FIELD OF APPLICATION

Embodiments described herein relate to a method for scheduling communications between satellites and ground stations, and a scheduler for ground stations in communication with satellites. In particular, the embodiments disclosed herein relate to a method and a scheduler in which communications occur with satellites in low and medium Earth orbit.

BACKGROUND ART

The use of satellites placed in orbit around the Earth for the diffusion of useful signals, for example, for telecommunications, audiovisual or for the Internet throughout the world, has been known for some time.

The satellites in orbit are placed in communication with service platforms, or ground stations, located on Earth and which can be organized into networks distributed over large areas of the Earth's surface so as to communicate with their satellites as much as possible. Each period in which a satellite is above the local horizon of a ground station, and thus visible therefrom, a so-called passage is obtained.

The communications between satellites and ground stations are managed by schedulers, whose task is to solve the assignment of satellite passages on ground stations. This passage assignment task is known as the Satellite Range Scheduling Problem SRSP.

One concept on which the scheduling is based is that which is known as conflict, i.e., that a ground station cannot perform more than one activity at a time, that is, it cannot consider more than one passage at a time. Similarly, each satellite cannot communicate with more than one ground station at a time.

Satellite operators, which manage the schedulers, ground stations and satellites, propose a passage booking system to their customers: the customer must purchase one or more passages useful thereto, choosing them from a list of available passages.

The classical satellites utilized, for example used for telecommunications, are called large and can weigh even a few tons. These satellites are so large and heavy that the operations for launching them entail costs which make them accessible only to some national or supranational agencies.

Such classical satellites used for telecommunications are in high orbit, i.e., at more than 30,000 km of elevation with respect to the surface of the Earth, so as to have a visibility of most of the planet. This allows the satellites to stay in the same portion of the sky for a long time, i.e., to have long time visibility windows, which facilitates their connection with the ground stations, allowing a constant communication with each satellite.

In order to make the satellites accessible to several entities or companies, what is known as nano-satellites or micro-satellites have been developed, the weight of which is usually between 10 and 100 kg. Such satellites of small dimensions, whose market is constantly expanding, are launched in low orbit, i.e., from about 200 to 600 km in elevation, which makes them visible to a ground station for a reduced time window.

One drawback of the known schedulers is that they are not capable of managing the communications with satellites in low orbit in an acceptable manner, due to the reduced communication time window.

Another drawback of the known schedulers is that they are programmed to manage communications with satellites which always operate in the same sectors (communications and the like), but cannot manage communications with satellites of different operators from different environments or sectors. This problem will be felt more as the number of active satellites in low and medium orbit grows.

A further drawback of the known schedulers is that they function optimally with a single ground station but have limitations when it comes to managing a configuration with multiple satellites and multiple ground stations.

Another problem which arises for customers is in the booking system, which they must pass through to obtain the useful passages to manage communications with their satellites.

WO-A1-2021/195687 discloses a system and a method for managing communications between objects and ground stations in which the allocation is made over time, instead of allocating different frequencies for different objects. The objects can thus use the same frequency if it is ensured that there are no communication overlaps in time and space. The optimization is linked to the fact of avoiding the interferences between objects with the same frequency as much as possible and in the meantime minimizing the latency between successive communication windows. This document does not describe a complete automation of the method of scheduling multi-mission satellite communication windows over a network which is distributed in space and heterogeneous, starting from mission requirements of individual satellites.

US-B-11096188 discloses a method for transmitting data from satellites using a primary ground station and a set of secondary ground stations. The scheduling concerns a plan of transmission windows with the set of secondary stations and is communicated to the satellite by the primary antenna, which also has the task of calculating a transmission plan on a set of secondary antennas and transmitting the calculated plan to the satellite. This document describes the optimization of the capacity and quality of the downlink of each individual satellite on dedicated antennas, but does not describe a complete automation of the method of scheduling multi-mission satellite communication windows over a network which is distributed in space and heterogeneous, starting from mission requirements of individual satellites.

CN-A-113783600 and US-A1-2018/054251 are related, respectively, to connectivity routing technologies in multi-node networks applied to a large constellation of satellites intended for the implementation of internet connectivity, and to a technique for a point to multipoint PTMP communication in the context of high elevation objects, such as drones, in which multiple ground stations are connected to the object at elevation.

