Navigation satellite, especially for a medium earth orbit

Abstract

Navigation satellite, especially for a Medium Earth Orbit (MEO), with - a case (12), - a navigation antenna (16) connected to the housing (12) for radiating navigation signals in a preferred direction, - an optical communication device (22) for unidirectional or bidirectional signaling with at least one other satellite leading in the direction of movement in the direction of a first communication axis (19) and / or with at least one other satellite trailing in the direction of movement in the direction of a second communication axis (21) and - a holder which is rotatably arranged on the housing (12) about an axis of rotation (17), wherein the axis of rotation (17) is substantially perpendicular to the first communication axis (19) and the second communication axis (21), wherein the holder is directly rotatable about this axis of rotation (17), - wherein the optical communication device (22) is arranged on the holder.

Description

[0001] The invention relates to a navigation satellite, preferably intended for a Medium Earth Orbit (MEO).

[0002] Optical links between navigation satellites allow for highly accurate clock synchronization (so-called composite clock), precise distance measurement, and the transmission of important data without external support. To enable this, the navigation satellites must be equipped with optical transmitting and receiving devices.

[0003] Maintaining the orientation of a navigation satellite in Medium Earth Orbit (MEO) presents a unique challenge. The orientation is determined by the fact that the navigation antenna must always point towards the Earth's center (nadir pointing), and the solar arrays must always be aligned with the sun. The first requirement defines the orientation except for a rotation around the nadir axis, i.e., the line connecting the centers of gravity of the Earth and the satellite. Two further axes are needed to align the solar arrays. One is the nadir axis, and the other is the axis on which the arrays are mounted and around which they can rotate. Since the satellite has a cold and a warm side, it is also rotated 180 degrees around the nadir axis near its closest and farthest points from the sun. The first rotation is called the "noon turn," and the second the "midnight turn."

[0004] The state of the art consists of more or less directional optical transmit and receive systems, e.g., TESAT LCT (see, e.g., BENZI, E. [et al.]: Optical Inter-Satellite Communication: the Alphasat and Sentinel-1A in-orbit experience. In: Proc. SpaceOps 2016 Conference, May 16-20, 2016, Daejeon, Korea, (AlAA 2016-2389). AlAA [online], DOL: 10.2514 / 6.2016-2389 [accessed June 14, 2018]). These are, firstly, complex to implement and, secondly, require a variable geometry between the optical systems and the phase center of, for example, the L-band antenna. The navigation antenna of current navigation satellites is rigidly connected to the satellite structure, i.e., the housing (also called the satellite bus), so that rotation of the satellite around its vertical axis also affects the antenna's orientation. Rotation of the navigation antenna and its radiation pattern is meant.

[0005] US-A-2017 / 0005719 describes a constellation of numerous satellites for communication with each other and with ground stations.

[0006] US-A-2004 / 0179847 concerns a satellite with a mirror optic for communication with other satellites.

[0007] EP 0876 013 concerns free-space optical transmission systems for uninterrupted links between individual satellites of globally accessible low-Earth orbiting satellite communication systems. For this purpose, four miniaturized optical terminals are geometrically arranged on the satellite body such that their azimuth axes intersect a reference plane perpendicular to the flight direction normal at four points, forming a quadrilateral. The initial orientations of these four optical terminals are oriented parallel to the flight direction tangent. Furthermore, the initial orientations of any two of these four optical terminals are directed with and against the satellite's flight direction tangent, respectively.

[0008] DE 21 65 282 concerns the design and operation of a high-performance communications satellite whose main body is located centrally within a large-area solar generator consisting of one or more sub-areas.

[0009] The object of the invention is to create a navigation satellite that, despite the two rotations of 180° each required during an orbit around the Earth, makes it possible to maintain an optical communication link to neighboring satellites in the same orbit.

[0010] To solve this problem, the invention proposes a navigation satellite, in particular for a Medium Earth Orbit (MEO), which is equipped with - a housing (i.e., a satellite structure or a satellite bus) - a navigation antenna connected to the housing for transmitting navigation signals in a preferred direction, - an optical communication device for unidirectional or bidirectional signaling with at least one other satellite leading in the direction of movement in the direction of a first communication axis and / or with at least one other satellite trailing in the direction of movement in the direction of a second communication axis and - a holder which is rotatably arranged on the housing around a pivot axis, - wherein the optical communication device is arranged on the holder.

