Method of and device for providing a reference timescale to a user equipment

Abstract

A method of providing a reference timescale to a user equipment is disclosed. The method is performed by a processing device and comprises the steps of: receiving, from a first ground station, timing measurement of a satellite signal, wherein the first ground station has a known geolocation and a ground station clock synchronized to the reference timescale, wherein timing measurement of a satellite signal is generated by the first ground station by time-tagging a time of arrival of the satellite signal comprising a time synchronization marker transmitted by the satellite, the time synchronization marker is known to the first ground station and the user equipment; calculating time of transmission of the satellite signal based on the received timing measurement; encoding the time of transmission and position information of the satellite in the relevant satellite signal; and transmitting the relevant satellite signal encoding the time of transmission and the position information of the satellite to the user equipment. Unlocking insights from Geo-Data, the present invention further relates to improvements in sustainability and environmental developments: together we create a safe and liveable world.

Description

METHOD OF AND DEVICE FOR PROVIDING A REFERENCE TIMESCALE TO A USER EQUIPMENTFIELD OF THE INVENTION

[0001] The present disclosure generally relates to distributing a reference timescale, and more specifically, to a method of and a device for providing a reference timescale to a user equipment. Unlocking insights from Geo-Data, the present invention further relates to improvements in sustainability and environmental developments: together we create a safe and liveable world.BACKGROUND OF THE INVENTION

[0002] Modem societies depend on access to very precise time to function in a much wider sense than most people realize. Time synchronization is being used in day-to-day operation of Power grids, Telecommunications, Water and Gas networks, financial trading, the Internet, Data Centres, etc. Precise timekeeping is a foundational element that supports the functioning and efficiency of various infrastructures in the societies. Without precise timekeeping, many systems people rely on daily would face significant challenges and inefficiencies.

[0003] Currently most applications needing a precise time source use the American Global positioning system, GPS or nowadays global navigation satellite system GNSS, a term which also includes the European Galileo, the Chinese BeiDou and Russian GLONASS.

[0004] While GNSS provides important services for navigation and timing, reliance on the GNSS introduces vulnerabilities and limitations, which can be attributed to a number of reasons.

[0005] First of all, GNSS signals are relatively weak and can be easily jammed by malicious transmitters, which overwhelm the receiver with noise, preventing it from receiving satellite signals. More sophisticated than jamming, spoofing involves broadcasting fake GNSS signals to deceive receivers into calculating incorrect positions or times. This can have serious implications for navigation and timing applications.

[0006] Moreover, there is various interference to GNSS signals. As an example, solar flares and other space weather phenomena can disrupt GNSS signals, leading to degraded performance or complete outages. Besides, urban environments with tall buildings (which will cause multipath effect), electromagnetic interference from other devices, and intentional signal interference can affect GNSS accuracy and reliability.

[0007] Therefore, though GNSS is widely and successfully used in all kinds of applications requiring access to a precise time source, this dependence on GNSS is identified as a vulnerability,particularly in light of the persistent GNSS interference problems reported these days. The potential exists for GNSS synchronization to be disrupted by a state actor over large geographical areas, or in localized areas by malicious or reckless operation of jamming devices by individuals.

[0008] Thus, both in public regulations and private industry, there is motivation and drive towards GNSS independent alternatives which can operate even when large geographical areas are denied GNSS coverage. As an example, a report published by National Institute of Standards and Technology, NIST, of the United States in November 2021 concludes that Critical Infrastructure would benefit from improvements in both resilience and diversity.

[0009] European, Union, EU, has published a report in 2023 called “Assessing Alternative Positioning, Navigation, and Timing Technologies for Potential Deployment in the EU” and showed that alternatives are available for distribution of UTC time, but all have limitations, be it the need for Fiber Optic links, need for large roll-out of new infrastructure or currently no availability in Europe or other parts of the world.

[0010] A currently available method of transferring time, referred to as the signal of opportunity approach, is a method of transferring precise time by utilizing existing signals that are already being transmitted for other purposes. These signals can come from a variety of sources such as radio, television broadcasts, cellular networks, or even satellite communications that are not originally intended for time synchronization.

[0011] The signal of opportunity approach, while useful for time transfer and synchronization, has several disadvantages that can impact its reliability and accuracy.

[0012] First of all, the approach relies on the availability of external signals, such as radio, TV broadcasts, or cellular signals. If these signals are not present or are weak, the receiver cannot perform synchronization .

[0013] Besides, as the signals are not specifically designed for time transfer, they may not have the necessary accuracy or stability required for precise time synchronization. There can also be unknown and variable delays in the transmission path of the signal, leading to inaccuracies in the extracted timing information.

[0014] Moreover, designing a receiver that can effectively use signals of opportunity for time synchronization can be complex, requiring sophisticated algorithms to extract and process the timing information accurately. Different types of signals (e.g., AM, FM, digital TV) may require different processing techniques, adding to the complexity of the receiver design.

[0015] Furthermore, the exact structure of the signals of opportunity may not be known, requiring the exchange of large amounts of data, such as raw radio frequency data, necessitating a large communications bandwidth.

[0016] Based on the above, it is desirable that an alternative way of distributing precise time is available, which would enable transfer of time independent of GNSS readily without significantly increasing the complexity of receiver design.BRIEF SUMMARY OF THE INVENTION

[0017] In one aspect of the invention there is provided a method of providing a reference timescale to a user equipment. The method is performed by a processing device and comprises the steps of:

[0018] receiving, from a first ground station, timing measurement of a satellite signal received by the first ground station from a satellite, wherein the first ground station has a known geolocation and a ground station clock synchronized to the reference timescale, wherein timing measurement of a satellite signal is generated by the first ground station by time-tagging a time of arrival of the satellite signal comprising a time synchronization marker transmitted by the satellite, the time synchronization marker is known to the ground station and the user equipment;

[0019] - calculating time of transmission of the satellite signal based on the received timing measurement,

[0020] - encoding the time of transmission and position information of the satellite in the relevant satellite signal; and

[0021] transmitting the relevant satellite signal encoding the time of transmission and the position information of the satellite to the user equipment.

[0022] The above method is based on the inventors’ insight that a precise reference timescale can be maintained and transferred to a user equipment by using observable satellite signals and beacons.

[0023] Specifically, the reference timescale is maintained by a first ground station. The first ground station has a known geolocation and a clock that is synchronized to a reference timescale. The first ground station generates timing measurement by time-tagging an arrival time of a satellite signal, which includes a time synchronization marker transmitted by the satellite. This marker is a known reference point for both the ground station and the user equipment, ensuring that the time measurement is precise and consistent.

[0024] Additional timing measurements can be generated from at least one second ground station, which also has a known geolocation. These further measurements are obtained in a similar manner, with the second ground station time-tagging the arrival of the same satellite signal. By having multiple timing measurements from different ground stations, the method can account for variations and improve the accuracy of the calculated time of transmission and in addition estimate the satellite’s position.

