A system for a space-based asset

Abstract

This application is concerned with a system 100 for determining one or more positional parameters of a space-based asset, such as a satellite, which the system forms part of. The system receives a plurality of monodirectional communication signals, S…Sn, from a reference network 120. The system performs a set of calculations and a timing synchronisation based on the received monodirectional communication signals to determine the one or more positional parameters of the space-based asset in a frame of reference. The space-based asset uses the one or more positional parameters to point an antenna beam to be transmitted from the space-based asset to a target location 130. The spaced-based asset is preferably a GEO solar power satellite, which transmits energy to a ground energy receiver (GER) 130. The system preferably includes a plurality of sub-assets SB,…SBn, each comprising three or more receivers in a triangular configuration, for receiving the monodirectional communication signals.

Description

The present disclosure relates to a system for determining the value of one or more positional parameters of a space-based asset and synchronising the space-based asset to a reference network. In particular, the system is adapted for use on a spacebased asset whereby the value of one or more positional parameters relate to one or more parameters of the space-based asset in a frame of reference, such as position and / or orientation. BACKGROUND Determining the position and orientation of a space-based asset as well as synchronising the space-based asset to a reference network requires a complex system and communication network configured to carry information signals. The complexity of the system increases as the physical size of the space-based asset increases. Typically, the determination for such properties for large space-based assets may be achieved by relying on inter-satellite links or long internal transmission pathways to carry the information. Such an implementation introduces delays into the system, thereby reducing the accuracy of the determination of position, orientation and timing synchronization. Further, inter-satellite link systems required an increased number of transmitters and receivers as well as increased processing power on board the space-based asset further increasing the complexity of the system. In low-Earth orbits (LEO), global navigation satellite system (GNSS) has been demonstrated to determine positioning. However, such measurements have been of low-accuracy. Orientation of large space-based assets is usually performed using star, Sun and Earth trackers. Internal synchronisation of spacebased assets typically use a central clock, for large structures this introduces a level of delay which is unacceptable for high-precision beamforming. In the US7512505B a system for determining ranging information of a spacecraft was disclosed. The system uses bi-directional communication between the satellite and ground-based reference stations. The ground-based reference stations then calculate the ranging measurements for the satellites. This system therefore requires the use of downlink signals which increases the number of communication signals required to carry information between the satellites and the ground reference stations. This can further introduce latency to the system reducing the accuracy of the measurements. For a physically large space-based asset, multiple downlinks would be required from a number of locations across the space-based asset which would be challenging to implement in practice. In CA2940652C discloses a system architecture for improving the performance and cost of global navigation satellite systems (GNSSJ. The system is distributed to decrease likelihood of failures, interference, and attacks. It utilises both downlinks to transmit information from the satellites to ground reference nodes as well as crosslinks to transmit information between satellites. However, the crosslinks between satellites only allow for relative measurements on position and altitude to be determined. It is therefore desirable to provide an improved system for determining one or more properties of a space-based asset, such as the position, orientation and / or timing synchronisation of the space-based asset. SUMMARY According to a first aspect of the disclosure, there is provided a system for determining one or more positional parameters of a space-based asset in a frame of reference, the system configured to: receive a plurality of monodirectional communication signals from a reference network; perform a set of calculations and a timing synchronisation based on the received monodirectional communication signals; and determine the one or more positional parameters of the space-based asset in a frame of reference based on the set of calculations; wherein the spacebased asset uses the one or more positional parameters to point an antenna beam to be transmitted from the space-based asset to a target location. Optionally, the one or more positional parameters comprise at least one of: position; and / or orientation angle; of the space-based asset. Optionally, the frame of reference is an Earth-centred Earth-Fixed frame of reference. Optionally, the system comprises one or more sub-assets configured to receive the plurality of monodirectional communication signals from the reference network. Optionally, each sub-asset comprises three or more receivers. Optionally, the three or more receivers are arranged in a geometric configuration. Optionally, the geometric configuration is a triangular configuration. Optionally, the plurality of monodirectional communication signals are encoded with a set of information. Optionally, the set of information comprises at least one of: a security tag; an acquisition code; reference network coordinates; and / or a time of transmission. Optionally, the reference network is configured to generate and transmit the plurality of monodirectional communication signals. Optionally, the reference network transmits the plurality of monodirectional communication signals continuously to the system. Optionally, the reference network comprises four or more reference nodes each comprising a transmission chain, whereby each reference node is configured to generate and transmit at least a single monodirectional communication signal. Optionally, the transmission chain comprises: a digital signal generator for generating a monodirectional communication signal; a timing reference coupled to the digital signal generator; a signal amplifier; and a transmission antenna; wherein the timing reference is configured to encode the monodirectional communication signal with a time of transmission. Optionally, the timing reference is one of: a caesium beam, a hydrogen maser clock, a frequency comb or an optical clock. Optionally, each monodirectional communication signal in the plurality of monodirectional communication signals further comprises a phase. Optionally, the one or more reference nodes are Earth-based. Optionally, the one or more reference nodes are space-based. Optionally, the set of calculations comprises at least one of: a single differencing algorithm; a double differencing algorithm; a triple differencing algorithm; a state estimation algorithm; a transformation algorithm; and a trigonometric vector algorithm. Optionally, the system comprises: one or more sub-assets; and one or more controllers coupled to each sub-asset; whereby each controller is configured to perform the set of calculations and the timing synchronisation based on the received monodirectional communication signals. Optionally, each monodirectional communication signal in the plurality of monodirectional communication signals comprises: a phase and a time of transmission. Optionally, each controller comprises a memory configured to store one or more types of data generated by the set of calculations. Optionally, the memory is a random-access memory. Optionally, the set of calculations comprises at least one of: a single differencing algorithm; a double differencing algorithm; a triple differencing algorithm; a state estimation algorithm; a transformation algorithm; and a trigonometric vector algorithm. Optionally, each sub-asset comprises three or more receivers, whereby each receiver is configured to receive the plurality of monodirectional communication signals from the reference network. Optionally, the three or more receivers are arranged in a geometric configuration. Optionally, the geometric configuration is a triangular configuration. Optionally, each sub-asset further comprises an oscillator coupled to each of the three or more receivers. Optionally, the oscillator is configured to generate a reference signal. Optionally, the oscillator is a chip-scale atomic clock or an oven controlled crystal oscillator. Optionally, the reference signal comprises a phase. Optionally, the controller is configured to receive the reference signal and the plurality of monodirectional communication signals. Optionally, the controller is configured to generate, for each receiver in the subasset, a list of comparison phases, wherein each comparison is the difference between the phase of the reference signal and the phase of each of the monodirectional communication signals. Optionally, if the calculation being performed is the single differencing algorithm, the controller of a given sub-asset executes the following steps: a) retrieve the list of