Methods and apparatus for generating and distributing orbital elements for Doppler, elevation angle, and pseudorange estimation

Abstract

An ephemeris server executes a method including the following: (a) (i) obtaining broadcast ephemeris information by accessing a broadcast information source on a wide area communication network; (ii) extracting orbit trajectory information of each satellite from the broadcast ephemeris information; and (iii) creating modified orbit trajectory information of each satellite from the extracted orbit trajectory information and storing it in a real-time database; (b) at each of a plurality of specified time points, updating the modified trajectory information in the real-time database based on an evaluation of an update criterion; and (c) providing the modified trajectory information to each of a plurality of devices, each device having a GNSS receiver capable of processing a broadcast signal of a satellite using the modified trajectory information.

Description

Technical Field

[0001] The present invention relates to a Global Navigation Satellite System (GNSS). The present invention relates to the processing and handling of ephemeris data used in the determination of position, velocity, and time (PVT) using GNSS. Background Art

[0002] To reduce acquisition time, prior information about the satellite trajectories in GNSS (e.g., position and velocity; "ephemeris") is provided to the GNSS receiver. For example, such information can be obtained from a terrestrial source or from the navigation messages broadcast by each satellite in GNSS. The ephemeris of a satellite allows the receiver to estimate the elevation, Doppler, and pseudorange of the satellite at any given time, which enables the receiver to narrow its frequency and delay search space when the satellite appears above the receiver's horizon. In this regard, a relatively narrow frequency window of 200 Hz allows a Doppler estimation error of approximately 100 Hz. However, the provided ephemeris becomes inaccurate over time because satellite trajectories are not completely predictable. For a GNSS receiver, as the provided satellite ephemeris becomes less accurate, the estimation errors of the Doppler, pseudorange, and elevation derived from it increase. Therefore, the time and energy required to acquire and track a satellite may increase significantly over time. Typically, GNSS updates the ephemeris information ( "system update") broadcast in each satellite navigation message between 1 - 2 hours.

[0003] When performing Doppler, pseudorange, and elevation estimations, a typical Medium Earth Orbit (MEO) satellite in GNSS takes approximately 120 seconds to fly over 1 degree around the Earth's center (i.e., the change in the elevation of the satellite ( "elevation") is approximately 1 / 2 degree per minute), so the estimation error of the elevation is relatively small. However, during this period, the velocity of the satellite may change by 60 m / s (i.e., up to ~1%), so the estimation errors of the Doppler and pseudorange may be large.

[0004] To determine the trajectory of a satellite in the Earth-centered-Earth-fixed (ECEF) coordinate system, six Keplerian orbital elements and the Earth's rotation rate are required These Keplerian orbital elements are: (i) the orbital eccentricity (e), (ii) the length of the semi-major axis of the orbit (A), (iii) the inclination at the reference epoch (i0), (iv) the longitude of the ascending node in the orbital plane at the reference epoch (Ω0), (v) the argument of perigee (ω), and (vi) the mean anomaly at the reference epoch (M0).

[0005] For the NAVSTAR Global Positioning System (“GPS”), Tables 20-II, 20-III, and 20-IV in Section 20.3.4 of the publication “NAVSTAR GPS Space Segment / Navigation User Segment Interfaces” (IS-GPS-200D; “Interface Specification”) list the definitions, data formats, and compensations of ephemeris parameters for the ECEF coordinate system. Most of these ephemeris parameters are related to the Keplerian orbit parameters. In GPS, through the compensation of the ECEF coordinate system, the provided ephemeris parameter values provide the best trajectory fit for each specific fitting interval.

[0006] The interface specification adds the following nine ephemeris parameters: (i) mean motion difference (Δn) based on the calculated value, (ii) rate of change of right ascension (iii) rate of change of inclination (IDOT), (iv) amplitudes of the sine and cosine harmonic correction terms for the argument of latitude (c us ,c uc ), (v) amplitudes of the sine and cosine harmonic correction terms for the orbital radius (c rs ,c rc ) and (vi) amplitudes of the sine and cosine harmonic correction terms for the inclination (c is ,c ic ). These additional ephemeris parameters are referred to as “gravitational harmonic correction terms, rates, and rate corrections” to the Keplerian parameters. These additional GPS parameters are also used in China's BeiDou system (BDS), Europe's Galileo system (GAL), and Japan's QZSS. In GPS, the length of the navigation message is 1500 bits, and the ephemeris transmitted therein is usually valid for no more than a few days.

