Factor graph optimization-based positioning method and device using multiple low earth orbit satellites

Abstract

The present disclosure relates to a positioning method of a terminal, using multiple low earth orbit (LEO) satellites. The positioning method may comprise the steps of: receiving a signal from each of one or more LEO satellites; acquiring ephemeris data of each of the LEO satellites; determining the Doppler shift on the basis of the ephemeris data and the signal; determining state variables and at least one factor to be used in factor graph optimization; and acquiring a positioning result using the factor graph optimization on the basis of the Doppler shift.

Description

Positioning method and device based on factor graph optimization using multiple low-orbit satellites

[0001] The present disclosure relates to positioning using multiple low-orbit satellites, and to a positioning method and device based on factor graph optimization (FGO).

[0002] The Global Navigation Satellite System (GNSS) uses satellites to provide information on the location, altitude, and velocity of terrestrial objects. GNSS-based positioning methods offer several advantages: they can be accessed regardless of the user's geographic location, their compact size, and the ability to obtain real-time output, allowing for mobile operation. However, GNSS is not suitable for precise positioning because it relies on pseudoranges, which are estimated distances that take into account factors such as propagation delay and synchronization errors. Furthermore, its low signal power makes it particularly difficult to provide accurate positioning results in urban canyons, where tall buildings are densely packed. Networks comprised of numerous low-Earth orbit (LEO) satellites can more easily secure line-of-sight (LOS) conditions and offer shorter delays, making them suitable for more precise positioning.

[0003] The present disclosure provides a method and device for performing precise positioning based on factor graph optimization (FGO) using multiple low-orbit satellites.

[0004] The present disclosure provides a method and device for determining factors of a factor graph in a positioning method.

[0005] The present disclosure provides a method and device for determining a delta range factor and a Doppler velocity factor in a positioning method.

[0006] The present disclosure provides a method and device for connecting state variables by Doppler velocity factors in a positioning method.

[0007] The present disclosure provides a method and device for performing positioning based on covariance and factor graph optimization in a positioning method.

[0008] According to one embodiment of the present disclosure, a method for positioning a terminal using multiple low-orbit satellites may include the steps of receiving signals from at least one low-orbit satellite, obtaining ephemeris data of each of the low-orbit satellites, determining a Doppler shift based on the ephemeris data and the signal, determining a state variable and at least one factor to be used in factor graph optimization, and obtaining a positioning result using the factor graph optimization based on the Doppler shift.

[0009] According to one embodiment of the present disclosure, the at least one factor may include a delta range factor and a Doppler velocity factor.

[0010] According to one embodiment of the present disclosure, the state variables may be linked by a Doppler velocity factor.

[0011] According to one embodiment of the present disclosure, the error function value of the delta range factor may be determined based on the difference between the delta range measurement vector and the observation function for measuring the Doppler shift, and the error function value of the Doppler velocity factor may be determined based on the difference between the velocity measurement vector and the observation function for measuring the velocity.

[0012] According to one embodiment of the present disclosure, the covariance matrix of the delta range factor and the Doppler velocity factor can be determined based on the elevation angle and carrier-to-noise ratio of the low-orbit satellite.

[0013] According to one embodiment of the present disclosure, the positioning result can be determined based on an error function value of the delta range factor, an error function value of the Doppler velocity factor, and a covariance matrix.

[0014] According to one embodiment of the present disclosure, the state variable may include at least one of a position of the terminal, a velocity of the terminal, or a receiver clock bias.

[0015] According to one embodiment of the present disclosure, the celestial data may include at least one of the status of each of the low-orbit satellites, the age of the data, a clock correction factor, or a parameter value regarding the orbit.

[0016] According to one embodiment of the present disclosure, a terminal using satellite communication may include a transceiver and at least one processor connected to the transceiver, wherein the at least one processor receives signals from at least one low-orbit satellite, obtains ephemeris data of each of the low-orbit satellites, determines a Doppler shift based on the ephemeris data and the signal, determines state variables and at least one factor to be used in factor graph optimization, and obtains a positioning result using the factor graph optimization based on the Doppler shift.

[0017] The following effects may be achieved by embodiments based on the present disclosure.

[0018] According to the present disclosure, a terminal can perform precise positioning using multiple low-orbit satellites.

[0019] According to the present disclosure, a terminal can perform precise positioning based on a Doppler velocity factor by using multiple low-orbit satellites.

[0020] The effects that can be obtained from the embodiments of the present disclosure are not limited to the effects mentioned above, and other effects that are not mentioned can be clearly derived and understood by those skilled in the art to which the technical configuration of the present disclosure is applied, from the description of the embodiments of the present disclosure below. In other words, unintended effects resulting from implementing the configuration described in the present disclosure can also be derived by those skilled in the art from the embodiments of the present disclosure.

[0021] FIG. 1 is a diagram illustrating an example of a network supporting satellite communication according to one embodiment of the present disclosure.

[0022] FIG. 2 is a diagram illustrating another example of a network system supporting satellite communication according to one embodiment of the present disclosure.

