above the target area is referred to as staying time of the artificial satellite <NUM>.

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

The satellite control device <NUM> sets each of the usage altitude Hb and a viewing angle of the monitoring device <NUM> so that the target area fits within the field of view while each artificial satellite (210A to 210C) 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 to 210C) 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 to 210C) can monitor the target area from an altitude lower than the usage altitude Hc.

The satellite control device <NUM> sets resolution of the monitoring device <NUM> so 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 to <FIG> and <FIG>.

The long axis of the elliptical orbit of the artificial satellite <NUM> is correlated with the satellite altitude. For this reason, by finely adjusting the altitude of each artificial satellite (210A to 210C), 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 to 210C), 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 constellation <NUM> while the relative angle of the elliptical orbit is maintained and while the sun-synchronization of the elliptical orbit is maintained.

The satellite control device <NUM> generates a command for adjusting the altitude of each artificial satellite (210A to 210C). Further, the satellite control device <NUM> generates a command for adjusting the orbit inclination angle of each artificial satellite (210A to 210C). Then, the satellite communication device <NUM> transmits these commands to each artificial satellite (210A to 210C).

For each artificial satellite (210A to 210C), the monitoring control device <NUM> adjusts the satellite altitude and the orbit inclination angle according to these commands. Specifically, the monitoring control device <NUM> controls the propulsion device <NUM> according to these commands. The propulsion device <NUM> can adjust the satellite altitude and the orbit inclination angle by changing satellite speed.

In <FIG>, a black circle depicted inside the Earth <NUM> represents the North Pole.

When flight speed of the artificial satellite <NUM> accelerates, the altitude of the artificial satellite <NUM> rises. Then, when the altitude of the artificial satellite <NUM> rises, ground speed of the artificial satellite <NUM> decelerates.

When the flight speed of the artificial satellite <NUM> decelerates, the altitude of the artificial satellite <NUM> lowers. Then, when the altitude of the artificial satellite <NUM> lowers, the ground speed of the artificial satellite <NUM> accelerates.

As illustrated in <FIG>, if the propulsion device <NUM> generates thrust in a direction orthogonal to the orbit surface at a point (equinox) where the artificial satellite <NUM> crosses in the sky above the equator, the orbit inclination angle can be finely adjusted effectively.

With rotation of the Earth <NUM>, the target area moves independently of the orbit surface of each artificial satellite (210A to 210C). Further, each artificial satellite (210A to 210C) 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 to 210C) 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 to 210C) in the orbit surface, the three artificial satellites (210A to 210C) can cooperate with each other in a timely manner to constantly monitor the target area.

Then, the satellite control device <NUM> generates a command for accelerating or decelerating each artificial satellite (210A to 210C) in the orbit surface. Then, the satellite communication device <NUM> transmits the generated command to each artificial satellite (210A to 210C).

After that, the satellite control device <NUM> generates the commands for adjusting the satellite altitude and the orbit inclination angle of each artificial satellite (210A to 210C). Then, the satellite communication device <NUM> transmits the generated commands to each artificial satellite (210A to 210C).

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

A position of the target area and a position of each artificial satellite (210A to 210C) can be managed by using a common coordinate system. Then, by using the common coordinate system, each artificial satellite (210A to 210C) 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 device <NUM> can 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 device <NUM> generates a command indicating the optimum pointing condition for each artificial satellite (210A to 210C). Then, the satellite communication device <NUM> transmits the generated command to each artificial satellite (210A to 210C).

The monitoring control device <NUM> controls the pointing function of the artificial satellite <NUM> according to the command from the ground facility <NUM>.

The monitoring control device <NUM> may control the attitude control device <NUM>, or control the pointing mechanism of the monitoring device <NUM>.

For monitoring the target area, it is effective to shorten a relative distance between the target area and each artificial satellite (210A to 210C). 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 device <NUM> generates a command for adjusting a flight position of each artificial satellite (210A to 210C). Then, the satellite communication device <NUM> transmits the generated command to each artificial satellite (210A to 210C).

After that, the satellite control device <NUM> generates commands for adjusting the satellite altitude and the orbit inclination angle of each artificial satellite (210A to 210C). Then, the satellite communication device <NUM> transmits the generated commands to each artificial satellite (210A to 210C).

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

The sun-synchronous orbit will be described with reference to <FIG>. A black circle depicted in the Earth <NUM> represents the North Pole. A line depicted through a center of the Earth <NUM> represents 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 satellite <NUM> is the sun-synchronous orbit, the sun incident angle relative to the orbit surface of the artificial satellite <NUM> does not change throughout a year.

