Space based position navigation and timing method

Abstract

A method and system suitable for orbit determination and time synchronisation is provided. The method 200 comprises a satellite broadcasting a first signal comprising a first time (T1) according to a

Description

Technical Field of the Invention The invention relates to space based Position Navigation and Timing (PNT) systems, specifically a method and system suitable for orbit determination and time synchronisation. Background to the Invention Satellite systems may be used to provide Position Navigation and Timing services. To achieve good accuracy and precision to the users of these services it is important that the Satellite's position and on-board clock offset with respect to some reference are monitored, this process is known as orbit determination and time synchronisation (ODTS). The process requires that the satellite’s clocks and positions are referenced to a known time scale and coordinate reference frame, typically this reference, for an Earth based system is located on the surface of the Earth or on the ground. Traditionally to ensure the accuracy of the system, monitoring stations are distributed across the Earth to provide maximum geographic coverage such that breaks in the ability to observe satellites, and hence maintain precise synchronisations of clocks and position, are minimised. This has the drawback that there is a significant geographic or global footprint for the ground monitoring systems or stations. This issue is known and different techniques have been considered to provide sufficiently accurate systems whilst minimising the footprint on Earth, for example by restricting the ground stations to certain geographic regions and the use of inter-satellite links for space segment autonomous generation of ephemeris and clock offsets. The present invention provides an improved method and system for provision of ODTS requiring a reduced trans-global presence and an easily implemented means of eliminating unobservable rotations using ground stations. Summary of the Invention According to a first aspect, the invention provides a method for orbit determination and time synchronisation for a position navigation and timing system, the method comprising the steps of, a satellite transmitting a first signal package to an anchoring element, the first signal package comprising a first time according to a first clock associated with the satellite, and the satellite registering the first signal package, the anchoring element receiving the first signal package at a second time, according to a second clock associated with the anchoring element and the anchoring element registering the first signal package and second time, the anchoring element transmitting a second signal package to the satellite at a third time according to the second clock, wherein the second signal package comprises the first time, second time and the third time, the satellite receiving the second signal package at a fourth time according to the first clock and the satellite registering the second signal package and the fourth time; wherein the satellite is configured to utilise the first time, second signal package and fourth time to calculate a time offset between the first clock and the second clock and estimate a range between the satellite and the anchoring element, such that the orbit determination and time synchronisation is established. Position navigation and Timing (PNT) systems are systems which provide a user within a service volume with the means to estimate their own position and the time relative to some reference time an example is the Global Positioning System (GPS). The satellite takes its usual meaning, orbiting a body such as the Earth, for example taking a Geostationary Earth Orbit (GEO), Low Earth Orbit (LEO), Medium Earth Orbit (MEO). The satellite transmits a first signal package, transmits takes its normal meaning, in that the signal is emitted or broadcast by the satellite, for example using Radio Frequency (RF) radiation or alternatively it may be transmitted using some other means such as light. A signal package may comprise a single signal or alternatively may comprise a plurality of signals. These signals may have different characteristics such as frequency, modulation or phase and may be encoded with information. The first signal package comprises a first time, according to a first clock aboard the satellite, at which the first signal package is transmitted. The clock may be any suitable clock such as a Rubidium Atomic Frequency Standard or Caesium Beam Tube. The first time is with reference to this first clock. Clocks are known to drift or change relative to a reference time, it is this relative change that is required to be corrected to improve the accuracy of the time synchronisation and thus orbit determination. The first time at which the first signal package is transmitted is recorded on board the satellite, for example in computer memory. The satellite transmits the signal package to an anchoring element. The anchoring element is any system or device which provides a reference time at a known location. For example it may be Earth or ground based or may be another satellite. A reference time is system time for the navigation system which is typically a known offset from a recognised time scale such as Coordinated Universal Time, known as UTC. The anchoring elements receive the transmitted first signal package and records the time with respect a second clock associated with the anchoring element. The second clock may be any suitable clock such as that is able to provide a reference time, this may include an ensemble of atomic clocks such as caesium beam tubes or active hydrogen masers. The anchoring element records or registers the second time along with the first signal package. The anchoring element then transmits a second signal package at a third