[0023] FIG. 3 is a diagram illustrating an example of a wireless device in a system supporting satellite communication according to one embodiment of the present disclosure.

[0024] FIG. 4 is a diagram showing a path along which a satellite and a terminal perform communication in an urban canyon according to one embodiment of the present disclosure.

[0025] FIG. 5 is a diagram showing an error distribution in a line of sight (LOS) environment according to one embodiment of the present disclosure.

[0026] FIG. 6 is a diagram showing an error distribution in a non-LOS environment according to one embodiment of the present disclosure.

[0027] FIG. 7 is a diagram showing an environment of a positioning method using a low-orbit satellite according to one embodiment of the present disclosure.

[0028] FIG. 8 is a diagram illustrating the structure of a factor graph according to one embodiment of the present disclosure.

[0029] FIG. 9 is a diagram illustrating an example in which a Doppler shift occurs according to one embodiment of the present disclosure.

[0030] FIG. 10 is a diagram illustrating a procedure for performing positioning using low-orbit satellites according to one embodiment of the present disclosure.

[0031] The terms used in these examples are selected from widely used, common terms, taking into account the functions they serve. However, these terms may vary depending on the intentions of engineers working in the field, precedents, the emergence of new technologies, etc. Furthermore, in certain cases, terms may be arbitrarily selected by the applicant, in which case their meanings will be described in detail in the relevant section. Therefore, the terms used in these examples should not be defined simply as names, but rather based on the meanings they embody and the overall content of these examples.

[0032] These embodiments may be modified in various ways and may take on various forms. Therefore, some embodiments are illustrated in the drawings and described in detail. However, this is not intended to limit these embodiments to a specific disclosed form, but rather to encompass all modifications, equivalents, and alternatives falling within the spirit and technical scope of these embodiments. The terminology used herein is solely for the purpose of describing the embodiments and is not intended to limit these embodiments.

[0033] Unless otherwise defined, the terms used in these examples have the same meaning as commonly understood by those of ordinary skill in the technical field to which these examples pertain. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an idealized or overly formal sense unless explicitly defined in these examples.

[0034]

[0035] FIG. 1 is a diagram illustrating an example of a network supporting satellite communication according to one embodiment of the present disclosure. Referring to FIG. 1, the satellite network includes a terminal (110), satellites (120-1, 120-2), and a gateway (130). The terminal (110) is a user device and may be mobile or fixed. The terminal (110) may support variable service bands and operation information depending on the capabilities and application operation of the terminal (110). In addition, the terminal (110) may be operated in a fixed form, a form specialized for mobility, or in various forms depending on the characteristics of the terminal (110). The terminal (110) may be referred to as a 'UE (user equipment)'. The satellites (120-1, 120-2) fly / operate in a fixed orbit and form a beam toward the ground, thereby providing a cell with a certain size of coverage. The gateway (130) provides a link to the satellites (120-1, 120-2) to access the network.

[0036] According to an example of the present disclosure, the link between the terminal (110) and the satellite (120-1) is referred to as a service link, and the link between the satellites (120-1, 120-2) and the gateway (130) is referred to as a feeder link. Here, the link may be a link based on the NR standard. Alternatively, a link newly defined in an evolved next-generation wireless communication system may be adaptively applied, or a link based on various interfaces of a communication system introduced by industry needs rather than the NR standard may be applied. In addition, an inter-satellite link (ISL) may be mainly used in the case of regenerative satellites.

[0037] For example, in the case of a transparent satellite based on the NR-RAN architecture, the satellite radio interfaces of the feeder link and service link may be NR-Uu. For transparent satellites, the satellite performs radio frequency filtering, frequency conversion, and amplification functions. For regenerative satellites, onboard functions are built into the satellite, allowing the satellite to perform some or all of the base station functions, such as switching and routing, coding and modulation, and decoding and demodulation, in addition to radio frequency filtering, frequency conversion, and amplification.

[0038] FIG. 2 is a diagram illustrating another example of a network system supporting satellite communication according to one embodiment of the present disclosure. FIG. 2 illustrates an example of an NTN that provides non-terrestrial connectivity to a terminal (210) using an NTN payload (220) and an NTN gateway (230). Here, the link between the NTN payload (220) and the terminal (210) is a service link and may be based on a Uu interface. The link between the NTN payload (220) and the NTN gateway (230) is a feeder link. The link between the NTN gateway (230) and an AMF / UPF (240) may be based on an NG interface. The NTN payload (220) may transparently forward a wireless protocol received from the terminal (210) to the NTN gateway (230) via the service link. Similarly, the NTN payload (220) can transparently forward a wireless protocol received from the NTN gateway (230) via a feeder link to the terminal (210).