None of these documents concern the complete automation of the method of scheduling multiple missions over a heterogeneous distributed network, consisting of ground stations with different features from each other, or the support of a large amount of mission requirements, fully manageable and definable by a single operator.

There is therefore a need to improve a scheduler for ground stations in communication with satellites as well as a method to manage communications with satellites which can overcome at least one of the drawbacks of the art.

An object of the present invention is to devise a method to manage communications with satellites which is capable of better planning the short passages of the satellites.

A further object of the present invention is to devise a method which allows the customer to no longer have to book the passages upstream of the scheduling.

A further object is to provide a scheduler and a software application capable of implementing the above method.

The Applicant has studied, tested and realized the present invention to overcome the drawbacks of the prior art, and to obtain the above as well as further objects and benefits.

DISCLOSURE OF THE INVENTION

The present invention is expressed and characterised in the independent claims. The dependent claims show other features of the present invention or variants of the main solution proposed.

In accordance with the aforesaid objects and to solve the aforesaid technical problem in a new and original manner, also obtaining considerable advantages with respect to the prior art, a method according to the present invention for scheduling communications between a first predetermined number of ground stations and a second predetermined number of satellites comprises an orbital calculation step, in which all the passages of the satellites on the ground stations are calculated, and a scheduling step, in which the calculated passages are organized into an optimized plan, commonly called a schedule.

Thereby, at least the advantage of the complete automation of the method of scheduling multiple missions over a heterogeneous distributed network, consisting of ground stations with different features from each other or the support of a large amount of mission requirements, fully manageable and definable by a single operator.

According to embodiments, the ground stations are different from each other, i.e., at least two ground stations are different from each other.

Obviously, the scheduling step is carried out taking into account the concept of conflict, i.e., that one ground station cannot communicate with more than one satellite at a time and, conversely, one satellite cannot communicate with more than one ground station at a time.

The first number and the second number above can be any integer greater than or equal to 1. Therefore, the scheduler is configured to schedule communications between one or more satellites and one or more ground stations. Preferably, the scheduler is configured to schedule communications between multiple satellites and multiple ground stations.

The orbital calculation step is performed considering, as input parameters, one or more first parameters related to the orbits of the satellites, one or more second parameters related to the ground stations and possibly one or more third parameters related to the satellites. It is preferable that the one or more third parameters linked to the satellites are considered as input parameters in this orbital calculation step.

The one or more first parameters can comprise data, understood as coordinates, of the orbit of each of the supported satellites, and data useful for predicting future positions of the same satellites. For example, such data can be expressed in a format known as Two-Line Element Set TLE.

The one or more second parameter(s) can comprise data related to the ground stations, for example one or more of ground station geographic data, a minimum elevation and possibly a maximum elevation of the passages given by the local legislation of the ground station, a preparation time between two successive communications of a ground station with the satellites (known as reposition time), and an elevation mask which takes into account physical and anthropological conformations around each ground station, such as the presence of reliefs, mountains, trees, constructions or other antennas around the ground stations.

The one or more third parameters can comprise one or more of a minimum number of passages of the satellites to be scheduled, a latency between two consecutive passages of a same satellite, lighting conditions, non-communication time windows, a minimum elevation of the passages and a minimum duration of the passages. Advantageously, the one or more third parameters are chosen by a user, in particular a customer of the satellite operator which manages the scheduler.

Favourably it is also provided, after the orbital calculation step and before the scheduling step, a filtering step in which the passages which do not satisfy at least one of either the first parameters, the second parameters and/or the third parameters are rejected. If the one or more third parameters are not considered as input parameters in the aforesaid orbital calculation step, they are taken into account as input parameters in this filtering step. Preferably, the one or more third parameters comprise a minimum number of passages of the satellites, and the method provides, immediately after the filtering step, a control step in which it is checked that the minimum number of passages can be satisfied for all the satellites. In the event of a negative result, a notification step of such a negative result is preferably provided, for example in the form of an error.