time, the signal package comprising the second time and a third time with respect the second clock. The signal may also include other information such as the location or identification number of the anchor element, almanac data for a satellite to broadcast to users or any other navigation data to be included in a broadcast signal. As with the first signal package the signals may have different characteristics such as frequency, modulation or phase and may be encoded with additional information. The second signal package is received by the satellite at a fourth time relative to the first clock and the satellite registers the second signal and fourth time. The satellite is configured to utilise the first time, second signal package and fourth time to calculate a time offset between the first clock and the second clock and estimate a range or distance between the satellite and the anchoring element, such that the orbit determination and time synchronisation is established on board the satellite, this calculation is understood in the art. This advantageously reduces the burden and reliance on the anchor element to maintain accurate ODTS. In certain embodiments the method further comprises the step of estimating the total electron count, comprising the steps of, transmitting between the satellite and anchor element a first and second signal comprising respectively a first and second frequency, wherein the first and second frequency are different, calculating the ionospheric delay from said first and second signal, such that the total electron count between the satellite and the anchor element can be estimated. It is known that when a transmitted signal has to pass through an atmosphere the Total Electron Count (TEC) in the ionosphere contributes to variation or delay in the time it takes for a signal to travel from a satellite. As such by calculating the ionospheric delay it is possible to estimate the total electron count, this process is known, thus time delays caused by the TEC can be corrected reducing the errors, which in turn improves performance of the system. A first and second signal are required having a first and second frequency, they may form part of the first or second signal package or may be separate signals. These frequencies are selected to be different. In some embodiments the first frequency is separated from the second frequency by at least 0.13 times the first frequency. Advantageously this separation helps to minimise the error associated with the time it takes the signal to propagate between the satellite and anchor element and hence the distance and ranging calculations. In certain embodiments of the method comprising the step of estimating the total electron count, the first and second signal are transmitted from the satellite to the anchor element. This advantageously allows the anchor element to receive the required signals and either record or transmit this information or alternatively undertake the calculations and store or transmit this information. In some embodiments of the method comprising the step of estimating the total electron count the first and second signal are transmitted from the anchor element to the satellite. This allows the satellite to receive the required signals and either record or transmit this information or alternatively undertake the required calculations on-board the satellite. In certain embodiments of the method comprising the step of estimating the total electron count the first and second signal comprises broadcast signals provided by the satellite to transmit navigation information to users. A broadcast signal is intended to be a signal which is already part of the system or satellite functionality. For example a satellite being used for PNT applications will be providing data or signals to users of the system, these signal may be termed broadcast signals. Users are those users receiving or using the signals. These signals may comprise two frequencies which can be used in the required calculations. For example a GPS satellite broadcasts on 1575.42 MHz and 1227.6 MHz, as such these signals may be used to perform the calculations and corrections. This advantageously means that that no additional hardware is required to send or receive the signals as these signals are already present in the system and that the frequency range or bandwidth used by the system is also minimised. In some embodiments of the method the total electron count is applied at the anchor element. Applying the total electron count means applying the required correction such that time delays can be corrected for this delay making and thus reducing the errors, which in turn improves performance of the system. Completing this at the anchor element advantageously removes the need to do this at the satellite. In other embodiments of the method the total electron count is applied at the satellite. Applying the total electron count means applying the required correction such that time delays can be corrected for this delay making and thus reducing the errors, which in turn improves performance of the system. Completing this on the satellite advantageously removes the need to do this at the anchor element reducing the complexity of this element. In certain embodiments of the method the first and or second signal package comprises dedicated multi-frequency signals for orbit determination and time synchronisation. Dedicated means that the signals are provided for the purpose of the method. This advantageously allows the signal package to be tailored to operate in a specific band or frequency. In some embodiments of the method the steps implemented on board the satellite are implemented in a constellation of satellites. Each of the satellites in the constellation may communicate individually with the same anchor element or different anchor elements. This advantageously allows for a constellation of satellites to