[0039] To this end, the following connectivity may be supported by the NTN payload (220): A base station may service multiple NTN payloads; an NTN payload may be serviced by multiple base stations;

[0040] The NTN payload (220) can change the carrier frequency before retransmitting data on the service link. That is, the NTN payload (220) can use different carrier frequencies on the service link and the feed link. For the NTN, as a network identifier, at least one of an AMF name, an NR cell global identifier (NCGI), a CgNB identifier (ID), a global gNB ID, a tracking area identity (TAI), a Single Network Slice Selection Assistance information (S-NSSAI), a Network Slice AS Group (NSAG), a Network Identifier (NID), a Closed Access Group (CAG) ID, and a local NG-RAN node ID (Identifier) ​​can be used, and additionally, a Mapped Cell ID can be further used. Here, the tracking area can correspond to a fixed geographical area.

[0041] Non-geosynchronous orbits (NGSOs) include low Earth orbits at altitudes of about 300 km to 1500 km and medium Earth orbits at altitudes of about 7000 km to 25000 km.

[0042] Service links can be categorized into three types: earth-fixed, quasi-earth-fixed, and earth-moving. The earth-fixed type provides beam(s) that continuously cover the same geographic area at all times. For example, a satellite in a geosynchronous orbit (GSO) can provide an earth-fixed service link. The quasi-earth-fixed type provides beam(s) that continuously cover the same geographic area for a limited period of time, and provides beams that cover different geographic areas for different periods of time. For example, a satellite in a non-earth-synchronous orbit can provide a quasi-earth-fixed service link using steerable beams. The earth-moving type provides beams whose coverage area slides over the surface of the Earth. For example, a satellite with a non-Earth-synchronous orbit could provide an Earth-mobile type service link using fixed or steerable beams.

[0043] Using satellites in non-Earth-synchronous orbits, a base station can provide quasi-Earth-fixed cell coverage or Earth-mobile cell coverage. Using satellites in geosynchronous orbits, a base station can provide Earth-fixed cell coverage. In non-Earth-synchronous orbits, a change in the service link may refer to a change in the serving satellite.

[0044] Pre-compensation by the terminal can be performed as follows. For the serving cell, the network can broadcast common timing advance (TA) parameters and ephemeris information. Here, the common TA means an offset corresponding to the RTT between the NTN payload and the reference point (RP). Accordingly, before connecting to the NTN cell, the terminal will have information about the satellite orbit and the common TA, and further, will have a valid global navigation satellite system (GNSS) position. To achieve synchronization, before and during connecting to the NTN cell, the terminal calculates the round trip time (RTT) of the serving link based on the GNSS position and the satellite orbit, and the frame time difference between the downlink and uplink (e.g., T TA ) can be pre-compensated.

[0045] The terminal can calculate the frequency Doppler shift considering the terminal's position and satellite orbit. If the terminal does not have a valid GNSS position and / or a valid satellite orbit, the terminal will not be able to communicate with the network until it acquires a valid GNSS position and a valid satellite orbit. In connected mode, the terminal can continuously update the TA and frequency pre-compensation. The terminal can be configured to report the TA during a random access procedure or in connected mode. In connected mode, event-triggered TA reporting can be supported.

[0046] While pre-compensation of instantaneous Doppler shifts experienced on the service link is performed by the terminal, management of Doppler shifts and transponder frequency errors experienced on the feeder link can be implemented by the satellite network.

[0047] The O&M (operations and maintenance) requirements are as follows. The following NTN-related parameters may be provided to a base station providing non-terrestrial connectivity by O&M. For example, orbital information describing the coordinates or orbital trajectory information of an NTN satellite may be provided. The orbital information may be provided upon request of the base station or on a regular basis. As for the format of the orbital information, two different sets may be supported. The first set includes satellite position and velocity state vectors, i.e., position and velocity. The second set may include at least one of the semi-major axis, eccentricity, argument of periapsis, longitude of ascending node, inclination, and mean anomaly at epoch time to. Additionally, additional information may be provided to enable location information of NTN gateways and base station operation for feeder / service link switches.

[0048] Information related to the satellite's orbit and the NTN gateway's location can be used for at least one of uplink timing and frequency synchronization. Furthermore, information related to the satellite's orbit and the NTN gateway's location can also be used for mobility management purposes and random access. The NTN-related parameters provided to the base station by the O&M may depend on the type of service link supported (e.g., Earth-fixed beam, quasi-Earth-fixed beam, moving beam, etc.).

[0049]

[0050] FIG. 3 is a diagram illustrating an example of a wireless device (300) in a system supporting satellite communication according to an embodiment of the present disclosure. The wireless device (300) included in the positioning system according to an embodiment of the present disclosure may be a mobile terminal such as a smartphone, tablet PC, or wearable device, but is not limited thereto.

[0051] Referring to FIG. 3, the wireless device (300) may include at least one control unit (310), at least one memory (320), at least one power unit (330), at least one transceiver unit (340), at least one input unit (350), at least one output unit (360), and / or at least one antenna (370).