Preferably, the scheduling step provides, as an input parameter, a list of passages obtained from the previous steps, that is, a list of calculated and available passages. The scheduling step provides, as an output result, an optimized plan of the passages.

The generation of the optimized plan can be iterative, i.e., it can be performed in different iterations in each of which a new plan is generated starting from the previous iteration plan. Therefore, the scheduling step can provide, as an additional input parameter, a previously generated optimized plan.

Advantageously, the orbital calculation, filtering (if provided), control (if provided) and/or scheduling steps are performed automatically. Any notification step of the negative outcome of the control (if provided) is preferably carried out in a non-automated manner.

According to an aspect, a scheduler configured to perform the above method is provided.

According to another aspect, a software application for implementing the above method is also provided when the software application is executed on a data processing system.

ILLUSTRATION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will become clear from the following embodiment disclosure, given by way of example only, with reference to the accompanying drawings in which:

- fig. 1 is a schematic view of a satellite network containing a scheduler;

- fig. 2 is a flowchart representing a method to manage communications with satellites according to the present invention;

- fig. 3 is a depiction of the latencies of the satellites during a plan generated by simulation;

- fig. 4 shows the latencies measured in the simulation which gave the depiction referred to in fig. 3; and - fig. 5 is a graphic depiction of a day of a plan generated by simulation.

It should be noted that in the present description the phrases and terminology used, such as the terms horizontal, vertical, front, rear, high, low, internal and external, with their variations, have the sole function of better illustrating the present invention with reference to the figures of the accompanying drawings and must not be used in any way to limit the scope of the invention itself, or the scope of protection defined by the appended claims.

Furthermore, those skilled in the art will recognize that certain dimensions, or features, in the figures may have been enlarged, deformed, or shown in an unconventional, or non-proportional manner to provide a version of the present invention which is easier to understand. When the dimensions and/or the values are specified in the description below, the dimensions and/or the values are provided for illustrative purposes only and are not to be construed as limiting the scope of protection of the present invention, unless such dimensions and/or values are present in the appended claims.

To facilitate understanding, identical reference numbers have been used, where possible, to identify identical common elements in the figures. It should be noted that elements and features of an embodiment can be conveniently combined or incorporated into other embodiments without further clarification.

DESCRIPTION OF SOME EMBODIMENTS

With reference to figure 1, a satellite network 10 comprises a plurality of ground stations 11 configured to communicate with a plurality of satellites 12 orbiting around the Earth. The ground stations 11 , which can be of several types different from each other, are connected to a scheduler 13 according to the present invention, configured to schedule communications between the ground stations 11 and the satellites 12. The ground stations 11 form a geographically distributed and heterogeneous network.

In particular, the satellites 12 are in low orbit, corresponding to an orbital period of 128 minutes or less and an orbital eccentricity less than 0.25. They can be in any number, from one to tens or hundreds of satellites. In these conditions, the communication time windows of each satellite 12 with a ground station 11 are short, in practice they have a duration of the order of 15 minutes.

It should be noted that, even if schematically depicted close to one another, the ground stations 11 are preferably very distant from one another, so as to cover as wide a geographical area as possible so as to optimize communication between ground stations 11 and satellites 12. For example, the ground stations 11 are advantageously located in different countries, possibly also on different continents. They can be in any number, from one to tens or even hundreds depending on requirements.

The scheduler 13 is configured to implement a communication scheduling method between the ground stations 11 and the satellites, which will be explained in detail below, based on input parameters 20 divided into three different groups 21, 22, 23 (fig. 1). At the end of its processing, the scheduler 13 provides an optimized plan 30, also called a schedule, including the passages of all satellites 12 on all the ground stations 11 of the orbital network 10. The plan 30 is optimized to contain as many passages as possible. The satellites which are not part of the satellite network 10 are obviously excluded from the calculations of the scheduler 13.

As is well known to the person skilled in the art, the main and essential parameter taken into account by the scheduler 13 is the concept of conflict, i.e., that each satellite 12 can communicate with only one ground station 11 at a time, and each ground station 11 can communicate with only one satellite at a time.