[0011] The main feature of the navigation satellite according to the invention is a holder or carrier for an optical communication device, wherein the carrier or holder is rotatably arranged on / in the housing about an axis.

[0012] The term "satellite housing" refers to the satellite structure (without the payload). This is also known as the satellite bus. The payload would therefore be the cradle containing the optical communication device and the navigation antenna. The optical communication device typically comprises two optical communication units, each defining an optical communication axis. The communication axis of the first optical communication unit is directed, for example, towards a satellite that is ahead of the navigation satellite in its direction of travel, while the optical communication axis of the second optical communication unit points towards a satellite that is lagging behind.

[0013] This makes it possible to establish and maintain permanent optical connections to neighboring satellites in the same orbital plane.

[0014] In a preferred embodiment of the invention, the navigation antenna can serve as a holder for the optical communication device or be arranged on the holder and thus be rotatable. By simultaneously using the holder for the optical communication device as a navigation antenna, no additional support elements for the optical communication device are required in the satellite structure. Rather, the existing navigation antenna is used as a support or holder for the optical communication device and thus for the optical communication units.

[0015] The first and second optical communication units should advantageously be arranged in a pivotable and thus movable manner on the holder or on the navigation antenna, so that the navigation satellite in question can enter into communication with the neighboring satellite in the same orbit and maintain this communication link.

[0016] In a further advantageous embodiment of the invention, the navigation antenna can have a navigation signal emission side and a rear side facing away from the emission side and towards the housing, and the optical communication device can be arranged on the rear side of the navigation antenna. By arranging the optical communication device and its components on the rear side of the navigation antenna, its emission side can be designed and used as previously known.

[0017] As mentioned above, optical communication links between navigation satellites are advantageous. The optical communication device of a navigation satellite can send and / or receive signals for time transfer and time synchronization (keyword: composite clock), and / or for distance measurement and / or communication with at least one of the leading and / or at least one of the trailing satellites.

[0018] According to a further aspect of the invention, which can be implemented independently of the foregoing and without implementing the previously described features of the invention in a navigation satellite, the optical communication device comprises a third optical communication unit for unidirectional or bidirectional signaling with a satellite arranged in a lower orbit, in particular in a Low Earth Orbit (LEO). This measure makes it possible to establish and maintain a permanent optical connection between the navigation satellite in a first orbit and a satellite in a lower second orbit, despite the two 180° rotations required per orbit. The navigation satellite according to the invention is expediently designed for a Medium Earth Orbit (MEO), while the lower-flying satellite, which need not necessarily also be a navigation satellite, could, for example, be a satellite in a lower orbit.is located in a Low Earth Orbit (LEO).

[0019] It is advantageous if the additional optical communication unit is also arranged on the rotatable holder.

[0020] The third optical communication unit is also advantageously located on the rear side of the rotatable navigation antenna facing the satellite housing, i.e., the satellite structure, provided that the antenna serves as a mount for the optical communication device. This ensures that the optical axis defined by the third optical communication unit extends through the navigation antenna. It is advantageous for the third optical communication unit to be pivotably mounted on or within the navigation antenna, for example, by means of a ball joint in which the transmitter and / or receiver of the third optical communication unit is located. This allows the optical communication axis of the third optical communication unit to be aligned accordingly in order to communicate with a lower-flying satellite.

[0021] It is further advantageous if the additional optical communication device is arranged above the holder and its optical communication axis is directed through an opening in the navigation antenna (specifically at the phase center of the navigation antenna). The rotation axis of the navigation antenna is advantageously designed as a hollow axis. This hollow axis can then, for example, facilitate both the transmission of navigation signals from the satellite to the navigation antenna and the integration of the third optical communication unit for optical communication between, for example, the MEO navigation satellite and a LEO satellite.