[0052] The control unit (310) can control the memory (320) and / or the transceiver (340), and can be configured to implement the descriptions, functions, procedures, proposals, methods, and / or operation flowcharts disclosed in this document. The memory (320) can be connected to the control unit (310) and can store various information related to the operation of the control unit (310). For example, the memory (320) can perform some or all of the controls controlled by the control unit (310), or store software code including commands for performing the descriptions, functions, procedures, proposals, methods, and / or operation flowcharts disclosed in this document. The transceiver (340) can be connected to the control unit (310) and can transmit and / or receive wireless signals via at least one antenna (370). The transceiver (340) can include a transmitter and / or a receiver. The transceiver (340) may include a receiver that receives signals from a low-orbit satellite.

[0053] At least one control unit (310) may be referred to as a controller, a microcontroller, a microprocessor, or a microcomputer. At least one control unit (310) may be implemented by hardware, firmware, software, or a combination thereof. For example, at least one application specific integrated circuit (ASIC), at least one digital signal processor (DSP), at least one digital signal processing device (DSPD), at least one programmable logic device (PLD), or at least one field programmable gate array (FPGA) may be included in at least one control unit (310). The descriptions, functions, procedures, proposals, methods, and / or operation flowcharts disclosed in this document may be implemented using firmware or software, and the firmware or software may be implemented to include modules, procedures, functions, etc. The descriptions, functions, procedures, proposals, methods, and / or operation flowcharts disclosed in this document may be included in at least one control unit (310), or may be stored in at least one memory (320) and driven by at least one control unit (310). The descriptions, functions, procedures, suggestions, methods and / or flowcharts disclosed in this document may be implemented using firmware or software in the form of code, instructions and / or sets of instructions.

[0054] At least one transceiver (340) can transmit user data, control information, wireless signals / channels, etc. mentioned in the methods and / or operation flowcharts of this document to at least one other device. At least one transceiver (340) can receive user data, control information, wireless signals / channels, etc. mentioned in the descriptions, functions, procedures, proposals, methods and / or operation flowcharts disclosed in this document from at least one other device. For example, at least one transceiver (340) can be connected to at least one control unit (310) and can transmit and receive wireless signals. For example, at least one control unit (310) can control at least one transceiver (340) to transmit user data, control information, or wireless signals to at least one other device. In this document, at least one antenna (370) can be a plurality of physical antennas or a plurality of logical antennas (e.g., antenna ports). At least one transceiver (340) can convert received user data, control information, wireless signals / channels, etc. from RF band signals to baseband signals in order to process the received user data, control information, wireless signals / channels, etc. using at least one control unit (310). At least one transceiver (340) can convert processed user data, control information, wireless signals / channels, etc. from baseband signals to RF band signals using at least one control unit (310).

[0055] The input unit (350) is a configuration for obtaining information such as user input, video, and audio, and may include various input means such as various mechanical / electronic input means, cameras, and microphones. The output unit (360) is for providing information to users by generating output related to sight, hearing, or touch, and may include a display, a speaker, a vibration module, and the like. The wireless device (300) supplies power through the power supply unit (330), and the power supply unit (330) may include a wired / wireless charging circuit, a battery, and the like.

[0056] The wireless device (300) may be a mobile device such as a mobile robot, a vehicle, a train, an aerial vehicle (AV), a ship, etc. In this case, the device may further include at least one of a driving unit including at least one of an engine, a motor, a power train, wheels, brakes, and a steering unit of the device, a sensor unit that supplies power and senses status information, environmental information, and user information around the device, an autonomous driving unit that performs functions such as path maintenance, speed control, and destination setting, and a position measurement unit that obtains mobile location information through a global positioning system (GPS) and various sensors.

[0057] Current GNSS-based positioning methods can suffer from low accuracy in urban canyon environments. In urban canyon environments, satellite signals are blocked from reaching the terminal directly, forcing the terminal to receive satellite signals via multiple paths. Consequently, current GNSS-based positioning estimates can be subject to significant errors.

[0058] FIG. 4 is a diagram illustrating a path for communication between a satellite and a terminal (410) in an urban canyon according to one embodiment of the present disclosure. GNSS can perform globally referenced positioning in outdoor environments. When using GNSS, the terminal (410) can perform positioning based on pseudoranges. As shown in FIG. 4, pseudoranges can be affected by non-line-of-sight (NLOS) and multipath. Furthermore, the strength of GNSS signals can be reduced in indoor environments and urban canyons. On the other hand, an inertial navigation system (INS) can be less sensitive than GNSS under changing environmental conditions. INS can be implemented by measuring linear acceleration and angular velocity in the high-frequency range. However, INS has the disadvantage of accumulative errors increasing over time. Therefore, a GNSS-INS integration method can be an alternative method in environments where GNSS is blocked. At this time, a Kalman filter (KF), an extended Kalman filter (Extended KF), and an unscented KF, which are types of Bayesian filters, can be used. A first-order Markov chain can be applied to these methods, and noise can be modeled as a Gaussian distribution. For example, the signal transmitted by the first satellite (420#1) can only experience a line-of-sight (LOS) environment. As a result, the error distribution of the measurement data may appear similar to a Gaussian distribution, as shown in FIG. 5. Therefore, the terminal (410) can perform relatively accurate positioning. However, if the actual error distribution differs from the Gaussian distribution, the positioning quality may deteriorate. For example, in an urban canyon environment, the signal transmitted by the second satellite (420#2) can experience both an LOS environment and a non-LOS environment.Therefore, the error distribution of measurement data can vary depending on the satellite's position. As a result, the error distribution of GNSS measurement data can exhibit a non-Gaussian distribution, as shown in Figure 6. In addition, since the environment changes over time for non-geostationary satellites, the error distribution in urban canyons can exhibit high temporal correlation. In other words, if noise has a non-Gaussian distribution (e.g., if data is contaminated by extreme values), the results from the positioning method can also be biased or inaccurate. Therefore, for precise positioning, it may be necessary to process extreme measurement results or explore temporal correlations. Factor graph optimization can be used to simultaneously explore temporal correlations between past measurements. In this case, LEO-based communications may have several advantages over traditional medium Earth orbit (MEO) and geostationary (GEO) satellite-based communications.