benefit from the sharing of data to further improve the accuracy or reduce errors in the position and time data. In certain embodiments implemented in a constellation of satellites the method further comprises the steps of the constellation of satellites communicating data via inter-satellite links. Inter-satellite links allow communication between satellites. Communication methods may include RF or light for example using visible or infrared light sources and receivers. This advantageously allows the sharing of information and data including about the time and ranging thus improving the accuracy of the system and also providing redundancy in the event that a satellite is unable to communicate with a ground station or anchor element. According to a second aspect the invention provides a position navigation and timing system for implementing the method of the first aspect, comprising, a satellite comprising a first clock, suitable for providing a first time and a signal transmitting means and signal receiving means and a data processing means configured for calculation of orbit determination and time synchronisation on board the satellite, and an anchor element, comprising a signal transmitting means and signal receiving means, a second clock suitable for providing a second time, and the anchor element further provided with a data processing means. The satellite takes its normal meaning orbiting a body such as the Earth, for example taking a Geostationary Earth Orbit (GEO), Low Earth Orbit (LEO), and Medium Earth Orbit (MEO). The first clock may be a clock having relatively low accuracy or stability when compared to a reference clock. This advantageously means that the satellite can be provided with a simpler, lower power, weight and cost clock then for example the anchor element, and still deliver improved accuracy. The satellite of the system further comprises a signal transmitting means and signal receiving means, these are configured to be able to send and receive signals for example RF signals. The transmitting and receiving means are configured to send and receive signals which may contain a plurality of frequencies or signals which are modulated, for example in amplitude or phase. The satellite further comprises a data processing means configured for calculation of orbit determination and time synchronisation on board the satellite. The data processing means is for processing the signals and data sent and received by the satellite and may include data or information storage. It may for example be a computer or field programmable gate array. Having the data processing means on-board the satellite advantageously allows the processing to be done on-board thus moving the processing burden away from the anchor element and also reducing reliance on ground segments. The system further comprises an anchor element, this element provides a reference location and time to the satellite. It may be another satellite for example a geostationary satellite having a fixed position, or may equally be a ground based system where the anchor element is on the surface of the body which the satellite is orbiting, for example the Earth. The anchor element has a signal transmitting means and signal receiving means, to send and receive signals from the satellite or satellites where there are more than one. This may be any suitable system such as an RF transmitter I receiver. The anchor element may also send and receive signals to other anchor elements. The transmitting and receiving means are configured send and receive signals which may contain a plurality of frequencies or signals which are modulated, for example in amplitude or phase. The anchor element has a second clock suitable for providing a second time, wherein the second time is a reference time. This clock is a high stability and high accuracy clock or ensemble of clocks, having an accurately measured offset to an internationally recognised reference time scale for example Coordinated Universal Time. The anchor element is further provided with a data processing means, for processing the signals and data sent and received by the anchor element and may also include data or information storage. It may for example be a computer or field programmable gate array. Advantageously the inventor has shown that the anchor element of the system may be simplified when used in the system of the present invention, when compared to existing systems. In certain embodiments there may be a plurality of anchor elements having different spatial or geographic locations. Preferably the system comprises two anchor elements as this advantageously provides improved performance over a single anchor element, more preferably the system comprises three anchor elements showing a further improvement in performance whilst still minimising the geographic or spatial footprint. In some embodiments of the second aspect the first clock is a compact lightweight clock relative to the second clock, having relatively low accuracy or stability when compared to a reference clock examples include a Chip Scale Atomic Clock or Miniature Atomic clock. These clocks advantageously reduce the cost and weight requirements of this aspect of the satellite providing great budgets for other aspects, for example electronics, power or sensor systems. In certain embodiments of the second aspect the system comprises a constellation of satellites comprising a communication means for inter-satellite links. A constellation takes the normal meaning in that it is a plurality of satellites which may be arranged in different configurations in relation to each other or the object they are orbiting. Each of the satellites in the constellation may communicate individually with the same anchor element or different anchor elements. The inter-satellite links allow the satellites in the constellation to communicate with each other without having to go via