comparison phases for one receiver and the list of comparison phases for another receiver and calculate the difference between the comparison phases of each of the monodirectional communication signals for the two receivers to generate a list of single differenced values; b) repeat step (a) until each combination of receivers has been iterated through; c] store the generated lists of single differenced values in the memory. Optionally, if the calculation being performed is the double differencing algorithm, the controller executes the following steps: a) retrieve a list of single differenced values for one pair of receivers; b) calculate the difference between the single differenced values in the list for different monodirectional communication signals to generate a list of double differenced values; c) repeat step (b) until all lists of single differenced values have been iterated through; d] store the generated lists of double differenced values in the memory. Optionally, the position of the sub-asset can be determined by applying the state estimation algorithm to the lists of double difference values. Optionally, the state algorithm is an extended Kalman filter. Optionally, the orientation angle of the sub-asset can be determined using the position of the sub-asset and the transformation algorithm wherein the transformation algorithm calculates the orientation angle of the sub-asset relative to a reference axis. Optionally, the reference axis is the earth-centred earth-fixed axis. Optionally, the triple differencing algorithm comprises calculating a difference between the list of comparison phases over two consecutive epochs. Optionally, if the timing synchronisation is being performed, the controller executes the following steps: a) calculate, using the position values, a time of flight for a given monodirectional signal; b) sample the received monodirectional communication signals at a sampling rate to retrieve the time of transmission; c) calculate, using the time of transmission and the time of flight, a local time stamp for the sub-asset for a given reference node; d) repeat steps (a) through (c) for all monodirectional communication signals; e) average over all local time stamps for the sub-asset to generate a derived subasset reference time; f) calculate the difference between the local time-stamp and the derived subasset reference time to derive a time reference correction value; g) apply the time reference value to the oscillator.; whereby once the time reference value has been applied, the oscillator is synchronised to the reference network. For instance, the sampling rate could be a Nyquist sampling rate. Optionally, the space-based asset is in an orbit around the Earth. Optionally, the orbit is a geosynchronous orbit or a low-Earth orbit. Optionally, the space-based asset is a solar power satellite system and the antenna beam is a power beam. Optionally, the solar power satellite system uses the determined position and orientation angle values and the timing synchronisation of the one or more subassets to angle and adjust the power beam to be sent to an Earth based energy receiver. Optionally, the target location is an Earth-based location. Optionally, the target location is a space-based location. Optionally, the antenna beam is configured to transfer at least one of: data or power to the target location. According to a second aspect of the disclosure there is provided a method for determining one or more positional parameters of a space-based asset in a frame of reference using the system according to the first aspect of the disclosure, the method comprising: receiving, at the system, a plurality of monodirectional communication signals from a reference network; using the system to perform a set of calculations and a timing synchronisation based on the plurality of monodirectional communication signals; and determining the one or more positional parameters of the space-based asset in a frame of reference based on the set of calculations; whereby the space-based asset uses the one or more positional parameters to point an antenna beam to be transmitted from the space-based asset to a target location. Optionally, the one or more positional parameters comprise at least one of: position; and / or orientation angle. Optionally, the space-based asset is a solar power satellite system. It will be appreciated that the method of the second aspect may include providing and / or using features set out in the first aspect and can incorporate other features as described herein. BRIEF DESCRIPTION OF THE DRAWINGS The disclosure is described in further detail below by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a diagram of an example embodiment of a system for determining one or more positional parameters of a space-based asset in a frame of reference according to the present disclosure; Figure 2 is a diagram of an example embodiment of a reference network configured to generate and transmit a plurality of monodirectional communication signals to the system of figure 1 according to the present disclosure; Figure 3 is a diagram of an example embodiment of the system of figure 1 comprising one or more sub-assets according to the present disclosure; Figure 4 is a diagram of an example embodiment of the system of figure 1 showing an example configuration of three or more receivers; Figure 5 is a flow chart showing a method of determining one or more positional parameters of a space-based asset according to the present disclosure; Figure 6 is a diagram showing high-level architecture for an exemplary embodiment of a system for determining one or more positional parameters of a space-based asset in a frame of reference according to the present disclosure; Figure 7 is a diagram of an exemplary embodiment of a control and management subsystem for the space-based asset of figure 6; Figure 8(a) is an exemplary embodiment of a reference network configured to generate and transmit a plurality of monodirectional communication signals to the system of figure 6; Figure 8(b) is a diagram of an exemplary embodiment of a reference node; Figure 8(c) is a diagram showing an exemplary embodiment of a timing reference for use with the reference node of figure 8(b); Figure 9 is a diagram showing the plurality monodirectional communication signals being transmitted to the space-based asset for the exemplary system in figure 6; Figure 10(a) is a diagram of an exemplary embodiment of a sub-asset comprising three or more receivers receiving a single monodirectional communication signal; Figure 10(b) is a diagram of an exemplary embodiment the sub-asset comprising three or more receivers receiving a plurality of monodirectional communication signals; Figure 11(a) is a diagram showing an exemplary embodiment of an oscillator generating a reference signal for the sub-asset in figure 10(a); Figure 11(b) is an exemplary set of equations used to generate a list of comparison signals is generated using the reference signal; Figure 12 is a diagram showing an exemplary embodiment of a single differencing algorithm and a double differencing algorithm that can be used by the system of figure 6; Figure 13 is a diagram showing an exemplary method of using a state estimation algorithm that can be used to calculate an position of a space-based asset; Figure 14 is a diagram showing an exemplary set of angles that used with a transformation algorithm to calculate an orientation of a space-based asset; Figure 15 is a diagram showing an exemplary method of synchronising the space-based asset to the reference network; Figure 16 is a diagram showing how the exemplary system of figure 6 uses the position and orientation to generate a coherent beam; and Figure 17 is a diagram showing an example embodiment of relationships between different entities. The present techniques will be described more fully hereinafter with reference to the accompanying drawings. Like numbers refer to like elements throughout. Parts of the space based asset system are not necessarily to scale and may just be representative of components of the space based system, or other described entities. DETAILED DESCRIPTION Figure 1 is a diagram of an example embodiment of a system 100 for determining one or more positional parameters of a space-based asset 110 in a frame of reference, in accordance with a first embodiment of the present disclosure. The one or more positional parameters comprise at least one of a position, an orientation angle and / or timing synchronisation of the space-based asset 110. The frame of reference may be, for example, an Earth-Centred Earth-Fixed frame of reference. The system 100 is adapted for use on the space-based asset 110 and is configured to receive a plurality of monodirectional communication signals S to Sn from a reference network 120. The system 100 may comprise one or more sub-assets configured to receive the plurality of monodirectional communication signals S to Sn from the reference network 120. Each of the one or more sub-assets may comprise three or more receivers arranged in a geometric configuration, for example, a triangular configuration. The plurality of monodirectional communication signals S to Sn may be encoded with a set of information. This set of information may comprise one or more types of information. For example, the system may be encoded with, a security tag, reference network coordinates, an acquisition code and / or a time of transmission. The system 100 is further configured