[0025] The time measurements from the first, and optionally the at least one second ground stations, are received at a processing device, which performs the method of the present disclosure. With the timing measurement from the first ground station (and optionally the timing measurement from theat least one second ground stations), the method calculates the time of transmission of the satellite signal. This calculation involves synchronizing the timing measurement(s) and accounting for the known position of the ground station and the satellite. By doing so, the method can accurately determine when the satellite signal was transmitted.

[0026] The calculated time of transmission and position information of the satellite are encoded into the relevant satellite signal. This encoded information is then transmitted to the user equipment. The user equipment, knowing the time synchronization marker and its own position, can decode this information to derive a precise reference timescale. This enables the user equipment to synchronize its own clock accurately with the reference timescale, which is essential for applications requiring precise timing.

[0027] This method is practical and enabling because it leverages existing satellite communication technology and ground station infrastructure. Existing GNSS ground stations can be arranged to track additional satellite signals and perform precise time measurements. Additionally, the encoding and transmission of timing and positional information are well within the capabilities of current satellite correction broadcast systems.

[0028] The method of the present disclosure provides an alternative way of providing precise reference timescales without relying on existing GNSS technologies. It can be implemented with existing technology. This makes the method applicable for a variety of applications that depend on accurate timing.

[0029] In an example of the present disclosure, the time synchronization marker comprises a pattern usable as a reference epoch for timing purposes.

[0030] In the context of the present disclosure, the time synchronization marker is used to ensure precise time measurement and synchronization. This marker comprises a pattern that serves as a reference epoch for timing purposes. The term "reference epoch" refers to a specific point in time that is universally recognized and used as a starting point for time measurements. By using a known pattern, both the user equipment and the ground stations have a common reference for synchronizing their clocks and measurements.

[0031] The pattern is embedded within the satellite signal and is known to all relevant parties, including the ground stations and the user equipment. When the satellite transmits its signal, this pattern acts as a recognizable timestamp. Upon receiving the signal, the ground stations use the pattern to precisely time-tag the arrival of the signal. This process ensures that the timing measurements are consistent and can be accurately compared across different ground stations.

[0032] By having a standardized pattern, the system eliminates ambiguities and errors that could arise from using arbitrary or varying markers. This consistency is beneficial for calculating the exact time of transmission of the satellite signal. The pattern acts as a reliable and repeatable reference point, allowing the ground stations to perform highly accurate timing measurements.

[0033] In an example of the present disclosure, the pattern comprises a pre-defined pattern, such as a unique word having a defined length occurring at a defined position, such as the start, of every frame transmitted by the satellite.

[0034] The predefined pattern maybe for example a unique word modulated using a known modulation technique. The use of a pre-defined pattern as a time synchronization marker simplifies the design and implementation of the system. Ground stations and user equipment can be programmed to recognize this pattern and use it to synchronize their internal clocks with the reference timescale. This capability is particularly important in applications requiring high precision, such as telecommunications and scientific research.

[0035] In contrast, in signal of opportunity, SOP, approaches the choice of pattern to time-tag a signal with is typically chosen by the reference station upon every measurement and then transmitted to the user. With limited or no assumptions about the signal the SOP approach can be very bandwidth intensive that would not be suitable for the capacity of many satellite broadcast channels and therefore cost prohibitive.

[0036] Incorporating a unique word with a defined length at the start of every frame transmitted by the satellite provides a robust and precise method for timing synchronization. This approach enhances the accuracy of time-tagging by ground stations, facilitates consistent and reliable synchronization across the system, and ensures that the user equipment can derive a precise reference time scale.

[0037] Furthermore, a timestamp is provided in the communication channel and identifies an absolute time, in a reference timescale, of the pre-defined pattern and thereby eliminating the time ambiguity inherent in a repeated marker. The timestamp essentially locks each marker instance to a specific time, ensuring that the receiver knows exactly which time reference the marker is linked to.

[0038] In an example of the present disclosure, the step of calculating the time of transmission of the satellite signal comprises:

[0039] - deriving a time of flight needed for the satellite signal to travel from the satellite to the first ground station;

[0040] - deriving at least one further time of flight needed for the satellite signal to travel from the satellite to at least one second ground station;

[0041] - obtaining a first time of transmission by subtracting the derived time of flight from the timing measurement of the satellite signal received by the first ground station;

[0042] - obtaining at least one further time of transmission by respectively subtracting the derived at least one further time of flight from at least one further timing measurement of the satellite signal received by the at least one second ground station, the at least one further timing measurement of the satellite signal being received by the processing device from the at least one second ground station;

[0043] - combining the first time of transmission and the at least one further time of transmission to derive the time of transmission of the satellite signal and optionally the satellite position.

[0044] As can be understood by those skilled in the art, with the first ground station having a known geolocation and a ground station clock synchronized to the reference timescale, when the satellite position is known, the time of transmission of the satellite signal can be calculated based on the positions of both the first ground station and the satellite and the timing measurement of the time arrival of the satellite signal at the first ground station.

[0045] Alternatively, combining the times of transmission from multiple ground stations, as described in this example, enhances the accuracy and reliability of the timing calculation. By using timing measurements from different locations, any individual measurement errors or timing discrepancies due to local conditions are mitigated and the satellite position can also be calculated.

[0046] This multi-station approach provides a more robust and precise determination of the satellite signal's time of transmission. It ensures that the derived time of transmission is not overly influenced by anomalies or inaccuracies at any single ground station, resulting in a highly accurate reference timescale that can be reliably used by the user equipment for synchronization purposes.

[0047] In an example of the present disclosure, the relevant satellite signal encoding the time of transmission and the position information of the satellite is transmitted through a network connecting the processing device and the user equipment.

[0048] In another example of the present disclosure, the relevant satellite signal encoding the time of transmission and the position information of the satellite is transmitted through the same satellite as the synchronization markers.

[0049] The network connecting the processing device and the user equipment can include fibre optics, internet, or other high-speed data connections, which ensures that the encoded signal reaches the user equipment with minimal delay and high data integrity. It can employ existing infrastructure, making it potentially more cost-effective and scalable, especially in environments with robust network coverage. Additionally, it allows for easy updates and maintenance of the communication system without relying solely on satellite transmission, thereby increasing the overall flexibility and adaptability of the system.

[0050] As an alternative solution, the relevant satellite signal is transmitted through the same satellite as the synchronization markers. Here the key advantage is the integration and consistency of the signal path. By using the same satellite to transmit both the synchronization markers and the encoded time and position information, the system ensures that the timing data remains synchronized with the satellite's operational context. This approach simplifies the user equipment's receiving process, as it only needs to monitor a single source for all necessary data, reducing the potential for errors and mismatches. This is also beneficial in remote or mobile scenarios where terrestrial network connectivity is unreliable or unavailable, ensuring continuous and consistent service directly from the satellite.

