System for receiving earth observation data

Abstract

One aspect of the invention relates to a system for receiving Earth observation data transmitted from a satellite in low orbit, the system comprising a set of ground stations, wherein each ground station is configured to receive data transmitted by the satellite in low orbit using a frequency higher than 12 GHz, wherein each ground station comprises an antenna of respective diameter less than or equal to 2 metres, wherein each ground station is associated with a predefined respective minimum elevation angle greater than or equal to 15 degrees, and wherein each ground station receives data from the satellite only when an elevation angle of the satellite with respect to the ground station is greater than or equal to the minimum elevation angle associated with the ground station.

Description

DESCRIPTION TITLE: 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 a low Earth observation satellite 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) refers to 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 below 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 harmful effects of atmospheric friction.

[0004] Low Earth orbit satellites are therefore satellites that circulate in orbits located 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 relative 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 used to perform geophysical and geographic observations of the Earth from their orbit, and include remote sensing satellites (weather, Earth imaging, or intelligence satellites). When an Earth observation satellite passes over a ground station (with each orbit), the satellite transmits its data to the ground station, meaning it sends the collected data to the ground station via a downlink communication link.

[0006] Today, downlink communications 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 achieved by parabolic antennas typically 3 to 5 meters in diameter. Due to the increased use of satellites in this band, their sheer number combined with 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, with values ​​above 12 GHz, such as the Ka band (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 susceptible 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 significant radio signal attenuation in high humidity conditions, the proposed solution uses ground stations (also called earth 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, these location constraints currently prevent widespread deployment of this solution or its general use for receiving data from low-Earth orbit satellites operating in 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 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] in which each ground station is configured to receive data transmitted by the low-Earth orbit satellite using a frequency greater than 12 GHz,

[0014] in which each ground station comprises an antenna with a respective diameter less than or equal to 2 meters,

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

[0016] in which each ground station receives data from the satellite 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 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] "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, among other things, when the satellite should transmit the data). In other words, such satellites are configured to offload their data, 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] A "ground station" (also called an Earth station or ground station) is understood to be an installation located on the Earth's surface and configured to communicate with a low-Earth orbit satellite. Specifically, 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 low-Earth orbit satellite, 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 this 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 diameter of the antenna can be on the order of a few tens of cm, 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] The term "diameter" refers to a characteristic dimension of an antenna, which can be the diameter in the mathematical sense for a circular antenna, or the equivalent diameter for a non-circular antenna (e.g., 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 either 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 directly overhead relative to the ground station). During the satellite's trajectory along its 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 point 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 some embodiments, two separate ground stations can be associated with two distinct predefined values ​​(i.e., two distinct minimum elevation angles).

[0024] Within the framework 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° makes it possible to compensate for the attenuation due to rain associated with the use of frequencies above the X-band, and also to reduce the diameter of the ground station antenna compared to conventional X-band ground stations (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, the time during which the ground station receives data from the satellite. Generally, 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 for miniaturization of the antenna for a ground station operating in a band of higher frequency than the X band, while maintaining equivalent performance compared to a ground station operating in the X band.

[0027] This allows the use of 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 3 m to 7 m diameter antennas used in current systems), and very easy to install. This makes it possible to increase the number of ground stations in the communication system, and thus 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] Using a relatively high minimum elevation angle (at least 15°) also offers greater flexibility during installation, as such an angle significantly reduces blocking problems caused by obstacles such as trees or buildings. Furthermore, this minimum elevation angle minimizes 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) significantly reduces 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 have shown that a communication system according to the invention allows for a reduction in the cost per gigabyte transmitted (i.e., the ratio between the data volume for coverage at 5° elevation by the infrastructure cost) by a factor of 6 to 12 (depending on the minimum minimum elevation angle used).

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

[0034] This ease of adding ground stations allows the use of 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 severely attenuated in the area where one of the ground stations is located, another ground station can compensate for it. Areas of intense rainfall, such as 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 significantly reduced.

[0035] It is noted that this addition of ground stations is made possible and feasible 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 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.

[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 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, particularly between 3 m and 6 m). Each ground station is thus configured so that the system performance remains similar to that of 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 is the area within which a ground station receives data transmitted by the satellite. To compensate for the reduced reception area (due to the increased minimum elevation angle), one solution is to use multiple ground stations. This is made possible by the significantly lower cost of a ground station according to the invention, as well as the ease of installation of 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 zone of reception 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, its associated reception area, and then, based on the reception areas obtained, to determine the number and location of ground stations required 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 (the "downlink" volume of the data).