to perform a set of calculations and a timing synchronisation based on the monodirectional communication signals S to Sn received at the three or more receivers. The system is then configured to determine the one or more positional parameters of the space-based asset 110 in the frame of reference based on these set of calculations. The one or more positional parameters could be, for example, a position or an orientation of the space-based asset 110 in the frame of reference. The set of calculations may comprise at least one of a single differencing algorithm, a double differencing algorithm, a triple differencing algorithm, a state estimation algorithm, a transformation algorithm and a trigonometric vector algorithm as shall be described in further detail herein. The space-based asset 110 uses the one or more positional parameters to point an antenna beam to be transmitted from the space-based asset to a target location. The target location could be, for example, Earth-based or space-based. The antenna beam is configured to transfer at least one of: data or power to the target location. The space-based asset 110 may be in an orbit around the Earth, for example a geosynchronous orbit (GSO) or a low-Earth orbit (LEO). The space-based asset 110 may be, for example, a solar power satellite system which uses the position orientation values and the timing synchronisation determined by the system 100 to angle and adjust a power beam to be sent to an Earth-based energy receiver. Figure 2 is a diagram of an example embodiment of a reference network 120 configured to generate and transmit a plurality of monodirectional communication signals S to Sn to a system 100 according to a second embodiment of the present disclosure. The reference network 120 is configured to transmit the plurality of monodirectional communication signals S to Sn continuously to the system 100. The reference network 120 comprises four or more reference nodes RN to RNn. Each reference node RN to RNn is configured to generate and transmit a single monodirectional communication signal S to Sn. For example, reference node RNn will transmit monodirectional communication signal Sn. The reference nodes RN to RNn each comprise a transmission chain. The transmission chain may comprise a digital signal generator, a timing reference coupled to the digital signal generator, a signal amplifier and a transmission antenna. A digital signal generator for a given reference node is configured to generate the monodirectional communication signal for that reference node. For example, the digital signal generator for reference node RNn will generate monodirectional communication signal Sn. The timing reference is configured to encode the monodirectional communication signal with a time of transmission. The timing reference may be, for example, a caesium beam, a hydrogen maser clock, a frequency comb or an optical clock as known in art. Each monodirectional communication signal in the plurality of monodirectional communication signals further comprises a phase. The one or more reference nodes RN to RNn may either be Earth-based reference nodes or space-based reference nodes. Figure 3 is a diagram of an example embodiment of a system 100 for a space-based asset 110, the system 100 comprising one or more sub-assets SB to SBn according to a third embodiment of the present disclosure. The system 100 may comprise, for example, one or more sub-assets SB to SBn and one or more controllers CN to CNn coupled to each subarray. Therefore, each subasset is coupled to its own controller and may act independently from the other subassets. For example, sub-asset SBn may be coupled to controller CNn. Each of the controllers of the one or more controllers CN to CNn is configured to perform the set of calculations and the timing synchronisation based on the received monodirectional communication signals S to Sn and hence determine the one or more positional parameters of the space-based asset 110 in the frame of reference based on these set of calculations. The one or more positional parameters could be, for example, an position and / or an orientation of the space-based asset 110 in the frame of reference. The set of calculations may comprise at least one of a single differencing algorithm, a double differencing algorithm, a triple differencing algorithm, a state estimation algorithm, a transformation algorithm and a trigonometric vector algorithm. Each controller of the one or more controllers CN to CNn may also comprise a memory M configured to store one or more types of data generated by the set of calculations. The memory may be, for example, a random-access memory. Figure 4 is a diagram of an example embodiment of a system 100 for a space-based asset 110, the system 100 comprising one or more sub-assets SB to SBN wherein each of the one or more sub-assets comprises three or more receivers A, B, C according to a fourth embodiment of the present disclosure. Each sub-asset in the one or more sub-assets SB to SBn of the system 100 may comprise three or more receivers A, B, C whereby each receiver is configured to receive the plurality of monodirectional communication signals S to Sn transmitted from the reference network 120. Whilst only the receivers of sub-asset SB have been labelled, it is understood that the receivers of the other sub-assets have the same labelling. For example, sub-asset SBn also comprises three or more receivers A, B, C whereby each of these receivers is configured to receive the plurality of monodirectional communication signals S to Sn. The three or more receivers A, B, C may be arranged in a geometric configuration, for example, a triangular configuration. Due to the geometric configuration of the three or more receivers, each receiver will receive a given monodirectional signal slightly differently from the other receivers. For example, receiver A of sub-asset SB may receive monodirectional signal S as Sa whereas receiver B will receive the same signal S as Sb. Each sub-asset in the one or more sub-assets SB to SBn further comprise an oscillator O coupled to each of the three or more receivers A, B, C. Whilst only the oscillator of sub-asset SB has been labelled, it is understood that the oscillators of the other sub-assets have the same labelling. For example, sub-asset SBn also comprises an oscillator 0 coupled to each of the three or more receivers A, B, C. The oscillators O of each sub-asset SB to SBn are configured to generate a reference signal Rx. The reference signal Rx comprises a phase. The oscillator O is a local oscillator for a given sub-asset. Each sub-asset generates its own unique reference signal with its own unique phase. For example, sub-asset SBn may generate reference signal Rx’. The oscillators O may be, for example a chip-scale atomic clock or an oven controlled crystal oscillator as known in the art. The reference signal Rx generated by each of the oscillators O is passed onto the controllers CN to CNn. Each of the three or more receivers A, B, C also pass on the plurality of monodirectional communication signals S to Sn that they have received. For example, receiver A of sub-asset SBn will pass on received monodirectional communication signals Sa to SnA to its given controller CNn. The controllers C to CNn are configured to generate, for each receiver in their respective subarray, a list of comparison phases. Each comparison phase 0 is the difference between the phase of reference signal Rx and the phase of one of the received monodirectional communication signals. The comparison signal 0 may also be referred to as, for example, a carrier phase measurement. For example, for receiver A of sub-asset SB, the comparison signal <p% is the difference between the phase of Rx and the phase of Sa. Therefore, each receiver for each sub-asset will have its own list of comparison signals. For example, receiver A of sub-asset SB will have a list of comparison signals starting with and ending with These lists of comparison signals are then stored in the memory M of the controllers CN to CNn. The controllers CN to CNn then use these lists of comparison signals for each receiver in their respective sub-asset to perform the set of calculations and the timing synchronisation. Hence the one or more positional parameters of the spacebased asset 110 can be determined based on these set of calculations. The one or more positional parameters could be, for example, a position and / or an orientation of the space-based asset 110. The set of calculations may comprise at least one of a single differencing algorithm, a double differencing algorithm, a triple differencing algorithm, a state estimation algorithm, a transformation algorithm and a trigonometric vector algorithm. If the calculation being performed is the single differencing algorithm, then the controllers CN to CNn are configured to execute the following steps: a) Retrieve (from the memory M) the list of comparison phases for one receiver and the list of comparison phases for another receiver and calculate the difference between the comparison phases of each of the monodirectional communication signals for the two receivers to generate a list of single differenced values; b] Repeat step (a) until each combination of receivers for each sub-asset has been iterated through; and c) Store the generated lists of single differenced values in the memory M of the controller. For example, for sub-asset SB, the controller CN will take the list of comparison phases for receiver A and the list of comparison phases for receiver B and calculate the difference between each pair of comparison phases in the lists. In other words, the controller will compute to generate single differenced value for monodirectional communication signal S and will repeat this until reaching calculation 0fn - (ppn to generate single differenced value (p^ for monodirectional communication signal Sn. The controller will repeat this calculation for receivers A and C and so on until single differenced values for all pairs of receivers for a given sub-asset have been calculated. If the calculation being performed is the double differencing algorithm, then the controllers CN to CNn are configured to execute the following steps: a) Retrieve a list of single differenced values for one pair receivers ; b) Calculate the difference between each of the single difference values for the pair of receivers in the list to generate a list of doubled differenced values; c] Repeat step (b) until each list of single differenced values have been iterated through; d] Store the lists of double differenced values in the memory M. For example, for a given sub-asset SB, the controller CN will retrieve each list of single differenced values. In other words, for a sub-asset of three receivers A, B and C it will retrieve three lists: List 1 : ... List 2 :0gc ...