[0059] Current GNSS constellations are inefficient in providing high-quality geometric LOS vectors. In contrast, mega-LEO satellite constellations, deploying thousands of space vehicles (SVs), can provide numerous LOS vectors. Therefore, research into LEO-PNTRC (positioning, navigation, timing, remote sensing, and communications) based on LEO satellites is actively underway.

[0060] A large number of low-Earth orbit (LEO) satellite constellations have close orbits to the Earth. Therefore, LEO satellite constellations can serve as reliable sources for measuring Doppler frequency shifts. In traditional global navigation satellite systems (GNSS), high-frequency shifts can be used as an alternative to pseudorange-based methods. Factor graph optimization (FGO), based on positioning methods, can handle high time-correlation and non-Gaussianity in urban canyons, and simultaneously explore time-correlations between past measurements.

[0061] This disclosure proposes a factor graph optimization method based on a positioning method utilizing robust Doppler shift measurements using a constellation of low-Earth orbit satellites. In performing factor graph optimization, the delta range factor and the Doppler velocity factor, which can serve as error functions, can be considered. The delta range factor can indicate the error between Doppler shift measurements and observations. The Doppler velocity factor can indicate the error between Doppler velocity measurements and observations between two consecutive epochs. In factor graph optimization, estimation states can be linked using the Doppler velocity factor. Note that communication with a satellite may be required to measure the Doppler shift.

[0062]

[0063] Figure 7 is a diagram illustrating an environment for a positioning method using a low-orbit satellite according to one embodiment of the present disclosure. Low-orbit satellites can approach the Earth relatively closer than geostationary and medium-orbit satellites of GNSS. Because low-orbit satellites can have an orbital altitude of 780 km, they can transmit signals that are 20 dB stronger than GNSS signals to a terminal. A low-orbit satellite constellation can provide a sufficient variety of LOS vectors. A terminal can select the most suitable satellite for network communication and registration procedures among multiple satellites over the horizon. Low-orbit satellite signals can penetrate indoors, and low-orbit satellite communication systems can provide good coverage even in deep urban canyon environments. Furthermore, communication delay times can be reduced from 700 ms to 100 ms, and low-orbit satellites close to Earth's orbit can provide strong signal power and a high Doppler frequency shift range (±40 kHz). Therefore, Doppler shift-based positioning technology for low-Earth orbit satellite signals can be an alternative to current pseudorange-based GNSS. In this disclosure, a factor graph optimization system based on Doppler shift measurements using a low-Earth orbit satellite constellation can be proposed, without using data from other types of sensors (INS, IMU, etc.).

[0064] In factor graph optimization, Doppler shift measurements of LEO satellite signals can be utilized. In factor graph optimization, each factor can act as an error function, and delta range factors and Doppler velocity factors can be considered. The delta range factor can indicate the error between Doppler measurements and observations. The Doppler velocity factor can indicate the error between Doppler velocity measurements and observations between two consecutive epochs. In factor graph optimization, estimation states can be linked using the Doppler velocity factor.

[0065] A factor graph is a bipartite undirected graph and can be used to model large functions with many variables by factoring them into smaller local subsets. Since the joint probability distribution can be expressed as the product of multiple single factors, as shown in Equation 1 below, large functions can be more easily analyzed.

[0066]

[0067] In [Equation 1], stands for joint probability distribution, is a variable means a local subset of, represents a factor of a joint probability distribution. In addition, factor graphs can be used to express the relationship between unknown states and factors.

[0068] FIG. 8 is a diagram illustrating the structure of a factor graph according to one embodiment of the present disclosure. A factor graph is a visual representation of unknown state variables (e.g., variable nodes) and factors that are functions of the state variables (e.g., factor nodes). As an example of the present disclosure, factors such as a delta range factor and a Doppler velocity factor may be considered. The Doppler velocity factor may be used to connect state variables. The number of delta range factors may be connected to each state variable equal to the number of satellites. The Doppler velocity factor may refer to a factor related to the position of the terminal, and the delta range factor may refer to a factor related to the Doppler shift, which is the frequency difference between a signal transmitted from 7 satellites and a signal received by the terminal. In FIG. 8, four delta range factors are connected to each state node, assuming a situation where the number of satellite signals considered is four. The number of satellite signals may vary depending on the performance of the terminal and the target positioning accuracy.