A first group 21 of input parameters 20 consists of parameters related to the orbit of each of the satellites 12, also called orbital data. More precisely, the parameters 20 of the first group 21 comprise information on the orbit itself and data which allow, at all times, to predict the positions of each satellite 12 in the future, particularly in the near future. The orbital data are very useful for calculating the passages on one or more ground stations 11.

The orbital data can be expressed in the TLE format, known per se, which divides the orbital data into two different rows and which allows, based on the orbital data of a satellite 12 at a given instant, to calculate any position of the satellite 12 in the future but also in the past with respect to the given instant to which the orbital data correspond.

The parameters 20 of the second group 22 relate to the ground stations 11. They include, for example, geo-location data of each ground station 11 , minimum and maximum elevation data, reposition time, and an elevation mask. The geo-location data of the ground stations 11 comprise the geographical position of each ground station 11, i.e., latitude, longitude and elevation, so as to accurately calculate the passages of the supported satellites 12.

The minimum elevation data, even if apparently connected to the satellites 12, are present in this second group 22 because they are dictated by the local legislation of the place where each ground station 11 is arranged. For example, local legislation can establish a minimum elevation below which the passages are not to be considered, in order to prevent the communication from being disturbed.

The maximum elevation is instead established to take into account the movement limits of the antenna of the ground station 11 , so as to prevent the satellites 12 from passing in areas where the antenna does not have visibility (keyhole problem).

The reposition time instead concerns the period of time which the ground stations 11 need to prepare for a passage after having just made one. In particular, this data serves to avoid performing a passage while the ground station 11 considered is not yet ready to communicate with a satellite 12.

The elevation mask relates to the physical and anthropological conformations in the vicinity of each ground station 11, such as the presence of reliefs, mountains, trees, other antennas, constructions or the like, which could mask the visibility of the passages.

The parameters 20 of the third group relate to the satellites 12, in particular to their behaviour within the plan 30 which will be obtained. These parameters 20 of the third group 23 can be set by a customer of the satellite operator managing the satellite network 10.

These parameters 20 of the third group 23 comprise the number of passages, the latency, the lighting, the non-communication windows, the minimum elevation and the duration.

Going into more detail, the number of passages, understood as the number of passages each day, provides a minimum number and/or a maximum number of daily passages, defined by the customer. This number of daily passages is intended to ensure a minimum communication time between satellites 12 and ground stations 11 each day, controllable with the minimum number of passages, without providing passages which were not requested, which is controllable with the maximum number of passages.

If it is impossible to meet the minimum number of passages requested, it can be provided that the method implemented by the scheduler fails. If instead the number of passages available is greater than the maximum number requested, the passages beyond the maximum number are excluded from the plan 30.

Latency represents a given time distance between two successive passages of the same satellite 12. This parameter can be chosen for several reasons, for example because a satellite 12 needs a certain period of time to collect sufficient data before communicating it to the ground by means of a ground station 11. Such a parameter can be expressed in terms of time (for example in hours) or in number of revolutions, the latter unit is more advantageous since it is independent of the orbit of the satellite 12, contrary to time which depends on the shape of the orbit followed.

Lighting concerns the lighting conditions of the area of the ground visible to the satellite, for example to discriminate between day and night. This parameter can be chosen based on mission requirements, for example to acquire images of certain areas of the ground during certain lighting conditions.

Non-communication windows are time windows during which communications from a given satellite 12 are not to be taken into account. The corresponding passages are to be excluded from the scheduling. This parameter can also be chosen to meet particular customer needs.

The minimum elevation is the minimum elevation at which a given satellite 12 can communicate with a ground station 11. Each passage of the satellite 12 performed below this elevation is to be ignored in the processing of the optimized plan 30. Unlike the minimum elevation data of the second group 22, this minimum elevation of the third group 23 is not dictated by local legislation of the place where a ground station 11 is located, but by customer needs.

Duration is the minimum duration which a passage must have to be included in the optimized plan 30.

The present invention also relates to a method of scheduling communications with satellites 12, more precisely between the ground stations 11 and the satellites 12 of a satellite network 10.

With reference to fig. 2, the method comprises a first step 40 dedicated to orbital calculation, which provides calculating all the possible future passages of the satellites 12 over the ground stations 11 during a day. This first step 40, also called the orbital propagation step, can only exploit part of the parameters 20 described above, in particular only parameters 20 of the first group 21 and the second group 22, but it is preferable that the parameters 20 of all three groups 21, 22, 23 are taken into account.

