Earth observation data receiving system

Abstract

One aspect of the invention relates to a system for receiving Earth observation data transmitted from a low-Earth orbit satellite. The system comprises a set of ground stations, wherein each ground station is configured to receive data transmitted by the low-Earth orbit satellite using a frequency above 12 GHz, wherein each ground station comprises an antenna with a diameter of 2 meters or less, wherein each ground station is associated with a predefined minimum elevation angle of 15 degrees or more, and wherein data transmission from the satellite to each ground station is carried out as long as the elevation angle of the satellite relative to said ground station is greater than or equal to the minimum elevation angle associated with the ground station. Figure to be published with the abstract: Figure 7

Description

Title of the invention: Earth observation data reception system TECHNICAL FIELD OF THE INVENTION

[0001] The technical field studied is that of satellite communications, in particular downlink communications from an Earth observation satellite in low orbit to one or more ground stations.

[0002] In particular, the invention relates to a network of ground stations configured to receive signals from a low-orbit Earth observation satellite, in a frequency band above the X band, i.e. a frequency band above 12 GHz, for example in the Ka band, the Q band, the W band or the E band. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] Low Earth Orbit (LEO) is an orbit relatively close to the Earth's surface, located less than 2000 km, and generally less than 1200 km or even less than 1000 km. In Very Low Earth Orbit (VLEO), satellites can even orbit at altitudes of less than 450 km, generally between 250 and 350 km, or even as low as 160 km above the Earth's surface, but rarely below, in order to avoid the adverse effects of atmospheric friction.

[0004] Low Earth orbit satellites are therefore satellites which circulate on orbits located on orbits relatively close to the Earth, ranging for example from 160 km to 1000 km altitude, or even 1200 km altitude, and generally less than 2000 km altitude relative to the Earth's surface.

[0005] Unlike geostationary satellites, located at an altitude of approximately 36,000 km, which orbit with zero inclination (parallel to the Earth's equator) and have an orbital period that coincides with the Earth's rotation period (meaning the satellite appears stationary from the perspective of a ground station), low Earth orbit satellites have an orbital period shorter than the Earth's rotation period and can follow a trajectory along a plane inclined to the Earth's equator. Therefore, more trajectories are available for low Earth orbit satellites, making them a particularly attractive option for Earth observation applications.Earth observation satellites are satellites used to perform geophysical and geographic observations of the Earth from their orbit, and notably include remote sensing satellites (satellites). meteorological, terrestrial imaging or intelligence). When a ground station is overflown by the Earth observation satellite (at each revolution of the satellite), the latter "offloads" its data to the ground station, that is to say it sends the collected data to the ground station, via a downlink communication link.

[0006] Today, downlink communication links (i.e. from the satellite to the ground station) between most low-Earth-orbit Earth observation satellites and ground stations are made using the X band, which is a frequency range from 8.025 GHz to 8.4 GHz for Earth observation applications.

[0007] The reception of this data is carried out by parabolic antennas typically 3 to 5 meters in diameter. Due to the increased use of satellites in this band, their number combined with a limited bandwidth (only 375 MHz for Earth observation applications), congestion has become a serious problem for the downlink of X-band data from a low-Earth orbit satellite to a ground station.

[0008] Frequency bands above the X band, having values ​​above 12 GHz, such as the Ka band for example (between 25.5 GHz and 27 GHz for Earth observation applications), provide access to wider bandwidths (1.5 GHz for the Ka band for Earth observation applications), but they are also more sensitive to signal degradation due to attenuation caused by the presence of water in the atmosphere (in the form of clouds, gases, rain and scintillation) - also called "rain attenuation".

[0009] For example, the article "Ka-Band and the Future of Big Data from Space," by KSAT (Kongsberg Satellite Services) and Astro Digital, presents a solution for receiving data from a low-Earth orbit satellite using the Ka-band. Due to the significant attenuation of the radio signal under high humidity conditions, the proposed solution uses ground stations (also called terrestrial stations) located in very specific geographical areas, namely Svalbard (at latitude 78° North) and the Troll Antarctic base (at latitude 72° South), where dry conditions allow for robust Ka-band transmission without interference from rain or snow. Such location constraints currently preclude widespread deployment of this solution or its general use for receiving data from low-Earth orbit satellites using the Ka-band.

[0010] There is therefore a need for new solutions to use bands above the X-band for Earth observation applications using low-orbit satellites. Summary of the invention

[0011] The invention offers a solution to the problems mentioned above, by proposing to use small ground stations operating in Ka band with a minimum elevation angle of at least 15° (whereas traditional solutions use much lower minimum elevation angles, generally 5°, to optimize the contact time with the satellite).

[0012] One aspect of the invention thus relates to a system for receiving Earth observation data transmitted from a satellite in low Earth orbit, the system comprising a set of ground stations,

[0013] wherein each ground station is configured to receive data transmitted by the low-orbit satellite using a frequency greater than 12 GHz,

[0014] wherein each ground station comprises an antenna of respective diameter less than or equal to 2 meters,

[0015] wherein each ground station is associated with a respective predefined minimum elevation angle greater than or equal to 15 degrees,

[0016] wherein data transmission from the satellite to each ground station is carried out as long as an elevation angle of the satellite relative to said ground station is greater than or equal to the minimum elevation angle associated with the ground station.

[0017] By "low Earth orbit satellite" is meant a satellite moving in an orbit located less than 2000 km from the Earth's surface.

[0018] The term "Earth observation data" refers to data transmitted from an Earth observation satellite, that is, a satellite used for Earth observation applications from its orbit (espionage, environmental monitoring, meteorology, mapping, etc.), for example, an Earth imaging satellite or a remote sensing satellite. Such satellites are configured to transmit data "blindly" according to a predetermined transmission plan received in advance by the satellite (the transmission plan indicating, in particular, when the satellite should transmit the data). In other words, such satellites are configured to offload their data, and not to establish a two-way communication link with a ground station. The data transmitted by the Earth observation satellite is received by any ground station from which the satellite is visible.

[0019] By "ground station" (also called earth station or ground station), we mean an installation located on the Earth's surface and configured to communicate with the low-Earth orbit satellite. In particular, a ground station is configured to at least receive signals (and therefore data) from the low-Earth orbit satellite (it may also be configured to transmit signals to the orbiting satellite). (low, but this is not mandatory). To communicate with the satellite, the ground station includes an antenna (for example, a parabolic antenna). In the context of the present invention, the antenna of each ground station has a diameter of 2 meters or less. It is understood that the antenna diameter is not necessarily the same from one ground station to another. It is also understood that, in certain embodiments, the antenna diameter can be on the order of a few tens of centimeters, for example, between 50 cm and 80 cm, which makes it possible to drastically reduce (by a factor of more than 10) the cost of the infrastructure.

[0020] By "diameter" is meant a characteristic dimension of an antenna, which may be the diameter in the mathematical sense of the term for a circular antenna, or the equivalent diameter for a non-circular antenna (for example, elliptical or square). The "equivalent diameter" of a non-circular antenna corresponds to the diameter that a circular antenna of the same surface area would have. Throughout this description, the term "diameter" is used to refer to the diameter or the equivalent diameter of the antenna.

[0021] The term "elevation angle" refers to the angle formed between the horizontal plane where the ground station is located and the line extending from the ground station to the satellite. The elevation angle ranges from 0° (when the satellite is on the horizontal plane) to 90° (when the satellite is at its zenith relative to the ground station). During the satellite's trajectory along an orbit, the elevation angle increases until it reaches the peak elevation of the pass (90° maximum) and then decreases.

[0022] The term "minimum elevation angle" refers to a predefined minimum value for the satellite's elevation angle relative to a ground station, at which the ground station receives data from the satellite. In other words, the ground station does not receive data from the satellite until the minimum elevation angle is reached. As soon as the satellite's elevation angle relative to the ground station reaches the predefined value (i.e., the minimum elevation angle), the satellite transmits data, which is then received by one or more ground stations.