A sun-synchronous circular orbit will be described with reference to <FIG>.

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 <NUM> kilometers to about <NUM>,<NUM> kilometers. At about <NUM> kilometers or lower, since effect of atmospheric resistance cannot be ignored, the sun-synchronization cannot be maintained. At about <NUM>,<NUM> 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> illustrates attribute values of the circular orbit at which the number of circulations of the artificial satellite <NUM> in a day is an integer. Each attribute value is an approximate value. That is, each numerical value indicated in <FIG> is an approximate number including a rounding error.

The attribute values of the circular orbit illustrated in <FIG> are examples of the attribute values for realizing the sun-synchronization. The number of circulations of the artificial satellite <NUM> in 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 <NUM>,<NUM> kilometers.

The radius of the sun-synchronous circular orbit required for making the artificial satellite <NUM> circulate seven rounds in a day is about <NUM>,<NUM> kilometers. This radius is calculated by adding the radius (about <NUM>,<NUM> kilometers) of the Earth to the altitude-from-ground-surface (about <NUM>,<NUM> kilometers) of the circular orbit. Therefore, the long axis length of the sun-synchronous elliptical orbit required for making the artificial satellite <NUM> circulate seven rounds in a day is about <NUM>,<NUM> kilometers. This long axis length is calculated by doubling the radius (about <NUM>,<NUM> 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 <NUM>,<NUM> kilometers and the perigee altitude is about <NUM> kilometers, the long axis length of the elliptical orbit is the above length (about <NUM>,<NUM> 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 satellite <NUM> circulate seven rounds in a day.

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

Main characteristics of the first embodiment will be described.

The satellite constellation <NUM> includes three or more artificial satellites <NUM> that monitor the target area of the Earth.

Each of the three or more artificial satellites <NUM> circulates 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 <NUM> ° 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 facility <NUM> includes the satellite control device <NUM> and the satellite communication device <NUM>, and controls the satellite constellation <NUM>.

The satellite control device <NUM> generates adjustment commands for each artificial satellite <NUM> of the satellite constellation <NUM>. The adjustment commands are commands for adjusting the altitude of the artificial satellite <NUM> and the orbit inclination angle of the elliptical orbit of the artificial satellite <NUM>.

The satellite communication device <NUM> transmits the generated adjustment commands to the artificial satellite <NUM> for each artificial satellite <NUM> of the satellite constellation <NUM>.

For each artificial satellite <NUM> of the satellite constellation <NUM>, the altitude of the artificial satellite <NUM> and the orbit inclination angle of the elliptical orbit of the artificial satellite <NUM> are adjusted according to the adjustment commands. Consequently, the sun-synchronization of the elliptical orbit of each artificial satellite <NUM> is maintained, and also, relative angles between the long axis of the elliptical orbit of each artificial satellite <NUM> and the long axes of the elliptical orbits of the other artificial satellites <NUM> are maintained.

The satellite control device <NUM> generates a control command for controlling the propulsion device of the artificial satellite <NUM> for each artificial satellite <NUM> of the satellite constellation <NUM>. The control command is a command for adjusting the position of the artificial satellite <NUM> on the elliptical orbit of the artificial satellite <NUM>.

The satellite communication device <NUM> transmits the generated control command to the artificial satellite <NUM> for each artificial satellite <NUM> of the satellite constellation <NUM>.

For each artificial satellite <NUM> of the satellite constellation <NUM>, the position of the artificial satellite <NUM> on the elliptical orbit of the artificial satellite <NUM> is adjusted according to the control command. Consequently, each artificial satellite <NUM> cooperate with the other artificial satellites <NUM> to constantly monitor the target area.

For each artificial satellite <NUM> of the satellite constellation <NUM>, the adjustment commands are executed after the control command is executed.

Further, the satellite control device <NUM> generates for each artificial satellite <NUM> of the satellite constellation <NUM>, the control command for controlling the propulsion device of the artificial satellite <NUM>. The control command is a command for adjusting the speed of the artificial satellite <NUM>.

For each artificial satellite <NUM> of the satellite constellation <NUM>, the speed of the artificial satellite <NUM> is adjusted according to the control command. Consequently, a relative position of the artificial satellite <NUM> towards the target area of the Earth is adjusted during a target time range assigned to the artificial satellite <NUM>.

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

Each artificial satellite <NUM> of the satellite constellation <NUM> has the pointing function for changing the monitoring direction.