For example, the data which can be exploited during the first orbital calculation step 40 are the orbital data, the positions of the orbital stations 11 and the minimum elevation given by the local legislation of the place where the ground stations 11 are located. However, it is possible to provide for exploiting other input parameters 20, for example parameters of the third group 23.

During the first step 40, the scheduler 13 advantageously uses a propagation algorithm, i.e., a series of calculations which allows the position of a satellite 12 to be predicted at a given time in the future with a predefined precision. The propagation algorithm can be of the SGP4 type.

Subsequently, a filtering step 50 is provided for rejecting, among all the passages calculated during the first step 40, those which do not meet one or more of the input parameters 20, in particular parameters 20 of the third group 23 and of the second group 22 (fig. 2). If the parameters 20 of the third group 23 have not been taken into account in the first step 40, they are entered as parameters in this filtering step 50. Thereby, the parameters of the third group 23 are nevertheless taken into account throughout the method as a whole.

The filtering step 50 then takes, as input parameters 20, all possible passages calculated during the first step 40 and the parameters of the third group 23 which reflect the needs of those who must exploit the satellites 12. During the filtering step 50, the passages which do not respect the maximum number of passages, any possible lighting conditions, the minimum elevation or the minimum duration, or the passages which occur during the non-communication windows can be rejected.

Advantageously, the filtering step can also exploit input parameters 20 of the second group 22, related to the ground stations 11 , in particular the maximum elevation and the elevation mask.

Immediately after the filtering step 50, the method can provide a control step 60, during which it is checked that the number of passages not rejected of each satellite 12 meets the minimum number of passages requested. If the number of passages not rejected is less than the minimum number of passages, this parameter 20 is not satisfied and a notification step 70 (fig. 2) proceeds, in which the negative outcome is communicated to the customer and to the satellite operator which manages the satellite network 10, so as to solve this situation, applying appropriate corrective actions, for example modifying the parameters 20 taken into account in the filtering step 50.

After rejecting the non-executable passages, the stored passages are inserted in a list and a scheduling step 80 is provided in which it is generated, starting from the aforesaid list of stored passages.

It is during this step that the concept of conflict explained above is applied.

Taking into account this concept and the list of stored passages, the scheduler 13 generates an optimized plan 30 containing as many passages as possible in a day.

Particularly advantageously, the scheduling step 80 can be iterative, i.e., the plan 30 can be iteratively generated.

In such a case, a first plan so is generated based on the list of executable passages p _av obtained during the previous steps 40, 50, 60, 70, choosing a first passage pi and adding it to the plan so. All the remaining passages of the passage list p _av which are in conflict with pi are removed from the passage list p _av . This thereby ensures that the passages kept in the passage list p _av can be added in successive iterations without conflicting with the passages of so.

In subsequent iterations, the passages kept in the list p _av are subsequently added to so, until the passage list p _av is empty.

At this point a plan optimization step begins. The scheduler 13 returns to the initial passage list p _av , i.e., as obtained during the orbital calculation, filtering 50, control 60 and notification 70 steps, and selects the passages pi to insert them iteratively in the plan so. The passages pt can be selected in a guided manner to satisfy a parameter 20 above, e.g., the minimum number of passages for each satellite 12, or randomly if the minimum number of passages is already satisfied for all the satellites 12.

In the first optimization iteration, a passage pt is selected and added in so, and all the passages already present in so which are in conflict with the passage pi are removed from so and re-inserted in the passage list p _av . Thereby, the passages removed from so can be re-added to so in a subsequent iteration.

The updated plan thus generated is compared to the initial plan so so as to define the best plan s _m between the two. The best plan s _m is the plan which contains the most passages.

At each optimization iteration an updated plan is generated by inserting another passagea pi taken from the updated list p _av ,i and removing the passages in conflict with the passage pi. The updated plan Si is then compared with the best plan s _m . If the newly generated plan s _t is better than the plan s _m , the latter is updated with Si. If s _m is better than st, s _m is kept and st is rejected.