[0023] It is understood that each station is associated with a respective minimum elevation angle, which means that, in certain embodiments, two distinct ground stations can be associated with two distinct predefined values ​​(i.e., two distinct minimum elevation angles).

[0024] In the context of the invention, the minimum elevation angle is greater than or equal to 15° for all ground stations. As detailed below, a minimum elevation angle of at least 15° compensates for the attenuation due to rain associated with the use of frequencies above the X band, and also allows for a reduction in the diameter of the ground station antenna compared to conventional ground stations. operating in X band (whose antennas generally have a diameter of around 4 to 6 meters).

[0025] It is noted that in traditional solutions, the minimum elevation angle is instead set to the lowest possible value, in order to maximize the contact time between the satellite and the ground station, that is to say, the time during which the ground station receives data from the satellite. In general, the minimum elevation angle is 5°.

[0026] According to the invention, the minimum elevation angle is set at a significantly higher value, at least 15°, because such values ​​allow miniaturization of the antenna for a ground station operating in a frequency band higher than the X band, while maintaining equivalent performance compared to a ground station operating in the X band.

[0027] It is therefore possible to use small ground stations, which have the advantage of being very inexpensive (approximately €10,000 to €30,000 per antenna for diameters between 60 cm and 1 m, compared to €400,000 to €1 million per antenna for diameters of 3 m to 7 m used in current systems), and very easy to install. It is thus possible to increase the number of ground stations in the communication system, and thereby increase the total reception area.

[0028] The communication system according to the invention therefore allows the use of frequencies higher than the frequencies of the X band, in particular higher than 12 GHz, for the reception of Earth observation data from a low orbit satellite, with small ground stations (therefore associated with a low infrastructure cost), while overcoming the attenuation problems which made this frequency band unusable in current systems.

[0029] The system according to the invention thus benefits from the advantages of the Ka band, namely a wider bandwidth, and less used (which reduces disturbances and congestion).

[0030] The use of a sufficiently high minimum elevation angle (at least 15°) also offers greater flexibility in installation, as such an angle significantly reduces blocking problems due to the presence of obstacles such as trees or buildings. Furthermore, such a minimum elevation angle reduces the impact of other terrestrial radio communication systems (such as interference from terrestrial systems using the same band).

[0031] The use of ground stations with small antennas (maximum diameter 2 m) allows for a significant reduction in manufacturing time and cost compared to conventional antennas (3 m to 7 m in diameter). Installation time and cost are also greatly reduced.

[0032] Simulations carried out have made it possible to evaluate that a communication system according to the invention allows a reduction in the cost per gigabyte transmitted (i.e. the ratio between the volume of data for coverage at 5° elevation by the cost of the infrastructure) by a factor of 6 to 12 (depending on the minimum elevation angle used).

[0033] Furthermore, the diameter of the antennas of the ground stations of the communication system of the invention allows for great modularity and flexibility in their installation. It is therefore very easy to move, or even to add one or more ground stations to the communication system.

[0034] This ease of adding ground stations makes it possible to use a technique called site diversity, which consists of linking several ground stations receiving the same signal (and therefore located sufficiently close to each other, for example, a few tens of kilometers apart). Thus, if the signal is strongly attenuated in the area where one of the ground stations is located, another ground station can compensate for it. Areas of intense rainfall, for example, supercells, generally only extend over a few kilometers. Therefore, by placing ground stations a few tens of kilometers apart, particularly in regions especially prone to rainfall, the attenuation due to rain in the downlink signal is greatly reduced.

[0035] It is noted that this addition of ground stations is made possible and conceivable due to the possible miniaturization of the antennas of the proposed system compared to existing systems.

[0036] In addition, due to their small size, ground stations can be advantageously installed as close as possible to the Internet backbone, for example on the roofs of data centers, thus reducing the costs and latency associated with connectivity to computer networks.

[0037] In one or more embodiments, each minimum elevation angle can be between 15 and 30 degrees.

[0038] In one or more embodiments, each ground station can be configured to receive data transmitted by the low-orbit satellite using a respective frequency between 25.5 GHz and 27 GHz.

[0039] Such a frequency range corresponds to the Ka band for Earth observation applications. One advantage of such a frequency range is its technological maturity, but it is understood that the invention is not limited to this frequency range.

[0040] In one or more embodiments, each ground station may include an antenna with a diameter between 30 cm and 1.5 m.

[0041] In one or more embodiments, for each ground station, the antenna diameter and minimum elevation angle may be such that a signal-to-noise ratio associated with the transmission of data from the satellite to the ground station is at least equivalent to a signal-to-noise ratio associated with a transmission of data from the satellite to a reference ground station operating at a reference frequency within the X band and associated with a minimum reference elevation angle of 5 degrees.

[0042] The reference ground station can typically have an antenna diameter of more than 3 m, for example 4 m.

[0043] In these embodiments, certain characteristics of the ground station, namely the antenna diameter and the minimum elevation angle, are determined so that the transmission quality is the same as for a ground station operating in X-band with a minimum elevation angle of 5° (which generally has an antenna diameter significantly greater than 2 m, in particular between 3 m and 6 m). Each ground station is thus configured so that the system performance remains similar to that of an X-band transmission, but with a significantly reduced infrastructure cost.

[0044] In one or more embodiments, the set of ground stations may comprise a plurality of ground stations.

[0045] Using a minimum elevation angle of at least 15° reduces the contact time with the satellite and the reception area, which corresponds to the area in which a ground station receives data transmitted by the satellite. To compensate for the reduction in the reception area (due to the increased minimum elevation angle), one solution is to use a plurality of ground stations. This is made possible by the significantly reduced cost of a ground station according to the invention, as well as by the simplicity of installing such a ground station.

[0046] In particular, for each ground station among the plurality of ground stations, the minimum elevation angle associated with the ground station can define a reception area in which the ground station receives signals from the satellite, and a union of the terrestrial reception areas of the plurality of ground stations can cover a predetermined percentage of a reference reception area.

[0047] According to the invention, it is thus possible to determine, for each ground station, the reception area associated with it, and then to determine, based on the reception areas obtained, the number of ground stations and their location to cover (at least partially) a reference reception area. This ensures a certain volume of data transmitted by the satellite and received by at least one ground station (data "downlink" volume).

[0048] For example, the predetermined percentage may be greater than or equal to 80%.

[0049] In one embodiment, the reference reception area can be a geographical area (for example, the surface of a country or a group of countries, or even a continent).

[0050] In another embodiment, the reference reception area may correspond to a reception area of ​​a single ground station associated with a minimum elevation angle of 5 degrees.

[0051] Another aspect of the invention relates to an Earth observation data transmission architecture comprising a low-orbit Earth observation satellite and a communication system as defined above.

[0052] In one or more embodiments, the satellite may include a steerable beam antenna.

