Patent Description:
A satellite orbit propagator is a device that estimates the current position and velocity of a satellite in orbit (or predicts the position and velocity of a satellite in the near future) by fusing a satellite orbit model with observation data.

The satellite orbit propagator may be implemented in software (SW) and mounted on a satellite or ground station system. The satellite orbit propagator mounted on the ground station can be used to predict the orbit for the next <NUM> to <NUM> hours and determine the time and attitude for imaging a target. A satellite orbit propagator mounted on the satellite may determine the position and velocity of the satellite to generate a reference coordinate system for imaging the target.

A conventional satellite orbit propagator uses position and velocity information of a satellite acquired from a satellite-mounted GPS receiver as observation data, where the GPS receiver requires four or more GPS satellite signal observations. However, the conventional satellite orbit propagator has a problem in that it is impossible to acquire observation data when the GPS receiver malfunctions or the GPS signal reception rate decreases.

For example, <CIT> describes a satellite surveillance system based on a database that contains information on the AIS transmitters for ships and buoys. However, <CIT> does not disclose the features of present invention as claimed in the characterizing portion of the independent claims.

Accordingly, an object of the present disclosure to provide a satellite orbit propagator using ship automatic identification information and a control method thereof.

In order to accomplish the technical objectives mentioned above, a method according to the present disclosure includes acquiring ship automatic identification system data (AIS data) during a predetermined period of time, performing a validation test on the AIS data acquired during the predetermined period of time, converting the AIS data determined to be suitable through the validation test, from an earth coordinate system into an inertial coordinate system, and estimating a position and a velocity of a satellite by using the AIS data converted into the inertial coordinate system and a predetermined orbit propagator model.

The method further includes calculating a circular dispersion for the AIS data acquired during the predetermined period of time.

A condition for determining suitability in the validation test includes a test criterion for a circular dispersion in which the calculated circular dispersion is determined to be equal to or greater than a predetermined reference value.

The condition for determining suitability in the validation test may further include a test criterion for the number of ships in which the number of ships for which the AIS data is acquired during the predetermined period of time is determined to be equal to or greater than a predetermined reference value.

A reference value of the test criterion for circular dispersion and a reference value of the test criterion for number of ships may be increased as a success rate of determining suitability in the validation test for the AIS data increases.

In order to accomplish the technical objectives mentioned above, a device as set forth in claim <NUM> is provided.

According to the present disclosure, there is an advantage in that the orbit can be determined using the automatic identification system (AIS) data when a satellite-mounted GPS receiver malfunctions or a GPS satellite signal reception rate decreases. In particular, while the precision is lower than that of the existing GPS-based orbit propagator, there is an advantage that it is useful for determining the initial orbit after launching a satellite.

Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings so that those with ordinary knowledge in the art can easily achieve the present disclosure.

<FIG> is a block diagram illustrating a configuration of a satellite orbit propagator according to an embodiment of the present disclosure.

Referring to <FIG>, the satellite orbit propagator according to the present disclosure may include an AIS reception unit <NUM>, a data acquisition unit <NUM>, a circular dispersion calculation unit <NUM>, a control unit <NUM>, a validation test unit <NUM>, a coordinate conversion unit <NUM>, and a satellite orbit propagator unit <NUM>.

The AIS reception unit <NUM> is mounted on a satellite, and the data acquisition unit <NUM>, the circular dispersion calculation unit <NUM>, the control unit <NUM>, the validation test unit <NUM>, the coordinate conversion unit <NUM>, and the satellite orbit propagator unit <NUM> may be mounted on the satellite or a ground station system.

When the data acquisition unit <NUM> or the like is mounted on the ground station system, the AIS data received by the AIS reception unit <NUM> may be downlinked from the satellite to the ground station system.

The AIS reception unit <NUM> is mounted on the satellite and receives automatic identification system data (AIS data) that is transmitted wirelessly from a ship.

The ship automatic identification device mounted on the ship may wirelessly transmit the AIS data including ship operation information at predetermined intervals. The AIS data may include ship information and cruise information (time, position, velocity, direction angle, and the like).

<FIG> illustrates a valid reception area of a satellite-mounted AIS receiver and AIS observation data on the Korean Peninsula.

As illustrated in <FIG>, the valid reception area of AIS data is <NUM> in the center radius of the satellite. On the other hand, as illustrated in <FIG>, the average number of AIS-equipped ships entering Korea is at least <NUM> ships/day, which has the advantage that the number of AIS observation data is sufficient.

The data acquisition unit <NUM> acquires the AIS data received by the AIS reception unit <NUM>.

When the data acquisition unit <NUM>, the circular dispersion calculation unit <NUM>, the control unit <NUM>, the validation test unit <NUM>, the coordinate conversion unit <NUM> and the satellite orbit propagator unit <NUM> are mounted on the ground station system, the AIS data received by the AIS reception unit <NUM> may be transmitted to the ground station system through satellite communication.