The satellite control device <NUM> generates a pointing command for each artificial satellite <NUM> of the satellite constellation <NUM>. The pointing command is a command for controlling the pointing function of the artificial satellite <NUM>.

The satellite communication device <NUM> transmits the generated pointing command to the artificial satellite <NUM> for each artificial satellite <NUM> of the satellite constellation <NUM>.

For each artificial satellite <NUM> of the satellite constellation <NUM>, the pointing function of the artificial satellite <NUM> is controlled according to the pointing command. Consequently, the monitoring direction of the artificial satellite <NUM> is directed to the target area of the Earth during the target time range assigned to the artificial satellite <NUM>.

Each artificial satellite <NUM> has the pointing function for changing the monitoring direction.

Each artificial satellite <NUM> includes the monitoring control device <NUM>.

The monitoring control device <NUM> directs the monitoring direction to the target area of the Earth by controlling the pointing function.

Each artificial satellite <NUM> includes the monitoring device <NUM> and the monitoring control device <NUM>.

The monitoring device <NUM> has the resolution variable function.

The monitoring control device <NUM> adjusts the resolution of the monitoring device <NUM> by controlling the resolution variable function of the monitoring device <NUM>.

The monitoring device <NUM> has an autofocus function.

The monitoring control device <NUM> sets a focal point of the monitoring device <NUM> to the target area by controlling the autofocus function of the monitoring device <NUM>.

Each artificial satellite <NUM> includes the communication device <NUM> that communicates with the ground facility <NUM>.

The communication device <NUM> has a dynamic range corresponding to change in the relative distance between the ground facility <NUM> and the artificial satellite <NUM>.

Since the three artificial satellites (210A to 210C) 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 to 210C) alternately pass in the vicinity of the perigee, it is possible to observe the target area with high resolution.

For a mode in which each long axis of three elliptical orbits of the satellite constellation <NUM> is tilted at equal intervals from each other in an elevation direction, mainly matters different from the first embodiment will be described with reference to <FIG>.

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 <NUM> rounds/day can be found by finely adjusting each of the apogee altitude at <NUM>,<NUM> kilometers and the perigee altitude at <NUM> kilometers.

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

<FIG> and <FIG> illustrate a sun-synchronous elliptical polar orbit.

<FIG> illustrates an elliptical polar orbit when viewed from the sky above the North Pole. A black circle depicted in the Earth <NUM> represents the North Pole. In the following diagrams, the black circle depicted in the Earth <NUM> represents the North Pole.

<FIG> illustrates the elliptical polar orbit when viewed from the sky above the equator. A line depicted in the Earth <NUM> represents the equator. In the following diagrams, the line depicted in the Earth <NUM> represents the equator.

In the sun-synchronous elliptical polar orbit, the northernmost end of the orbit surface crosses directly below the Sun <NUM> at <NUM>:<NUM> pm.

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

The elliptical orbit for <NUM> rounds/day is an orbit similar to a so-called polar orbit since the elliptical orbit for <NUM> rounds/day has the orbit inclination angle of about <NUM> degrees. That is, the elliptical orbit for <NUM> rounds/day is similar to the elliptical polar orbits illustrated in <FIG> and <FIG>.

<FIG> and <FIG> illustrate the satellite constellations <NUM> in which each artificial satellite (210A to 210C) circulates on the elliptical orbit with LST <NUM>:<NUM>.

The elliptical orbit with LST <NUM>:<NUM> is an elliptical orbit whose LST on the orbit surface is <NUM>:<NUM> pm (see <FIG>).

The satellite constellation <NUM> is constituted of the three artificial satellites (210A to 210C), and each artificial satellite (210A to 210C) circulates on the elliptical orbit. Each long axis of the three elliptical orbits is tilted evenly by <NUM> degrees from each other in an elevation direction (latitude direction, north-south direction) (see <FIG>). Consequently, the artificial satellite <NUM> that monitors from a perigee side and the artificial satellite <NUM> that monitors from an apogee side alternately fly in the sky above the target area of the Earth <NUM>.

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

In the elliptical orbit with LST <NUM>:<NUM>, <NUM>:<NUM> pm is basically an optimum monitoring time.

Before and after <NUM>:<NUM> pm, there are monitoring opportunities at around <NUM>:<NUM> and around <NUM>:<NUM>. 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 <NUM>:<NUM> 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 to 210C) fly in the sky above the target area, by changing the LSTs of the three elliptical orbits.