[0051] In an example of the present disclosure, the time of transmission information includes a measurement of the carrier phase at the time of transmission, including information on a number of carrier cycles that have accumulated between measurement epochs.

[0052] The carrier phase refers to a specific point in a wave cycle of the signal when it is transmitted and includes a continuous count of complete carrier cycles that have elapsed, which provides a continuous count of how many full cycles have passed between two time points (epochs) during the signal's transmission. This detailed carrier phase information allows the user equipment to synchronize not just based on the arrival time of the signal but also based on the carrier phase of the signal at the time of transmission, leading to improved accuracy.

[0053] It will be understood by those skilled in the art that the number of carrier cycles can be a floating point number of carrier cycles.

[0054] The advantage of this approach is that it significantly enhances the system's accuracy, as the carrier phase can be measured with higher accuracy than the pre-defined synchronization marker timing information. This results in a more stable and accurate reference timescale.

[0055] In an example of the present disclosure, ionospheric and / or tropospheric delay is considered in deriving the time of flight.

[0056] By accounting for these delays, the system can correct for the time lags introduced as the signal passes through the ionosphere. This correction ensures that the calculated time of flight is more precise, leading to a more accurate determination of the time of transmission. Consequently, this improves the overall synchronization and reliability of the reference timescale provided to the user equipment.

[0057] In an example of the present disclosure, timing measurement of the satellite signal is generated by each ground station after accumulating energy from a number of symbols of the time synchronization marker in a decision directed manner.

[0058] By using a decision-directed approach, the system can iteratively refine its timing measurements, leading to more precise synchronization. This technique ensures reliable performance and high precision in various environmental conditions, ultimately providing a more accurate reference timescale for the user equipment.

[0059] In an example of the present disclosure, symbols comprising the time synchronization marker within the satellite signal are generated in a coherent manner with the timescale of a reference system.

[0060] This coherence allows for precise alignment of the marker with the reference timescale, reducing timing errors and improving the accuracy of the synchronization process.

[0061] In another example of the present disclosure, carrier-phase of the satellite signal is generated coherently with the time synchronization marker and thereby is generated in a coherent manner with the timescale of a reference system.

[0062] This makes both the carrier-phase and the symbols of the signal synchronized and aligned with the same time reference. Having the carrier phase and the symbols both aligned with the reference timescale helps the receiving station to derive more accurate and stable timing information.

[0063] In an example of the present disclosure, the satellite comprises geostationary satellites and non-geostationary satellites.

[0064] The method of the present disclosure is not limited to a specific kind of satellite, it is versatile and both geostationary satellites and non-geostationary satellites can be used.

[0065] A second aspect of the present disclosure provides a method of obtaining a reference timescale by a user equipment with a known user equipment location, the method comprising the steps of:

[0066] receiving the relevant satellite signal encoding the time of transmission of the satellite signal and the position information of the satellite transmitted according to the first aspect of the present disclosure;

[0067] - determining a time of reception of the time synchronization marker in the original satellite signal, in a user time frame local to the user equipment; and

[0068] - deriving the reference timescale using the known user equipment location, the time of reception, and the time of transmission of the satellite signal and the position of the satellite in the received relevant satellite signal.

[0069] By combining the timing information and the position of the satellite, the user equipment can effectively use the reference timescale to correct its own local time, allowing it to maintain highly accurate timing.

[0070] In an example of the present disclosure, the method further comprises a step of calculating a time offset between the user time frame and the derived reference timescale.

[0071] The time offset can be used to correct a local system clock of the user equipment.

[0072] In an example of the present disclosure, the method further comprises the steps of:

[0073] receiving the carrier phase at the time of transmission;

[0074] - recording a carrier phase at time of reception;

[0075] - smoothing an estimate of time difference between the time of transmission and the time of reception using an estimate of a phase difference between transmission and reception;

[0076] wherein the reference time scale is derived using the smoothed estimate of time difference between the time of transmission and the time of reception.

[0077] As can be contemplated by those skilled the art, by taking into account the carrier phase at the time of transmission, the accuracy of the derived reference timescale is improved.

[0078] In an example of the present disclosure, at least one of ionospheric and tropospheric delay is considered in deriving the time of flight of the original satellite signal from the satellite to the user equipment.

[0079] A third aspect of the present disclosure provides a system for providing a reference timescale to a user equipment, the system comprising a computing device having a processor arranged to perform the method according to the first aspect of the present disclosure.

[0080] The system further comprises:

[0081] a first ground station with a known geolocation and a ground station clock synchronized to the reference timescale, the first ground station arranged to generate timing measurements of a satellite signal received by the first ground station from a satellite, by time-tagging a time of arrival of the satellite signal comprising a time synchronization marker transmitted by the satellite, the time synchronization marker is known to the ground station and the user equipment;

[0082] a user equipment with a known user equipment location and arranged to receive the relevant satellite signal encoding the time of transmission and the position information of the satellite and to derive a time offset between a user time frame and the reference time frame using the known user equipment location, a time of reception of the synchronization marker in the original satellite signal in a user time frame local to the user equipment, and the time of transmission of the received relevant satellite signal.

[0083] A fourth aspect of the present disclosure provides a device for providing a reference timescale to a user equipment, the device comprising a processor for performing the method according to the first aspect of the present disclosure.

[0084] A fifth aspect of the present disclosure provides a computer program product, comprising a computer readable storage medium storing instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to the first aspect of the present disclosure.

[0085] The above mentioned and other features and advantages of the disclosure will be best understood from the following description referring to the attached drawings. In the drawings, like reference numerals denote identical parts or parts performing an identical or comparable function or operation.BRIEF DESCRIPTION OF THE DRAWINGS

[0086] In order to describe the manner in which the above-recited and other advantages and features of the disclosure can be obtained, a more particular description of the principles briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only exemplary embodiments of the disclosure and are therefore not to be considered to be limiting of its scope, the principles herein are described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0087] FIG. 1 schematically illustrates a system for providing a reference timescale to a user equipment;

[0088] FIG. 2 illustrates a modulation scheme used by Inmarsat L-band signals.

[0089] FIG. 3 illustrates a frame of INMARSAT L-band signals;

[0090] FIG. 4 schematically illustrates, in a flow chart, a method of providing a reference timescale to a user equipment in accordance with the present disclosure;

[0091] FIG. 5 schematically illustrates using a same clock to drive carrier demodulation and tracking modules and a local time reference of a receiver;

[0092] FIG. 6 schematically illustrates a Hatch fdter for smoothing timing estimates with carrier phase measurements; and

[0093] FIG. 7 shows test results of time transfer using the method of the present disclosure between two test sites over three days.DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0094] Embodiments contemplated by the present disclosure will now be described in more detail with reference to the accompanying drawings. The disclosed subject matter should not be construed as limited to only the embodiments set forth herein. Rather, the illustrated embodiments are provided by way of example to covey the scope of the subject matter to those skilled in the art.