[0007] In recent years, many applications have been developed for small mobile devices or sensors where the location of such devices is relevant. In many such applications (e.g., “Internet of Things” (IoT) applications), the location of such devices is obtained through an on-board GNSS receiver module. IoT devices must operate on battery power and communicate using wireless communication. In fact, in many such applications, IoT devices must operate under very low power requirements and communicate using a remote, very low data rate wireless communication system. Summary of the Invention

[0008] According to an embodiment of the present invention, an ephemeris server performs a method for providing ephemeris information of satellites from at least one GNSS. The method includes (a) (i) obtaining broadcast ephemeris information by accessing a broadcast information source on a wide area communication network; (ii) extracting orbit trajectory information of each satellite from the broadcast ephemeris information; and (iii) creating modified orbit trajectory information of each satellite from the extracted orbit trajectory information and storing it in a real-time database; (b) at each of a plurality of specified time points, updating the modified trajectory information in the real-time database based on an evaluation of an update criterion; and (c) providing the modified trajectory information to each of a plurality of devices, each device having a GNSS receiver capable of processing broadcast signals of satellites using the modified trajectory information.

[0009] The present invention can be better understood by considering the following detailed description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 A system 100 having an ephemeris server 101 and devices 102-1 to 102-n, each device having a GNSS receiver, is shown according to an embodiment of the present invention.

[0011] Figure 2 is a table showing each field of an LK ephemeris according to an embodiment of the present invention.

[0012] Figure 3 Shows the number of updates and the average length of each bitstream for various simulation results under Policy 1, Policy 2, or a policy other than these (“current policy”). DETAILED DESCRIPTION

[0013] By Figure 1 the system 100 of the present invention is illustrated in the detailed description. Figure 1Fig. 0 shows a system 100 with an ephemeris server 101 and devices 102-1 to 102-n according to an embodiment of the present invention, each device having a GNSS receiver. In system 100, devices 102-1 and 102-n both perform activities related to satellite navigation (e.g., determining position, velocity, or time), activities related to satellite ranging, or any other suitable activities using their on-board GNSS receivers, but rely on the ephemeris server 101 to provide them with satellite ephemeris information to facilitate navigation-related activities. The ephemeris server 101 accesses or receives system updates from a conventional source 103 (e.g., NASA's CDDIS) via a wide area network 104 (e.g., the Internet), for example. Devices 102-1 to 102-n (some of which may be mobile devices and sensors) communicate with the ephemeris server 101 via one or more wide area communication networks 105 (e.g., a low power wide area network (LPWAN), such as a LoRa-based network). For these devices, power savings and efficient use of low bandwidth are crucial. Therefore, it is highly desirable that the ephemeris information provided by the ephemeris server 101 be updated infrequently, and each update message should have as few bits as practically possible.

[0014] As already mentioned above, the duration for which the ephemeris in a system update remains valid depends on the subsequent behavior of the satellites. For high-performance applications, the available lifetime of the ephemeris in a system update may be as short as a few hours. The present invention extends the lifetime of the satellite ephemeris used in the on-board GNSS receivers of low-power devices by providing such device-selected ephemeris parameter values that are modified to have a longer lifetime ("persistent Kepler ephemeris" or "LK ephemeris") and are updated in the GNSS receiver upon request or based on one or more performance criteria, rather than when a system update is available. For example, the ephemeris server 101 can monitor the Doppler or pseudorange estimation error between the Doppler or pseudorange estimations obtained using the latest system update and the Doppler or pseudorange obtained using the LK ephemeris. The ephemeris server 101 re-evaluates the LK ephemeris only when the monitored quantity exceeds a predetermined threshold ("update criterion").

[0015] Typically, the threshold for the Doppler estimation error is set to 100 Hz (i.e., the effective ephemeris is expected to produce a Doppler estimation error within ±50 Hz). To reduce communication costs, the LK ephemeris contains only the selected ephemeris parameters of the system update. The ephemeris server 101 sends the LK ephemeris to each GNSS receiver in a message encoded in an efficient manner.

[0016] The inventors have observed the following:

[0017] (i) Perturbations in the satellite motion do not result in large Doppler estimation errors, and thus the harmonic terms (i.e., the magnitudes of the sine and cosine harmonic correction terms for the argument of latitude (c us ,cuc ) The magnitudes (c rs , c rc ) of the sine and cosine harmonic correction terms for the orbital radius and the magnitudes (c is , c ic )) of the sine and cosine harmonic correction terms for the inclination do not need to be included in the LK ephemeris.

