Multi-static spatial system

Abstract

A multistatic space system provides a Formation of cooperative satellites (1, 11, 12, 13), deployed in a low Earth orbit, in which said Formation comprises at least three satellites (1, 11, 12, 13) logically and physically connected to one another by means of first cables (8, 9 10). The first cables (8, 9, 10) comprise unwound portions and wound portions and allow the exchange of signals and data between the satellites in the system. In the system, there is a main or Hub satellite (1 ) and at least two secondary or Companion satellites (11, 12, 13). Inside said first cables (8, 9, 10) the synchronization signals generated by the main satellite or Hub (1 ) are also transmitted. The synchronization signals are shared among all satellites (1, 11, 12,13) of the Formation to ensure a common time and phase / frequency reference. The secondary or Companion satellites (11, 12, 13) operate jointly and coherently due to their synchronization and the geometry of the whole system. System geometry is optimized before and during system operation according to the mission of the system by varying the length of the deployed portions of the first cables (8, 9, 10) in addition to the use of position and attitude control means. In the system, the reciprocal and absolute positions of the Formation of satellites (1,11,12,13) are monitored in real time with respect to the reference integral to the Earth. Furthermore, there is also high-precision geometric control, achieved by connecting the secondary or Companion satellites (11, 12, 13) of the Formation to one another by means of second cables (14, 15, 16), in which the second cables (14, 15, 16) comprise unwound portions and wound portions. The first cables (8, 9, 10) and second cables (14, 15, 16) create a mesh structure, which implements particularly accurate connection redundancy and control of the geometry and distribution of the masses of the Formation, including the masses of the unwound and wound portions of the first cables (8, 9, 10) and second cables (14, 15, 16). In particular, the means of position and attitude control include ionic motors and use centrifugal force to keep said first cables (8, 9, 10) and said second cables (14, 15, 16) taut.

Description

[0001] “MULTI-STATIC SPATIAL SYSTEM”

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of spatial systems for Earth observation and surveillance. In this context, radar techniques allow operation at any time and independent of solar illumination. Recently, several government agencies and private industries have been putting in orbit constellations of numerous and small artificial satellites.

[0004] In particular, the solution proposed herein uses a constellation, called a Formation of interconnected artificial satellites for Earth observation and surveillance, whose operation is programmed, coordinated and controlled to achieve specific objectives.

[0005] PRIOR ART

[0006] Some acronyms used in such a way that the description is easier to understand are given hereinafter.

[0007] • AESS: IEEE Aerospace and Electronic Systems Society,

[0008] • Bi-static: With a transmitting element and a receiving element;

[0009] • Companion: For a satellite system it is a non-main part or element of the system connected to the Hub',

[0010] • DEM: Digital Elevation Model;

[0011] • DLR: Deutsches Zentrum fur Luft- und Raumfahrt, the German Aerospace Authority;

[0012] • GNSS: Global Navigation Satellite System;

[0013] • GPS: Global Navigation System of the United States of America

[0014] • GPa: Giga Pascal;

[0015] • Hub: For a satellite system it is the center or main element of the system;

[0016] • Hub-and-Spoke: Literally, center and hub (of a wheel), that is, central and peripheral element of a connected system;

[0017] • IEEE: Institute of Electrical and Electronics Engineers;

[0018] • LED: Light Emitting Diode',

[0019] • LEO: Low Earth Orbit',

[0020] • MirrorSAR: Particular multi-static SAR system with satellites in Formation, proposed by the German Aerospace Center DLR;

[0021] • Multistatic: With multiple receiving and / or transmitting elements;

[0022] • MIMO: Multiple Input Multiple Output',

[0023] • MPa: Mega Pascal;

[0024] • NRT: Noise Radar Technology - radar technology providing the transmission of random signals;

[0025] • Pa: Pascal - pressure unit;

[0026] • PRF: Pulse repetition frequency (radar);

[0027] • SAR: Synthetic Aperture Radar;

[0028] • SIMO: Single-Input-Multiple-Output;

[0029] • Swath: Strip of ground illuminated by a synthetic antenna radar (SAR for imaging;

[0030] • TanDEM-X: TerraSAR-X twin satellite, a German Earth observation satellite using SAR technology; and

[0031] • Tethered (for a satellite system): Connected with cables or tethers.