[0095] The new technology presented in the present disclosure has the benefit of being wirelessly available almost anywhere. It would be available from space from several satellites with redundant global coverage. Such a technology will be attractive for end users and infrastructure providers of many different timing applications. It will reduce the reliance on GNSS and provide additional redundancy through GNSS independence. For national critical infrastructure this technology would be of significant value. The wireless similarities with GNSS will generally make it easy to install as an addition where GNSS already is used. The technology can largely re-use existing satellite broadcasting infrastructure.

[0096] The term "reference timescale" refers to a precise time standard provided by for example a ground network of the present disclosure. Based on the present disclosure, a precise time standard is maintained on the ground, which is transferred using observable satellite signal as a “beacon”.

[0097] Referring to Figure 1, a system 10 for providing a reference timescale to a user equipment is illustrated.

[0098] The system 10 comprises one or more satellites 11, one or more ground stations 12, which may also be referred to as observation stations. The system 10 further comprises any number of receiver stations 13, which may also be referred to user equipment or user receivers or simply receivers. The system 10 further comprises a processing device 14 for collecting information from the observationstations 12 and transmitting timing information about signals received by the observation stations 12 and the receiver stations 13 from the satellite 11.

[0099] The one or more satellites 11 are arranged to broadcast signals, which can be received by the ground stations 12 and the receiver stations 13, as indicated by arrows 16 of Figure 1.

[0100] The satellite 11 can comprise geostationary or non-geostationary satellites. The satellite 11 can be a communication satellite or an Earth observation satellite.

[0101] The signals transmitted by the satellite 11 has a modulation scheme and framing structure which are known to the ground stations 12 and receiver stations 13. As can be understood by those skilled in the art, this is realised by following a standardized protocol or proprietary schemes which are made known to the ground stations 12 and receiver stations 13.

[0102] The satellite 11 may comprise for example an Inmarsat satellite transmitting L-band signals. These signals are in the 1 to 2 GHz range, commonly used for mobile satellite services due to their robustness against atmospheric conditions. The modulation scheme, such as QPSK, and the framing structure ensure compatibility and reliable decoding by the ground stations 12 and the receiver stations 13. These protocols include synchronization bits and error correction codes to maintain data integrity and facilitate seamless communication.

[0103] Another example is a weather satellite, such as a Geostationary Operational Environmental Satellite, GOES. GOES satellites transmit in the L-band and S-band, using modulation schemes like BPSK and QPSK. They provide important meteorological data, including real-time weather monitoring and forecasting. The standardized protocols used ensure that ground stations and receiver stations can accurately process the weather data for analysis and dissemination.

[0104] In accordance with the present disclosure, a reference timescale is distributed from the ground network comprising the ground station 12 to the user equipment 13 by using satellite signals. The user equipment 13 can include user equipment found in applications as described in the background section.

[0105] In a typical signal of opportunity, SOP, scenario, the user equipment is not a “participating user” for the signal of interest, but essentially eavesdrops on that signal and extracts timing information from what it picks up in an ad hoc manner.

[0106] The present disclosure differs from the SOP approach in that the user equipment of the present disclosure is a receiver that already processes the signal of interest, that is, the signal transmitted via the satellite 11.

[0107] Moreover, the signal that the satellite 11 transmits is not specifically designed for time transfer. Therefore, there is no assumption about stability or accuracy of the satellite clock.

[0108] The present disclosure takes advantage of the user equipment’s prior knowledge of the satellite signal, including the modulation scheme and framing information. The framing information in particular is the source of a regular, defined pattern in the satellite signal which can be used as a“reference epoch” for timing purposes. The defined patern in the satellite signal therefore is considered or functions as a time synchronization marker which will be used by the present disclosure for time transfer from the ground network to the user equipment.

[0109] As a general example, for a communication satellite, the adopted modulation scheme may for instance comprise Binary Phase Shift Keying, BPSK, Quadrature phase-shift keying, QPSK, 8PSK, or 64QAM. For framing information, the satellite signal typically includes a unique word, payload data and control information. The unique word is a specific patern at the start of each frame to enable synchronization .

[0110] In the case the signal transmited by the satellite does not comprise a defined patern such as a unique word, a patern formed by a sequence of decoded symbols may be selected as the time marker, the patern is therefore broadcast to the user. This is still very bandwidth efficient compared to broadcasting raw RF data as used in the SOP approach.[OHl] In an example, for Inmarsat L-band signals, the modulation scheme is Raised Root Cosine (RRC)-shaped Binary Phase Shift Keying (BPSK), which is illustrated in Figure 2. The framing information of the INMARSAT L-band signals takes the form of a unique word, 64 symbols in duration, that occurs at the start of every frame (every 3.36 seconds for a 1200 baud signal, and every 1.68 seconds for a 2400 baud signal). A frame of the INMARSAT L-band signals is illustrated in Figure 3, where UW stands for unique word.

[0112] For earth observation satellites, generally well-documented downlink signals are used for data transmission, which might use standard protocols and modulation schemes to ensure compatibility with ground stations.

[0113] The ground station(s) 12 has a known location, which can be determined based on an approach known to those skilled in the art. In other words, the ground station 12 is precisely geo-located and exact geographical coordinates of the ground station 12 can be determined with a high degree of accuracy. This means the specific latitude, longitude, and altitude of the location of the ground station 12 can be identified within a small margin of error.

[0114] As an example, the location of the ground station 12 can be determined using GNSS satellite information obtained from a hand-held GPS receiver during installation of the ground station 12, through survey, or through the use of topological maps and so on.

[0115] Referring back to Figure 1, each ground station 12 has a local clock 121. At least one of the ground stations 12 has its local clock 121 synchronized to the reference time scale. In an embodiment of the present disclosure, local clocks 121 of one or more ground stations 12 are synchronized to the reference timescale. In another embodiment of the present disclosure, one ground station 12 has a local clock 121 synchronized to the reference timescale, local clocks 121 of other ground stations 12 are not necessarily synchronized to the reference timescale.

[0116] Each of the ground stations 12 is arranged to receive signals (telemetry, scientific data, communications, etc.) from the satellite 11 and time-tag time of arrival of the satellite signal comprising a time synchronization marker transmitted by the satellite 11.

[0117] The receiver station 13 is generally a fixed station with a known location. Each receiver station 13 is equipped with a local receiver clock 131. Each of the receiver station 13 is also arranged to receive, independently of the ground stations, signals from the satellite 11 and time-tag time of arrival of the satellite signal comprising a time synchronization marker transmitted by the satellite 11.