[0018] (ii) The mean motion difference Δn, the rate of change of right ascension , and the rate of change of inclination IDOT are related. These three terms depend largely on the constantly changing rotation rate of the Earth and the tilted polar axis. Since the rotation rate of the Earth changes very slowly, its impact on the satellite orbit is generally not prominent. Therefore, it may not be necessary to include these terms in the transmitted LK ephemeris. For example, for GPS satellites, the GNSS receiver can use IDOT = 0 and This is a good estimate of these terms in some applications.

[0019] (iii) The orbital eccentricity e, the inclination i0 at the reference epoch, the longitude Ω0 of the ascending node in the orbital plane at the reference epoch, the argument of perigee ω, and the mean anomaly M0 at the reference epoch are relatively stable parameters. Therefore, the updates required for these ephemeris parameters are not expected to be frequent.

[0020] However, the estimation error in the semi-major axis A - which is a factor in determining the satellite orbit period - may require the expiration of the LK ephemeris. Generally, the semi-major axis A of the satellite orbit can be estimated using the following equation:

[0021]

[0022] where n is the mean motion of the satellite, Δn is the estimation error of the mean motion, and μ is the gravitational constant of the Earth. Even for a relatively long time period (e.g., 60 days), the mean motion n can be modeled by a linear function in time. When the correlation |ρ| between the mean motion n and the linear function of time exceeds 0.5, and when there is at least 30 days of historical data for the mean motion n, the estimation of n can be obtained from the following equation

[0023]

[0024] where k and b are values that can be obtained from, for example, linear regression of historical data. When the correlation |ρ| between the mean motion n and the linear function of time does not exceed 0.5, and when there is at least 30 days of historical data for the mean motion n, the estimation of n can be provided by the following equation

[0025]

[0026] Estimation can be used to calculate the correction ΔM of the mean anomaly M0 of the satellite at the reference epoch t ref as follows:

[0027]

[0028] The estimated value Δt of the clock error in the satellite can be obtained using the following equation:

[0029]

[0030] The estimated Δλ of the Doppler estimation error can be obtained using the following equation:

[0031]

[0032] where a is the acceleration of the satellite along the line of sight at time t, and a max is the maximum acceleration of the satellite along the line of sight at time t. To ensure that the Doppler estimation error in the GNSS receiver is within the Doppler threshold (Δλ) max (e.g., 100 Hz), the value of is obtained by solving the following equation:

[0033]

[0034] where t maxΔM is the time at which the maximum estimated error (ΔM) of the mean anomaly M0 max occurs.

[0035] Then, a persistent estimate of the semi-major axis length of the satellite orbit can be obtained using the following equation

[0036]

[0037] The persistent mean anomaly ref at the reference epoch t can be derived from the mean anomaly M0 at the specified time t of the last system update according to the following equation t as follows:

[0038]

[0039] Similarly, the persistent longitude of the ascending node of the orbital plane is related to the longitude Ω0 of the ascending node of the orbital plane at the specified time t of the last system update according to the following equation t as follows:

[0040]

[0041] wherein, is the rotation rate of the Earth, and WN t are the GPS week of the reference epoch and the GPS week of the ephemeris time respectively.

[0042] According to an embodiment of the present invention, only the following ephemeris parameters are prepared by a processor in the ephemeris server 101 based on system updates obtained from at least one GNSS and provided in an ephemeris update from the ephemeris server 101: (i) system identification ("system ID"), (ii) satellite identification ("PRNID"), (iii) estimation of the semi-major axis length of the satellite orbit (iv) estimation of the orbital eccentricity (v) estimated inclination at the reference epoch (vi) estimated longitude of the ascending node in the orbital plane at the reference epoch (vii) estimated argument of perigee and (viii) mean anomaly at the reference epoch Items (iii)-(iv) together constitute an instance of the LK ephemeris. As shown below, the GNSS receiver can estimate the elevation angle, pseudorange, and Doppler within a reasonable predetermined threshold (e.g., 100 Hz Doppler threshold and 250 microsecond delay threshold) using only these LK ephemeris tables over an extended period (e.g., several weeks), thus avoiding frequent updates to the GNSS receiver.

[0043] Theoretically, if both the ephemeris server 101 and the GNSS receiver know the reference epoch, the ephemeris server 101 does not need to provide the ephemeris time of each LK ephemeris to the GNSS receiver. However, in practice, it is preferably explicitly provided because quantization errors may accumulate. In one embodiment, the ephemeris server 101 provides at least the GPS week of the reference epoch to the GNSS receiver.