The circular dispersion calculation unit <NUM> may calculate a circular dispersion for the ship automatic identification information data acquired during a predetermined period of time. In this example, the predetermined period of time may be determined using the received AIS data, to be an update cycle time corresponding to a cycle for updating the satellite position and velocity of the satellite orbit propagator. For example, when the satellite position and velocity are updated in the satellite orbit propagator every <NUM> minute, the update cycle time may be set to be <NUM> minute. Of course, depending on embodiments, the predetermined period of time may be determined from any time to the update time of the satellite orbit propagator.

The validation test unit <NUM> may perform a validation test on the AIS data acquired by the data acquisition unit <NUM> during a predetermined period of time to determine whether the AIS data is suitable or not.

The validation test unit <NUM> may determine whether the AIS data is suitable or not based on a predetermined suitability determination condition, and the suitability determination condition may include a test criterion value for number of ships and a test criterion value for circular dispersion. In addition, the test criterion value for number of ships and the test criterion value for circular dispersion of the suitability determination condition may be set differently according to the success rate of determining suitability, which will be described below.

The control unit <NUM> controls the overall operation of the satellite orbit propagator according to the present disclosure. The control unit <NUM> may calculate the success rate of determining suitability of the validation test for the AIS data, and automatically change the test criterion value for number of ships and the test criterion value for circular dispersion accordingly.

The coordinate conversion unit <NUM> may convert the AIS data determined to be suitable through the validation test, from the earth coordinate system to the inertial coordinate system.

While the satellite orbit is expressed in the inertial coordinate system, the position and velocity information of the ship in the received AIS data is expressed in the earth coordinate system. Accordingly, it is necessary to convert the position and velocity information of the ship in the received AIS data from the earth coordinate system to the inertial coordinate system. In order to calculate the direction cosine matrix for this purpose, correction values for observation time (polar motion, earth rotation, nutation, precession, bias, and the like) are required, and these values are regularly distributed by the International Astronomical Union (IAU)/Standards Of Fundamental Astronomy (SOFA). For reference, the precision of the direction cosine matrix may be determined according to the range of the correction values considered. The conversion of information expressed in the earth coordinate system to the inertial coordinate system is well known, and accordingly, a detailed description thereof will be omitted.

The satellite orbit propagator unit <NUM> may estimate the position and velocity of the satellite using the AIS data converted into the inertial coordinate system and a predetermined orbit propagator model. The orbit information of the satellite can be calculated using the position and velocity information converted into the inertial coordinate system and the orbit propagator model, and commonly used satellite orbit propagator models include Two Body Propagator, J2 Propagator, J4 Propagator, and Simplified General Perturbation (SGP) Model, and the like. Each of these models has a different amount of calculation, so there may be differences in precision performance. As described above, the satellite orbit propagators that estimate the current position and velocity of a satellite on the satellite orbit or predict the position and velocity of the satellite in the near future by fusing the satellite orbit model of the corresponding satellite and the observation data observed by the corresponding satellite, are already well known, and accordingly, a detailed description thereof will be omitted.

<FIG> is a flowchart provided for explaining an operation of a satellite orbit propagator according to an embodiment of the present disclosure.

Before providing description with reference to <FIG>, first, the following are the definition of the terms.

Tk: time of updating satellite orbit propagator using AIS data.

Nk: number of ships from which AIS data is acquired between times Tk-<NUM> and Tk.

<FIG> is a diagram illustrating the test criterion for number of ships according to an embodiment of the present disclosure, and <FIG> is a diagram illustrating the test criterion for circular dispersion according to an embodiment of the present disclosure.

As illustrated in <FIG>, fN(p) and FCE(P) may be monotonic increasing functions having a positive value as a function of the success rate (p) of determining suitability in the validation test, and may be designed according to the performance of the system. fN(p) and FCE(P) may be designed to be automatically determined using the success rate (p) of determining suitability in the validation test.

<FIG> illustrates a test result for number of ships according to an embodiment of the present disclosure.

Referring to <FIG>, when the number of ships from which the AIS data is received is small as illustrated in <FIG>, it may be determined that the number of observation data used in the satellite orbit propagator is insufficient and that it is not suitable to accurately estimate the position and velocity of the satellite. In addition, when the number of ships from which the AIS data is received is equal to or greater than a predetermined criterion as illustrated in <FIG>, it may be determined that the number of observation data used in the satellite orbit propagator is sufficient and it is suitable to accurately estimate the position and velocity of the satellite.

<FIG> illustrates a test result for circular dispersion according to an embodiment of the present disclosure.

Referring to <FIG>, when the positions of the ships from which the AIS data is received are not evenly distributed as illustrated in <FIG> and thus the circular dispersion is lower than a predetermined criterion, it may be determined that it is not suitable to estimate the position and velocity of the satellite. Conversely, when the positions of the ships from which the AIS data is received are dispersed by more than a predetermined criterion as illustrated in <FIG> and thus the circular dispersion is higher than the predetermined criterion, it may be determined that it is suitable to accurately estimate the position and velocity of the satellite.

Referring back to <FIG>, first, initialization is performed to set t = t0 (≥ <NUM>), p = p0 (≥ <NUM>) (S510).