[0053] Another aspect of the invention relates to a method for configuring a system for receiving Earth observation data transmitted from a satellite in low Earth orbit, the system comprising a set of ground stations, each ground station being configured to receive data transmitted by the satellite in low Earth orbit using a frequency greater than 12 GHz, each ground station comprising an antenna with a diameter less than or equal to 2 meters, each ground station being associated with a respective minimum elevation angle greater than or equal to 15 degrees, wherein data transmission from the satellite to each ground station is carried out only when an elevation angle of the satellite relative to said ground station is greater than or equal to the minimum elevation angle associated with said ground station, the method comprising, for one ground station among the set of ground stations:

[0054] - receive a geographical position from the ground station;

[0055] - receive a first attenuation value due to rain for the position geographical received, the first attenuation value due to rain being associated with a reference frequency in the X band and a reference elevation angle of the satellite relative to the ground station of 5°;

[0056] - receive a set of values ​​of the satellite's elevation angle relative to at the ground station, in which each value of the set of values ​​of the satellite's elevation angle relative to the ground station is greater than or equal to 15°;

[0057] - for each value among the set of values ​​of the satellite's elevation angle relative to the ground station:

[0058] receive a second rain attenuation value for the received geographic position, said second rain attenuation value being associated with the frequency used to receive, by the ground station, the data transmitted by the satellite, and with said value of the elevation angle of the satellite relative to the ground station;

[0059] calculate, from the first value of attenuation due to rain and the second value of attenuation due to rain, a miniaturization factor, the miniaturization factor corresponding to the factor by which a diameter of a ground station antenna can be reduced relative to a reference antenna of a reference ground station operating at the reference frequency and the reference elevation angle, while maintaining at least an equivalent signal-to-noise ratio during data transmission from the satellite to the ground station;

[0060] - to determine, from all the calculated miniaturization factors, a diameter of the ground station antenna and / or a minimum elevation angle, the minimum elevation angle belonging to the set of values ​​of the satellite's elevation angle relative to the ground station.

[0061] The term "configuration method" means a method for determining the characteristics of one or more ground stations, the characteristics including, in particular, the minimum elevation angle and / or the antenna diameter. Such a method allows for optimal design of the transmission system for the installation of the ground stations (for the antenna diameter) and for the operation of the system (for the minimum elevation angle). In some embodiments, the minimum elevation angle and the antenna diameter are determined.

[0062] The first attenuation value and / or the second attenuation values ​​can be obtained from any known prior art meteorological model, for example a model defined in the ITU-R recommendations.

[0063] The attenuation values ​​depend on several factors (in addition to the geographical location considered) related to the data transmission link, notably the frequency and the elevation angle. The first attenuation value here corresponds to a reference value, corresponding to transmission in band X with a reference elevation angle of 5°. The second set of attenuation values ​​are values ​​determined for a frequency belonging to a band higher than band X (the frequency band that we wish to use for the new transmission system), and for different values ​​of the minimum elevation angle. The aim is thus to determine, among the different values ​​of the minimum elevation angle tested, an "optimal" value of the minimum elevation angle (or of the antenna diameter, these two characteristics being related).According to the above procedure, the optimal value of the minimum elevation angle is determined from a miniaturization factor, defined as the factor (or coefficient) by which the diameter of an antenna in the new system must be divided relative to the diameter of an antenna in a reference system (in the X-band) to maintain the same signal-to-noise ratio during data transmission. Ideally, this factor should be greater than 1: this means that the antenna can be miniaturized relative to the reference system. reference angle. However, this is not always possible at low minimum elevation angles (on the order of 5°). In this case, it is possible to select a higher minimum elevation angle, which consequently increases the miniaturization factor (or coefficient).

[0064] The reference ground station may have, for example, an antenna diameter of 4 m.

[0065] In one or more embodiments, the above process can be implemented for each ground station among the set of ground stations.

[0066] In one or more embodiments, for a value a of the satellite's elevation angle relative to the ground station among the set of values ​​of the satellite's elevation angle relative to the ground station, the miniaturization factor MF can be:

[0067] MF = 10A ((G - AG - AA) / 20),

[0068] with G = 20xlogi0(fY / fx), AG = 20 x logi0(da / d5) and AA = AY - Ax;

[0069] where Ax corresponds to the first value of attenuation due to rain, AY corresponds to the second value of attenuation due to rain, fx corresponds to the reference frequency in the X band, fY corresponds to the frequency used to receive, by the ground station, the data transmitted by the satellite, d5 corresponds to a distance from the satellite to the ground station for an elevation angle of 5° and da corresponds to the distance from the satellite to the ground station for the value a of the elevation angle of the satellite relative to the ground station.

[0070] In one or more embodiments, the determination, based on all the calculated miniaturization factors, of the diameter of the ground station antenna and / or the minimum elevation angle may include:

[0071] - determine a subset of values ​​of the satellite's elevation angle by relative to the ground station, among the set of values ​​of the satellite's elevation angle relative to the ground station, for which the miniaturization factor exceeds a predetermined threshold;

[0072] - select, from the determined subset of values, a minimum value of the elevation angle of the satellite relative to the ground station.

[0073] In one or more embodiments, if no value of the satellite elevation angle relative to the ground station among the set of values ​​of the satellite elevation angle relative to the ground station exceeds the predetermined threshold, the determination, from the set of calculated miniaturization factors, of the diameter of the ground station antenna and / or the minimum elevation angle may include: selecting, from the set of values ​​of the satellite elevation angle relative to the ground station, a maximum value of the miniaturization factor.

[0074] In one or more embodiments, the method may further include an installation of the base station according to the diameter of the ground station antenna and / or the determined minimum elevation angle.

[0075] The invention and its various applications will be better understood upon reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0076] Other features and advantages of the invention will become apparent from the description, which can be read in conjunction with the figures. These figures are provided for illustrative purposes only and are not intended to limit the scope of the invention.

[0077] Fig. 1a represents the elevation angle of a satellite relative to a ground station.

[0078] Fig.lb represents the minimum elevation angle of the satellite for which the ground station is configured to receive signals from the satellite.

[0079] Fig. 2a represents a map of attenuation values ​​for a ground station operating in Ka band at a frequency of 26 GHz and at an elevation angle a of 5°.

[0080] Fig. 2b represents a map of the difference between the attenuation values ​​for a ground station operating in the Ka band at a frequency of 26 GHz and at an elevation angle α of 5° and the attenuation values ​​for a ground station operating in the X band at a frequency of 8.2 GHz and at an elevation angle α of 5° (assuming that the two satellite systems are equivalent, i.e. they have the same size ground station antenna, the same size satellite antenna and the same radio output power).

[0081] Fig. 2c represents a map of the miniaturization factor, i.e. the factor by which it is possible to reduce the diameter of the ground station antenna of the Ka-band system while maintaining the same signal-to-noise ratio as for the X-band system, for an elevation angle α of 5°.

[0082] Figure [Fig.3] represents the attenuation values ​​in Ka band and X band in a given geographical area (Dublin) for an example of satellite pass.

[0083] Fig. 4 represents the distance from the satellite to the ground station as a function of the elevation angle a and the difference in gain AG with respect to an elevation angle of 5° due to the decrease in free space attenuation (FSL).

[0084] Fig. 5a represents a map of the miniaturization factor for an elevation angle a of 15°.

[0085] Fig. 5b represents a map of the miniaturization factor for an elevation angle α of 30°.

[0086] Figure [Fig. 6] represents a flowchart of a method for configuring a ground station according to an embodiment of the invention.

[0087] Figure 7 represents a communication system according to one embodiment of the invention.

[0088] Fig. 8 represents an example of a communication system according to the embodiment of Fig. 7.

[0089] Figure 9a illustrates cell transfer in one embodiment of the invention.

[0090] Figure 9b illustrates cell transfer in another embodiment of the invention.

[0091] Fig. 10 represents a processing device configured to process satellite data received by one or more ground stations of the communication system, according to an embodiment of the invention. DETAILED DESCRIPTION

[0092] In a known manner and as shown in [Fig.1a], it is recalled that the elevation angle of a satellite 1 with respect to a ground station 3 corresponds to the angle a, between 0° and 90°, between the horizontal plane 2 on which the ground station 3 is located and the line 4 passing through the ground station 3 in the direction of the satellite 1.