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

[0049] In one embodiment, the reference reception area can be a geographical area (for example, the surface area 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 low-Earth orbit satellite. The system comprises a set of ground stations, each ground station configured to receive data transmitted by the low-Earth orbit satellite using a frequency above 12 GHz. Each ground station comprises an antenna with a diameter of 2 meters or less, and each ground station is associated with a minimum elevation angle of 15 degrees or greater. Each ground station receives data from the satellite only when the satellite's elevation angle relative to said ground station is within a certain range. 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 of the ground station;

[0055] - 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°;

[0056] - receive a set of values ​​for the elevation angle of the satellite relative to the ground station, in which each value in the set of values ​​for the elevation angle of the satellite 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] - 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 process" refers to a process 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 ground stations (for the antenna diameter) and for system operation (for the minimum elevation angle). In some embodiments, the minimum elevation angle and the antenna diameter are predetermined.

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

[0063] Attenuation values ​​depend on several factors (in addition to the geographical location) related to the data transmission link, including frequency and elevation angle. The first attenuation value here is a reference value, corresponding to X-band transmission with a reference elevation angle of 5°. The second set of attenuation values ​​are determined for a frequency in a band higher than X (the frequency band to be used for the new transmission system), and for different values ​​of the minimum elevation angle. The goal is to determine, from among the various tested minimum elevation angle values, an "optimal" minimum elevation angle (or antenna diameter, as these two characteristics are related).According to the method described above, 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. 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 larger 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 = 10 A ((G - AG - AA) / 20),

[0068] with G = 20xlogio(fy / fx), AG = 20 x log (da / ds) 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, ds 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 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;

[0072] - select, from the determined subset of values, a minimum value for 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 angle values elevation of the satellite 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 by 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] Figure 1a represents the elevation angle of a satellite relative to a ground station.

[0078] Figure 1b represents the minimum elevation angle of the satellite for which the ground station is configured to receive signals from the satellite.

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

[0080] Figure 2b represents a map of the difference between the attenuation values ​​for a ground station operating in Ka band at a frequency of 26 GHz and an elevation angle a of 5° and the attenuation values ​​for a ground station operating in X band at a frequency of 8.2 GHz and an elevation angle a 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] Figure 2c represents a map of the miniaturization factor, that is, 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 a of 5°.

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

[0083] Figure 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 compared to an elevation angle of 5° due to the decrease in free space attenuation (FSL).

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

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

[0086] Figure 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] Figure 8 represents an example of a communication system according to the embodiment of Figure 7.

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

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

[0091] Figure 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] As is known and shown in Figure 1 a, it is recalled that the elevation angle of a satellite 1 relative 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, representing 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 typically configured to transmit data when its elevation angle relative to the ground station exceeds this 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 that the satellite transmits its data without first establishing a communication link with a ground station.The ground station, for its part, receives the data emitted by the satellite when it is clearly visible from the ground station.

[0094] The minimum elevation angle amin of the satellite is shown in Figure 1b. More precisely, Figure 1b illustrates the trajectory of a satellite 1 passing over a ground station 3 over time. The satellite's elevation angle relative to ground station 3 increases 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 of 90° corresponds 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 Figure 1b that the lower the value of the minimum elevation angle amin, the higher the contact time, i.e. the time interval [ti ; tz] during which a communication link is established between the ground station 3 and the low 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 angle The minimum elevation 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, such as the Ka-band, can be much smaller (some with antenna diameters less than one meter), and thus represent a significantly 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 as attenuation increases, and it increases as 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 (including 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 band X (i.e. a frequency band whose lower bound is higher than the upper bound of band X, for example, above 12 GHz) is equal to:

[0103] SNRY = SNRx + 20xlogio(f Y / 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] - a gain of 20xlogio(fY / fx) at the satellite antenna;

[0109] - a loss of 20xlog (fY / fx) due to free space loss (FSL); and

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

[0111] In the following, the calculations are carried out for a Ka band, but a similar reasoning applies to other frequency bands higher than 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 20xlog(26 / 8.2) dB ≈ 10 dB.

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

[0114] SNR Ka= 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] SNRKS = SNRx + 10 dB - AA (3).