(p^ List3 -.(pcA-<PcA Then for List 1, the controller CN will calculate <ppB — (p%p to generate a double differenced value of <pAps. It will iterate through the list of single differenced values until all possible combinations have been computed. Hence for List 1 it will generate a list of double differenced values starting from (ppps and ending with The controller CN will then repeat this for List 2 and List 3. The generated lists of double differenced values are then stored in the memory M. The position of a given sub-asset can then be determined by applying a state estimation algorithm to the lists of double differenced values to generate a position matrix. The state estimation algorithm may be an extended Kalman filter as known in the art and the position matrix may take the form of: \XA XB Ya Yb Yc za ZB Zc Where Xa, Ya, and Za are the X, Y and Z positions of receiver A for a given sub-asset SB. Once the position of a given sub-asset has been determined, this value can be used to determine the orientation angle of the given subarray. This can be achieved by applying a transformation algorithm to the position matrix wherein the transformation algorithm calculates the orientation angle of the sub-asset relative to a reference axis. The reference axis could be, for example, the Earth-centred Earth-fixed (ECEF) axis. Finally, if the timing synchronisation is being performed, the controller executes the following steps: a] calculate, using the position values, a time of flight for a given monodirectional signal; b) sample the received monodirectional communication signals at a sampling rate to retrieve the time of transmission; c) calculate, using the time of transmission and the time of flight, a local time stamp for the sub-asset for a given reference node; d) repeat steps (a) through (c) for all monodirectional communication signals; e] average over all local time stamps for the sub-asset to generate a derived subasset reference time; f) calculate the difference between the local time-stamp and the derived subasset reference time to derive a time reference correction value; g) apply the time reference value to the oscillator. When calculating the position, the controller samples the received monodirectional communication signals at a sampling rate. The sampling rate may be, for example, a Nyquist sampling rate. Using the position, a time of flight can be derived. The time of flight is the time it took for the monodirectional communication signal to be transmitted to the sub-asset. The time of flight, the time of transmission (as encoded in the monodirectional communication signal] and the known errors and time delays due to hardware are used to calculate a local time-stamp for the sub-asset from a given reference node. By averaging over all time-stamps for all reference nodes in the reference network, a derived sub-asset reference time can be calculated. The difference between the local time-stamp and the derived sub-asset reference time gives a time reference correction value. The differential of the time reference correction value gives a frequency correction value. Using the frequency correction value and the list of comparison phases allows for correction of the oscillator such that it is synchronised with the reference network. The adjustment of the oscillator works similar to a phase-locked loop (PLL). In the embodiments of the present disclosure, the loop filter will be a software controlled integrated circuit which uses software to compensate for the frequency and phase changes of the oscillator compared to the reference network by applying a voltage variation. Once the position and orientation angle of each sub-asset in the one or more subassets SB to SBn of system 100 is determined and the timing synchronisation has been applied, the position and orientation angle can be used by the space-based asset 110. For example, if the space-based asset is a solar power satellite system then the position, orientation and timing synchronisation values of the one or more sub-assets SB to SBn are used to angle and adjust a plurality of power beams to be sent to an Earth-based energy receiver such that the power beams constructively interfere at the energy receiver to provide maximum energy density. Figure 5 is a flow chart showing a method 500 of determining one or more positional parameters of a space-based asset using a system according to a fifth embodiment of the present disclosure. The system that can be used to execute the method 500 may be any of the systems described in the present disclosure. At step 510, the system receives a plurality of monodirectional communication signals from a reference network. Then at step 520 the system performs a set of calculations and a timing synchronisation based on the plurality of monodirectional communication signals. Then, at step 530, the one or more positional parameters are determined based on the set of calculations performed in the previous step. Finally, at step 540 the space-based asset uses the one or more positional parameters to point an antenna beam to be transmitted from the space-based asset to a target location. The system that can be used to execute the method 500 is adapted for use on the space-based asset. The one or more positional parameters could be, for example, a position and / or an orientation of the space-based asset. The set of calculations may comprise at least one of a single differencing algorithm, a double differencing algorithm, a triple differencing algorithm, a state estimation algorithm, a transformation algorithm and a trigonometric vector algorithm. The space-based asset may be in an orbit around the Earth, for example a geosynchronous orbit (GSO) or a low-Earth orbit (LEO). The space-based asset may be, for example, a solar power satellite system which uses the position and orientation values and the timing synchronisation determined by the system to angle and adjust a power beam to be sent to an Earth-based energy receiver. In the following is described an exemplary embodiment for implementing the system and method of the present disclosure. It should be understood that the system may be used for other embodiments and uses, in accordance with the understanding of the skilled person. Figure 6 is a diagram showing high-level architecture for an exemplary embodiment of a system 100 for determining one or more positional parameters of a space-based asset according to the present disclosure. The system 100 is adapted for use on a space-based asset. In this exemplary embodiment, the space-based asset is a Solar Power Satellite (SPS) and the one or more positional parameters are position, orientation angle and timing synchronisation of the sub-assets of the SPS. The SPS uses the one or more positional parameters of each of the sub-assets to point an antenna beam to a target location. The SPS of this exemplary embodiment is positioned above the Earth in a geosynchronous orbit (GSO). The system 100 comprises one or more sub-assets SB to SBn configured to receive a plurality of monodirectional communication signals S to Sn from a reference network 120. Each of the one or more sub-assets comprise three or more receivers arranged in a geometric configuration. In this exemplary embodiment, the receivers are antennae and the geometric configuration is triangular. The reference network 120 comprises four or more reference nodes GRNi to GRNZ. In this exemplary embodiment, the references nodes are ground reference nodes GRN. They are a specific implementation of the reference nodes RN to RNn of figures 2, 3 and 4. Each GRN is configured to generate and transmit a single monodirectional communication signal S to Sn. For example, reference node GRNi will generate and transmit monodirectional communication signal Si. Each reference node GRNi to GRNZ comprises a transmission chain which is described with figure 8(b). Each reference node has a stable timing reference with a known offset to Coordinated Universal Time (UTC). The reference nodes GRNi to GRNZ generate a unique navigation message to be transmitted to the one or more sub-assets SB to SBn. Additional reference