the anchor element, this advantageously allows for the sharing of data which can be used to further improve the accuracy of position and timing data. The data may include the time offset between a satellite clock and the reference time of the anchor element, which in turn allows for each satellite in the constellation to calculate its own time offset position and range or distance in the constellation. This further reduces reliance on communication with the anchor element, so for example where the anchor element is ground based, fewer ground based anchor elements are required. The satellites may also communicate with each other in different modes of operation. For example they may communicate “in-plane” meaning they communicate with satellites in the same orbit plane, or may equally communicate “cross-plane” meaning they communicate with satellites in a different orbit, or alternatively they may communicate with a combination of in-plane and cross-plane. Furthermore the satellites may be configured to communicate to the nearest neighbour or alternatively the next nearest neighbour. This advantageously allows for efficient transmission of signals whilst also improving redundancy in the system. In certain embodiments of second aspect having a constellation of satellites with inter-satellite communication means, the communication means is an antenna for transmitting and receiving signals. An antenna takes its normal meaning being configured to send and receive Radio Frequency signals in a suitable band or frequency. The antenna may be configured to send, transmit or broadcast the signal directionally, having a preferred direction or “lobe” of energy, or be omnidirectional, radiating substantially equally in all directions. This provides configurability to the system to allow it to operate in different modes depending on the system requirements. In some embodiments of second aspect having inter-satellite communication means, the antenna is a steerable antenna. Steerable means that the radiation pattern of the antenna can be directed or steered into a chosen direction or volume. It may be physically or mechanically steered by movement of the antenna or may equally be electronically steered, for example using a phased array. As the beam is steerable it means the energy being radiated can be directed toward the receiving satellite, this advantageously increases the time over which the satellites remain in communication as they move relative each other, and further allows the energy to radiated into a smaller volume or “lobe”. It can also allow a satellite to direct the radiation to more than one satellite, for example in plane or cross plane, thus greatly improving the flexibility and performance of the system. In some embodiments of the second aspect comprising inter-satellite communication means the communication means comprises a light emitting and light receiving means. In this context light includes any suitable part of the electromagnetic spectrum, for example visible light, but may equally be ultraviolet, Near InfraRed (NIR) or InfraRed (IR) light. This advantageously provides a highly directional method of communication. In certain embodiments of the second aspect the anchor element is configured for simultaneously transmitting and receiving a signal comprising a plurality of frequencies, or alternatively a plurality of polarisations. The anchor element is configured to send and receive signals via having certain hardware and software which allows for a signal to be both sent and received at the same time. For example this may be achieved by having a plurality of antennae configured to send and receive signals having different characteristics such as frequency or polarisation. Simultaneous takes its normal meaning in that it occurs at the same time. The signals may be modulated, for example in phase or amplitude. This advantageously allows the time between the sent and received signals to be minimised thus further reducing errors and thus providing improved accuracy in the system. In certain embodiments of the second aspect the anchor element is ground based. The anchor element being ground based advantageously provides a fixed position relative the satellite thus improving the accuracy of the time and position data. Ground based means on the surface of the planet or object which the satellite(s) are in orbit. For example where a satellite system is orbiting Earth, ground based, means it is located on the surface of the Earth. Any feature in one aspect of the invention may be applied to any other aspects of the invention, in any appropriate combination. In particular device aspects may be applied to method or use aspects and vice versa. The invention extends to a method and system substantially as herein described, with reference to the accompanying drawings. In all aspects, the invention may comprise, consist essentially of, or consist of any feature or combination of features. Brief Description of the Drawings The invention will now be described, purely by way of example, with reference to the accompanying drawings, in which; Figure 1 is a illustration of the prior art; Figure 2 is an illustration of method of the invention; Figure 3 is a illustration of the method implemented in a system of the second aspect of the invention; Figure 4 is an illustration of the method implemented in an alternative embodiment of the second aspect; Figure 5 is an illustration of the method implemented in an embodiment of the second aspect. Figure 6 is an illustration of an alternative embodiment of the second aspect. The drawings are for illustrative purposes only and are not to scale. Detailed Description Figure 1 illustrates the prior art, an example Global Navigation Satellite System (GNSS). The system (100) uses a constellation of satellites (101) in orbit around the Earth (102) which broadcast ranging signals (103) modulated by a set