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

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

[0119] MF = 10 A ((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 satellite's elevation angle: the lower the elevation angle, the higher the attenuation. Thus, in the preceding equations, the values ​​AKa, A y , Ax and AA are defined for a given angle of elevation.

[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 Figure 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 great 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 Figure 2c can be obtained from:

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

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

[0127] Such attenuation maps can be generated from a meteorological model. Figure 2a shows 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. Such models are well-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 X band to obtain an AA attenuation difference map such as that shown in Figure 2b.

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

[0130] The mapping of Ka-band attenuation values ​​(Figure 2a) shows that attenuation values ​​are generally higher in the intertropical zone, that is, the area between the Tropic of Cancer and the Tropic of Capricorn, and particularly near the equator. This is mainly due to the significant presence of water vapor in these areas.

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

[0132] Due to this significant difference in attenuation, it appears in Figure 2c that the regions where 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 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 was possible to miniaturize the diameter of the ground station antenna in the white area of ​​the map in Figure 2c;

[0135] - if it was possible to further miniaturize the diameter of the ground station antenna in locations outside the white area of ​​the mapping in Figure 2c.

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

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

[0138] - the distance between the satellite and the ground station decreases as 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 the X-band and the Ka-band. If the elevation angle is not the same, the distance between the satellite and the ground station is also not the same, which has an impact on free-space path loss, as detailed below.

[0140] The first property is illustrated in Figure 3, which shows curve 5 of Ka-band attenuation values ​​in clear weather in a given geographical area (here, the city of Dublin) and curve 7 of X-band attenuation values ​​in clear weather for the same geographical area. Curve 6 represents the satellite's 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 values ​​is significant. (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 Figure 4. Curve 8 in Figure 4 represents the distance (in km) from the satellite to the ground station as a function of the elevation angle a. Curve 9 in Figure 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 path loss decreases with the square of the distance from the satellite to the ground station. Therefore, we have: AG = 20 x log (da / ds), where ds 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 α.

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

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

[0146] with AG = 20 x log (da / ds).

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

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

[0149] with G = 20xlogio(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] Figure 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 (except the wettest regions). The miniaturization factor is greater than 3 in most regions, and even greater than 4 in the driest. This means that in most regions, a standard 4-meter diameter antenna operating in the X-band can be replaced, with an equivalent signal-to-noise ratio, by an antenna less than 1.5 meters in diameter, or even less than 1 meter, operating in the Ka-band.

[0157] Figure 5b represents a map 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 (except the wettest regions). The miniaturization factor is greater than 5 in most regions, and even greater than 6 in the driest. This means that in most regions, a standard 4-meter diameter antenna operating in the X-band can be replaced, with 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-orbit satellite to a plurality of ground stations, each configured to receive signals emitted by the low-orbit satellite using frequencies in a band above the X-band, for example the Ka-band, at elevation angles less than or equal to 15°. More precisely, each ground station is associated with a respective minimum elevation angle and receives signals when the satellite's elevation angle 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 of at least 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 for 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, 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 (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 antenna diameter can be determined based on the geographical location where it is planned to be installed, according to the method shown in Figure 6. The method in Figure 6 is implemented for one ground station. When there are multiple ground stations, the method in Figure 6 can be implemented, successively or in parallel, for each ground station among the plurality of ground stations.

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

[0165] A rain attenuation value, referred to here 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 shown in the attenuation map in Figure 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 can include 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 ​​can include two values: 15° and 30°.

[0167] During step 630, a rain attenuation value, referred to here as the second rain attenuation value, associated with the geographic 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 weather model, as described above with reference to Figure 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 angle elevation, 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 the goal is 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:

[0173] Assuming that these values ​​were obtained relative 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 for miniaturization of the ground station antenna, while an elevation angle of 30° allows a miniaturization factor of 2, which means 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 a miniaturization factor of 2 (corresponding to a 2 m antenna), while an elevation angle of 30° allows a miniaturization factor of 4 (corresponding 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 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 elevation angle for which the miniaturization factor is maximal;

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

[0180] - If several miniaturization factors are greater than 1, determine whether, for one or more elevation angles, the equivalent antenna diameter (i.e., the antenna diameter obtained by multiplying the diameter of the reference ground station antenna by the obtained miniaturization factor) is less than a predetermined threshold - or, equivalently, whether 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 diameters If equivalent antennas 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 under consideration.

