EP2731193B1

EP2731193B1 - Space borne antenna system

Info

Publication number: EP2731193B1
Application number: EP12007610.4A
Authority: EP
Inventors: Thomas Fügen; Michael VÖLKER; Rainer Klein; Jean-Christophe Angevain
Original assignee: Airbus Defence and Space GmbH
Current assignee: Airbus Defence and Space GmbH
Priority date: 2012-11-08
Filing date: 2012-11-08
Publication date: 2019-07-24
Anticipated expiration: 2032-11-08
Also published as: CA2829633C; EP2731193A1; CA2829633A1

Description

The invention relates to a space borne antenna system. In particular, the invention relates to a multiple antenna system for space borne applications like radar, synthetic aperture radar (SAR), interferometric SAR or moving target indication (MTI).
Known antenna systems, for example an interferometric SAR instrument, often comprise two or more radiating apertures (i.e. antenna arrangements) which are mounted on a single platform, e.g. a spacecraft or a satellite, with a certain spatial sensor separation. A large separation, i.e. baseline, between the radiating apertures is reached by mounting at least one of the antennas on the tip of a boom like structure. The boom like structure (e.g. arm or mast) may be fixed or deployable. According to the partly high weight of the antenna mounted at the tip of the boom, the boom has to be adapted to have a sufficient, predetermined stiffness. However, this may result in a heavy and mechanically complex boom. Furthermore, the distance between platform and antenna may influence the performance of the antenna system.
US 6 424 314 B1 discloses a support for a deployable reflector for use on a modular satellite antenna assembly being constructed of an elongated boom supported at both ends by a pair of two axis actuators. The boom is attached at its inboard end to the satellite structure in close proximity to the point of attachment of the associated signal feed assembly to minimize the differential thermal stress throughout the antenna assembly.
Ludwig M et al: "Imaging Ka-band SAR interferometer" discloses a study into the feasibility of an interferometric Ka-Band SAR instrument with high resolution capabilities.
EP 1 119 072 A2 discloses a method and apparatus for producing contiguous spot beam communications coverage on the Earth's surface. The apparatus comprises an antenna system including two wide scan antennas and two narrow scan antennas. The two wide scan antennas are disposed substantially opposite each other, and the two narrow scan antennas are disposed substantially opposite each other and substantially normal to the wide scan antennas. The first wide scan antenna, second wide scan antenna, and first narrow scan antenna produce a first beam pattern on a planetary surface and the first wide scan antenna, second wide scan antenna, and second narrow scan antenna produce a second beam pattern on the planetary surface.
D.M. Simpson: "The Snapdragon Family" discloses spacecraft configuration called "Snapdragon" being a repackaging of existing hardware on a new structure.
It is therefore an object of the present invention to provide a space borne antenna system which is structural and/or functional improved.
This object is solved by an antenna system according to the features of claim 1. Preferred embodiments are set out in the dependent claims.
The invention is based on the consideration that separation of the feed arrangement and the radiation element enables a more performing antenna system. The reason for the better performance is the fact that no cables between the platform and the tip of the boom have to be installed. In known antenna systems, for the exchange of control commandos or RF signals between the platform and an antenna being arranged on the tip of the boom, the signals have to be routed along the boom by cables or waveguides which is complex. RF signal quality suffers from losses due to cable or waveguide length. Furthermore, in contrast to the known antenna systems, no calibration effort and thermal control of the antennas remote from the platform is necessary.
In the antenna system according to the invention, the first radiating element may be adapted to receive electromagnetic waves from the first feed arrangement and to redirect them in a different direction. Hence this first antenna arrangement represents a transmit (Tx) antenna. Alternatively, the first radiating element may be adapted to receive electromagnetic waves and to redirect them in a different direction to the first feed arrangement. In this case the first antenna arrangement is adapted to be a receive (Rx) antenna. Furthermore, the first radiating element may be adapted to receive electromagnetic waves from the first feed arrangement and to redirect them in a different direction and to receive electromagnetic waves from the different direction and to redirect them to the first feed arrangement. Hence, this first antenna arrangement represents a receive and transmit (Rx/Tx) antenna.