of navigation data. User equipment (104) on the ground receives signals (103) from at least four satellites (101) and can calculate the time of arrival according to the user equipment (104) clock (not shown) and using the navigation data to ascertain the time of broadcast and position of the satellites can calculate both the position of the user equipment and the time offset of user equipment (104) clock from the system time of the GNSS. Critical to the functioning of existing GNSS is the precise estimation and prediction of the orbits of the satellites and synchronisation of the clocks on board each of the satellites. This Orbit Determination and Time Synchronisation (ODTS) process relies on the extensive network of monitoring stations on the ground (105) in precisely known locations, which for global system need to be distributed across the Earth (102). The orbits and time offsets of the satellite clocks are calculated centrally and then uploaded to each of the satellites (101) in the constellation. The periodicity of the required uploads is once every 24 hours for each satellite. The need for ground monitoring stations (105) for ODTS dictates that conventional GNSS require a large geographic ground infrastructure. Figure 2 illustrates the method of the invention (200), a satellite broadcasts a first signal or signal package using a Radio frequency signal, the signal package comprising a first time (T1) according to a first clock (not shown) (201). The first signal package is received by an anchoring element being a ground station at a second time (T2) according to a second clock providing a reference time at the ground station the anchoring element registers or records this second time (T2) and the first signal package including first time (T1) (202). The anchoring element transmits a second signal package to the satellite at a third time (T3) according to the second clock wherein the second signal package comprises the second time (T2) and the third time (T3), (203). The satellite receives the second signal package at a fourth time (T4) according to the first clock and the satellite registering the second signal package and fourth time (T4), (204); wherein the satellite is configured to utilise the first time (T1), second signal package to calculate the time offset. The time offset is calculated using the following method; Atgnd ~ T2 — T1 + R Atsat ~ T1 — T2 + R Atsat - Atgnd -T2-T1+ R-T1 + T2-R = 2(T2- Tit) Toffset ~ (Atsat "Atgnd) / 2 Where: R - The propagation delay due to the satellite to anchor element range Atgnd = The propagation delay measured at the anchor element Atsat = The propagation delay measured at the satellite T1 = The satellite time according to the first clock T2 = The anchor element time according to the second clock Toffset = The offset between Satellite time and anchor element time Figure 3 illustrates a position navigation and timing system (300) of the second aspect of the invention comprising, a satellite (301) comprising a first clock (not shown), suitable for providing a first time and a signal transmitting means (302) and signal receiving means (302) and a data processing means (not shown) configured for calculation of orbit estimates and time offsets on board the satellite (301), and an anchor element (303), comprising a signal transmitting and receiving means (304) a second clock (not shown) suitable for providing a second time used as a reference time, and the anchor element further provided with a data processing means (not shown). The signal (305) is transmitted from the satellite (301) to the anchor element (304), and a return signal (306) is sent back from the anchor element to the satellite. Figure 4 illustrates a system (400) of the second aspect implemented within a constellation of satellites (401) in a low earth orbit utilising Inter Satellite Links (ISL)(not show), with an anchor element on the ground (402). The satellites (401) receive a reference time (403) from the anchor element (402) and propagate this reference time (403) throughout the constellation (401) using ISL in plane. The ISL is a non-steerable directional antenna (not shown) which transmits the Radiofrequency signals including the reference time (403). The reference time (403) is shared using a two-way time transfer, with the time transfer being performed in both directions around the orbit plane to further reduce errors and improve redundancy. One satellite (404) in the constellation is not functioning, but due to the two way transfer round the orbital plane the satellites either side of the faulty satellite are still able to be communicated with and maintain performance. Figure 5 illustrates a further embodiment of the second aspect of the invention wherein the system (500) comprises a constellation of satellites (501) having ISL. The satellites communicate in plane (502) and cross plane (503). Figure 6 illustrates a further embodiment of the second aspect of the invention wherein the system (600) comprises a satellite (601) communicating with an anchor element (602) wherein the satellite (601) is a GPS satellite and transmits a broadcast signal (603) comprising dual frequencies of 1575.42 MHz and 1227.6 Mhz. The signal is received by the anchor element (602) which uses the two frequencies, to calculate the ionospheric delay to calculate an estimated of the Total Electron Count, which it then transmits via a signal (604) comprising a single frequency to the satellite (601). It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention. Moreover, the invention has been described with specific reference to Position Navigation and Timing services. It will be understood that this is not intended to be limiting and the invention may be used more generally. For example, the invention may be used more generally in communication constellations. Additional applications of the invention will occur to the skilled person.