[0093] The minimum elevation angle amin is a predefined value, which corresponds to the minimum elevation angle at which a downlink communication link is established from the low-Earth orbit satellite to the ground station. In practice, the satellite is generally configured to transmit data when its elevation angle relative to the ground station exceeds the predefined minimum elevation angle.This configuration is achieved using a transmission plan sent upstream to the satellite, which specifies a transmission period during which the satellite must transmit data. This transmission period is determined based on the position of the ground station, the satellite's trajectory, and the desired minimum elevation angle (this corresponds to the time interval during which the satellite is "visible" from the ground station, and during which the satellite's elevation angle relative to the ground station is greater than or equal to the predefined minimum elevation angle). It should be noted that in the case of an Earth observation satellite, data transmission is "blind," meaning the satellite transmits its data without first establishing a communication link with a ground station.The ground station, for its part, receives the data transmitted by the satellite when it is clearly visible from the ground station.

[0094] The minimum elevation angle amin of the satellite is shown in [Fig. 1b]. More specifically, [Fig. 1b] illustrates the trajectory of a pass of the satellite 1 over a ground station 3 over time. The elevation angle of the satellite relative to a ground station 3 increases up to its maximum value (this maximum value, called the "peak elevation", depends on the satellite's trajectory and is less than 90°, a peak elevation value of 90° corresponding to a very specific case where the trajectory of satellite 1 passes through the zenith of ground station 3) and then decreases again. The low-Earth orbit satellite 1 transmits data (received by ground station 3) as long as the elevation angle a of satellite 1 relative to ground station 3 is greater than or equal to the minimum elevation angle amin. It appears from [Fig. 1b] that the lower the value of the minimum elevation angle amin, the longer the contact time, i.e., the time interval [t1; t2] during which a communication link is established between ground station 3 and the low-Earth orbit satellite 1.

[0095] For this reason, the minimum elevation angle amin is traditionally chosen to be as low as possible to maximize this contact time. Thus, the minimum elevation angle amin is generally set around 5° (a lower value being difficult to use in practice, due to the obstruction of the communication link between the ground station 3 and the low-orbit satellite 1 by obstacles such as buildings, mountains or trees).

[0096] Currently, ground stations used to receive data unloaded by Earth observation satellites in low orbit operate mainly in the X band, at frequencies between 8.025 GHz and 8.4 GHz, and with a minimum elevation angle of 5°.

[0097] Due to the limited bandwidth (only 375 MHz between 8.025 GHz and 8.4 GHz), the capacity of current X-band systems remains quite limited, and congestion is significant. Furthermore, X-band ground stations are large (at least 4 m antenna diameter) and represent a very high cost (between €400,000 and €1 million per ground station).

[0098] Conversely, ground stations operating in higher bands, for example in the Ka band, can be much smaller (less than one meter in antenna diameter for some of them), and thus represent a much lower cost. Furthermore, the bandwidth is greater than that offered by the X band (1.5 GHz for Earth observation applications in the Ka band).

[0099] However, as mentioned previously, the attenuation due to the presence of water in the atmosphere is greater the higher the frequency band in which the ground station operates. For example, the attenuation due to the presence of water is significantly greater in the Ka, Q, or W bands than in the X band.

[0100] However, the signal-to-noise ratio of a downlink transmission depends on the attenuation and the diameter of the ground station antenna. More specifically, the signal-to-noise ratio decreases when the attenuation increases, and it increases when the diameter of the ground station antenna increases.

[0101] The inventors of the present invention therefore sought to determine to what extent the diameter of the ground antenna could be reduced by using a frequency band higher than the X band compared to a ground antenna operating in the X band, while maintaining the same signal-to-noise ratio.

[0102] For similar atmospheric conditions (in particular similar humidity conditions) and similar transmission conditions (same size of ground station antenna, same size of satellite antenna and same radio output power), the signal-to-noise ratio (SNRY) in a frequency band Y higher than the X band (i.e. a frequency band whose lower bound is higher than the upper bound of the X band, for example above 12 GHz) is equal to:

[0103] SNRy = SNRx + 20xlog10(fY / fx) - (AY - Ax)

[0104] where SNRX represents the signal-to-noise ratio in the X band, AY corresponds to the attenuation (also called "weakening") of the signal in the Y band, Ax corresponds to the attenuation of the signal in the X band, fx corresponds to the frequency used in the X band and fY corresponds to the frequency used in the Y band.

[0105] More specifically, we have:

[0106] SNRy = SNRx + G-(Ay-Ax)(1)

[0107] where the gain G comes from:

[0108] - with a gain of 20xlogi0(fY / fx) at the satellite antenna;

[0109] - a loss of 20xlogi0(fY / fx) due to free space attenuation (FSL, for "Free Space Loss" in English); and

[0110] - with a gain of 20xlogi0(fY / fx) at the ground station antenna.

[0111] In the following, the calculations are performed for a Ka band, but a reasoning A similar principle applies to other frequency bands above the X band (K, Q or W band for example).

[0112] It is assumed that the frequency used in the X band is 8.2 GHz and that the frequency used in the Ka band is 26 GHz. Thus, the gain G is equal to 20xlogi0(26 / 8.2) dB ~ 10 dB.

[0113] The signal-to-noise ratio SNRKa in the Ka band is therefore equal to:

[0114] SNRKa = SNRx + 10 dB - (AKa - Ax) (2)

[0115] Denoting AA = AKa - Ax as the difference in attenuation in the Ka band and in the X band, we have:

[0116] SNRKa = SNRX + 10 dB - AA (3).

[0117] Thus, in Ka band, the diameter of the ground station antenna can be reduced (this is called "miniaturization of the ground station"), while maintaining the same signal-to-noise ratio, as long as the AA attenuation difference remains below 10 dB.

[0118] More specifically, the miniaturization factor of the ground station, i.e. the factor by which the diameter of the antenna of the Ka-band ground station can be reduced compared to an X-band ground station while maintaining the same signal-to-noise ratio, is equal to:

[0119] MF = 10A ((10 - AA) / 20) (4)

[0120] For example, if the miniaturization factor is equal to 2, this means that it is possible to replace a ground station with a 4 m diameter antenna operating in X band with a ground station with a 2 m antenna operating in Ka band, while maintaining the same signal-to-noise ratio.

[0121] It is noted that the attenuation is related to the elevation angle of the satellite: the lower the elevation angle, the higher the attenuation. Thus, in the preceding equations, the values ​​AKa, Ay, Ax and AA are defined for a given elevation angle.

[0122] From equation (4), it is possible to establish a map of the miniaturization factor, representing the miniaturization factor in different terrestrial geographical areas, at a predefined elevation angle a. Such a map is shown in [Fig. 2c], for an elevation angle of 5°.

[0123] Figure 2c thus represents a world map, on which the values ​​of the miniaturization factor appear as a function of the location on the map. The areas left in white represent the regions of the world in which the attenuation at 5° in the Ka band is so significant that it is not possible to reduce the diameter of the ground antenna while maintaining the same signal-to-noise ratio as in the X band (miniaturization factor MF < 1 - an MF factor strictly less than 1 even meaning that it would be necessary to increase the diameter of the ground antenna to maintain the same signal-to-noise ratio as in the X band).

[0124] A map of the miniaturization factor such as that shown in [Fig. 2c] can be obtained from:

[0125] - a map of attenuation values ​​worldwide for a ground station operating in X-band and at a fixed elevation angle; and

[0126] - a map of attenuation values ​​worldwide for a ground station operating in Ka band and at the same elevation angle a.

[0127] Such maps of attenuation values ​​can be established from a meteorological model. Figure 2a thus represents a map of attenuation values ​​worldwide for a ground station operating in the Ka band at a frequency of 26 GHz and an elevation angle α of 5°, obtained using the clear-weather (i.e., in the presence of only clouds and gas) meteorological model provided by the ITU (International Telecommunication Union), described in ITU-R Recommendation P.618-14 of August 2023 (for "ITU- Radiocommunication). Such models are known and readily available, and are not detailed here. It is understood that any meteorological attenuation model can be used to obtain such maps.