Until time t = Tk (S530-Y), the AIS reception unit <NUM> continuously receives the AIS data wirelessly transmitted from the ship (S520).

The data acquisition unit <NUM> may acquire the received AIS data while repeating operations at S520 and S530. The data acquisition unit <NUM> may acquire AIS data ( <MAT>) between the previous update time Tk-<NUM> of the satellite orbit propagator and the new update time Tk.

Next, the circular dispersion calculation unit <NUM> may calculate a circular dispersion CEk by using the AIS data acquired by the data acquisition unit <NUM> (S540). At S540, the circular dispersion calculation unit <NUM> may calculate CEk by using <MAT>.

The circular dispersion CEk can be defined as follows. For example, a circular dispersion may be defined as a radius of a circle including points of a predetermined ratio from a center point (or center position) of a cluster of points corresponding to the longitudinal and latitudinal positions of the ship for which AIS data is acquired between Tk-<NUM> and Tk. For example, when the predetermined ratio is <NUM>% and AIS data corresponding to <NUM> ships is acquired, the radius of a circle including <NUM> points from the center of a cluster of points corresponding to the positions of the <NUM> ships may be calculated as a circular dispersion CEk.

Next, the validation test unit <NUM> may perform a validation test on the AIS data to determine whether it is suitable or not (S550).

The validation test may include a test of number of ships and a test of circular dispersion.

The test of number of ships may be implemented to determine it suitable when the number of AIS data acquired during the predetermined period of time is equal to or greater than a predetermined reference value. That is, when Nk ≥ fN(p), the test of number of ships may determine it suitable.

The test of circular dispersion may determine it suitable when CEk ≥ fCE(p).

When it is determined to be suitable by both the test of number of ships and the test of circular dispersion at S550 (S550-Y), the control unit <NUM> may transmit the AIS data ( <MAT>) to the coordinate conversion unit <NUM> (S563).

Next, the coordinate conversion unit <NUM> may receive the AIS data ( <MAT>, <MAT>) determined to be suitable through the validation test, and convert it from the earth coordinate system into the inertial coordinate system (S570).

In addition, the satellite orbit propagator unit <NUM> may estimate the position and velocity of the satellite using the AIS data converted into the inertial coordinate system and a predetermined orbit propagator model (S580).

Meanwhile, when it is determined unsuitable by one of the test of number of ships and the test of circular dispersion at S530 (S550-N), the control unit <NUM> may delete the AIS data ( <MAT>) (S567).

The control unit <NUM> may calculate the success rate (p) of determining suitability in the validation test by Equation <NUM> below when the validation test is suitable, and may calculate it by Equation <NUM> when the validation test is not suitable (S591). In addition, the control unit <NUM> may change fN(p) and fCE(p) according to the newly calculated success rate (p) of determining suitability. The operation at S591 may be performed between S550 and S593. <MAT> <MAT>.

In Equations <NUM> and <NUM>, Δp is a positive number and may be determined as a design parameter.

That is, as the success rate of determining suitability in the validation test for AIS data increases, the test criterion value FCE(P) for circular dispersion and the test criterion value fN(p) for number of ships also increase, resulting in strengthened criterion for validation test.

In addition, depending on embodiments, it is also possible to calculate the success rate of determining suitability by Equation <NUM> below. <MAT> where, Ns may be defined as the number of times of determining suitability in validation test, and NF may be defined as the number of times of determining unsuitability in validation test.

The success rate of determining suitability may be defined in a manner other than that described herein.

Next, k is increased by <NUM>, and the process moves to S520 to perform a procedure according to the next update time Tk+<NUM>.

Embodiments which do not form part of the claimed invention include a computer-readable recording medium including program instructions for performing various computer implemented operations. The recording medium records a program for executing the methods described above. The recording medium may include program instructions, data files, data structures, and so on, either alone or in combination. Examples of such recording medium include a magnetic medium such as hard disk, floppy disk and magnetic tape, an optical recording medium such as CD and DVD, a magneto-optical medium, and a hardware device configured to store and carry out program instructions, such as ROM, RAM, flash memory, and so on. Examples of program instructions include high-level language codes that may be executed by a computer using an interpreter, and so on as well as machine language codes such as those generated by a compiler.

Claim 1:
A method comprising:
acquiring ship automatic identification system data (AIS data) during a predetermined period of time, wherein the AIS data are received at a satellite;
performing a validation test on the AIS data acquired during the predetermined period of time (S550);
converting the AIS data determined to be suitable through the validation test, from an earth coordinate system into an inertial coordinate system (S570); and
estimating a position and a velocity of the e satellite by using the AIS data converted into the inertial coordinate system and a predetermined orbit propagator model (S580),
characterized in that
the method further comprises
calculating a circular dispersion for the AIS data acquired during the predetermined period of time (S540); and
wherein a condition for determining suitability in the validation test includes a test criterion for circular dispersion wherein the calculated circular dispersion is determined to be equal to or greater than a predetermined reference value.