[0118] The processing device 14 is arranged to receive timing measurements of the satellite signals from the ground station(s) 12, to process the receiving timing measurements to obtain time of transmission of the satellite signals, and to convey the obtained time of transmission of the satellite signals to the user equipment 13, such that the user equipment 13 can obtain the reference timescale, that is a precise time, which can be used for applications run by the user equipment 13.

[0119] The processing device can be an integral part of the ground station 12. Alternatively, the processing device can be independent of the ground station 12. When the processing device is a separate from the ground station 12, the location of the ground station 12 is available to the processing device, for example by receiving it from the respective ground station 12 or have such information stored locally in the processing device.

[0120] The method of providing a reference timescale to a user equipment of the present disclosure will be described in detail in the following.

[0121] To effect a time transfer from the reference timescale to the user equipment, two pieces of information need be provided to the user:

[0122] 1. The precise time, in the reference time frame, at which the signal epoch (start of frame) occurs at the transmitter. The transmitter can be for example a GEO satellite in the case of Inmarsat L- band signals.

[0123] 2. The signal delay from the transmitter to the receiver.

[0124] Both of these pieces of information are provided by adding extra data to a same signal from the satellite that is being tracked for timing purposes.

[0125] For providing the reference timescale to the user equipment based on the present disclosure, the position of the satellite transmitting the signal is needed as well. This information may already be available for the satellite 11, or may be obtained by methods known to those skilled in the art: for example from a network of observation stations; satellite laser ranging etc.

[0126] In an example, the observation stations 12 may form a part or the whole of the network of observations stations used for satellite orbit estimation. Using the time of flight estimates, which are equivalent to pseudoranges in GNSS, the “inverse GNSS” problem can be solved to determine the satellite location.

[0127] Figure 4 schematically illustrates, in a flow chart, a method 40 of providing a reference timescale to a user equipment in accordance with the present disclosure. The method is performed by for example the processing device 14 of Figure 1.

[0128] At step 41, the processing device 14 receives timing measurement of a satellite signal from a first ground station, such as the ground stations 12 in Figure 1. The first ground station has a known geolocation and a first ground station clock synchronized to a reference timescale, such as one UTC.

[0129] A timing measurement of a satellite signal is generated by the first ground station 12 by time-tagging time of arrival of the satellite signal comprising a time synchronization marker transmitted by the satellite, the time synchronization marker is known to the ground station and the user equipment. The time synchronization marker may comprise for example a pattern usable as a reference epoch for timing purposes. In an example, the pattern can be a pre-defined pattern, which for example comprises a unique word with a defined length occurring at a defined location, such as a start, of every frame transmitted by the satellite.

[0130] With a pre-defined pattern, more bandwidth is saved and the workings of the user receiver is simplified as it does not need to remember the symbols recently received and their arrival time until the selected time marker symbols information would be received over the correction channel after the fact.

[0131] Alternatively, the synchronization marker can consist of a pattern that is first transmitted by the satellite and subsequently communicated to the user.

[0132] In an example, the first ground station 12 continuously time-tags satellite signals that it receives from the satellite. In another example, the first ground station 12 time-tags the received satellite signals for a predetermined period of time.

[0133] Time measurements of the satellite signals may also be generated by one or more second ground stations with known geolocations and transmitted by the second ground stations to the processing device.

[0134] In the present disclosure, the ground station(s) time-tag the arrival of the known unique words of the satellite signals. Therefore, there is no need to save the samples or broadcast these to the user. The user equipment also knows the “unique word” as it is part of the public standard, and it can start tracking these and time-tag the arrival independently of the ground station acting as the base (reference) receiver.

[0135] The timing measurements of the satellite signal from the first ground station and optionally from the second ground stations are received by the processing device 14.

[0136] At step 42, the processing device 14 calculates time of transmission of the satellite signal based on the timing measurements received from the first ground station, optionally also based on the timing measurements from the second ground stations. The time of transmission of the satellite is expressed in the reference system timescale.

[0137] Specifically, the time of transmission of the satellite can be derived by first deriving a time of flight needed for the satellite signal to travel from the satellite to the first ground station and then subtracting the derived time of flight from the timing measurement of the satellite signal received by the first ground station.

[0138] The time of transmission of the satellite signal may be calculated based on timing measurements from multiple ground stations including the first ground station and one or more second ground station, which helps to get a better resolution.

[0139] As an example, the time of transmission of the satellite signal may be calculated in the following way.

[0140] At a step, a time of flight needed for the satellite signal to travel from the satellite to the first ground station is derived. At least one further time of flight needed for the satellite signal to travel from the satellite to at least one second ground station may also be derived.

[0141] Based on the derived time of flight, a first time of transmission is obtained by subtracting the derived time of flight from the timing measurement of the satellite signal received by the first ground station, and at least one further time of transmission is derived by respectively subtracting the derived at least one further time of flight from the at least one further timing measurement of the satellite signal received by the at least one second ground station.

[0142] By combining the first time of transmission and the at least one further time of transmission, the time of transmission of the satellite signal can be derived. This approach allows a more reliable time of transmission to be determined.

[0143] It is noted that when timing measurements from several ground stations are available, the satellite position may also be optionally derived. This is very helpful when the position of the satellite is not known.

[0144] At step 43, the processing device 14 encodes the derived time of transmission and position information of the satellite to relevant satellite signal.

[0145] The derived time of transmission and position information of the satellite can be encoded and modulated onto the same satellite signal that is used for time transfer. This is done for example by adding the derived information to the data part of the satellite signal.

[0146] At step 44, the relevant satellite signal encoding the time of transmission and position information of the satellite is transmitted to the user equipment receiving the same satellite signal, which the user equipment will use to derive the reference timescale to correct its local clock.

[0147] In other words, once the user equipment obtains this information, it can use it to transfer the time of the reference epoch in the satellite time frame to the user local time frame. In a sense, timetag information referencing the satellite signal range to the reference timescale is transmitted to the user equipment via the encoded satellite signal. This is then used to obtain an estimate of an offset between the local time and the system time.

[0148] The signal carrying the position and time of transmission of the satellite can be transmitted to the user equipment in different ways. In an example, referring to Figure 1, the signal carrying the time of transmission and position information of the satellite is transmitted from the processing device 14 via an uplink station 15 to the satellite 11, and then to the user equipment 13.

[0149] The signal carrying the time of transmission and position information of the satellite can be transmitted to the user equipment via a telecommunication network as well, without going via the satellite. In this case, the processing device 14 and the user equipment 13 are connected to the same network (not shown in Figure 1). The processing device 14, upon generating the signal carrying the time of transmission and position information of the satellite, will have the same signal transmitted to the user equipment via the telecommunication network.