[0128] It is then possible to subtract the attenuation maps in Ka band and in X band to obtain a map of the AA attenuation difference such as that shown in [Fig.2b].

[0129] The mapping of the miniaturization factor of [Fig.2c] can be obtained from equation (4) above, in which the values ​​of AA are those of [Fig.2b].

[0130] It appears from the mapping of attenuation values ​​in the Ka band ([Fig. 2a]) that the attenuation values ​​are generally higher in the intertropical zone, i.e. the area between the Tropic of Cancer and the Tropic of Capricorn, and in particular near the equator. This is due in particular to the significant presence of water vapor in these areas.

[0131] In these regions, the difference in attenuation between the Ka band and the X band can be very significant (see [Fig. 2b]). And even outside these regions, the difference in attenuation can exceed 10 dB.

[0132] Due to this significant difference in attenuation, it appears on [Fig.2c] that the regions in which miniaturization is possible (regions located outside the white zone) are quite limited and generally only allow a miniaturization factor between 1 and 2.9. This means that, in these regions, it is possible to use a Ka-band ground station with an antenna diameter reduced by a factor of 1 to 2.9 compared to an X-band ground station, while maintaining the same signal-to-noise ratio.

[0133] The inventors of the present invention sought to determine:

[0134] - if it were possible to miniaturize the diameter of the ground station antenna in the white area of ​​the mapping of [Fig.2c];

[0135] - if it were possible to further miniaturize the diameter of the station's antenna at soil in locations outside the white area of ​​the map in [Fig.2c].

[0136] The inventors determined that this miniaturization was possible by operating at higher minimum elevation angles than existing systems (generally 5°). This is mainly due to two properties:

[0137] - the difference in attenuation between the Ka band and the X band decreases drastically when the angle of elevation is greater than or equal to 15°;

[0138] - the distance between the satellite and the ground station decreases when the elevation angle increases, which decreases the free space attenuation (FSL), and consequently increases the gain G in equations (1) to (4) above.

[0139] Indeed, equations (1)-(4) above correspond to the case where the elevation angle is the same in band X and in band Ka. If the elevation angle is not the same, the The distance between the satellite and the ground station is also not the same, which has a consequence on the free space attenuation, as detailed below.

[0140] The first property is illustrated in [Fig. 3], which shows a curve 5 of the Ka-band attenuation values ​​in clear weather in a given geographical area (here, the city of Dublin) and a curve 7 of the X-band attenuation values ​​in clear weather for the same geographical area. Curve 6 represents the satellite elevation angle as a function of time.

[0141] When the elevation angle is small (between 5° and 15°), the Ka-band attenuation values ​​(curve 5) are more stable (the variations are much less abrupt) than when the elevation angle is greater than or equal to 15°. Furthermore, when the elevation angle is less than 15°, the difference between the Ka-band attenuation (curve 5) and the X-band attenuation (curve 7) is much greater than for elevation angles greater than or equal to 15°.

[0142] The second property is illustrated in [Fig. 4]. Curve 8 in [Fig. 4] represents the distance (in km) from the satellite to the ground station as a function of the elevation angle α. Curve 9 in [Fig. 4] represents the difference in gain AG with respect to an elevation angle of 5°, due to the decrease in free space attenuation (FSL).

[0143] Indeed, the free-space attenuation decreases with the square of the distance from the satellite to the ground station. We therefore have: AG = 20 x logi0(da / d5), where d5 corresponds to the distance from the satellite to the ground station for an elevation angle of 5° and da corresponds to the distance from the satellite to the ground station for an elevation angle a.

[0144] Thus, equation (4) above, giving the miniaturization factor, becomes, when the Ka-band ground station operates with an elevation angle α > 5°:

[0145] MF = 10A ((10 - AG - AA) / 20) (5)

[0146] with AG = 20 x logio(da / d5).

[0147] In general, for a ground station in a frequency band higher than the X band and operating with an elevation angle α > 5°, we have:

[0148] MF = 10A ((G - AG - AA) / 20) (6)

[0149] with G = 20xlogi0(fY / fx) as defined previously.

[0150] Figure 4 corresponds to a satellite located at an altitude of 550 km above the Earth's surface. It can be observed on curves 8 and 9 that:

[0151] - for an elevation angle of 5°, the distance from the satellite to the ground station is 2200 km;

[0152] - for an elevation angle of 15°, the distance from the satellite to the ground station is 1500 km, which results in an AG reduction of approximately -3.2 dB;

[0153] - for an elevation angle of 30°, the distance from the satellite to the ground station is 1000 km, which results in an AG reduction of approximately -6.8 dB.

[0154] From equation (6), it is possible to determine maps of the miniaturization factor for different values ​​of the elevation angle.

[0155] Fig. 5a thus represents a map of the miniaturization factor for a frequency of 26 GHz (in Ka band, therefore) and an elevation angle of 15°.

[0156] At such an elevation, it is possible to miniaturize the diameter of the ground station antenna by a factor of at least 1.5 in virtually all regions of the world (with the exception of the wettest regions). The miniaturization factor is greater than 3 in most regions of the world, and even greater than 4 in the driest regions. This means that in most regions of the world, a standard 4 m diameter antenna operating in the X-band can be replaced, for an equivalent signal-to-noise ratio, by an antenna less than 1.5 m in diameter, or even less than 1 m, operating in the Ka-band.

[0157] Fig. 5b represents a mapping of the miniaturization factor for a frequency of 26 GHz (in Ka band, therefore) and an elevation angle of 30°.

[0158] At such an elevation, it is possible to miniaturize the diameter of the ground station antenna by a factor of at least 3 in virtually all regions of the world (with the exception of the wettest regions). The miniaturization factor is greater than 5 in most regions of the world, and even greater than 6 in the driest regions. This means that in most regions of the world, a standard 4 m diameter antenna operating in the X-band can be replaced, for an equivalent signal-to-noise ratio, by an antenna less than 80 cm, or even less than 70 cm, operating in the Ka-band.

[0159] According to one aspect of the invention, a system for transmitting Earth observation data from a low-Earth orbit satellite to a ground station configured to receive signals emitted by the low-Earth orbit satellite using a frequency band above the X band, for example the Ka band, at an elevation angle of at least 15° is proposed. The ground station includes an antenna with a diameter of 2 meters or less, or even 1 meter or less.

[0160] Another aspect of the invention relates to a system for transmitting Earth observation data from a low-Earth orbit satellite to a plurality of ground stations, each configured to receive signals emitted by the low-Earth orbit satellite using frequencies in a band above the X-band, for example the Ka-band, at elevation angles of at least 15°. More specifically, each ground station is associated with a respective minimum elevation angle and receives signals when the elevation angle of the satellite relative to the ground station is greater than or equal to the minimum elevation angle. For each ground station, the minimum elevation angle has a predefined value at least equal to 15°, for example, between 15° and 30°. Each ground station has an antenna with a diameter of 2 meters or less, or even 1 meter or less. Depending on the embodiment, the frequency at which each ground station receives data transmitted from the satellite may or may not be the same for all ground stations.

[0161] The minimum elevation angle can vary from one ground station to another. In particular, the minimum elevation angle of a ground station can advantageously be determined based on the humidity level of the geographical area in which the ground station is located. Typically, the higher the humidity level of the geographical area in which the ground station is located, the higher the minimum elevation angle can be. It is therefore possible to assign lower values ​​for the minimum elevation angle to ground stations located in areas with lower humidity levels (typically outside the intertropical zone), and thus increase the contact time for these ground stations.

[0162] The antenna diameter can also vary from one ground station to another. In particular, the antenna diameter of a ground station can be advantageously determined based on the humidity level of the geographical area in which the ground station is located. Typically, the higher the humidity level of the geographical area in which the ground station is located, the larger the antenna diameter of the ground station can be.