[0044] In one embodiment, the ephemeris server 101 can monitor the estimated pseudorange For example, in GPS, the C / A code has 1023 chips transmitted within 1 millisecond. Therefore, for a pseudorange prediction threshold of 0.25 ms, the code delay is expected to be within a range of 1 / 4 of the 1023 possible delays. Generally, like the 100 Hz Doppler threshold of MEO satellites, the 0.25 ms pseudorange prediction threshold provides a validity period of about several months. In this case, the LK ephemeris table generated by the method of the present invention provides a suitable pseudorange estimate. However, for a smaller pseudorange prediction threshold (e.g., 0.02 millisecond), the validity period is much shorter (e.g., about several days). For this shortened validity period, the length estimate of the semi-major axis of the orbit based on historical data may not be appropriate. However, in this case, the ephemeris parameters from the most recent system update squared can be used as a length estimate in the LK ephemeris Pseudorange estimates of 10 kilometers or longer should also include the contributions of the satellite clock bias (AF0) and clock drift (AF1).

[0045] Elevation, pseudorange, and Doppler estimates based on the LK ephemeris inevitably deteriorate over time because the trajectories of GNSS satellites are completely unpredictable. Accordingly, in one embodiment of the present invention, the ephemeris server 101 monitors these predictions. When the estimation error of the Doppler or pseudorange exceeds the corresponding preset threshold of the satellite, the ephemeris server 101 updates its LK ephemeris for that satellite.

[0046] To support its activities, the ephemeris server 101 maintains a real-time database of LK ephemerides organized in the form of "snapshots". Each snapshot corresponds to a set of most recently generated LK ephemeris tables for the satellites supported by the ephemeris server 101. (The supported satellites can be selected from more than one GNSS.) In one embodiment, the GNSS receiver can request the most recent snapshot relative to a specified time. (For example, the specified time can be represented as UTC time.) The GNSS receiver can also request a bitstream encoding the updates from the specified snapshot (most likely, the snapshot currently implemented in the GNSS receiver) to the most recent snapshot.

[0047] In one embodiment, each snapshot is tagged with a timestamp. Similarly, each LK ephemeris is also tagged with a timestamp in the form of a sequence number. The sequence number can be an integer t, starting from the initialization time (e.g., t = 0 at 00:15 on January 1, 2017), which increments by 1 at each re-evaluation time (e.g., each UTC hour). Thus, these sequence numbers label consecutive epochs of a UTC hour. In this description, the snapshot at time t is denoted as s(t), and the LK ephemeris created for satellite j (j = 1, …, n) at time t is denoted as keph(j,t). Initially, i.e., at initialization, the ephemeris server 101 creates the snapshot s(0) based on the most recent system updates of all monitored satellites. Thus, the snapshot s(0) includes the LK ephemeris tables keph(0,0), keph(1,0), …, keph(n,0).

[0048] At the next re-evaluation time point (i.e., t = 1), the ephemeris server 101 determines which of the LK ephemerides kep(0,0), kep(1,0), …, kep(n,0) in the snapshot s(0) need to be updated based on the update criteria for each satellite. The ephemeris server 101 creates a new LK ephemeris only when the update criteria are met. For example, at time t = 1, if the LK ephemerides kep(0,0) and kep(25,0) need to be updated, the ephemeris server 101 creates the LK ephemerides kep(0,1) and kep(25,1). The ephemeris server 101 creates a new snapshot at the re-evaluation time point only when at least one LK ephemeris needs to be updated. Thus, the snapshot s(1) includes the LK ephemerides kep(0,1), kep(1,0), …, kep(24,0), kep(25,1), kep(26,0), …, kep(n,0). If there are no long-term Kepler ephemerides in the snapshot s(0), the ephemeris server 101 will not create the snapshot s(1). This set of actions is repeated at each re-evaluation time point. Note that in this embodiment, both the snapshot and the LK ephemeris are indexed by their respective creation times.

[0049] As described above, the update criteria evaluation uses the magnitude difference between the estimated pseudorange Doppler or elevation obtained using the latest system update and the corresponding estimated pseudorange Doppler or elevation obtained using the LK ephemeris. The update criteria are met when this difference exceeds a preset threshold. In one embodiment, at each re-evaluation time point, the ephemeris server 101 checks this update criteria for each monitored satellite at each of approximately 100 locations distributed around the world. When the update criteria for a satellite are met at one or more of the 100 locations, a new LK ephemeris for the satellite is created.