[0022] The two main variants according to the invention can also be described by the following feature groups, wherein individual features of each feature group can also be combined with individual features of another feature group: 1. Navigation satellite, especially for a Medium Earth Orbit (MEO), with - a case, - a navigation antenna arranged in and / or on the housing with a radiation axis for transmitting navigation satellites in one radiation direction, - an optical communication device for unidirectional or bidirectional communication with at least one other satellite leading in the direction of movement and / or with at least one other satellite following in the direction of movement and - a holder which is rotatably arranged on the housing about an axis of rotation extending in the direction of radiation, - wherein the optical communication device is arranged on the holder. 2. Navigation satellite according to item 1, wherein the navigation antenna is designed as a plate-shaped holder for the optical communication device. 3. Navigation satellite according to paragraph 2, wherein the navigation antenna has a navigation signal emitting side and a rear side facing away from the navigation signal and towards the housing, and wherein the optical communication device is arranged on the rear side of the navigation antenna. 4. Navigation satellite according to any one of the numbers 1 to 3, wherein the optical communication device comprises a first and a second optical communication unit, each defining an optical communication axis which is oriented substantially orthogonal to the axis of rotation. 5. Navigation satellite according to paragraph 4, wherein the two optical communication units are arranged essentially diametrically opposite each other with respect to the axis of rotation of the navigation antenna or the holder. 6. Navigation satellite according to any one of the numbers 1 to 5, wherein the optical communication device has a third optical communication unit for unidirectional or bidirectional communication with a satellite arranged in a lower orbit, in particular in a Low Earth Orbit (LEO). 7. Navigation satellite according to paragraph 6, wherein the third optical communication unit defines an optical communication axis extending in the direction of radiation from the navigation antenna. 8. Navigation satellite according to paragraph 7, wherein the optical communication axis of the third optical communication unit extends through an opening in the navigation antenna or holder. 9. Navigation satellite according to paragraph 7 or 8, wherein the third optical communication unit is mounted on and / or in the housing or on and / or in the navigation antenna so as to be pivotable one- or two-dimensionally about the radiation axis of the navigation antenna. 10. Navigation satellite according to paragraph 8 or 9, wherein the opening is located at the phase center of the navigation antenna. 11. Navigation satellite named after one of the numbers 1 to 10, wherein the axis of rotation of the navigation antenna is designed as a hollow wave. 12. Navigation satellite, especially for a Medium Earth Orbit (MEO), with - a case, - a navigation antenna arranged in and / or on the housing with a radiation axis for radiating navigation signals in a preferred radiation direction and - an optical communication device for unidirectional or bidirectional communication with another satellite advancing in the direction of movement and / or with another satellite following in the direction of movement, - wherein the navigation antenna is rotatably arranged on the housing about an axis of rotation extending in line with the radiation axis and - the optical communication device is attached to the rotatable navigation antenna.

[0023] The invention is explained in more detail below with reference to two exemplary embodiments and the drawing. Specifically, the drawing shows: Fig. 1 a satellite subconstellation consisting of several MEO navigation satellites and several lower-flying LEO satellites, Fig. 2 a schematic representation of a navigation satellite according to an embodiment of the invention, wherein a part of the navigation satellite is shown enlarged, and Fig. 3 a second embodiment of a navigation satellite according to the invention, wherein a part of the satellite is also shown enlarged.

[0024] In the first embodiment of the invention, the optical transmitting and receiving systems are arranged on a rotatable support above the navigation antenna. The rotatable support creates an additional axis that allows the two optical systems to be kept aligned with the preceding or tracking satellite; they "look" past the navigation antenna, and the two optical beams and the nadir direction lie in the same plane. Only minor corrections, which can be made with a movable mirror, are necessary for this. Additionally, the angle in the orbital plane can be adjusted to another position to illuminate the satellite two satellites ahead, should the nearest one fail. The navigation antenna can be mounted on the support. This keeps the azimuth angle, measured around the nadir axis, fixed. In particular, the noon turn and the midnight turn do not affect the signal transmission of the navigation antenna.Of particular importance is the fixed relationship between the optical alignment and the antenna alignment that results from this arrangement.

[0025] Another embodiment of the invention relates to an optical transmitting and receiving system located above the navigation antenna, arranged such that the beam "looks" through a hole in the center of the antenna. It can be specifically aligned to one of several possible LEO satellite positions (approximately + / - 18 degrees). The optical axis intersects the nadir axis. Ideally, this occurs at the phase center of the navigation antenna.