[0163] In particular, for each ground station, the minimum elevation angle and the antenna diameter can be determined according to the geographical location at which its installation is planned, using the method shown in [Fig. 6]. The method in [Fig. 6] is implemented for one ground station. When there are multiple ground stations, the method in [Fig. 6] can be implemented, successively or in parallel, for each ground station among the plurality of ground stations.

[0164] During a step 610, data relating to the geographical position of the ground station is received. This data can be, for example, geographical coordinates.

[0165] A rain attenuation value, referred to herein as the first rain attenuation value, associated with the geographic location of the ground station, is then received in step 620. This first rain attenuation value corresponds to the rain attenuation value under given meteorological conditions (e.g., clear weather) for a reference frequency in the X band and a reference elevation angle of 5°. The first attenuation value received in step 620 may, for example, correspond to the attenuation value in the attenuation map of [Fig. 2a] at the point corresponding to the geographic location received in step 610.

[0166] Then steps 630 and 640 are implemented for each value in a set of values ​​for the elevation angle of the satellite relative to the ground station in question. Thus, for each value in the set of values, the sequence of steps (630, 640) can be implemented successively or in parallel. The set of values ​​for the elevation angle may comprise one or more values. Each of these values ​​is greater than or equal to 15°, and perhaps, in some embodiments, less than or equal to 30°. For example, the set of values ​​may comprise two values: 15° and 30°.

[0167] During step 630, a rain attenuation value, referred to herein as the second rain attenuation value, associated with the geographical location of the ground station is received. This second rain attenuation value corresponds to the rain attenuation value under the same meteorological conditions as in step 620, for a frequency above 12 GHz (e.g., the Ka band), and for the current elevation angle. This second attenuation value can typically be determined from a meteorological model, as described above with reference to [Fig. 2a].

[0168] From the first attenuation value and the second attenuation value, it is possible to determine, in step 640, the value of the miniaturization factor at the location considered for the current value of the elevation angle, as described above. In particular, this value of the miniaturization factor can be calculated from equation (6) above.

[0169] It is understood that determining a minimum antenna diameter would be equivalent to determining a miniaturization factor. For example, if one wishes to maintain a signal-to-noise ratio equivalent to that of a 4 m diameter antenna operating in the X-band, a miniaturization factor of 2 corresponds to a minimum antenna diameter of 2 m for the ground station.

[0170] Following the implementation of the sequences of steps (630, 640) for each value of the set of values ​​of the elevation angle, we thus obtain a set of values ​​of the miniaturization factor, each associated with an elevation angle among the set of values ​​of the elevation angle.

[0171] From this set of miniaturization factor values, it is possible to determine a pair of values ​​(amin, D) for the ground station, where amin corresponds to the minimum elevation angle associated with the ground station and D corresponds to the diameter of the ground station antenna (step 650). This pair of values ​​is determined so that it is effectively possible to miniaturize the diameter of the ground antenna compared to a conventional X-band antenna (typically on the order of 4 m).

[0172] For example, at the end of steps 630 and 640, the following values ​​may have been determined: a = 15° a = 30° Station 1 0.5 2 Station 2 2 4 Station 3 4 6.5

[0173] Assuming that these values ​​were obtained with respect to a reference ground station operating in the X-band and having an antenna diameter of 4 m, this means that:

[0174] - for station 1, an elevation angle of 15° does not allow miniaturization the ground station antenna, while an elevation angle of 30° allows a miniaturization factor of 2, meaning that it is possible to use a 2 m antenna to obtain a signal-to-noise ratio equivalent to that which would be obtained with the reference ground station. In this case, a minimum elevation angle of 30° can be chosen for station 1;

[0175] - for station 2, an elevation angle of 15° allows for a miniaturization factor of 2 (which corresponds to a 2 m antenna), while an elevation angle of 30° allows a miniaturization factor of 4 (which corresponds to a 1 m antenna). In this case, it may be advantageous to choose an elevation angle of 30° for station 2, as the cost of a 2 m antenna is significantly higher than the cost of a 1 m antenna (even if the coverage area is smaller, as detailed below);

[0176] - for station 3, an elevation angle of 15° allows for a miniaturization factor of 4 (corresponding to a 1 m antenna), while an elevation angle of 30° allows a miniaturization factor of 6.5 (corresponding to an antenna of approximately 60 cm). In this case, it may be advantageous to choose an elevation angle of 15° for station 3, as the gain in miniaturization is relatively small compared to the loss of coverage.

[0177] According to one embodiment, step 650 of determining the pair of values ​​(amin, D) for the ground station can be implemented according to the following rules:

[0178] - if no miniaturization factor is greater than 1, select the angle elevation for which the miniaturization factor is maximal;

[0179] - if any single miniaturization factor is greater than 1, select the angle elevation for which the miniaturization factor is greater than 1;

[0180] - if several miniaturization factors are greater than 1, determine if, for If one or more elevation angles are present, the equivalent antenna diameter (i.e., the antenna diameter obtained by multiplying the diameter of the reference ground station antenna by the miniaturization factor) is less than a predetermined threshold—or, equivalently, if the miniaturization factor exceeds a predetermined threshold. If only one equivalent antenna diameter meets this criterion, select the elevation angle for which the criterion is met. If several equivalent antenna diameters meet this criterion, select, among the associated elevation angles, the lowest elevation angle.

[0181] Of course, it is possible to set other criteria for selecting the minimum elevation angle amin for the ground station considered.

[0182] Such a transmission system makes it possible to use higher frequencies than those of the X band and therefore to benefit from a much wider and much less used bandwidth (few systems currently use such frequencies, due to the problem of attenuation due to rain), thus avoiding the problems related to congestion of the X band, while overcoming the problem of loss of quality of the associated link.

[0183] Figure 7 represents an Earth observation data transmission system from a low-orbit satellite to a set of ground stations, according to one embodiment of the invention.

[0184] The communication system of [Fig.7] comprises a plurality of ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g. Each ground station 10a, 10b, 10c, lOd, 10e, lOf, 10g comprises an antenna with a diameter of less than or equal to 2 meters, and is configured to receive signals emitted by the low-orbit satellite using frequencies above the X band, in particular above 12 GHz, at minimum elevation angles of at least 15°. It is thus understood that each ground station 10a, 10b, 10c, lOd, 10e, lOf, 10g receives Earth observation data from the satellite only when the elevation angle of the satellite relative to the ground station is greater than or equal to the minimum elevation angle associated with the ground station 10a, 10b, 10c, lOd, 10e, lOf, 10g. For example, the minimum elevation angle of each ground station 10a, 10b, 10c, lOd, 10e, lOf, 10g may have been determined according to the method in [Fig. 6].

[0185] As mentioned previously, the different ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g may or may not have the same antenna diameter. The different ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g may or may not be associated with the same minimum elevation angle.

[0186] Each ground station 10a, 10b, 10c, lOd, 10e, lOf, 10g is associated with a respective receiving zone 20a, 20b, 20c, 20d, 20e, 20f, 20g. The term "receiving zone" (or "viewing zone") means an area encompassing all portions of orbits for which the ground station can receive signals emitted from low Earth orbit satellites, when those satellites are located within those portions. The outer contour of this zone may be represented by a circle or an ellipse, which represents the isoelevation circle associated with the minimum elevation angle of the ground station. For example, the portion of the trajectory of a satellite on a particular orbit represented on the [Fig.lb] corresponds to a diameter of the outer contour (i.e., a segment joining two points on the outer contour, the two points being symmetrical with respect to the center of the outer contour) of the reception area. In other words, as soon as the satellite passes inside this reception area, it transmits data that the ground station can receive.

[0187] The reception area depends on the minimum elevation angle amin (the smaller the minimum elevation angle amin, the more extensive the reception area, as shown in [Fig.lb]) and the diameter of the ground station antenna (the larger the diameter of the ground station antenna, the more extensive the reception area).