[0069] The state variable may include at least one of the terminal's position, the terminal's velocity, and the terminal's clock bias. Accordingly, the state variable may be configured as in [Mathematical Formula 2] below.

[0070]

[0071] In [Equation 2], represents the state of the terminal (receiver) at the kth epoch, means the location of the terminal, refers to the speed of the terminal, refers to the clock drift of the terminal.

[0072] By configuring the state variables as in [Mathematical Formula 2], the Doppler velocity factor can be determined based on two consecutive state variables, and thus can be a factor related to velocity. The states of all terminals and the positions of satellites can be considered based on the earth-entered earth-fixed (ECEF) frame. Since the state variables include the velocity and clock drift along with the terminal position, the velocity of the terminal can be considered when performing positioning, and more precise positioning can be achieved. Referring to Fig. 8, the terminal receives a signal from a satellite, measures the Doppler shift, determines the delta range factor and the Doppler velocity factor based on the measured Doppler shift, and performs optimization based on the factor graph, thereby determining the set of state variables x, and the specific method can be used as described below.

[0073] FIG. 9 is a diagram illustrating an example of a Doppler shift occurring according to one embodiment of the present disclosure. In the present disclosure, the Doppler shift refers to a frequency change of an electromagnetic signal due to relative movement between a terminal (910) and a low-orbit satellite (920). As shown in FIG. 9, it can be assumed that the terminal (910) and the low-orbit satellite (920) are moving. and At this point, the positions of the low-orbit satellites (920) are respectively and . P and Q are respectively and At this point, it refers to the sub-satellite point of the low-orbit satellite (920). The LOS vector r between the terminal (910) and the low-orbit satellite (920) is indicated by a dotted line.

[0074] The Doppler shift can be expressed as in [Mathematical Formula 3] below.

[0075]

[0076] In [Equation 3], means Doppler shift, means the LOS vector change rate, refers to the wavelength of the signal transmitted by the low-orbit satellite (920).

[0077] The frequency of the signal transmitted by the low-orbit satellite (920) About, can be expressed as in [Mathematical Formula 4] below.

[0078]

[0079] In [Mathematical Equation 4], c represents the speed of light.

[0080] The LOS vector r between the terminal (910) and the low-orbit satellite (920) can be expressed as in [Mathematical Formula 5] below.

[0081]

[0082] In [Equation 5], refers to the location of a low-orbit satellite (920), means the location of the terminal (910).

[0083] Since it can be viewed as the change in LOS vector r with respect to time, it can be expressed as in [Mathematical Formula 6] below.

[0084]

[0085] In [Equation 6] refers to the speed of a low-orbit satellite (920), refers to the speed of the terminal (910).

[0086] Therefore, by applying [Mathematical Formula 4] and [Mathematical Formula 6] to [Mathematical Formula 3] described above, it can be expressed as [Mathematical Formula 7] below.

[0087]

[0088] Meanwhile, the clock used in the terminal (910) may not be as accurate as a GNSS satellite having an atomic clock. Therefore, the clock of the terminal (910) may have an error (bias). The clock error of the terminal (910) may be referred to as receiver clock drift. The clock drift of the terminal (910) If so, the error occurring in the estimation of the frequency of the received signal can be expressed as in [Mathematical Formula 8] below.

[0089]

[0090] Frequency of the signal received by the terminal (910) and the frequency of the signal transmitted by the low-orbit satellite (920) The difference can be expressed as in [Mathematical Formula 9] below.

[0091]

[0092] In [Equation 9], means measurement noise.

[0093] Apply [Equation 7] and [Equation 8] to [Equation 9], and on both sides If multiplied by , it can be expressed as [Mathematical Formula 10] below.

[0094]

[0095] The left side of [Equation 10] is the delta range If defined as , it can be expressed as [Mathematical Formula 11] below.

[0096]

[0097] The terminal (910) can extract multiple signals from multiple different low-orbit satellites. Accordingly, the Doppler shift for each signal from each of the multiple low-orbit satellites can be calculated. Accordingly, the delta range for n low-orbit satellites can be expressed as in [Mathematical Equation 12] below.

[0098]

[0099] In [Equation 12], means the delta range of the kth low-orbit satellite, means the position of the kth satellite, represents the velocity of the kth satellite.

[0100] For example, four low-orbit satellites can be used, such as GNSS. However, the number of low-orbit satellites is not limited to a specific number, and the number of low-orbit satellites received can be implemented differently depending on the terminal's performance and purpose. [Mathematical Equation 12] Based on the actually measured delta range In theory, it can be identical to the calculation of the right-hand side of [Equation 12]. Since the terminal can know the velocity and position of the satellite through celestial data, the right-hand side of [Equation 12] can be viewed as a function of the position, velocity, and clock drift of the terminal. In other words, the right-hand side of [Equation 12] can be viewed as a value that can change depending on the result estimated by the terminal of the position, velocity, and clock drift of the terminal, and an observation function can be defined based on this.