[0032] In the prior art, it is known to provide satellite systems using radar technology, in particular in the so-called “synthetic antenna”, briefly SAR, embodiment, in which a group of satellites is enabled for transmission and a group of satellites is enabled for reception. Such solutions suffer from problems related to the synchronization of the various satellites forming the system.

[0033] In addition, systems have been proposed and used formed by two SAR satellites which transmit and receive alternatively. This type of solution also suffers from synchronization problems and also requires longer times for communications since the individual satellites transmit in turn.

[0034] Documents describing some of the most widely used known techniques and solutions in the field of space systems for Earth observation and surveillance are cited below.

[0035] For example, the SAR technique is explained, with particular attention to small satellites, in the document by Anthony Freeman: “Design Principles for Smallsat SARS”, Small satellites Conference, August 2018, Tuesday SSC18-V-01 , document available on DigitalCommons@USU - Small Satellites Conference: Design Principles for Smallsat SARS or on researchgate.net. The main SAR techniques such as polarimetry, interferometry and differential interferometry, as well as emerging techniques such as polarimetric SAR interferometry, tomography, holographic tomography, digital beamforming, MIMO Multiple-Input Multiple-Output systems, and bi-static and multi-static configurations, are presented, among others, in document A. Moreira, P. Prats-lrapola, M. Younis, G. Krieger, I. Hajnsek and K. P. Papathanassiou: “A tutorial on synthetic aperture radaF, in IEEE Geoscience and Remote Sensing Magazine, vol. 1 , no. 1 , pp. 6-43, March 2013, Doi: 10.1 109 / MGRS.2013.2248301 . This document is available on https: / / ieeexplore.ieee.org / document / 6504845.

[0036] However, the bi-static and multi-static spatial SAR presents some important design problems and considerable operational challenges such as phase / frequency and time synchronization, signal and data transfer and processing, disturbances on the orbits of the satellites with the need to detect and control the geometry of the Formation.

[0037] Further information on these systems is available in the document by Gerhard Krieger and Alberto Moreira: “Spaceborne Bi- and Multistatic SAR: Potential and Challenges" , IEE proceedings - Radar Sonar and Navigation 153 - August 2013: 184-198, DOI: 10.1 049 / ip-rsn:20045111 . Document available on the website https: / / www.researchgate.net / publication / 3357926_Spaceborne_bi- _and_multistatic_SAR_potential_and_challenges.

[0038] The progress of SAR systems tends to improve both the image resolution and the imaging area per unit of time, which is equivalent to enlarging the strip of the acquired earth / sea images (or “Swath”) in an image shot. Said requirements conflict because of the radar ambiguities resulting from the pulse operation mode, which is currently used in SARs. A solution to the conflict, which leads to systems called High Resolution-Wide Swath (HRWS), uses two (bi-static SAR) or more satellites (multi-static SAR) in close-up formation. For this subject, some references are:

[0039] 1 ) M. Rodriguez-Cassola, P. Prats-lrapola, G. Krieger, A. Reigber, and A.

[0040] Moreira: “Bistatic SAR Image Formation: A Systematic Approach", proceedings of the IEEE Conference IGARSS 2014, pp. 3945-3948, code: 978-1- 4799-5775-0 / 14 / $31 .00, document available on the website: https: / / www. researchgate. net / pub i ication / 263663084 B i stat i c SAR Image Formation A Systematic Approach;

[0041] 2) M. Rodriguez-Cassola, P. Prats, G. Krieger and A. Moreira: “Efficient Time-Domain Focusing for General Bistatic SAR Configurations: Bistatic Fast factored Backprojection" , 8th European Conference on Synthetic Aperture Radar, 2010, pp. 1 -4, Document available on the website: btiPg-ZZtggQxptorejeee.orq / yocummt^758912?amumber^5758912:

[0042] 3) Lopez Dekker, Pau Prats-lraola, Francesco De Zan, Daniel Schulze, Gerhard Krieger, Alberto Moreira: “Tandem X first DEM acquisition: A crossing orbit experiment’, IEEE Geoscience and Remote Sensing Letters 8(2011 -09-01 ): 943- 947-August 2013 DOI: 10.1109 / LGRS.2011 .2127444.