[0150] In a specific example, the signal carrying the position and time information of the satellite is transmitted as a Networked Transport of RTCM via Internet Protocol.

[0151] As for deriving the reference timescale, specifically, assuming that T3ysis the system time at the transmit epoch (at the satellite), 7Xis the receiver time at the receive epoch (at the user equipment) and TOF is the time of flight, 7X=is a receiver clock error. The receiver calculates the system time at the receive epoch T2sysfrom the T3ysand TOF, T^'3= T3ys_|_ 'pQ T|1Cdifferencebetween the system time Ty 3at the receive epoch and the receiver time 7Xat the receive epoch gives the receiver clock error at the receive epoch ATx= 7X— Tys.

[0152] The time of flight measurement corresponds to the pseudorange concept from GNSS expressed in units of time rather than distance. With a sufficient diversity of satellites, these measurements can be used to solve for the user position via trilateration.

[0153] In the present disclosure, the processing performed by the ground stations 12 is precisely the same as that performed by the user equipment: generating timing measurements for the observed signal epochs relative to the local time scale. There is no impact on the structure of the satellite signal or the broadcast infrastructure as such. In addition, possible correction for path delay by both the ground station and the user equipment is also handled in the same manner.

[0154] In practice, a receiver tracks the symbols received from the satellite, and detects the presence of the unique word in order to correctly frame the symbols for data demodulation and Viterbi decoding. To use the service as provided by the method of the present disclosure, the user equipment 13 needs to extract precise timing information from the existing (or modified) tracking loops, and to link the timing epochs (start of frame for example) to its local time reference.

[0155] This can be achieved using techniques such as driving the receiver clock from the same source as the local time reference, and recording the receiver time offset between the observed epoch (start of frame) and a local time epoch (such as the local UTC second transition). The observed epoch refers to a specific event or point in the received satellite signal, such as the start of a frame indicated by a known pattern like a unique word.

[0156] Using the same clock to drive both the carrier demodulation and tracking modules as well as the local time reference of a user equipment ensures that all timing-related processes within the user equipment are synchronized. This synchronization helps to achieve accurate signal processing and precise time measurements.

[0157] The user equipment records the time offset between the observed epoch (e.g., the start of a frame) and a local time epoch (e.g., the exact moment a UTC second transitions). This offset provides a precise measurement of when the signal was received in the user equipment's local time.

[0158] Figure 5 schematically illustrates using a same clock to drive carrier demodulation and tracking modules and a local time reference of a user equipment.

[0159] In Figure 5, a local oscillator 531 is used to drive a carrier demodulation module 532, a tracking module 533 and a receive clock 534. The demodulation module 532 is arranged to demodulate satellite signals 510 received from the satellite 51 to output a detected signal 520. The tracking module 533 adjusts the local oscillator 531 to stay synchronized with the incoming signal 520, ensuring continuous and accurate demodulation.

[0160] The user equipment incorporates a model of the receiver oscillator 531 to account for the change in the offset with respect to the reference timescale that occurs during the delay between the observation of the reference epoch and the time at which the transmission time information is received from the system.

[0161] The user equipment is arranged to account for a latency between when the reference epoch occurs and when the user equipment receives the time tag relating to that epoch, either from the satellite signal itself or over the alternative communications channel.

[0162] Other processing performed by the receiver, including for example channelization, carrier wipe off, correlation, phase and symbol discrimination and so on, are known to those skilled in the art and will not be elaborated here.

[0163] The above describes the basic principle of the method of the present disclosure, which is distinctive from both the GNSS and the SOP approach.

[0164] It differs from GNSS in the sense that there is no knowledge or requirement for satellite clock stability. Transmission time is determined after the fact, measurements are made on communication symbols rather than a spreading code and no modifications need to be made to the chosen communication signal. Besides, satellite symbol clocks and carrier-phase clocks can be different. The user receiver needs to have a certain oscillator stability to bridge the gap between the arrival of the measured signal and the corresponding correction.

[0165] It differs from SOP in that there is knowledge about the modulation of the communication signal used and ability to receive the data symbols. Therefore, it is possible to use a predefined time marker (for instance the unique word) and no need to send information about the time marker to theuser receiver in the form of bandwidth hungry RF samples. Furthermore, corrections can be provided over the same channel.

[0166] With the method of the present disclosure, when geostationary satellite is used to transmit the signal, directional antennas can be used to receive the signal. In this case, interference signals from other directions in the sky will be attenuated and the desired signal amplified, making the signal reception very robust and resilient to interference and jamming unlike GNSS that uses omnidirectional antennas.

[0167] There are a number of additional elements that can be considered for greater accuracy, or better performance under challenging conditions, such as obstructed / occluded view of the transmitter, high interference / jamming, strong multipath or significant ionospheric activity.

[0168] A potential improvement is in modelling the signal delay from the transmitting satellite to the receiver. The simplest model is that the signal propagates at the speed of light, so knowing the location of the transmitter (satellite) at the time of transmission and the location of the receiver at the time of reception, the total time of flight can be computed.

[0169] However, the time of flight is also impacted by all the effects that may be accounted for in GNSS position, such as the atmosphere, notably the ionosphere and the troposphere, which each cause additional delays to the signal, and relativistic effects, etc. The ionospheric delay can be largely corrected using ionospheric model products, or by estimating it by forming a combination of two signals on separate radio frequencies.

[0170] These models predict the electron density distribution in the ionosphere that can be used to calculate corrections that can be applied to the signal. Examples of ionospheric model products that can be used are as follows.

[0171] International Reference Ionosphere, IRI, which is a widely used empirical model that provides monthly median values of ionospheric parameters such as electron density, electron temperature, and ion composition at different altitudes and geographic locations.

[0172] Global Ionospheric Maps, GIM, which are maps generated from observations made by a network of ground-based and satellite-based receivers. They provide real-time or near-real-time estimates of the ionospheric electron density distribution globally.

[0173] NeQuick: which is an ionospheric model developed by the European Space Agency (ESA). It is particularly useful for modeling the ionosphere's behavior in the equatorial and low-latitude regions.

[0174] The ionospheric delay can also be estimated and removed by forming a combination of two signals on two separate radio-frequencies. In principle, this can be employed if the satellite has two radio-signals that are transmitted with sufficient separation in frequency to allow for observing the ionosphere. The method may for instance be applied to dual -frequency GNSS observations.

[0175] A variation of this method would be to use signals from two or more single-frequency satellites each broadcasting on different frequencies. For the method to work the transmission time of these satellite signals need to be referenced to the same timescale. Furthermore, it assumes some model of the distribution of the ionosphere and take into account the different travel paths of the different satellite signals to the user. The model can include the assumption that the ionosphere is homogeneous, such that the only variable that needs to be accounted for is the different angles of interception of the two satellite signal paths.