nodes GRN’ may be used for redundancy and better angular diversity. Redundancy reference nodes are placed in a similar location to at least one of the reference nodes GRNi to GRNZ in case of a fault. For certain locations, the redundancy nodes are more important. For example, reference nodes located directly under the satellite may require a redundancy node. The plurality of monodirectional communication signals S to Sn are encoded with the unique navigation message which contains the reference node location, the precise Time of Transmission (TOT) and the ground energy receiver (GER) 130 location. The GER is the target location at which the SPS points the antenna beam in this exemplary embodiment. The one or more sub-assets SB to SBn of the system 100 has a disciplined oscillator (DO) on-board which is used by each of the receivers on the sub-asset to measure the navigation message. Each receiver decodes the navigation message and together calculate the orientation of the sub-asset using differential position. The differential position is the positions of each of the receivers with respect to a reference orientation. Each receiver on the one or more sub-assets SB to SBn receive all the monodirectional communication signals S to Sn. The one or more sub-assets SB to SBn then use the timing reference time of each reference node (GRN reference time) to generate a phase reference and steer the on-board oscillator. The steering of the on-board oscillator refers to the timing synchronisation of the oscillator to the reference network. Each sub-asset calculates the beam-steering phase required to deliver a coherent power beam to the GER 130. A transmitter sub-system is then used to transmit the power beam to the GER 130. Figure 7 is diagram showing an exemplary embodiment of a control and management subsystem for the space-based asset 110. The GRN transmitter subsystem block 710, generates the monodirectional communication signals (uplink signals) transmitted by the reference network. The SCMS block 720 decodes the information. The SCMS block 720 also performs the position and orientation measurements and calculations and the timing synchronisation. A downlink DL is sent to the satellite control centre 730. The downlink DL comprises information such as health data and the performance of the onboard oscillator. Figure 8(a) is an exemplary embodiment of the reference network 120 comprising ground reference nodes GRNi to GRNz that may be used with the exemplaiy system 100 of figure 6. The reference network is configured to generate and transmit a plurality of monodirectional communication signals to the exemplary system 100 of figure 6. For the exemplary system of figure 6, at least four ground reference nodes GRN,, GRNj, GRNk, GRNi should be used. However, in alternative embodiments up to z number of ground reference nodes may be used. Each of the ground reference nodes GRNi, GRNj, GRNk, GRNi are coupled to a timing reference 142. The timing reference 142 may also be referred to as a timing network. From the timing network 142 each of the ground reference nodes GRNj, GRNj, GRNk, GRNi can derive the time from UTC. Each ground reference node GRNi, GRNj, GRNk, GRNi have their own centrally disciplined time Igrn and their own phase Qgrn. For example, ground reference node GRNi will have centrally disciplined time ti and phase 0,. The reference network 120 transmits the plurality of monodirectional communication signals S to Sn continuously to the SPS in GSO. Figure 8(b) is a diagram of an exemplary embodiment of a reference node GRN of reference network 120 for use with the exemplary system of figure 6. The reference node GRN comprises a transmission chain 140. The transmission chain 140 comprises a digital signal generator 144a, a timing reference 142, a signal amplifier 146 and a transmission antenna 148. The timing reference 142 is coupled to the digital signal generator 144. The timing reference 142 is formed of two components. The first is coupled to the UTC time reference, similar to that of figure 8(a). The second component is an internal clock of the reference node GRN itself. This second component could be, for example, a hydrogen maser atomic clock, an oscillator a frequency comb or an optical clock as known in the art. The digital signal generator 144 is configured to generate the monodirectional communication signal. The plurality of monodirectional communication signals S to Sn may be encoded with a set of information. The digital signal generator 144 is used to encode this information into the monodirectional communication signal. This set of encoded information may include, for example, a security tag, reference network coordinates, an acquisition code and a time of transmission. The signal amplifier 146 may be, for example, a high-power amplifier. The transmission antenna 148 may be, for example, a high-gain transmit parabolic antenna, as known in the art. The transmission chain 140 may also be implemented as a satellite gateway with digital signal generation, up-conversion to transmission frequency, amplification and transmission through a high-gain parabolic antenna. Each reference node GRN will use the UTC-derived time as well as a unique acquisition code, identification and security information and an internal clock to generate a high-stability modulated waveform. This high-stability modulated waveform is the monodirectional communication signal for the exemplary embodiment described in figure 8(b). The monodirectional communication signal is then transmitted to the SPS and includes information on the time of transmission tGRN. Each monodirectional communication signal is transmitted with a known phase 0gw. Figure 8(c) is a diagram showing an exemplary embodiment of a timing reference 142 for use with the exemplary reference network 120. The timing reference 142 comprises a UTC timing reference 142a, an internal clock 142b, a Global Positioning System (GPS) common view system 142c, a GPS receiver for common view calculations 142d and a time distribution network 142e. The functions that the time distribution network 142e performs is shown by box 142f. The internal clock 142b may be, for example, a caesium beam or a hydrogen maser or an oscillator as known in the art. In alternative embodiments, the timing reference 142 may comprise a White Rabbit (PTP-WR) time synchronisation protocol and a combination of (ground-run fibre) links to global time sources, for example the National Institute of Standards and Technology (NIST) or the National Physics Laboratory (NPL). In other embodiments, a combination of PTP-WR with GPS common view may be used to link longer baselines or alternatively the timing reference 142 may use a different satellite network such as the Satelies Iridium Satellite Time and Location (STL) Service for synchronization. In this exemplary embodiment of the timing reference 142, timing differences between the internal clocks 142b of each reference node GRN can be tracked using global navigation satellite system (GNSS) satellites and a common view time transfer. The GPS common view system 142c feeds a given reference node GRN oscillator 142b into a high accuracy reference GPS receiver 142d. The measurement of GPS time with respect to the oscillator 142b for each reference node GRN is reported and stored in the storage network 142e. The common view measurement is the differential in GPS time between the two GRNs sampled at the same epoch for the same sub-asset of the SPS. The advantages of this system over GPS alone are significantly improved precision frequency and time transfer, and an improvement of measurement integrity. This must be done using post-processing and is not performed in real-time, however short latency can be made possible with rapid data transfer. Since the reference node GRN oscillator 142b will be high stability, it will not be drifting quickly and hence latency is not an issue. Hence, the timing of the oscillator 142b can be synchronised (steered) using this measurement. In alternative embodiments, the offset between a reference node clock and the UTC may be encoded in the navigation message as a fixed constant that can be accounted for in the processing algorithms onboard the SPS. Figure 9 is a diagram showing a plurality of monodirectional communication signals Si to SI being received at a space-based asset 110. The monodirectional communication signals Si to SI have been generated and transmitted by the reference network 120 and reference nodes GRNi to GRNZ as described in figures 8(a) and 8(b). The space-based asset 110 could be either a macro spacecraft, such as a SPS, or a spacecraft cluster. The plurality of monodirectional communication signals Si to SI from each reference node GRNi to GRNi arrive at the SPS 110 with a certain time and a certain phase. For example, monodirectional communication signal Si arrives at time ti with phase pi. Figure 10(a) is a diagram of an exemplary embodiment of a sub-asset SB comprising three or more receivers A, B, C receiving a single monodirectional communication signal SL The sub-asset SB is the same as the sub-asset of figure 6, therefore the same labelling has been kept and the components are taken to have the same functionality and meaning as for figure 6. The sub-asset SB also comprises an oscillator 0 which is coupled to each of the receivers A, B and C. The monodirectional communication signal SI from reference node GRNi arrives at the SB a phase pi. The three or more receivers A, B, C