The first radiating element can comprise a flat reflector, such as a reflecting plane, or a reflector or a reflect-array.
The radiating elements can be passive elements having a predetermined shape and reflecting properties according to a desired signal beam deflection.
The first feed arrangement comprises a first secondary antenna that can comprise at least one reflector antenna or reflect-array antenna or antenna elements.
The first secondary antenna system can comprise a feed system and one or more reflectors or one and more reflect-arrays. The feed system may comprise one or more antenna elements.
In other words, according to this invention the first feed arrangement is realized as focusing secondary antenna for the first radiating element. It may comprise of at least one ore more reflector antennas or reflect-array antennas. Likewise, the first feed arrangement may comprise at least one reflector antenna system, comprising or consisting of at least one or more feed elements and a one ore more reflectors. The first feed arrangement may alternatively comprise at least one reflect-array antenna system, comprising or consisting of one or more feed elements and one or more reflect-arrays. Hence, electromagnetic waves may be deflected by or within the feed arrangement. In such an antenna system, multiple antenna elements in the feed system may be used for beam squint in any direction. Likewise, one or more first secondary antennas may be used for beam squint in any direction.
An example for the first secondary antenna with more than one reflector comprises a cassegrain like secondary (hyperbolic) reflector which is illuminated with at least one focusing (parabolic) ternary reflector which is illuminated by a feed system with one or more antenna elements.
The first boom is a deployable or extractable structure (e.g. a mast or telescopic arm) with arbitrary shape and configuration. The shape and the configuration of the boom may be arbitrary, since it is not essential for the current invention as long as there is a baseline, i.e. distance, between the first feed arrangement and a corresponding first radiating element.
A further antenna or antenna system (in other words: an antenna arrangement) is arranged on or close to the platform, wherein the antenna or antenna system is adapted for reception and/or transmission of electromagnetic waves. As a result, the antenna system according to the invention comprises more than one antenna arrangements. For example, the further antenna arranged on or close to the platform may be used as a receive antenna, while the first antenna arrangement which has a respective first radiating element mounted at the tip of the first boom is a transmit antenna. The antenna system according to the invention further comprises a second antenna arrangement with the same features as the first antenna arrangement. In case the first and the second antenna arrangements are operating as transmit antennas, the antenna system may be used as an interferometric SAR antenna system.
An advantage of the antenna system according to the invention is that the radiating elements can be realized passive, i.e. no RF signals need to be routed along the boom(s) being connected to the platform. The weight of the passive radiating element at the end of the boom is low. This approach avoids laying RF waveguides along the deployable boom, and minimizes calibration efforts and electrical losses in these waveguides.
A further advantage of the proposed antenna system is that the feed system and its assigned electronics are mounted in close vicinity of the platform or directly into the platform where they are under tight thermal control.
The principle advantage is that the proposed feed system allows architectures with very long boom (boom length >> diameter of reflecting element at tip of boom) as the first secondary antenna illuminates the first radiating element.
All described configurations might be used for transmit (Tx) antennas and/or receive (Rx) antennas.
The invention will be explained in more detail with reference to the accompanying figures.