[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 in Figure 7 comprises a plurality of ground stations 10a, 10b, 10c, 10d, 10e, 10f, 10g. Each ground station 10a, 10b, 10c, 10d, 10e, 10f, 10g comprises an antenna with a diameter of 2 meters or less, 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, 10d, 10e, 10f, 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, 10d, 10e, 10f, 10g. For example, the minimum elevation angle of each ground station 10a, 10b, 10c, 10d, 10e, 10f, 10g may have been determined according to the method shown in Figure 6.

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

[0186] Each ground station 10a, 10b, 10c, 10d, 10e, 10f, 10g is associated with a respective reception zone 20a, 20b, 20c, 20d, 20e, 20f, 20g. The term "reception zone" (or "visibility zone") refers to an area encompassing all the Portions of orbits for which the ground station can receive signals transmitted from low-Earth orbit satellites, when these satellites are located within these portions. The outer boundary of this zone can 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 a satellite's trajectory in a particular orbit shown in Figure 1b corresponds to a diameter of the outer boundary (i.e., a segment joining two points on the outer boundary, the two points being symmetrical with respect to the center of the outer boundary) of the receiving zone. In other words, as soon as the satellite passes within this receiving zone, 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 larger the reception area, as shown in Figure 1b) and the diameter of the ground station antenna (the larger the diameter of the ground station antenna, the larger the reception area).

[0188] Figure 7 also depicts a ground station 100 (here 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, 10d, 10e, 10f, and 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, 10d, 10e, 10f, 10g of the communication system according to the invention.

[0190] In one or more embodiments, the plurality of ground stations 10a, 10b, 10c, 10d, 10e, 10f, 10g can be arranged so that the union of the receiving zones 20a, 20b, 20c, 20d, 20e, 20f, 20g of the ground stations 10a, 10b, 10c, 10d, 10e, 10f, 10g of the communication system according to the invention covers "in large "Part" refers to the reference reception area 200. "Largely" means that the union of reception areas 20a, 20b, 20c, 20d, 20e, 20f, and 20g covers at least a predetermined percentage of the reference reception area 200. This predetermined percentage could be, for example, 80% or greater. It should be noted that the union of two surfaces corresponds to a set containing all the elements belonging to either of the two surfaces.

[0191] In particular embodiments, the plurality of ground stations 10a, 10b, 10c, 10d, 10e, 10f, 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, 10d, 10e, 10f, 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" relative to the reference reception area 200) by the plurality of ground stations 10a, 10b, 10c, 10d, 10e, 10f, 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, 10d, 10e, 10f, 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, 10d, 10e, 10f, 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 in Figure 7, the communication system includes 7 ground stations, but of course, a communication system according to the invention can include 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 shows a specific example of the communication system in 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, and Linz, Netherlands). Austria, kyiv in Ukraine, Athens in Greece, Tabarka in Tunisia, Albufeira in Portugal and Brest in France). For the sake of readability, the references of the ground stations and reception areas are not indicated on Figure 8 - the ground stations are represented by solid-lined stars and the reception areas by solid-lined ellipses, as in Figure 7.

[0196] In this example, all ground stations 10a, 10b, 10c, 10d, 10e, 10f, 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] Figure 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 line "thin" represent the orbit 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 31 a, 31 b, 31 c, 31 d, 31 e, 31 f) represent the orbital portions 30a, 30b, 30c, 30d, 30e, 30f outside any reception zone 20a, 20b, 20c, 20d, 20e, 20f, 20g but inside the reference reception zone 200;

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

[0203] The portions shown in "thick" solid lines (portions 31a, 31b, 31c, 31d, 31e, 31f) 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, 10f, 10g does not cover the reference reception area 200 of the reference ground station 100. It can be seen in Figure 8 that these portions are very reduced compared to the reference reception area 200. Thus, it is possible to replace a large ground station quite effectively with 7 small ground stations - and this despite 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 some ground stations (particularly for ground stations located in low-humidity regions). Adding one or more additional ground stations is easily accomplished, notably due to the small size of the antennas used in the ground stations, 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, 10d, 10e, 10f, 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, 10d, 10e, 10f, 10g.

[0206] In one embodiment, all ground stations located in a given geographical area can send the received data to a single processing device, which could be, for example, a remote computer server (particularly a cloud-based one). For instance, each ground station could include an antenna and a digitizing device and could be configured to send the digitized spectrum of the data received from the satellite to the processing device. The processing device can centrally process all the data received from the ground stations (i.e., the digitized data spectra). 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 involving multiple stations, where the same message may be received by several stations, in whole or in part.