The number of optimization iterations is a parameter of the scheduler which is defined a priori by the scheduler manager.

Thereby, a new optimized plan 30 is generated with each iteration. Advantageously, optimization heuristics can also be applied to make sure that the plan 30 is improved with each iteration.

For this scheduling step 80, the scheduler 13 can utilize a Hill Climbing algorithm possibly coupled with a Tabu Search algorithm. The Hill Climbing algorithm is of an iterative type and evolves from an initial solution, making incremental changes to the solution, so as to make the solution increasingly optimal with each iteration. The Tabu Search algorithm implements mechanisms whereby it accepts changes which worsen the solution to broaden the space of the solutions explored.

The Hill Climbing algorithm evolves directly towards a maximum, which can be local, while the Tabu Search algorithm further explores the space of the solutions to try to find an absolute maximum.

It is possible to provide for further optimization of the plans created through what is known as a filling step.

Once a final plan Si is obtained, for example when the number of iterations provided in the scheduler 13 has been exhausted, there are also a plurality of passages remaining in the passage list p _av . Among these remaining passages, it is likely that at least a part has been excluded from the plan because it conflicts with other passages for a short time, for example because they overlap the reposition time of a certain passage of the plan by a few seconds. In this case, it is possible to try to add the passage of the list p _av in the plan st, slightly decreasing the duration of this passage and the passage of the plan before the reposition time, so that there is no more overlap with the reposition time. If this additional insertion is feasible (i.e., if the decreased additional passage and the decreased passage of the plan meet the parameters 20 taken into account, in particular the reposition time and the minimum duration of each passage), it is executed and the plan Si is updated with the additional passage. Otherwise, the entry is not executed and the plan remains unchanged.

In the method described above, all the steps, except the notification step in the event of a negative result of the control of the minimum number of passes, can be performed automatically.

EXAMPLE

A simulation of a satellite communications scheduling procedure was performed considering a fleet of 64 satellites divided into 8 plans (8 satellites per plan) and distributed via RAAN and true anomaly. The orbits are SSO at 600 km above sea level.

The simulation considers a network of 4 ground stations, located in Italy (45.593 latitude; 9.362 longitude), Spain (38.674; -4.162), Ireland (51.953; -8.176) and Lithuania (54.913; 23.997). The input parameters 20 set are summarized in table 1.

The maximum number of revolutions is not truly binding and if it is not satisfied it is still optimized and the plan is considered valid.

A plan 30 was generated for 5 days, the results are illustrated in figures 3, 4 and

5.

In fig. 3 the darker zones represent the zones where a satellite is in communication or has just concluded a communication with a ground station. Latency increases somewhat (up to 10 orbits in some cases) because all the ground stations are located in Europe and are not scattered around the world. But almost 66.6% of the passages have a latency of one orbit, as visible in fig. 4.

Fig. 5 instead shows a plan on a day, in which each vertical segment of different grey shades corresponds to a different satellite. The thickness of these segments increases with the duration of the passage.

The top pane shows the initial plan, and the bottom pane shows the plan after a filling step, where it can be seen that the segments are more numerous and sometimes more compact with respect to the top diagram.

There are 6,450 passages available on the five days, 1,655 are entered in the plan and with the filling process it increases to 2,211 passages, which corresponds to an increase of 33.3% in the number of passages in the plan.

The scheduling method was very efficient, taking about 60 seconds to generate the plan over 5 days considering 64 satellites and 4 ground stations. The filling process lasted about 120 seconds due to the need to control the conflicts

It is clear that modifications and/or additions of parts or steps can be made to the scheduler of communications between satellites and ground stations and to the method for scheduling communications between satellites and ground stations described so far, without departing from the scope of the present invention as defined by the claims.

It is also clear that, although the present invention has been described with reference to some specific examples, a person skilled in the art will be able to make many other equivalent forms of schedulers for scheduling communications between satellites and ground stations and the method for scheduling communications between satellites and ground stations, having the features expressed in the claims and therefore all of which falling within the scope of protection defined thereby.

In the following claims, the references in parentheses have the sole purpose of facilitating reading and must not be considered as limiting factors as regards the scope of protection defined by the claims themselves.