<FIG> and <FIG> illustrate the satellite constellation <NUM> in which each artificial satellite (210A to 210C) circulates on the elliptical orbits whose LST times are different from each other.

The artificial satellite 210A circulates on the elliptical orbit with LST <NUM>:<NUM>.

The artificial satellite 210B circulates on an elliptical orbit with LST <NUM>:<NUM>. The elliptical orbit with LST <NUM>:<NUM> is an elliptical orbit whose LST on the orbit surface is <NUM>:<NUM>.

The artificial satellite 210C circulates on an elliptical orbit with LST <NUM>:<NUM>. The elliptical orbit with LST <NUM>:<NUM> is an elliptical orbit whose LST on the orbit surface is <NUM>:<NUM>.

By making the long axis of the elliptical orbit with LST <NUM>:<NUM>, circle in an azimuth direction (longitude direction, east-west direction) by plus or minus <NUM> degrees, the elliptical orbit with LST <NUM>:<NUM> and the elliptical orbit with LST <NUM>:<NUM> are formed (See <FIG>).

Each long axis of the three elliptical orbits is evenly tilted by <NUM> degrees from each other in an elevation direction (latitude direction, north-south direction) (see <FIG>).

Consequently, the artificial satellite <NUM> which monitors from the perigee side and the artificial satellite <NUM> which monitors from the apogee side alternately fly directly above the target area of the Earth <NUM>.

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

Since the ground speed of each artificial satellite (210A to 210C) 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 to 210C) is large, it is possible to have an opportunity to monitor the target area about <NUM> to <NUM> times by the three artificial satellites (210A to 210C).

<FIG> and <FIG> illustrate the satellite constellation <NUM> including the artificial satellites (210A to 210C) each of which circulates on the elliptical orbits whose LST times are different from each other.

By making the long axis of the elliptical orbit with LST <NUM>:<NUM>, circle in an azimuth direction (longitude direction, east-west direction) by plus or minus <NUM> degrees, the elliptical orbit with LST <NUM>:<NUM> and the elliptical orbit with LST <NUM>:<NUM> are formed (see <FIG>).

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 <NUM>:<NUM> to <NUM>:<NUM>.

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

Main characteristics of the second embodiment will be described.

The satellite constellation <NUM> includes three or more artificial satellites <NUM> which monitor the target area of the Earth.

At least one of the three or more artificial satellites <NUM> circulates on the orbit on which the northernmost end of the orbit surface crosses directly below the Sun at <NUM>:<NUM> pm.

The orbit of each of the three or more artificial satellites <NUM> is 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 satellites <NUM> circulates on the orbit whose local sun time on the orbit surface is <NUM>:<NUM> pm.

The satellite constellation <NUM> includes three artificial satellites (210A to 210C).

One artificial satellite 210A circulates on a first elliptical orbit. The northernmost end of the orbit surface of the first elliptical orbit crosses directly below the Sun at <NUM>:<NUM> 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 <NUM> degrees or smaller.

Even if the elliptical orbit of the artificial satellite 210A is an orbit similar to a so-called polar orbit, the same effect as that in the first embodiment can be obtained.

As to running of the satellite constellation <NUM>, mainly matters different from the first embodiment and the second embodiment will be described.

In order to verify feasibility of the satellite constellation <NUM>, one artificial satellite <NUM> is manufactured (developed), and one manufactured (developed) artificial satellite <NUM> is put into an orbit. The ground facility <NUM> controls the one artificial satellite <NUM>.

Then, after the feasibility of the satellite constellation <NUM> is verified, the satellite constellation <NUM> by the three artificial satellites <NUM> is run. The ground facility <NUM> controls the three artificial satellites <NUM>.

During preparation of the satellite constellation <NUM>, one or two artificial satellites <NUM> may be prepared in advance. The ground facility <NUM> controls the one or two artificial satellites <NUM>.

The embodiments are examples of preferred modes, and are not intended to limit the technical scope of the present invention.

Claim 1:
A satellite constellation (<NUM>) comprising three or more artificial satellites (<NUM>) that include a visible light optical sensor and are configured to monitor a target area of the Earth,
wherein each of the three or more artificial satellites circulates on a non-frozen elliptical orbit having sun-synchronization and an orbit inclination angle,
characterised in that the elliptical orbits are non-circular and lie within a common planar surface referred to as the orbit surface, and
wherein the long axis of each elliptical orbit forms an equal angle with each long axis of two adjacent elliptical orbits on the orbit surface.