are arranged in a geometric configuration. In this exemplary embodiment, they are arranged in a triangular configuration. The triangular configuration of receivers A, B, C is an equilateral triangle with a baseline of As. In other words, the vector line between each of the receivers is of length As. For example, the length of vector CA (the vector from receiver C to receiver A) is As. This is the same for vectors AB and BC. In alternative embodiments, the length of vectors AB, BC, CA may not be the same. The phase received at each of the receivers A, B and C is plA , plB, plc. Each receiver A, B, C receives the monodirectional communication signal SI from reference node GRNi at different times. This is due to the geometric configuration of receivers resulting in a difference of Ar, also known as a range difference. The range difference between receiver C and receiver A is given by ArcA. The range difference between receiver C and receiver B is given by Arse. Figure 10(b) is a diagram of an exemplary embodiment of a sub-asset SB comprising three or more receivers A, B, C receiving a plurality of monodirectional communication signal Si to SI. The sub-asset SB is the same as the sub-asset of figure 10(a), therefore the same labelling has been kept and the components are taken to have the same functionality and meaning as for figure 10(a). The plurality of monodirectional communication signals Si to SI were transmitted from a reference network 120 comprising reference nodes GRNi to GRNi. Each monodirectional communication signal arrives at each receiver with a given phase. The phases received at receiver A are: The phases received at receiver B are: 0L ^Pb’ ^B' <Pb The phases received at receiver C are Figure 11(a) is a diagram showing an exemplary embodiment of an oscillator 0 generating a reference signal Rx. The reference signal Rx comprises a phase. The oscillator 0 is the same as the oscillator in figure 10(a), therefore the same labelling has been kept and the components are taken to have the same functionality and meaning as for figure 10(a). The oscillator O is coupled to each of the receivers A, B and C of the sub-asset SB. The oscillator O is configured to act as a common clock for each of the receivers A, B and C. The oscillator 0 is configured to generate a reference signal Rx which has a module phase (p^Bc- This module phase is used to measure the carrier phase measurement for each of the receivers. This is achieved by comparing the module phase (p^Bc with the phase of a monodirectional communication signal received at one of the receivers A, B or C. Figure 11(a) is an example showing the comparison of the reference signal Rx with the monodirectional communication signal SI received at receiver A. Figure 11(b) is a set of exemplary equations EQN1, EQN2, EQN3 used to generate a list of comparison signals using the reference signal Rx. The reference signal Rx is the same as the reference signal Rx of figure 11(a), therefore the same labelling has been kept and the components are taken to have the same functionality and meaning as for figure 11(a). The list of comparison signals generated is the carrier phase measurements for each receiver. They are generated by calculating the difference between the module phase (p^Bc °f the reference signal Rx and the phase of a monodirectional communication signal received at one of the receivers A, B or C. The list of comparison phases for receiver A is calculated using EQN1. The list of comparison phases for receiver B is calculated using EQN2. The list of comparison phases for receiver C is calculated using EQN3. From equations EQN1, EQN2, EQN3 the precise range of each receiver A, B, C to a reference node GRN can be derived after integer ambiguities are solved. Integer ambiguities are the uncertainty in the integer number of wavelengths between the ground reference node and the receiver due to the phase being cyclical. For example, the range of receiver A of sub-asset SB to reference node GRN; is calculated using the difference <Pabc — <Pa- Figure 12 is a diagram showing an exemplary embodiment of a single differencing algorithm ALG1 and a double differencing algorithm ALG2. The single differencing algorithm ALG1 and the double differencing algorithm ALG2 form part of the set of calculations that are used by the system 100 of figure 6 to calculate the position of the sub-assets of the SPS. From the double differencing algorithm ALG2, a double differenced range p can be calculated using ALG3. For the single differencing algorithm ALG1, the list of comparison phases of a monodirectional communication signal from a reference node GRN between the different receivers A, B and C are then compared to remove common errors. For example, the received phase of monodirectional communication signal Si from reference node GRN, is compared between receivers A and B to give: <Pb - <Pa = <PaB Where (plAB is the single differenced value. This will be repeated for all monodirectional communication signals to generate a list of single differenced values (p1^”"2. This calculation will be repeated for all combinations of receivers A, B, C. Such that three lists of single differenced values are generated: Computing these single differenced values between all combinations of receivers A, B, C remove any clock errors from the reference nodes GRNito GRNZ as well as any atmospheric delays as these will be the same between all receivers. After applying the single differencing algorithm ALG1, the only differences remaining will be those due to the range difference Ar. In order to account for error due to instantaneous jitter of either the clock on-board the sub-asset or reference node clock, the double differencing algorithm ALG2 is applied. For the double differencing algorithm ALG2, for each pair of receivers the single differenced values for one monodirectional communication signal is compared to the single differenced values for another monodirectional communication signal. For example, for the receiver pair A and B, the single differenced values (plAB for monodirectional communication signal Si will be compared to the single differenced values (pAB for monodirectional communication signal Sk to give: ^AB — ^AB = QaB Where is the double differenced value. This will be repeated for all combinations monodirectional communication signals to generate a list of double differenced values DD. This calculation will also be repeated for all combinations of receivers A, B, C. The double differencing algorithm ALG2 removes second order errors due to instantaneous jitter of either the clock on-board the sub-asset or reference node clock. Hence the subtraction removes them. Finally, the double-differenced range p can be calculated using the double differenced values DD and ALG3. For example, the double-differenced range for receiver pair A and B is given by rearranging the following: ^ABf^ “ (Pab) “ ^AB + ^noise Where fi is the frequency of the plurality of monodirectional communication signals and Afi is the wavelength of the plurality of monodirectional communication signals. The value represents the integer number of wavelengths between a given reference node GRN, to GRNZ and a given receiver, A, B, C. For example, if receiver A is 36,000 kilometres (km) from a reference node GRN transmitting with a frequency of 2.45 Gigahertz (GHz) and a wavelength of 0.122 meters (m), then receiver A would be 294,203,531.96477 wavelengths away. The integer number of wavelengths would be 294,203,531. The integer number of wavelengths of a phase is cyclical, meaning that the phase at 294,203,531.96477 wavelengths is the same as that at 294,203,532.96477 wavelengths. The value €noise is the combined error figure which is minimised by using the double-differencing algorithm ALG2 As long as the combined error figure is minimised, and the integer number of wavelengths can be resolved, then the range Ar can be determined accurately. Figure 13 is a diagram showing an exemplary method 1300 of using a state estimation algorithm. The state estimation algorithm forms part of the set of calculations that are used by the system 100 of figure 6 to calculate the position of the sub-assets of the SPS. The method 1300 comprises the following steps: At step 1310, the double differenced values DD are retrieved for a given sub-asset SB. In alternative embodiments, a triple differencing algorithm may also be applied. The triple differencing algorithm calculates differencing between two consecutive epochs to remove integer ambiguity. Then at step 1320 the state estimation algorithm is applied to produce a matrix at step 1330. The state estimation algorithm could be, for example, an extended Kalman filter (EKF) or may be achieved using machine learning techniques. The state estimation algorithm for the exemplary system of figure 6 is executed as follows: 1. Defining a model that describes the GSO of the sub-asset and the associated ranging estimates 2. Taking the time of arrival and phase measurements of the plurality of monodirectional communication signals over a set of epochs 3. Performing a statistical estimation (for example, a least squares or a recursive least squares) of the sub-asset position based on the measurements and the model to get an initial estimate 4. Generate a measurement matrix which comprises pseudorange values, single differenced values and double differenced values 5. Updating continuously to track position The matrix generated at step 1330 is a matrix ofthe estimated