[0176] Improvements in the quality of the timing measurements generated by the receivers can also be made, which involve improvements in accuracy for tracking under benign conditions and improvements in robustness for tracking under adverse conditions.

[0177] In terms of the improvements in accuracy, it is noted that that the satellite signal discussed in the present disclosure is not designed for time transfer and suffers a number of deficiencies for this purpose. To help overcome this shortcoming, multi -symbol integration may be performed during tracking. This consists of accumulating the energy from multiple (typically 8 or 16) symbols, in a decision-directed manner, prior to generating a timing error estimate. The accumulated energy of the multiple symbols contributes to a higher signal to noise ratio at an input to a non-linear timing discriminator, resulting in both better time and frequency estimation capability.

[0178] An advantage of the method of the present disclosure over SOP processing of the satellite signal is that, being a participating receiver, the receiver of the present disclosure performs continuous tracking, which provides measurements that are filtered over time, for a degree of accuracy that is greater than can be achieved in a “snapshot” processing mode.

[0179] Such signal tracking performance can be further improved upon if the symbol and carrier clocks are generated in a coherent fashion. Specifically, in signal processing, coherent clocks refer to a situation where a symbol clock (which dictates the timing of the data symbols) and a carrier clock (which dictates the timing of the carrier wave) are synchronized and derived from the same reference source. This coherence ensures that there is a fixed and predictable relationship between the two clocks, which helps to achieve accurate signal tracking and processing.

[0180] If symbol and carrier clocks are coherent, then a technique known as carrier-aiding, or carrier smoothing, can be used to filter raw symbol measurements using the highly precise, though ambiguous, carrier phase measurements.

[0181] Carrier-phase smoothing makes it possible to smooth the noisier time-tag measurements of the unique words over time. This smoothing with the carrier-phase can happen independently at the ground stations and the user equipment, and there is in principle no need to broadcast any specific data to facilitate it, provided that the symbol and carrier clocks are coherent.

[0182] Specifically, carrier-aiding involves the following steps:

[0183] 1. Continuously track the phase of the carrier wave with high precision.

[0184] 2. Measure the symbols as they are received, noting the timing and any noise present.

[0185] 3. Use the carrier phase measurements to filter the symbol measurements. This process reduces the noise and provides a more accurate estimate of the signal's path delay.

[0186] By applying carrier-aiding, the performance of the signal tracking system is significantly improved.

[0187] For the above to work, the modulator of an uplink ground station is configured to make use of coherent clocks for symbol and carrier generation. With this in place, direct carrier smoothing of the symbol measurements is possible in the user equipment.

[0188] If the symbol and carrier clocks are not coherent, it is also possible to enable carrier smoothing by providing extra information through the signal in space, delivering a carrier phase estimate and a carrier clock correction parameter, or the carrier signal versus symbol clock frequency difference, to the receiver to apply to enable this process.

[0189] The carrier phase estimate is an estimate of the phase of the carrier signal, typically obtained from a phase lock loop (PLL). Contrary to a typical PLL used for data demodulation, the carrier phase estimate ensures continuity over time by keeping track of accumulated carrier cycles.

[0190] With the carrier phase, in addition to the instantaneous phase from the signal, an integer cycle count is kept. This integer part is very useful for timing / positioning.

[0191] The carrier clock correction parameter provides information on how much the carrier clock needs to be adjusted to match the symbol clock. It helps to align the frequencies of the carrier and symbol clocks, reducing phase errors.

[0192] The carrier signal versus symbol clock frequency difference is a measure of the frequency difference between the carrier signal and the symbol clock. By knowing this difference, the receiver can apply a correction factor to compensate for any drift or offset between the two clocks.

[0193] To implement the carrier smoothing, the satellite or transmitting station can include a carrier phase estimate, carrier clock correction parameter, or the frequency difference information in the signal. This information can be embedded in the data stream or sent as auxiliary data.

[0194] The user equipment extracts this additional information from the incoming signal and applies a smoothing algorithm that uses the carrier phase and clock correction information to reduce the impact of noise and errors. The applied algorithm can involve techniques such as Hatch filtering to provide a stable and accurate time estimate.

[0195] Figure 6 schematically illustrates a Hatch filter, that combines coarse timing estimates 61 with fine carrier phase estimates 62 to produce smoothed timing estimates 63. The filter blends 620 a coarse timing estimate with a sum 611 of previous 612 filtered timing estimate and a difference 610 between the current and previous fine phase estimates. The blending operation 620 can be implemented as a weighted combination of the two inputs, where the weights sum to unity.

[0196] In the event that the symbol and carrier clocks are coherent, then the Hatch filter can be applied directly to the raw symbol timing and carrier phase estimates in the user equipment 13.

[0197] In the event that the symbol and carrier clocks are incoherent, then that Hatch filter of Figure 6 can be implemented in one of the following ways:

[0198] 1. Using the difference between the coarse time estimate from the user equipment and the timing information provided by the system as the coarse time estimate, and the difference between the carrier phase measurement at the user equipment and that provided by the system as the carrier phase measurement.

[0199] 2. Using the same measurements as the previous case, but corrected for the time of flight between the user equipment 13 and the satellite 11.

[0200] 3. Using the carrier clock correction or frequency difference information provided by the system to correct the raw timing or carrier phase measurement prior to using the Hatch filter directly as in the case of the coherent symbol and carrier clocks.

[0201] Figure 7 shows test results of time transfer using the method of the present disclosure between three test sites over three days. Tracking parameters were varied to observe impact on time transfer (narrower bandwidths shown for time-axis values between 45000 and 100000). All three sites are synchronized via GNSS in order to test the accuracy of time transfer of the present disclosure. The above line is the result of site one, the lower line is the result of site two, both of them showing time transfer from the third site. Timing accuracy is better than 0.5 microseconds.

[0202] Improvements in the robustness of the timing measurements in degraded scenarios can be achieved by taking advantage of the predictability of the first symbols in every frame. For INMARSAT U-band signals, a frame starts with the 64 symbol Unique Word, followed by a 32-bit service identifier and an 8-bit spare byte, which translate into 112 symbols (after convolutional encoding) that are perfectly predictable every frame. This corresponds to around 20 ms of data for the 2400 baud signal and around 40 ms of data for the 1200 baud signal.

[0203] In the event that the signal to noise (plus interference) level is too low to track the signal, a significant boost in robustness can be achieved by processing only these predictable symbols in a coherent fashion for a single snapshot measurement every frame. While this results in a reduction in accuracy compared to a continuous tracking implementation, it allows the receiver to continue to operate under conditions that would otherwise cause a complete loss of service.

[0204] The above describes the present disclosure with reference using timing measurement of satellite signals to convey the reference timescale. It will be understood by those skilled in the art that the signal used may also be received from a ground-based transmitter instead of from a satellite.