[0207] An example of such a treatment device is shown in Figure 10.

[0208] The processing device 700 of Figure 10 includes a computer with 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 board, 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 the 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 being configured to operate in a frequency band higher than the X band (in particular with frequencies higher than 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. This handover allows the connection to switch 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 Figure 9a. In this Figure 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 and 10b delimit the minimum elevation angle at which either station ground 10a, 10b receives data from satellite 50a, the dashed segments represent the direction of the beam - fixed - of 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] Within the framework 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 with an antenna whose beam orientation can be controlled (hereafter called a "beam steering antenna"), for example a flat electronically oriented antenna, as shown in Figure 9b.

[0217] In Figure 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 Figure 9b, with such a steerable beam antenna, satellite 50b constantly points towards the nadir, and the beam orientation is electronically controlled, so that satellite 50b does not need to perform any turning maneuvers during the cellular handover. Thus, the transition 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 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

35 DEMANDS

1. A system for receiving Earth observation data transmitted from a low-Earth orbit satellite, the system comprising an array of ground stations (10a, 10b, 10c, 10d, 10e, 10f, 10g), wherein each ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g) is configured to receive data transmitted by the low-Earth orbit satellite using a frequency greater than 12 GHz, wherein each ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g) comprises an antenna of a respective diameter less than or equal to 2 meters, wherein each ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g) is associated with an angle of a predefined minimum elevation greater than or equal to 15 degrees, in which each ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g) receives data from the satellite only when an elevation angle of the satellite relative to said ground station (10a, 10b, 10c, 10d, 10e, 10f,10g) is greater than or equal to the minimum elevation angle associated with the ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g).

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 (10a, 10b, 10c, 10d, 10e, 10f, 10g) 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 (10a, 10b, 10c, 10d, 10e, 10f, 10g) 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 (10a, 10b, 10c, 10d, 10e, 10f, 10g), 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 (10a, 10b, 10c, 10d, 10e, 10f, 10g) is at least equivalent to a signal-to-noise ratio associated with data transmission from the satellite to a reference ground station (100) 36 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 (10a, 10b, 10c, 10d, 10e, 10f, 10g) comprises a plurality of ground stations (10a, 10b, 10c, 10d, 10e, 10f, 10g).

7. System according to the preceding claim, wherein, for each ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g) among the plurality of ground stations, the minimum elevation angle associated with the ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g) defines a receiving area (20a, 20b, 20c, 20d, 20e, 20f, 20g) 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 (200).

8. Earth observation data transmission architecture comprising a low-orbit Earth observation satellite (50a, 50b) and a communication system according to any one of the preceding claims.

9. Architecture according to claim 8, wherein the satellite (50a, 50b) comprises a steerable beam antenna.

10. A method for configuring a system for receiving Earth observation data transmitted from a satellite in low Earth orbit, the system comprising an array of ground stations (10a, 10b, 10c, 10d, 10e, 10f, 10g), each ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g) being configured to receive data transmitted by the satellite in low Earth orbit using a frequency greater than 12 GHz, each ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g) comprising an antenna with a diameter less than or equal to 2 meters, each ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g) being associated with an elevation angle respective minimum of 15 degrees or greater, in which each ground station (10a, 10b, 10c, 10d, 10e, 10f, 10g) receives data from the satellite 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 a ground station among the set of ground stations: receiving (610) a geographical position of the ground station; - receive (620) 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 ​​for the elevation angle of the satellite relative to the ground station, in which each value in the set of values ​​for the elevation angle of the satellite relative to the ground station is greater than or equal to 15°; - for each value among the set of values ​​of the elevation angle of the satellite relative to the ground station: o receive (630) a second value of attenuation due to rain for the geographical position received, 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;to calculate (640), 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 (650), from the set of 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 elevation angle of the satellite relative to the ground station. [Claim 1 1] A method according to the preceding claim, wherein, for a value a 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, the miniaturization factor MF is: MF = 10 A ((G - AG - AA) / 20), with G = 20xlogio(fy / fx), AG = 20 x log (da / ds) and AA = AY - Ax; 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, ds 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. [Claim ^] A method according to claim 10 or 11, wherein the determination (650), 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.

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