[0101] Position and drift vector of the terminal (910) and delta range measurement vector For , the observation function for measuring the Doppler shift can be expressed as in [Mathematical Formula 13] below.

[0102]

[0103] The Jacobian matrix of can be expressed as [Mathematical Formula 14] below.

[0104]

[0105] In [Equation 14], silver It means, Is It means, Is It means, Is It means, Is It means, Is Meaning, Is It means.

[0106] The terminal measures the delta range based on the frequency shift measured from the satellite. can be determined, and the error function for the delta range factor is can be expressed as in [Mathematical Formula 15] below.

[0107]

[0108] In [Equation 15], means the covariance matrix, is the covariance matrix It refers to the Mahalanobis distance based on .

[0109] The satellite's elevation angle and carrier-to-noise ratio ( ) can be calculated based on. To determine the elevation angle, the terminal can estimate the initial position of the terminal by using the least squares method based on [Mathematical Formula 12] and [Mathematical Formula 14]. Since the position of the satellite can be obtained based on celestial data, the terminal can calculate the elevation angle based on the estimated initial position and the position of the satellite.

[0110] For time t, the observation model for the speed of the terminal (910) can be expressed as [Mathematical Formula 16] below.

[0111]

[0112] In [Equation 16], refers to a speed measurement vector determined based on the least square method using [Mathematical Formula 12] and [Mathematical Formula 14], means the observation function for speed measurement, refers to the noise associated with speed measurement.

[0113] Observation function for speed measurement can be expressed as in [Mathematical Formula 17] below.

[0114]

[0115] In [Equation 17], refers to the time interval between two consecutive epochs.

[0116] Delta range measurement vector Error function for Doppler velocity factor can be expressed as in [Mathematical Formula 18] below.

[0117]

[0118] In [Equation 18] means the covariance matrix, is the covariance matrix It refers to the Mahalanobis distance based on .

[0119] The satellite's elevation angle and carrier-to-noise ratio ( ) can be calculated based on the Doppler velocity factor. The Doppler velocity factor is determined based on two consecutive state variables. Therefore, as shown in Fig. 9, class The Doppler velocity factor in between can be connected to the edge.

[0120] Therefore, the objective function for positioning based on factor graph optimization can be expressed as a sum of error functions, and the terminal can perform positioning by estimating the state variable x that minimizes the value of the objective function as in [Mathematical Formula 19] below.

[0121]

[0122] FIG. 10 is a diagram illustrating a procedure for performing positioning using low-orbit satellites according to one embodiment of the present disclosure. Referring to FIG. 10, a terminal can estimate its current location by receiving a signal from at least one satellite.

[0123] In step S1001, the terminal receives signals from low-orbit satellites. The received signals may be for mobile communications, but are not limited to specific signals. At least one of the signals used for communication between the satellite and the terminal, or a separate reference signal for positioning, may be used. A terminal in an urban canyon can select a satellite within the low-orbit satellite constellation that is not obscured by the line of sight.

[0124] In step S1003, the terminal acquires ephemeris data from low-orbit satellites. This ephemeris data can be acquired in various ways. For example, low-orbit satellites can broadcast their ephemeris data to the terminal. Alternatively, the ephemeris data can be unicasted between the terminal and the low-orbit satellites via an RRC connection. The ephemeris data may include at least one of a week number, satellite accuracy and status, data age, satellite clock correction factor, and orbital parameter values. By periodically transmitting ephemeris data, the satellites can estimate the positions and velocities of the satellites. The ephemeris data can be provided in the form of a two-line element (TLE) file. Assisted GPS (A-GPS) technology can also be used to acquire the ephemeris data. To quickly acquire the satellite positions, the ephemeris data can be transmitted to the terminal via a terrestrial communication network.

[0125] In step S1005, the terminal measures the Doppler shift. By measuring the frequency of the received signal, the terminal can measure the Doppler shift based on the difference between the frequency actually transmitted by the satellite and the frequency measured by the terminal. The measured Doppler shift can be used to calculate an error function. The frequency of the signal transmitted by the satellite can be preset to a specific frequency. Therefore, the terminal can measure the Doppler shift by comparing the frequency of the received signal with the preset frequency, knowing the frequency. As another example, the satellite can transmit astronomical data including information about the frequency transmitted by the satellite to the terminal.

[0126] In step S1007, the terminal determines the delta range factor and the Doppler velocity factor. The terminal may first set the state variables to be used in the factor graph optimization method. For example, the state variables may include at least one of the position of the terminal, the velocity of the terminal, or the receiver clock bias. As described above, the delta range factor may be determined based on the state variables of the terminal, and the Doppler velocity factor may be determined based on two consecutive state variables. Therefore, two consecutive state variables may be connected by the Doppler velocity factor. The error function for the delta range factor may be determined based on the difference between the delta range measurement vector and the observation function for measuring the Doppler shift, as in [Mathematical Formula 15]. The error function for the Doppler velocity factor may be determined based on the difference between the velocity measurement vector and the observation function for velocity measurement, as in [Mathematical Formula 18]. The terminal may determine the elevation angle and carrier-to-noise ratio ( ) can be used to determine the covariance matrix, and the error function can be calculated using the Mahalanobis distance based on the covariance matrix.