Claims

1. A method for orbit determination and time synchronisation for a position navigation and timing system, the method comprising the steps of;a. a satellite transmitting a first signal package to an anchoring element, the first signal package comprising a first time according to a first clock associated with the satellite, and the satellite registering the first signal package;b. the anchoring element receiving the first signal package at a second time, according to a second clock associated with the anchoring element and the anchoring element registering the first signal package and second time;c. the anchoring element transmitting a second signal package to the satellite at a third time according to the second clock, wherein the second signal package comprises the first time, second time and the third time;d. the satellite receiving the second signal package at a fourth time according to the first clock and the satellite registering the second signal package and the fourth time; wherein the satellite is configured to utilise the first time, second signal package and fourth time to calculate a time offset between the first clock and the second clock and estimate a range between the satellite and the anchoring element;such that the orbit determination and time synchronisation is established.

2. The method of claim 1 further comprising the step of estimating the total electron count, comprising the steps of;a. transmitting between the satellite and anchor element a first and second signal comprising respectively a first and second frequency wherein the first and second frequency are different;b. calculating the ionospheric delay from said first and second signal;such that the total electron count between the satellite and the anchor element can be estimated.

3. The method of claim 2 wherein the first and second signal are transmitted from the satellite to the anchor element.

4. The method of claim 2 wherein the first and second signal are transmitted from the anchor element to the satellite.

5. The method of claim 3 wherein the first and second signal comprise broadcast signals provided by the satellite to transmit navigation information to users.

6. The method of claims 2-5 wherein the total electron count is applied at the anchor element.

7. The method of claim 2-5 wherein the total electron count is applied at the satellite.

8. The method of any preceding claim wherein the first and or second signal package comprises dedicated multi-frequency signals for orbit determination and time synchronisation.

9. The method of any preceding claim wherein the steps implemented on the satellite are implemented in a constellation of satellites.

10. The method of claim 9 further comprising the steps of the constellation of satellites communicating data via inter-satellite links.

11. A position navigation and timing system for implementing the method of claims 1-10, comprising, a satellite comprising a first clock, suitable for providing a first time and a signal transmitting means and signal receiving means and a data processing means configured for calculation of orbit determination and time synchronisation on board the satellite, and an anchor element, comprising a signal transmitting means and signal receiving means, a second clock suitable for providing a second time, and the anchor element further provided with a data processing means.

12. The system of claim 11 when the first clock is a compact lightweight clock, relative to the second clock.

13. The system of claim 11-12 further comprising a constellation of satellites comprising a communication means for inter-satellite links.

14. The system of claim 13 wherein the communication means is an antenna for transmitting and receiving signals.1015. The system of claim 14 wherein the antenna is a steerable antenna.

16. The system of claim 13 wherein the communication means comprises a light emitting and light receiving means.

17. The system of claims 11-15 wherein the anchor element is configured for simultaneously transmitting and receiving a signal comprising a plurality of frequencies.

18. The system of any proceeding claim wherein the anchor element is ground based.

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