[0188] Figure 7 also represents a ground station 100 (hereinafter referred to as the "reference ground station 100") operating in the X-band and having a diameter strictly larger than the diameters of the other ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g (typically on the order of 4 m for ground stations used in most current systems), and associated with a minimum reference elevation angle of 5° (the minimum elevation angle used in most current systems). It is understood that the reference ground station 100 is not part of the communication system according to the invention, but is shown here to illustrate an aspect of the invention.

[0189] The reference ground station 100 is associated with a reception area 200 (here referred to as the "reference reception area") which is much larger than the reception areas 20a, 20b, 20c, 20d, 20e, 20f, 20g of the ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g of the communication system according to the invention.

[0190] In one or more embodiments, the plurality of ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g can be arranged such that the union of the receiving zones 20a, 20b, 20c, 20d, 20e, 20f, 20g of the ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g of the communication system according to the invention covers "largely" the reference receiving zone 200. By "largely," it is understood that the union of the receiving zones 20a, 20b, 20c, 20d, 20e, 20f, 20g covers at least a predetermined percentage of the reference receiving zone 200. The predetermined percentage may be, for example, greater than or equal to 80%. It is recalled that the union of two surfaces corresponds to a set which contains all the elements belonging to one or the other of the two surfaces.

[0191] In particular embodiments, the plurality of ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g can be arranged so that the union of the receiving areas 20a, 20b, 20c, 20d, 20e, 20f, 20g of the ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g of the communication system according to the invention covers the entire reference receiving area 200.

[0192] Thus, the reference ground station 100 can be "replaced" (or approximately replaced - allowing some "gaps" in relation to the reference reception area 200) by the plurality of ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g.

[0193] For example, in the case where the union of the reception zones 20a, 20b, 20c, 20d, 20e, 20f, 20g of the ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g covers the entire reference reception zone 200, any data transmitted from the satellite can be received by at least one of the ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g of the communication system according to the invention (because the satellite is located in at least one of the reception zones 20a, 20b, 20c, 20d, 20e, 20f, 20g).

[0194] In the example of [Fig.7], the communication system comprises 7 ground stations, but of course, a communication system according to the invention can comprise any number N of ground stations, with N greater than or equal to 2 (or even N greater than or equal to 1 if the desired coverage area does not correspond to an "equivalent" coverage area of ​​a ground station operating in X band for elevation angles greater than 5°, but to a predefined geographical area that one seeks to cover).

[0195] Figure 8 represents a particular example of the communication system of Figure 7. Figure 8 represents the receiving areas 20a, 20b, 20c, 20d, 20e, 20f, 20g of seven ground stations 10a, 10b, 10c, 10d, 10e, 10f, 10g installed across Europe (specifically, in Bergen, Norway; Linz, Austria; Kyiv, Ukraine; Athens, Greece; Tabarka, Tunisia; Albufeira, Portugal; and Brest, France). For the sake of clarity, the references of the ground stations and receiving areas are not shown in Figure 8—the ground stations are represented by solid-lined stars and the receiving areas by solid-lined ellipses, as in Figure 7.

[0196] In this example, all ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g have an antenna with a diameter of less than 1 meter and operate at a minimum elevation angle of 30°.

[0197] Figure 8 also shows a reference ground station 100 (located here in Milan, Italy, represented by a dashed star) and the reference receiving area 200 (represented by a dashed ellipse). In this example, the reference ground station 100 has a 5-meter diameter antenna and operates at a minimum elevation angle of 5°.

[0198] Fig. 8 also represents orbits 30a, 30b, 30c, 30d, 30e, 30f, which therefore correspond to possible trajectories for satellites in low Earth orbit.

[0199] For each orbit 30a, 30b, 30c, 30d, 30e, 30f:

[0200] - the portion(s) in continuous "thin" line represent the orbital portions 30a, 30b, 30c, 30d, 30e, 30f in at least one of the reception areas 20a, 20b, 20c, 20d, 20e, 20f, 20g of the seven ground stations 10a, 10b, 10c, 10d, 10e, 10f, 10g;

[0201] - the portion(s) in continuous "thick" line (portions 31a, 31b, 31c, 31d, 31e, 3 If) represent the orbital portions 30a, 30b, 30c, 30d, 30e, 30f outside any reception area 20a, 20b, 20c, 20d, 20e, 20f, 20g but inside the reference reception area 200;

[0202] - the portion(s) in dashed lines represent the orbital portions 30a, 30b, 30c, 30d, 30e, 30f outside of any reception zone 20a, 20b, 20c, 20d, 20e, 20f, 20g and of the reference reception zone 200.

[0203] The portions shown in "thick" solid lines (portions 31a, 31b, 31c, 31d, 31e, 3f) thus represent the orbital portions 30a, 30b, 30c, 30d, 30e, 30f for which the communication system comprising the seven ground stations 10a, 10b, 10c, 10d, 10e, 1f, 10g does not cover the reference reception area 200 of the reference ground station 100. It can be observed in [Fig. 8] that these portions are very small compared to the reference reception area 200. Thus, it is possible to replace a large ground station quite effectively with 7 small ground stations – even with a fairly high minimum elevation angle (30° in this example).

[0204] Of course, for complete coverage of the reference reception area, it is possible to include more ground stations in the communication system, and / or to reduce the minimum elevation angle of certain ground stations (particularly for ground stations located in low-humidity areas). Adding one or more additional ground stations is easily accomplished, notably due to the small size of the antennas of the ground stations used, which are associated with a much lower cost than conventional antennas, and the simplicity and flexibility of installing such stations.

[0205] In one or more embodiments of the invention, each ground station 10a, 10b, 10c, lOd, 10e, lOf, 10g is configured to receive data and transmit it, via a communication link, for example a wired one, to one or more processing devices. Thus, the processing device(s) are configured to receive and process data received from the low-Earth orbit satellite through the ground stations 10a, 10b, 10c, lOd, 10e, lOf, 10g.

[0206] In one embodiment, all ground stations located in a given geographical area can send the received data to the same processing device, which may be, for example, a remote computer server (in particular, a cloud-based server). For example, each ground station may include an antenna and a digitizing device, and may be configured to send the digitized spectrum of the data received from the satellite to the processing device. The device The processing system can handle all the data received from ground stations (i.e., the digitized data spectra) in a centralized manner. This allows, in particular, the reconstruction of information or messages by assembling data from multiple stations. It also allows for the efficient management of cellular handover scenarios between multiple stations where the same message may be received by several stations, in whole or in part.

[0207] An example of such a processing device is shown in [Fig. 10].

[0208] The processing device 700 of [Fig. 10] comprises a computer comprising A memory 701 for storing data received from one or more ground stations. The processing device 700 further includes a circuit 702. This circuit can be, for example, a processor capable of interpreting instructions in the form of a computer program, an electronic card, or a programmable electronic chip such as an FPGA (Field-Programmable Gate Array) configured to process the received data. The processing device 700 also includes an input interface 703 for receiving data from at least one ground station 10a, and an output interface 704 for providing processed data.

[0209] Another aspect of the invention relates to a communication architecture comprising:

[0210] - an Earth observation satellite in low Earth orbit; and

[0211] - a set of ground stations each having an antenna with a maximum diameter of 2 m (or even 1 m), and each configured to operate in a frequency band higher than the X band (in particular with frequencies above 12 GHz), with minimum elevation angles greater than or equal to 15°, for example between 15° and 30°.

[0212] In any communication architecture between a satellite and ground stations, when the satellite leaves the reception area of ​​one ground station and enters the reception area of ​​another, a cellular handover is necessary. The cellular handover allows the connection to be transferred from a communication link between the satellite and one ground station to a communication link with another ground station.