[0127] The satellite's elevation angle can be determined based on the terminal's position and the satellite's position. The terminal can determine the satellite's position based on celestial data. The terminal's position can be initially estimated using the least squares method based on the calculated Doppler shift. For example, the initially estimated terminal's position can be estimated using the least squares method based on the delta range factor, as shown in Equations 12 and 14.

[0128] In step S1009, the terminal determines its position based on factor graph optimization. Depending on the position estimated by the terminal, the value calculated in the error function may change. Therefore, the terminal position estimation using the factor graph can be determined by finding a state x where the sum of the error function values ​​for the delta range factor and the error function values ​​for the Doppler velocity factor is minimized based on covariance, as in [Mathematical Equation 19].

[0129] Although the step of receiving celestial data (S1003) above is described after the step of receiving satellite signals (S1001), receiving celestial data is for obtaining basic data for determining the position and velocity of the satellite, and can be performed before the step of receiving satellite signals.

[0130]

[0131] Meanwhile, those skilled in the art will appreciate that the present invention can be implemented in modified forms without departing from the essential characteristics of the above-described description. Therefore, the disclosed methods should be considered illustrative rather than restrictive. The scope of the present disclosure is set forth in the claims, not the foregoing description, and all differences within the scope equivalent thereto should be construed as being encompassed by the present disclosure.

[0132] The above may also apply to other systems.

Claims

1. In a terminal positioning method using multiple low-orbit satellites, A step of receiving signals from at least one low-orbit satellite; A step of acquiring celestial data (ephemeris data) of the above low-orbit satellite; A step of determining a Doppler shift based on the above astronomical data and the above signal; A step of determining state variables and at least one factor to be used in factor graph optimization; and A terminal positioning method, comprising a step of obtaining a positioning result using the factor graph optimization based on the above Doppler shift.

2. In paragraph 1, A method for positioning a terminal, wherein at least one of the factors comprises a delta range factor and a Doppler velocity factor.

3. In paragraph 2, A terminal positioning method in which the above state variables are connected by the above Doppler velocity factor.

4. In paragraph 3, The error function value of the above delta range factor is determined based on the difference between the delta range measurement vector and the observation function for measuring the Doppler shift, A terminal positioning method, wherein the error function value of the above Doppler velocity factor is determined based on the difference between the velocity measurement vector and the observation function for velocity measurement.

5. In paragraph 4, A terminal positioning method, wherein the covariance matrix of the above delta range factor and the above Doppler velocity factor is determined based on the elevation angle and carrier-to-noise ratio of the low-orbit satellite.

6. In paragraph 5, A terminal positioning method, wherein the above positioning result is determined based on an error function value of the delta range factor, an error function value of the Doppler velocity factor, and a covariance matrix.

7. In paragraph 1, A method for positioning a terminal, wherein the state variable includes at least one of a position of the terminal, a velocity of the terminal, or a receiver clock bias.

8. In paragraph 1, A terminal positioning method, wherein the celestial data includes at least one of the status of each of the low-orbit satellites, the age of the data, a clock correction coefficient, or a parameter value related to the orbit.

9. For terminals using satellite communication, Transmitter and receiver; and comprising at least one processor connected to the transceiver; At least one processor, Receive signals from at least one low-orbit satellite, Obtain celestial data (ephemeris data) of the above low-orbit satellite, Determine the Doppler shift based on the above astronomical data and the above signal, Determine the state variables and at least one factor to be used in factor graph optimization; and A terminal that obtains positioning results using the above factor graph optimization based on the above Doppler shift.

10. In paragraph 9, A terminal, wherein at least one of the above factors comprises a delta range factor and a Doppler velocity factor.

11. In paragraph 10, The above state variables are connected by the Doppler velocity factor, terminal.

12. In paragraph 11, The error function value of the above delta range factor is determined based on the difference between the delta range measurement vector and the observation function for measuring the Doppler shift, The error function value of the above Doppler velocity factor is determined based on the difference between the velocity measurement vector and the observation function for velocity measurement.

13. In paragraph 12, The covariance matrix of the above delta range factor and the above Doppler velocity factor is determined based on the elevation angle and carrier-to-noise ratio of the satellite.

14. In paragraph 13, The above positioning result is determined based on the error function value of the delta range factor, the error function value of the Doppler velocity factor, and the covariance matrix.

15. In paragraph 9, A terminal, wherein the state variable includes at least one of the position of the terminal, the velocity of the terminal, or the receiver clock bias.

16. In paragraph 9, The terminal, wherein the celestial data includes at least one of the status of each of the low-orbit satellites, the age of the data, a clock correction coefficient, or a parameter value related to the orbit.

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