Fig. 1 shows a first example of a known antenna system in which a radiating element is carried out as a flat reflector, such as a reflecting plane, or a reflect-array.
Fig. 2 shows a second example of a known antenna system in which the radiating element is carried out as a curved reflector.
Fig. 3 shows a third example of an antenna system useful for understanding the invention, in which a feed arrangement is carried out as a secondary antenna acting as focusing element.
Fig. 4 shows a fourth example of an antenna system useful for understanding the invention, comprising two antenna arrangements being mounted at respective tips of two booms that are arranged on opposite sides of a platform of the antenna system.
Fig. 5 shows an embodiment of an antenna system according to the invention, comprising two antenna arrangements being mounted at the tips of two booms and an additional third antenna being arranged on or close to the platform.

Fig. 1 shows a first example of a known space borne antenna system. The antenna system comprises a platform 10, a boom 20, a feed system 30 and a passive radiating element 40a. The platform 10 may be, for example, a satellite or a spacecraft. The boom 20 can be, for example, a mast or a telescopic arm. The boom 20 comprises a first end 21 and a second end 22. While the first end 21 is attached to the platform 10, the second end 22 which constitutes a tip of the boom 20 bears the radiating element 40a. This means, the radiating element 40a is mounted at the tip of the boom.
In the example of Fig. 1, the radiating element 40a is a flat reflector, such as a reflecting plane, or a reflect-array which typically have a surface which is within one plane. The feed system 30 is arranged on or close to the platform 10. The feed system 30 may comprise one or more antenna elements. The combination of feed system 30 and radiating element 40a is called an antenna arrangement. An electronics or control unit of the feed system 30 which is not shown in the figures can be mounted in close vicinity of the platform 10 or directly into the platform 10.
The feed system 30 and the radiating element 40a are arranged such that the radiating element 40a can receive electromagnetic waves from the feed system 30 and redirect them in a different direction (beam direction 50). As a result, the antenna arrangement constitutes a transmit (Tx) antenna.
Furthermore, the antenna arrangement could be realized such that electromagnetic waves received by the radiating element 40a may be directed to the feed system to constitute a receive (Rx) antenna. Likewise, the antenna system may be adapted to both receive and transmit electromagnetic waves, such that the antenna system constitutes a receive and transmit antenna.
Fig. 2 shows a second known example of a space borne antenna system which differs from the example of Fig. 1 only in that, that the passive radiating element is a reflector 40b which may have a concave surface. The reflector may be a parabolic reflector or of any other shape.
In the third example useful for understanding the invention and illustrated in Fig. 3 the feed arrangement is a secondary antenna 44 consisting of a reflector 43 and a feed system 30 directed to the reflector or reflect-array. The feed system 30 may comprise one or more antenna elements. The reflector 43 may be realized alternatively as reflect-array. In this antenna system, the reflector 40b (or alternatively flat reflector, e.g. reflecting plane or reflect-array) at the tip of the boom 20 constitutes a primary antenna and the antenna 44 a secondary antenna. For transmission, the primary antenna 42 on the tip of the boom receives electromagnetic waves from the secondary antenna 44 and forwards them in the direction of intention (beam direction 50). For reception, the primary antenna element 42 receives electromagnetic waves from the beam direction 50 and directs them in the direction of the secondary antenna 44.
Providing a feed arrangement on or close to the platform 10 and the first end 21 of the boom, respectively, and a radiating element at the second end 22 of the boom 20 ensures that no RF (radio frequency) signals need to be routed along the boom via cables or waveguides. This approach avoids laying RF waveguides along the fixed or deployable boom, and minimizes calibration efforts and electrical losses in these waveguides.
An antenna system according to a fourth example illustrated in Fig. 4 comprises two antenna arrangements. The first antenna arrangement comprises a first boom 20-1, a first passive radiating element 40b-1 (constituting a first primary antenna 42-1) at the tip of the first boom 20-1 and a first feed arrangement (i.e. first secondary antenna 44-1) close to or arranged in the platform 10. The second antenna arrangement comprises a second boom 20-2, a second passive radiating element 40b-2 (constituting a second primary antenna 42-2) at the tip of the second boom 20-2 and a second feed arrangement (i.e. a second secondary antenna 44-2) close to or arranged in the platform 10. The first and second booms 20-1 and 20-2 are arranged on opposite sides of the platform 10. A baseline (distance) between the two radiators 40b-1 and 40b-2 is depicted with 60. Both feed arrangements (first secondary antenna 44-1 and second secondary antenna 44-2) consist of a reflecting element 43-1 and 43-2 as well as a feed system 30-1 and 30-2.
In this antenna system, one of the antenna arrangements may be used as transmit (Tx) antenna while the other antenna arrangement may be used as receive (Rx) antenna. Alternatively, at least one of the antenna arrangements might be adapted to be a receive and transmit antenna.
The embodiment of an antenna system according to the invention (Fig. 5) is based on the fourth example illustrated in Fig. 4. In addition, a further antenna 70 is mounted on or in close vicinity to the platform 10. The antenna 70 comprises a reflector or reflect-array 72 and a feed system 71. While the further antenna 70 is adapted to receive electromagnetic waves (receive Rx antenna), the two other antenna arrangements are configured to transmit electromagnetic waves (transmit Tx antennas). Alternatively, receive and transmit direction might be interchanged between the antenna arrangements.
An antenna system as illustrated in Figs. 4 and 5 may be used as across track and/or along track interferometric SAR.