positions of receivers A, B, C of sub-asset SB: (¾ XB Xci Ya Yb Yc Za ZB Zc The geometry of the receivers A, B, C in the sub-asset SB can then be used to calculate the position of the phase centre of the power transmission sub-asset of the SPS. The phase centre of the sub-asset is the point on which radiation to be emitted from the SPS is centred. When the sub-asset is functioning normally this is equivalent to the geometric centre. The phase centre is then used to determine the phase of the transmit elements.. Figure 14 is a diagram showing an exemplary set of angles a, p, y that are determined with a transformation algorithm. The transformation algorithm forms part of the set of calculations that are used by the system 100 of figure 6 to calculate the orientation angle ofthe sub-asset. Once the matrix of the positions of the receivers A, B and C has been generated, this matrix can be used to calculate the orientation angle of the sub-asset. The orientation angle of the sub-assets of the SPS is calculated relative to a reference axis. In the exemplary embodiment ofthe system of figure 6, the reference axes are the Earth-Centered Earth-Fixed (ECEF) axes. The matrix can be used to determine --------> --------> -------> the vectors AB, BC, CA in ECEF coordinates. A rotation matrix is applied to derive the set of angles a, p, y. In this exemplary embodiment the rotation matrix takes the form of: cos a cos / ? cos a sin / ? siny — sin a cosy cos a sin / ? cosy + sin a siny~ sin a cos / ? sin a sin / ? siny + cos a cosy sin a sin / 3 cosy — cos a siny . —sin / ? cos / ? sin y cos / ? cosy Where a is the yaw angle, p is the pitch angle and y is the roll angle of the sub-asset. By multiplying the measured coordinate vectors of each receiver by the inverse of the baseline coordinate vector (with no rotation applied) it is possible to derive the rotation matrix and calculate the roll, pitch and yaw of the sub-asset SB relative to the ECEF axes. The baseline coordinate vector is the vector containing reference coordinates for a single receiver relative to the geometric centre of the sub-asset. Figure 15 is a diagram showing an exemplary method of synchronising the spacebased asset 110 to the reference network 120. An on-board correlator coarse timestamps the phase measurement and time of arrival measurement of each monodirectional communication signal from each reference node. Each measurement has a time of transmission associated with it. If the correlator is cold starting - it can take the oscillator local time as the time of transmission. If the correlator is pre-coded with coarse position, for example GEO alt is 35,786 km, then the cold start time may be time of transmission + 35, 786 km / speed of light. If the correlator is warm starting, the oscillator 0 will be running with its own phase and timestamp. During the process of position determination, the pseudorange to the GRN is the sum of all delays and the time of flight from the GRN to the receiver. The position determination process will also determine the clock bias of the oscillator O, which can be used in the time synchronisation process. The true time of flight will be derived from the true position determination process, and will be the time of arrival (i.e. the measurement timestamp of the receiver) minus all oscillator 0 and GRN bias terms, all atmospheric delay terms and all hardware delays. The timestamp of the oscillator is then updated to the time of transmission plus the time of flight. The phase alignment comes from a similar model whereby the exact phase of the oscillator is determined during the positioning process. To ensure a baseline time which can be used by all sub-arrays, the derived time reference and phase will be calculated from the ensemble of GRN time references. For each timestamp, the phase is derived for the Oscillator 0 with respect to the ensemble GRN reference. The correction for the physical oscillator is the difference between local oscillator 0 reference time and phase, and the GRN reference time and phase. This value is sent to a microcontroller acting as a phase locked loop (PEL) for the oscillator. A correction voltage is applied to bring the oscillator back into phase with the GRN reference. The correct timestamp is applied at the oscillator, where it will continue to free run at its own rate until the next correction is applied. Figure 16 is a diagram showing how the exemplary system of figure 6 uses the position and orientation determined to generate a coherent beam 1510. The coherent beam 1510 is transmitted by the SPS 110. The coherent beam 1510 is formed from one or more power beams PB to PBn generated by the one or more sub-assets SB to SBn. Once the position and orientation angle (also known as pose) of the sub-assets is known, the azimuth and elevation required to point an antenna beam at a target Earth location 130 can then be calculated. This is calculated with trigonometry. The azimuth and elevation are the two components of the angle between the vector normal to the sub-assets of the SPS and the vector from the sub-assets of the SPS to the target site 130. There is no difference between this method being used for wireless power delivery and communications beamforming. The azimuth and elevation are then fed into a beamformer for element-wise relative phase generation. The exact location of the phase centre of the transmit antenna is used to calculate the spacecraft transmit phase, based on the time of transmission to the ground energy receiver 130. Th transmit phase of each sub-asset SB is dependent on its position within the SPS. With the accurately calculated pointing angles, times and phases of transmission for each sub-asset within the SPS a coherent beam 1510 is formed at the receiving site 130 on Earth. If the phases of the one or more power beams PB to PBn are in close alignment, then they constructively interfere at the receiving site 130 to deliver maximum energy density. Figure 17 is a diagram showing an example embodiment of relationships between different entities. It displays the cardinality and direction of data flow between system entities in the different system segments. The thin lines between entities describe the cardinality of relationships between them. For example, one-to-one, one-to-many, many-to-one. In some cases, an entity or relationship is optional, described by a combined symbol, such as many or zero for the space reference node. Arrows describe the direction of information flow between entities, with dashed arrows showing an optional relationship. The system 100 utilises a synchronised network 120 of ground and / or space-based radio-transmitters for accurate position, timing and phase acquisition of receivers in a GSO. The utility of several receivers onboard sub-asset of the SPS allows for a precise and accurate determination of position, orientation and timing across subassets of arbitrary size or across an arbitrary number of independent space-based assets. The system as described in the present disclosure allows for: Compensation of timing, frequency and phase errors. Orientation determination leading to beam forming calculations. For example power beams for space-based solar power or data transmission for satellite communications. High-accuracy position and orientation determination for attitude and orbit control. For example for space stations in GSO, cis-Lunar space or deep space. High stability frequency reference for transponders. Independent modular spacecraft design or cluster constellation tracking and position determination. The system described in the present disclosure enables time critical applications to be served from GSO. For example, space-based solar power or coherent communication from multi-satellite distributed arrays. In an alternative embodiment of the system, a GNSS space service volume can be utilised either independently or in addition to the reference network. The GNSS space service volume is the volume of space which can be served by GNSS satellites. In another alternative embodiment of the system, retrodirective steering beam or pilot beam can be used to augment the beam forming when the system is being used for power beaming applications. For example, for when the system is being utilised for space-based solar power. In another alternative embodiment of the system, a mothership may be used. The mothership may be configured to host a highly accurate timing source with a link to provide the space-based asset with a timing reference. The timing source could be, for example, a hydrogen maser or caesium beam. The link could be, for example, a radiofrequency link or an optical link. In another alternative embodiment of the system, a rolling delay may be incorporated into the plurality of monodirectional communication signals. This will allow for greater differences in observed signals between epochs. It will be appreciated that the system of the present disclosure may be configured for a solar power satellite system. Further embodiments may relate to other types of satellite systems for other applications and for other Earth orbits, in accordance with the understanding of the skilled person. 35 Various improvements and modifications may be made without departing from the scope of the disclosure. A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the disclosure. Accordingly, the above description of the specific embodiments is made by way of 5 example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims

1. A system for determining one or more positional parameters of a space-based asset in a frame of reference, the system configured to:receive a plurality of monodirectional communication signals from a reference network;perform a set of calculations and a timing synchronisation based on the received monodirectional communication signals; anddetermine the one or more positional parameters of the space-based asset in a frame of reference based on the set of calculations;wherein the space-based asset uses the one or more positional parameters to point an antenna beam to be transmitted from the space-based asset to a target location.

2. The system of claim 1, wherein the one or more positional parameters comprise at least one of: position; and / or orientation angle; of the space-based asset.

3. The system of claim 1, wherein the frame of reference is an Earth-centered Earth-Fixed frame of reference.

4. The system of claim 1, wherein the reference network comprises four or more reference nodes each comprising a transmission chain, whereby each reference node is configured to generate and transmit at least a single monodirectional communication signal.

5. The system of claim 4, wherein the transmission chain comprises:a digital signal generator for generating a monodirectional communication signal;a timing reference coupled to the digital signal generator;a signal amplifier; anda transmission antenna;wherein the timing reference is configured to encode the monodirectional communication signal with a time of transmission.

6. The system of claim 5, wherein each monodirectional communication signal in the plurality of monodirectional communication signals further comprises a first phase.

7. The system of claim 6, wherein the one or more reference nodes are Earth-based and / or space-based.

8. The system of claim 2, wherein the system comprises:one or more sub-assets; andone or more controllers coupled to each sub-asset;whereby each controller is configured to perform the set of calculations and the timing synchronisation based on the received monodirectional communication signals, the received monodirectional communication signals each comprising a first phase and a time of transmission.

9. The system of claim 8, wherein each controller comprises a memory configured to store one or more types of data generated by the set of calculations.

10. The system of claim 9, wherein the set of calculations comprises at least one of: a single differencing algorithm; a double differencing algorithm; a triple differencing algorithm; a state estimation algorithm; a transformation algorithm; and a trigonometric vector algorithm.

11. The system of claim 10, wherein each sub-asset comprises three or more receivers, whereby each receiver is configured to receive the plurality of monodirectional communication signals from the reference network.

12. The system of claim 11, wherein the three or more receivers are arranged in a geometric configuration.

13. The system of claim 11, wherein:each sub-asset further comprises an oscillator coupled to each of the three or more receivers;the oscillator being configured to generate a reference signal, the reference signal comprising a second phase.

14. The system of claim 13, wherein the controller is configured to receive the reference signal and the plurality of monodirectional communication signals.

15. The system of claim 14, wherein the controller is configured to generate, for each receiver in the sub-asset, a list of comparison phases, wherein each comparison is the difference between the first phase of each monodirectional communication and the second phase of the reference signal.

16. The system of claim 15, wherein if the calculation being performed is the single differencing algorithm, the controller of a given sub-asset executes the following steps:a) retrieve the list of comparison phases for one receiver and the list of comparison phases for another receiver and calculate the difference between the comparison phases of each of the monodirectional communication signals for the two receivers to generate a list of single differenced values;b] repeat step (a) until each combination of receivers has been iterated through;c) store the generated lists of single differenced values in the memory.

17. The system of claim 16, wherein if the calculation being performed is the double differencing algorithm, the controller executes the following steps:a] retrieve a list of single differenced values for one pair of receivers;b) calculate the difference between the single differenced values in the list for different monodirectional communication signals to generate a list of double differenced values;c] repeat step (b) until all lists of single differenced values have been iterated through;d] store the generated lists of double differenced values in the memory.

18. The system of claim 17, wherein the position of the sub-asset can be determined by applying the state estimation algorithm to the lists of double difference values.

19. The system of claim 35, wherein the orientation angle of the sub-asset can be determined using the position of the sub-asset and the transformation algorithm wherein the transformation algorithm calculates the orientation angle of the subasset relative to the frame of reference..

20. The system of claim 19, wherein the triple differencing algorithm comprises calculating a difference between the lists of comparison phases over two consecutive epochs.

21. The system of claim 13, wherein if the timing synchronisation is being performed, the controller executes the following steps:a) calculate, using the position values, a time of flight for a given monodirectional signal;b) sample the received monodirectional communication signals at a sampling rate to retrieve the time of transmission;c) calculate, using the time of transmission and the time of flight, a local time stamp for the sub-asset for a given reference node;d) repeat steps (a) through (c) for all monodirectional communication signals;e] average over all local time stamps for the sub-asset to generate a derived sub-asset reference time;f) calculate the difference between the local time-stamp and the derived subasset reference time to derive a time reference correction value;g) apply the time reference value to the oscillator.;whereby once the time reference value has been applied, the oscillator is synchronised to the reference network.

22. The system of claim 1, wherein the space-based asset is in an orbit around the Earth.

23. The system of claim 2, wherein the space-based asset is a solar power satellite system and the antenna beam is a power beam.

24. The system of claim 23, wherein the solar power satellite system uses the determined position and orientation angle values and the timing synchronisation of the one or more sub-assets to angle and adjust the power beam to be sent to the target location, the target location comprising an Earth based energy receiver.

25. A method for determining one or more positional parameters of a space-based asset in a frame of reference using the system of any preceding claim, the method comprising:receiving, at the system, a plurality of monodirectional communication signals from a reference network;using the system to perform a set of calculations and a timing synchronisation based on the plurality of monodirectional communication signals; anddetermining the one or more positional parameters of the space-based asset in a frame of reference based on the set of calculations;whereby the space-based asset uses the one or more positional parameters to point an antenna beam to be transmitted from the space-based asset to a target location.

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