[0213] Some satellites are equipped with a fixed beam antenna, which requires the satellite to be steered in attitude to point towards the ground station. Such a satellite is shown in [Fig. 9a]. In this [Fig. 9a], satellite 50a moves over time t: before time ts, it is in communication with a first ground station 10a, and after time ts it is in communication with a second ground station 10b (the solid line segments connecting satellite 50a to either of the ground stations 10a, 10b delimit the minimum elevation angle at which either of the ground stations 10a, 10b receives data from satellite 50a, the dashed segments represent the direction of the beam - fixed - from the antenna of satellite 50a).

[0214] At time ts, satellite 50a must therefore switch from communication with the first ground station 10a to communication with the second ground station 10b, and must therefore turn around to point towards the second ground station 10b. During this turnaround, communication is interrupted between satellite 50a and the two ground stations 10a, 10b.

[0215] In the context of the present invention, since the number of ground stations is significantly greater than in conventional architectures, this loss of communication is also significantly more frequent, which can lead to a decrease in the volume of data transmitted from the satellite 50a to the network of ground stations 10a, 10b.

[0216] To avoid an interruption of communication between the satellite and the plurality of ground stations, it is possible to use a 50b satellite having an antenna whose beam orientation can be controlled (hereafter referred to as a "beam steering antenna"), for example a flat electronically oriented antenna, as shown in [Fig.9b].

[0217] In [Fig.9b], the solid line segments connecting satellite 50b to either of the ground stations 10a, 10b delimit the minimum elevation angle at which either of the ground stations 10a, 10b receives data from satellite 50a and the dashed line segments represent the antenna beam of satellite 50b, directed towards the ground station 10a, 10b with which it is in communication.

[0218] As illustrated in [Fig. 9b], with such a steerable beam antenna, the satellite 50b is constantly pointed towards the nadir and the beam orientation is electronically controlled, so that the satellite 50b does not have to perform any turning maneuvers during the cellular handover. Thus, the handover from a communication link with a first ground station 10a to a communication link with a second ground station 10b is performed almost instantaneously (or even instantaneously, if the satellite has an antenna capable of generating two separate steerable beams), which avoids interruption of communication between the satellite 50b and the plurality of ground stations 10a, 10b.

[0219] Thus, in embodiments of the invention, the communication architecture comprises a plurality of ground stations as defined above and a satellite having a steerable beam antenna.

[0220] Of course, the present invention is not limited to the embodiments described above by way of example. It extends to other variants.

Claims

Demands

1. A system for receiving Earth observation data transmitted from a low-orbit satellite, the system comprising an array of ground stations, wherein each ground station is configured to receive data transmitted by the low-orbit satellite using a frequency greater than 12 GHz, wherein each ground station comprises an antenna of respective diameter less than or equal to 2 meters, wherein each ground station is associated with a respective predefined minimum elevation angle greater than or equal to 15 degrees, wherein data transmission from the satellite to each ground station is carried out as long as an elevation angle of the satellite relative to said ground station is greater than or equal to the minimum elevation angle associated with the ground station.

2. System according to claim 1, wherein each minimum elevation angle is between 15 and 30 degrees.

3. A system according to any one of the preceding claims, wherein each ground station is configured to receive data transmitted by the low-orbit satellite using a respective frequency between 25.5 GHz and 27 GHz.

4. System according to any one of the preceding claims, wherein each ground station comprises an antenna with a diameter between 30 cm and 1.5 m.

5. A system according to any one of the preceding claims, wherein, for each ground station, the antenna diameter and minimum elevation angle are such that a signal-to-noise ratio associated with data transmission from the satellite to the ground station is at least equivalent to a signal-to-noise ratio associated with data transmission from the satellite to a reference ground station operating at a reference frequency within the X band and associated with a minimum reference elevation angle of 5 degrees.

6. System according to any one of the preceding claims, wherein the set of ground stations comprises a plurality of ground stations.

7. A system according to the preceding claim, wherein, for each ground station among the plurality of ground stations, the minimum elevation angle associated with the ground station defines a receiving area in which the ground station receives signals from the satellite, and wherein a union of the terrestrial receiving areas of the plurality of ground stations covers a predetermined percentage of a reference receiving area.

8. Earth observation data transmission architecture comprising a low orbit Earth observation satellite and a receiving system according to any one of the preceding claims.

9. Architecture according to claim 8, wherein the satellite includes a steerable beam antenna.

10. A method for configuring a system for receiving Earth observation data transmitted from a low-Earth orbit satellite, the system comprising an array of ground stations, each ground station being configured to receive data transmitted by the low-Earth orbit satellite using a frequency greater than 12 GHz, each ground station comprising an antenna with a diameter less than or equal to 2 meters, each ground station being associated with a respective minimum elevation angle greater than or equal to 15 degrees, wherein the transmission of data from the satellite to each ground station is carried out only when an elevation angle of the satellite relative to said ground station is greater than or equal to the minimum elevation angle associated with said ground station, the method comprising, for one ground station among the array of ground stations: - receiving a geographic position of the ground station;- receive a first rain attenuation value for the received geographic position, the first rain attenuation value being associated with a reference frequency in the X band and a reference elevation angle of the satellite relative to the ground station of 5°; - receive a set of values ​​of the elevation angle of the satellite relative to the ground station, in which each value of the set of values ​​of the elevation angle of the satellite relative to the ground station is greater than or equal to 15°;

11. - for each value among the set of values ​​of the elevation angle of the satellite relative to the ground station: • receive a second value of attenuation due to rain for the received geographic position, said second value of attenuation due to rain being associated with the frequency used to receive, by the ground station, the data transmitted by the satellite, and with said value of the elevation angle of the satellite relative to the ground station; • calculate, from the first value of attenuation due to rain and the second value of attenuation due to rain, a miniaturization factor, the miniaturization factor corresponding to the factor by which a diameter of a ground station antenna can be reduced relative to a reference antenna of a reference ground station operating at the reference frequency and the reference elevation angle, while maintaining at least an equivalent signal-to-noise ratio during data transmission from the satellite to the ground station; - determine, from all the calculated miniaturization factors, a diameter of the ground station antenna and / or a minimum elevation angle, the minimum elevation angle belonging to the set of values ​​of the satellite's elevation angle relative to the ground station. A method according to the preceding claim, wherein, for a value 'a' of the satellite's elevation angle relative to the ground station among the set of values ​​of the satellite's elevation angle relative to the ground station, the miniaturization factor MF is: MF = 10A((G - AG - AA) / 20), with G = 20xlogi0(fY / fx), AG = 20xlogi0(da / d5) and AA = AY - Ax where Ax corresponds to the first attenuation value due to rain, Ay corresponds to the second attenuation value due to rain, fx corresponds to the reference frequency in the X band, fY corresponds to the frequency used to receive, by the ground station, the data transmitted by the satellite, d5 corresponds to a distance from the satellite to the ground station for an elevation angle of 5° and da corresponds to the distance from the satellite to the ground station for the value a of the elevation angle of the satellite relative to the ground station.

12. A method according to claim 10 or 11, wherein the determination, from all the calculated miniaturization factors, of the diameter of the ground station antenna and / or the minimum elevation angle comprises: - determine a subset of values ​​of the satellite's elevation angle relative to the ground station, from the set of values ​​of the satellite's elevation angle relative to the ground station, for which the miniaturization factor exceeds a predetermined threshold; - select, from the determined subset of values, a minimum value for the elevation angle of the satellite relative to the ground station.

13. A method according to the preceding claim, wherein, if no value of the elevation angle of the satellite relative to the ground station among the set of values ​​of the elevation angle of the satellite relative to the ground station exceeds the predetermined threshold, the determination, from the set of calculated miniaturization factors, of the diameter of the ground station antenna and / or the minimum elevation angle comprises: selecting, from the set of values ​​of the elevation angle of the satellite relative to the ground station, a maximum value of the miniaturization factor.