[0050] However, given the limited bandwidth of devices 102-1 and 102-n, each device receives an update to its onboard LK ephemeris only when it requests an update from the ephemeris server 101. To reduce the number of terms used in this detailed description, the set of LK ephemerides in a device is also referred to as a "snapshot". In some embodiments, since there is no coordinated activity between the ephemeris server and the devices in system 100 (e.g., as described below, the server can restore a previous ephemeris as the current snapshot for any of a variety of reasons), the sequence number of the LK ephemeris on the ephemeris server may be less than the sequence number of the corresponding LK ephemeris in the device. In this case, even if the snapshots on the ephemeris server 101 and the device have the same sequence number, these snapshots do not necessarily need to have the same set of LK ephemerides.

[0051] Along with the update request, the device sends a timestamp (i.e., a sequence number) associated with its last snapshot to the ephemeris server 101. The ephemeris server 101 then compares the received sequence number with the sequence numbers of each LK ephemeris table in its current snapshot. Those ephemeris tables in the current snapshot of the ephemeris server 101 that have a sequence number greater than the received sequence number are designated for the requested update. Based on this comparison, the ephemeris server 101 creates and sends to the requesting device a bitstream for updating those designated ephemeris tables. For example, assume that the device sends an update request at time t = 27, at which time its snapshot s(25) includes LK ephemeris tables keph(0,24), keph(1,24),..., keph(24,24), keph(25,24), keph(26,24), …, keph(n,24), while the ephemeris server 101 has a current snapshot s(27) that includes LK ephemeris tables keph(0,24), keph(1,24)),..., keph(24,24), keph(25,27), keph(26,23),..., keph(n,24). Thus, the update request from the device includes the sequence number 25. Upon receiving the update request, the ephemeris server 101 determines that only the sequence number of the LK ephemeris keph(25, 27) in its current snapshot s(27) is greater than the sequence number in the update request. Therefore, the device needs only the bitstream for updating the LK ephemeris keph(25,24) to keph(25,27). Using this bitstream, the device creates its snapshot s(27), which includes LK ephemeris tables keph(0,24), keph(1,24), …, keph(24,24), keph(25,27), keph(26,24), …, keph(n,24).

[0052] In the ephemeris server 101, the sequence number can have any suitable bit length (e.g., 29 bits, as in GPS time). However, in some embodiments, due to limited communication bandwidth and other reasons, the sequence numbers in some of the devices 102-1 to 102-n can be implemented with a shorter bit length than the sequence numbers in the ephemeris server 101. Naturally, the sequence numbers in such devices "roll over" more frequently than the sequence numbers in the ephemeris server 101. In other words, the sequence number on the device should be considered a modulo-2 m integer, where m is the bit length. When comparing the sequence number of an LK ephemeris in its snapshot (say, A) with the sequence number in the update request (say, B), the ephemeris server 101 represents B as the shorter of the directed distances from 0 modulo-2 m In particular, B is mapped to C, which takes the value B when B < 2 m-1 and otherwise takes the value (2 m-1-B). When (A - C) > 0, the serial number of the LK ephemeris in the snapshot of the ephemeris server 101 is greater than the serial number in the update request.

[0053] From time to time, the ephemeris server 101 may need to be reset to recover from a service interruption. In the recovery, a new server can be initialized - regenerating all LK ephemeris tables - to replace the server that has stopped service. Such an approach is impractical because it would cause the GNSS receivers of devices 102-1 to 102-n to update all of their snapshots, as the serial numbers of almost all LK ephemerides in the snapshot of the new server will be greater than those in the device's snapshot. The resulting workload would overload the communication bandwidth of the system 100.

[0054] The present invention provides a fast and cost-effective recovery method. In some embodiments, the real-time database of the ephemeris server 101 can be recovered from an off-site archive. When the archived database is recovered, the recovery process regenerates the LK ephemeris tables that are no longer valid in the recovery. For a short service interruption, the bandwidth required to service subsequent update requests from devices 102-1 to 102-n may be fully acceptable. For example, when the snapshot of the current epoch can be recovered from the archive and the recovery is completed before the next epoch (i.e., the next UTC hourly re-evaluation time point), the service interruption is minimized.

[0055] According to an embodiment of the present invention, if each system update is used to generate the LK ephemeris, the ephemeris server 101 also records the time of the system update. In this method, even when one or more snapshots are lost (either non-recoverable or because the service interruption spans one or more re-evaluation time points), all snapshots after the recovered snapshot but before the service interruption can also be recreated. For snapshots lost during the service interruption, they can be recreated using all system updates that occurred during the interruption time, such system updates being obtainable from sources external to the system 100.