[0205] In this case, the method of the present disclosure can be summarized as follows:

[0206] A method of providing a reference timescale to a user equipment, the method performed by a processing device and comprising the steps of:

[0207] receiving, from a first ground station, timing measurement of a signal received by the first ground station from a transmitter, wherein the first ground station has a known geolocation and a ground station clock synchronized to the reference timescale, wherein timing measurement of a signal is generated by the first ground station by time-tagging a time of arrival of the signal comprising a time synchronization marker transmitted by the transmitter, the time synchronization marker is known to the first ground station and the user equipment;

[0208] - calculating time of transmission of the signal based on the received timing measurement of the satellite signal,

[0209] - encoding the time of transmission of the signal and position information of the transmitter in the relevant signal; and

[0210] transmitting the relevant signal encoding the time of transmission and the position information of the transmitter to the user equipment.

[0211] The above description about the time synchronization marker also applies here, while the time synchronization marker may follow a different standard which allows its arrival to be time tagged by the ground station. The relevant signal encoding the time of transmission and the position information of the transmitter may also be transmitted via available communication channels as discussed above.

[0212] An example system that the alternative method applies to is VHF Data Exchange System (VDES), Automatic Identification System (AIS) standard that supports both base stations on ground and satellite transmissions, with which the time distribution principles described in the present disclosure herein can apply for. In this case a VHF transmitter is used to transmit signals carrying the time synchronization marker to the ground station described above. The ground station time tags the arrival of the signal and sends the time tagged signal to the processing device described above, which can then use the method of the present disclosure to distribute the reference timescale to user devices.

[0213] The invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art.

[0214] Further modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

Claims

23CLAIMS1. A method of providing a reference timescale to a user equipment, the method performed by a processing device and comprising the steps of: receiving, from a first ground station, timing measurement of a satellite signal received by the first ground station from a satellite, wherein the first ground station has a known geolocation and a ground station clock synchronized to the reference timescale, wherein timing measurement of a satellite signal is generated by the first ground station by time-tagging a time of arrival of the satellite signal comprising a time synchronization marker transmitted by the satellite, the time synchronization marker is known to the first ground station and the user equipment; calculating time of transmission of the satellite signal based on the received timing measurement of the satellite signal, encoding the time of transmission of the satellite signal and position information of the satellite in the relevant satellite signal; and transmitting the relevant satellite signal encoding the time of transmission and the position information of the satellite to the user equipment.

2. The method according to claim 1, wherein the time synchronization marker comprises a pattern usable as a reference epoch for timing purposes.

3. The method according to claim 2, wherein the pattern comprises predefined pattern, such as a unique word having a defined length occurring at a defined position, such as a start, of every frame transmitted by the satellite.

4. The method according to any of the previous claims, wherein the step of calculating the time of transmission of the satellite signal comprises: deriving a time of flight needed for the satellite signal to travel from the satellite to the first ground station; deriving at least one further time of flight needed for the satellite signal to travel from the satellite to at least one second ground station; obtaining a first time of transmission by subtracting the derived time of flight from the timing measurement of the satellite signal received by the first ground station; obtaining at least one further time of transmission by respectively subtracting the derived at least one further time of flight from at least one further timing measurement of the satellite signal received by the at least one second ground station, the at least one further timing measurementof the satellite signal being received by the processing device from the at least one second ground station; combining the first time of transmission and the at least one further time of transmission to derive the time of transmission of the satellite signal and optionally the satellite position.

5. The method according to any of the previous claims, wherein the relevant satellite signal encoding the time of transmission and the position information of the satellite is transmitted through a network connecting the processing device and the user equipment.

6. The method according to any of the previous claims, wherein the relevant satellite signal encoding the time of transmission and the position information of the satellite is transmitted through the same satellite as the synchronization markers.

7. The method according to any of the previous claims, wherein the time of transmission information includes a measurement of the carrier phase at the time of transmission, including information on a number of carrier cycles that have accumulated between measurement epochs.

8. The method according to any of the previous claims 4 to 7, wherein at least one of ionospheric and tropospheric delay is considered in deriving the time of flight.

9. The method according to any of the previous claims, wherein timing measurement of the satellite signal is generated by each of the ground station(s) after accumulating energy from a number of symbols of the time synchronization marker in a decision directed manner.

10. The method according to any of the previous claims, wherein the time synchronization marker is generated in a coherent manner with the timescale of a reference system.

11. The method according to any of the previous claims, wherein carrier-phase of the satellite signal is generated coherently with symbols comprising the time synchronization marker and thereby is generated in a coherent manner with the timescale of a reference system.

12. The method according to any of the previous claims, wherein the satellite comprises geostationary satellites and non-geostationary satellites.

13. A method of obtaining a reference timescale by a user equipment with a known user equipment location, the method comprising the steps of:receiving the relevant satellite signal encoding the time of transmission of the satellite signal and the position information of the satellite transmitted according to any of the previous claims 1 to 12; determining a time of reception of the synchronization marker in the original satellite signal, in a user time frame local to the user equipment; and deriving the reference time scale using the known user equipment location, the time of reception, and the time of transmission of the satellite signal and the position of the satellite in the received relevant satellite signal.

14. The method according to claim 13, further comprising a step of calculating a time offset between the user time frame and the derived reference timescale.

15. The method according to claim 13 or 14 dependent on claim 7, further comprising the steps of: receiving the carrier phase at the time of transmission; recording a carrier phase at time of reception; smoothing an estimate of time difference between the time of transmission and the time of reception using an estimate of a phase difference between transmission and reception; wherein the reference timescale is derived using the smoothed estimate of time difference between the time of transmission and the time of reception.

16. The method according to any of the previous claims 13 to 15, wherein at least one of ionospheric and tropospheric delay is considered in deriving the time of flight of the original satellite signal from the satellite to the user equipment.

17. A system for providing a reference time scale to a user equipment, the system comprising a computing device having a processor arranged to perform the method according to any of the previous claims 1 to 12; wherein the system further comprises: a first ground station with a known geolocation and a ground station clock synchronized to the reference timescale, the first ground station arranged to generate timing measurements of a satellite signal received by the first ground station from a satellite, by time-tagging a time of arrival of the satellite signal comprising a time synchronization marker transmitted by the satellite, the time synchronization marker is known to the ground station and the user equipment; a user equipment with a known user equipment location and arranged to receive the relevant satellite signal encoding the time of transmission and the position information of the satellite and to derive a time offset between a user time frame and the reference time frame using the known user equipment location, a time of reception of the synchronization marker in the original satellite signalin a user time frame local to the user equipment, and the time of transmission of the received relevant satellite signal.

18. A device for providing a reference timescale to a user equipment, the device comprising a processor for performing the method according to any of the previous claims 1 to 12.

19. A computer program product, comprising a computer-readable storage medium storing instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to any of the claims 1 to 12.

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