LIST OF REFERENCE SIGNS

10: platform
20: boom
20-1: first boom
20-2: second boom
21: first end of boom
22: second end of boom
30: feed system
30-1: first feed system
30-2: second feed system
40: radiating element
40a: reflecting plane / reflect-array
40b: reflector
40b-1: first reflector
40b-2: second reflector
42: primary antenna
42-1: first primary antenna
42-2: second primary antenna
43: reflector / reflecting plane / reflect-array
43-1: first reflector / reflecting plane / reflect-array
43-2: second reflector / reflecting plane / reflect-array
44: secondary antenna
44-1: first secondary antenna
44-2: second secondary antenna
50: beam direction
60: geometric baseline
70: antenna or antenna system
71: feed system
72: reflector / reflect-array
Rx: receive beam direction
Tx: transmit beam direction

Claims

A space borne antenna system, comprising
- a platform (10) having a first boom (20-1) and a second boom (20-2), wherein the first boom (20-1) extends from a first side of the platform (10) in a first direction and the second boom (20-2) extends from a second side of the platform (10) in a second direction, wherein the first and second sides are opposite sides of the platform (10) and the first direction is opposite to the second direction;

- a first antenna arrangement comprising following elements:
- a first feed arrangement (30-1) arranged on or close to the platform (10), wherein the first feed arrangement (30-1) comprises a first secondary antenna (43-1), and

- a first radiating element (40b-1) being mounted at a tip of the first boom, wherein the tip of the first boom is remote to the platform (10), wherein the first boom comprises a first end and a second end, wherein the first end of the first boom is attached to the platform (10) and the second end of the first boom is the tip of the first boom, wherein the first radiating element (40b-1) is adapted to receive electromagnetic waves from the first feed arrangement (30-1) and to redirect it in a different direction and/or vice versa, wherein said different direction is a main radiation direction of the first radiating element (40b-1);

- a second antenna arrangement comprising following elements:
- a second feed arrangement (30-2) arranged on or close to the platform (10), wherein the second feed arrangement (30-2) comprises a second secondary antenna (43-2), and

- a second radiating element (40b-2) being mounted at a tip of the second boom, wherein the tip of the second boom is remote to the platform (10), wherein the second boom comprises a first end and a second end, wherein the first end of the second boom is attached to the platform (10) and the second end of the second boom is the tip of the second boom, wherein the second radiating element (40b-2) is adapted to receive electromagnetic waves from the second feed arrangement (30-2) and to redirect it in a different direction and/or vice versa, wherein said different direction is a main radiation direction of the second radiating element (40b-2); and

- a further antenna (70) arranged on the platform (10) and adapted for reception and/or transmission of electromagnetic waves in a main radiation direction, wherein the first and second booms (20-1, 20-2) are deployable or extractable structures, and wherein a boom length of the first boom (20-1) is longer than a diameter of the first radiating element (40b-1), and wherein a boom length of the second boom (20-2) is longer than a diameter of the second radiating element (40b-2),
wherein the main radiating directions of the first and second radiating elements correspond to the main radiating direction of the further antenna (70),
wherein, in a first alternative, the further antenna (70) is adapted to receive electromagnetic waves, while the first and second antenna arrangements are adapted to transmit electromagnetic waves, or
wherein, in a second alternative, the further antenna (70), the first and the second antenna arrangements are adapted such that the receive and transmit directions are interchanged with respect to the first alternative.
The antenna system according to claim 1, wherein each of the first and second radiating elements (40b-1, 40b-2) comprises a reflecting plane or a reflector or a reflect-array.
The antenna system according to claim 1 or 2, wherein each of the first and second secondary antennas (44-1, 44-2) comprises a reflector antenna or a reflect-array antenna or antenna elements.
The antenna system according to any one of the foregoing claims, wherein the first feed arrangement (30-1) is arranged on or close to the first side of the platform (10), and wherein the second feed arrangement (30-2) is arranged on or close to the second side of the platform (10).
The antenna system according to any one of the foregoing claims, wherein the further antenna (70) is arranged on a third side of the platform (10) different from the first and second sides of the platform (10).
The antenna system according to any one of the foregoing claims, wherein the space born antenna system is synthetic aperture radar.