[0056] According to an embodiment of the present invention, the ephemeris server 101 can also recover by generating a special snapshot ("reference snapshot") from a previous system update (e.g., the system update current 24 UTC hours before the service interruption). The LK ephemeris tables from the reference snapshot are not used to update the LK ephemeris tables of devices 102-1 to 102-n. For example, at time a reference snapshot s(*) is created from a previous system update, including LK ephemeris tables keph(0,*), keph(1,*), …, keph(n,*). Thereafter, at i.e., p re-evaluation time points later, when its corresponding update criteria are met, the LK ephemeris tables keph(25,*), keph(26,*) are updated such that the snapshot including the LK ephemeris During an epoch an update request received at the ephemeris server can cause the LK ephemerides of satellites 25 and 26 to be updated in the requesting device. The remaining LK ephemerides are replaced in the normal course at subsequent re-evaluation time points - according to their respective update criteria - and can be used to update the corresponding LK ephemerides in devices 101-1 to 101-n. In this way, the limited bandwidth due to service restoration is not overloaded. In some embodiments, instead of selecting a system update current at a reference time for all LK ephemerides of a reference snapshot, different reference times - possibly randomly selected - can be used to generate LK ephemerides for each satellite.

[0057] Admittedly, the recovery method according to the present invention may cause a performance degradation in some of devices 102-1 to 102-n. For example, a device may have to use an expired LK ephemeris until the corresponding LK ephemeris from the reference snapshot is replaced in the normal course. However, the recovery method of the present invention ensures that system 101 recovers gracefully over time.

[0058] According to an embodiment of the present invention, a bitstream for LK ephemeris update has a header part and a body part. The header part includes a 15-bit "Data Issue, Ephemeris (IODE)" and an identification string (SID) listing the satellites whose LK ephemerides are updated. The IODE includes a sequence number of the update. The SID includes four 1-bit GNSS identification flags, which (in order) indicate GPS, BDS, GAL, and QZSS. If only one of the four GNSS identification flags is set, the next 8-bit word is an index of a list of satellites in the identified constellation. Otherwise, following the order of the GNSS identification flags, satellite flags of each GNSS including the LK ephemeris are followed. If the GPS flag is set, its satellite map includes 31 flags, each set flag representing a satellite whose LK ephemeris is included. The satellites are represented in the order of their pseudo-random number codes indexes ("PRN ID"). Similarly, for BDS, GAL, and QZSS, their respective satellite flag maps have 37 bits, 36 bits, and 8 bits respectively.

[0059] The body of the bitstream is a consecutive list of LK ephemerides (i.e., orbital and clock parameter values) in the order of the set flags in the satellite flag map in the header. Figure 2 is a table showing each field of the LK ephemeris according to an embodiment of the present invention. Note that when the "week flag" or " flag" is not set, its corresponding "week" field or The field does not appear in the bitstream. In some embodiments, the "orbit type" field is omitted because the orbit type of any identified satellite (i.e., MEO, GEO, or GSO) is known.

[0060] In some embodiments, the ephemeris server 101 determines whether to set a health flag in the LK ephemeris to control the update frequency.

[0061] In some embodiments, the current snapshot is pre-installed in each GNSS receiver before each GNSS receiver is released for operation. When starting operation in the system 101, the current snapshot can also be installed in each GNSS receiver.

[0062] The format of each LK ephemeris (e.g., fields and their corresponding bit lengths) can be determined empirically. According to one embodiment, 365 UTC days of actual system updates are collected and used in the simulation. The data for the first 70 days is used to adjust the bit lengths of each field in the LK ephemeris. For example, longer bit lengths provide higher precision in the Kepler parameter values. The more sensitive the update criteria for the Kepler parameter, the longer the bit length required. Due to the number of parameters in the LK ephemeris, adjusting the format of the LK ephemeris to obtain the desired bit length and high performance (i.e., relatively infrequent updates) may require a large number of trial-and-error manipulations, as well as educated guesses.

[0063] After obtaining the desired LK ephemeris format, the data for the remaining 295 UTC days is used for 31 satellites. The ephemeris server updates its LK ephemeris table according to one embodiment of the present invention as described above. Every 1200 UTC seconds, the ephemeris server evaluates the Doppler update criteria for all satellites at 100 randomly selected locations globally. In this simulation, the Doppler criteria for each satellite is a 90 Hz threshold. When this threshold is exceeded in any of the evaluated update criteria, an LK ephemeris is generated for the next snapshot.