[0026] Fig. Figure 1 shows a satellite subconstellation consisting of several MEO navigation satellites 10, 10', 10" and several LEO satellites 11. Each navigation satellite 10, 10', 10" comprises a satellite structure with, among other things, a housing 12 and solar generators 14 and is equipped with a rotatable navigation antenna 16 that maintains its orientation to the Earth. The navigation antenna 16 has a front radiating face 16' and a rear face 16". On the rear face 16" of the rotatable navigation antenna 16 (see the axis of rotation indicated at 17), optical communication units 18, 20 (with optical communication axes 19 and 21, respectively) of an optical communication device 22 are mounted, which ensure optical inter-satellite communication with the preceding and following satellites 10', 10".

[0027] Fig. Figure 2 shows a more detailed representation of the structure of the rotatable navigation antenna 16 together with the optical communication units 18, 20. The navigation antenna 16 is rotated, for example, at its phase center 24 about a hollow axis 26, which enables both the transmission of signals from the satellite to the navigation antenna 16 and the integration of another optical communication unit for communication between a MEO satellite 10, 10', 10" and LEO satellite 11.

[0028] The previously described additional third optical communication unit is located in the Fig. The exemplary embodiment of the MEO navigation satellite 10 shown in Figure 3 is shown. Insofar as the elements of the satellite are as shown in Figure 3, the following applies: Fig. 3 those of the satellite according to Fig. Accordingly, they are designated with the same reference numeral. The additional third optical communication unit 28, for example, is mounted above the navigation antenna 16 dh on its rear side 16". Its optical communication axis 30 passes through the phase center 24 of the navigation antenna 16. By using a compact design, it can be ensured that the required opening 32 in the navigation antenna 16 does not affect its radiation pattern and that an optical communication link to a lower-flying LEO satellite 11 can be established. Depending on the orbital geometry, the third optical communication unit only needs to cover a limited angular range and can therefore be integrated within the navigation antenna 16, as shown in Fig. 3 is shown. REFERENCE MARK LIST 10 navigation satellites 10' Navigation satellites 10" navigation satellites 11 LEO satellite 12 cases 14 solar generators 16 Navigation antenna 16' Radiating side of the navigation antenna 16" back of the navigation antenna 17 axis of rotation 18 first optical communication unit 19 optical communication axis 20 second optical communication unit 21 optical communication axis 22 Optical communication device 24-phase center 26 Hollow axle 28 third optical communication unit 30 Communication axis 32 Opening in the phase center

Claims

[1] Navigation satellite, especially for a Medium Earth Orbit (MEO), with - a case (12), - a navigation antenna (16) connected to the housing (12) for radiating navigation signals in a preferred direction, - an optical communication device (22) for unidirectional or bidirectional signaling with at least one other satellite leading in the direction of movement in the direction of a first communication axis (19) and / or with at least one other satellite trailing in the direction of movement in the direction of a second communication axis (21) and - a holder which is rotatably arranged on the housing (12) about an axis of rotation (17), wherein the axis of rotation (17) is substantially perpendicular to the first communication axis (19) and the second communication axis (21), wherein the holder is directly rotatable about this axis of rotation (17), - wherein the optical communication device (22) is arranged on the holder. [2] Navigation satellite according to claim 1, characterized by that the navigation antenna (16) serves as a holder for the optical communication device (22) or is arranged on the holder. [3] Navigation satellite according to claim 2, characterized by , that the navigation antenna (16) has a navigation signal emitting side (16') and a rear side (16") facing away from this, which is facing the housing (12), and that the optical communication device (22) is arranged on the rear side (16") of the navigation antenna (16). [4] Navigation satellite according to any one of claims 1 to 3, characterized by, that the optical communication device (22) sends signals to at least one of the leading satellites and / or to at least one of the trailing satellites and / or receives such signals from at least one of the leading satellites and / or from at least one of the trailing satellites for time transfer and time synchronization and / or for distance measurement and / or for communication. [5] Navigation satellite according to any one of claims 1 to 4, characterized by a further optical communication device (22) for unidirectional or bidirectional signaling with a satellite arranged in a lower orbit, in particular in a Low Earth Orbit (LEO), in the direction of a third communication axis (28). [6] Navigation satellite according to claim 5, characterized by , that the further optical communication device (28) is also arranged on the rotatable holder. [7] Navigation satellite according to claim 5 or 6 and claim 3, characterized by , that the further optical communication device (28) is arranged above the holder and is directed with its communication axis (28) through an opening in the navigation antenna (16) for uni- or bidirectional signaling. [8] Navigation satellite according to claim 7, characterized by , that the opening is located at the phase center of the navigation antenna (16).

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