[0064] In the use Figure 2 of an LK ephemeris format in a 365 UTC day simulation, some LK ephemeris tables did not expire. The most frequently updated satellite had 3 updates. For the last 295 UTC day duration of the simulation, a total of 41 updates were required.

[0065] In a second simulation, 500 UTC days of system updates and different pseudorange update criteria were used. The results are consistent with the hypothesis that higher update frequencies result from narrower pseudorange thresholds (e.g., 0.25 ms vs. 0.02 ms). In this simulation, a 50 Hz Doppler threshold was used, which does not appear to have a substantial impact on pseudorange performance. (That is, increasing the 50 Hz threshold did not substantially reduce the update frequency for each pseudorange threshold.)

[0066] In one embodiment (“Strategy 1”), the ephemeris server 101 does not send a bitstream for update unless the number of LK ephemerides included in the bitstream exceeds a predetermined number (e.g., 5).

[0067] In another embodiment (“Strategy 2”), both a wide pseudorange threshold and a narrow pseudorange threshold are monitored, where the narrow pseudorange threshold is the product of the wide pseudorange threshold and a scale factor less than 1.0. In this method, when any satellite meets the update criteria based on the wide pseudorange threshold, the update LK ephemeris that meets its update criteria is updated for the satellite based on its narrow pseudorange threshold. Figure 3 The number of updates and the average length of each bitstream for various simulation results under Strategy 1, Strategy 2, or a strategy other than these (“Current Strategy”) are shown respectively. Under Strategy 2, various scale factors are simulated. This simulation is performed using ephemeris data of 56 satellites from GPS, GAL, BDS, and QZSS.

[0068] The above detailed description is provided to illustrate specific embodiments of the present invention and is not restrictive. Many variations and modifications within the scope of the present invention are possible. The present invention is set forth in the appended claims.

Claims

1. An ephemeris information server, comprising: A processor; A first communication interface; A second communication interface with a wide area network; And A real-time database, wherein the processor (a) (i) obtains broadcast ephemeris information by accessing a broadcast information source on a wide area communication network using the first communication interface, the broadcast ephemeris information being related to satellites in at least one GNSS; (ii) extracts orbital trajectory information of each satellite from the broadcast ephemeris information; and (iii) creates modified orbital trajectory information for each satellite based on the extracted orbital trajectory information and stores the modified orbital trajectory information in the real-time database; (b) at each of a plurality of specified time points, updates the modified trajectory information in the real-time database based on an evaluation and satisfaction of an update criterion; and (c) provides the modified trajectory information to each of a plurality of devices via the second communication interface, each device having a GNSS receiver capable of processing broadcast signals of satellites using the modified trajectory information, wherein the update criterion is based on an estimated error between a parameter value calculated using the broadcast ephemeris information and the parameter value calculated using the modified trajectory information.

2. The ephemeris information server according to claim 1, wherein, The modified trajectory information is provided to the device on demand.

3. The ephemeris information server according to claim 1, wherein The second communication interface accesses a wide area low power communication network LPWAN.

4. The ephemeris information server according to claim 3, wherein The LPWAN is LoRa-based.

5. The ephemeris information server according to claim 1, wherein, The calculated parameter value is one or more of the following: elevation angle, pseudorange, and Doppler.

6. The ephemeris information server according to claim 1, wherein When the update criterion is not satisfied, the modified trajectory information of each satellite is not updated.

7. The ephemeris information server according to claim 1, wherein The modified trajectory information includes Keplerian ephemeris information of each satellite.

8. The ephemeris information server according to claim 7, wherein, The Keplerian ephemeris information includes an estimate of the semi-major axis of one of the satellites.

9. The ephemeris information server according to claim 8, wherein, The semi-major axis is estimated using the mean motion of the satellite.

10. The ephemeris information server according to claim 9, wherein, The mean motion is modeled by a linear function of time.

11. The ephemeris information server according to claim 7, wherein, The Keplerian ephemeris information includes a parameter value related to the mean anomaly of the orbit of one of the satellites.

12. The ephemeris information server according to claim 7, wherein, The Keplerian ephemeris information further includes an estimate of a parameter related to the longitude of the ascending node associated with the orbital plane of one of the satellites.

13. The ephemeris information server according to claim 1, wherein The modified trajectory information is transmitted to one of the devices in a bit stream related only to the modified trajectory information that the device does not yet have.

14. The ephemeris information server according to claim 13, wherein, The bit stream includes modified trajectory information of two or more satellites.

15. The ephemeris information server according to claim 1, wherein, The update criterion includes a first performance criterion and a second performance criterion, wherein (i) at each specified time point, the update criterion is considered satisfied only when at least one satellite satisfies the first performance criterion, and (ii) when the update criterion is considered satisfied, the modified trajectory information of those satellites that satisfy the second performance criterion is updated.

16. The ephemeris information server according to claim 1, wherein, The specified time points occur at regular time intervals.

17. The ephemeris information server according to claim 1, wherein, A copy of the real-time database is stored at one or more locations remote from the ephemeris information server at different times, enabling the ephemeris information server to be restored to a previous state.

18. The ephemeris information server according to claim 17, wherein, The ephemeris information server also records, in the real-time database, the creation time associated with each creation instance of the modified trajectory information, to allow the real-time database to be restored to a desired state after being restored to the previous state.

19. The ephemeris information server according to claim 1, wherein, The ephemeris information server restores the real-time database by creating a current modified trajectory information as a reference set of the modified trajectory information at a selected previous time, and wherein such modified trajectory information is provided to the device only after updating the modified trajectory information in the reference set after the selected time.

20. A method for providing ephemeris information of satellites from at least one GNSS, the method comprising, in an ephemeris information server: (a) (i) obtaining broadcast ephemeris information by accessing a broadcast information source on a wide-area communication network; (ii) extracting orbital trajectory information of each satellite from the broadcast ephemeris information; and (iii) creating modified orbital trajectory information for each satellite based on the extracted orbital trajectory information and storing the modified orbital trajectory information in a real-time database; (b) at each of a plurality of specified time points, updating the modified trajectory information in the real-time database based on an evaluation and satisfaction of an update criterion; and (c) providing the modified trajectory information to each of a plurality of devices, each device having a GNSS receiver capable of processing broadcast signals of the satellites using the modified trajectory information, wherein the update criterion is based on an estimated error between a parameter value calculated using the broadcast ephemeris information and the parameter value calculated using the modified trajectory information.

21. The method according to claim 20, wherein, The modified trajectory information is provided to the device on demand.

22. The method according to claim 20, wherein, The device and the ephemeris information server communicate via a wide-area low-power communication network LPWAN.

23. The method according to claim 22, wherein, The LPWAN is LoRa-based.

24. The method according to claim 20, wherein, The calculated parameter value is one or more of the following: elevation angle, pseudorange, and Doppler.

25. The method according to claim 20, wherein, When the update criterion is not satisfied, the modified trajectory information of each satellite is not updated.

26. The method according to claim 20, wherein, The modified trajectory information includes Keplerian ephemeris information of each of the satellites.

27. The method according to claim 26, wherein, The Keplerian ephemeris information includes an estimate of the semi-major axis of one of the satellites.

28. The method according to claim 27, wherein, The semi-major axis is estimated using the mean motion of the satellite.

29. The method according to claim 28, wherein, The mean motion is modeled by a linear function of time.

30. The method according to claim 26, wherein, The Keplerian ephemeris information includes a parameter value related to the mean anomaly of the orbit of one of the satellites.

31. The method according to claim 26, wherein The Keplerian ephemeris information further includes an estimate of a parameter related to the longitude of the ascending node associated with the orbital plane of one of the satellites.

32. The method according to claim 20, wherein The modified trajectory information is transmitted to one of the devices in a bit stream related only to the modified trajectory information that the device does not yet have.

33. The method according to claim 32, wherein, The bit stream includes modified trajectory information of two or more satellites.

34. The method according to claim 20, wherein, The update criterion includes a first performance criterion and a second performance criterion, wherein (i) at each specified time point, the update criterion is considered satisfied only if at least one satellite satisfies the first performance criterion, and (ii) when the update criterion is considered satisfied, the modified trajectory information of those satellites that satisfy the second performance criterion is updated.

35. The method according to claim 20, wherein, The specified time points occur at regular time intervals.

36. The method according to claim 20, wherein, Copies of the real-time database at different times are stored at one or more locations remote from the ephemeris information server, enabling the ephemeris information server to be restored to a previous state.

37. The method according to claim 36, wherein, The ephemeris information server also records in the real-time database the creation time associated with each creation instance of the modified trajectory information to allow the real-time database to be restored to a desired state after being restored to the previous state.

38. The method according to claim 20, wherein, The ephemeris information server restores the real-time database by creating, at a selected previous time, the current modified trajectory information as a reference set of the modified trajectory information, and wherein such modified trajectory information is provided to the device only after the modified trajectory information in the reference set is updated after the selected time.

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