[0043] The bi-static / multi-static configuration can be used to:

[0044] - (i) implement the above-mentioned High Resolution-Wide Swath (HRWS) mode of operation with spacecraft close together along the orbit (i.e. along the track),

[0045] - (ii) measure the elevation of the soil, adding a third spatial dimension to the SAR images with a technique called interferometry, with the under cases of interferometry along the track and transverse to the track, and

[0046] - (iii) for detecting moving objects on the earth or sea surface for surveillance purposes.

[0047] The case (i) has already been explained above; as regards the case (ii) a reference is the patent: US 6 552 678 B1 : “Satellite method for using radar interferometry to establish a digital terrain model".

[0048] The following documents are also to be considered as known art:

[0049] - M. Zink et al.: “TanDEM-X: A single-pass SAR interferometer for global DEM generation and demonstration of new SAR techniques", 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), 2015, pp. 2888-2891 , Doi: 10.1109 / IGARSS.2015.7326418,

[0050] - G. Krieger et al.: “Interferometric synthetic Aperture Radar (SAR) Missions Employing Formation Flying”, proceedings of the IEEE (June 2010):816 - 843, DOI: 10.1109 / JPROC.2009.2038948, document available on the website: https: / / www.researchgate.net / pubHcation / 224124596 interferometric Synthetic Aperture Radar SAR Missions Employing Formation Flying.

[0051] With regard to case (iii), three references follow on the description of techniques for the detection of mobile targets:

[0052] - S. V. Baumgartner and G. Krieger: “Dual-Platform Large Along-Track Baseline GMTI” in IEEE transactions on Geoscience and Remote Sensing, VOL 54, no. 3, pp. 1554-1574, March 2016, document available on the website: http: / / dx.dgLorg / 10.1109 / TGRS.2015.2483019;

[0053] - Christoph H. Gierull, Ishuwa Sikaneta, Delphine Cerutti-Maori: “Two-Step Detector for RAD ARSAT-2’s Experimental GMTI Mode” in IEEE transactions on Geoscience and Remote Sensing, January 2013, a document available on the web site: http: / / dx.doi.Org / 10.1109 / TGRS,2012.2201729.

[0054] - Patent application WO 2022056638 A1 : “System, method, and satellites for surveillance imaging and earth observation using synthetic aperture radar imaging”.

[0055] Within the field of the prior art, there are space systems which form a distributed array for radar surveillance (primary and / or secondary) of air traffic, as in the patent application US4843397 (A) “Distributed-array radar system including an array of interconnected elementary satellites", in the article by E. Brookner: “Derivation of a satellite radar architecture for air surveillance”, IEEE Eascon - 1983 (IEEE Cat. No. 0531 -6863 / 83), pp 465-475 and in the article by R.C. Hermiller, J.E. Belyea, P.G. Tomlinson: “Distributed Array Radar1’, IEEE transactions on Aerospace & Electronic Systems Vol. AES-19 No. 6 November 1983, pp. 831 -839. Also, to the known art belong MIMO systems for aerial surveillance from space, as in the patent application: US2020233080 (A1 ) “Smallsat Surveillance constellations using Mimo Radar1’.

[0056] A key problem in bi-static or multi-static spatial radars is that of synchronization, that is, of the real-time distribution of the time and phase / frequency references (necessary for the correct operation) to the various spatial vehicles involved.

[0057] A possible solution uses disciplined oscillators of the Global Navigation Satellite System (GNSS).

[0058] Another solution is that of MirrorSAR which provides multi-static (and as a particular case, bi-static) synthetic aperture radar system (SAR) configurations using a transmitting satellite and one or more satellites, of reduced complexity and cost, which operate as “transponders”, i.e. as virtual “mirrors” which simply retransmit the radar echoes received by them. It should be noted, however, that an implicit assumption in the MirrorSAR solution is the extremely precise and accurate knowledge of the distances between the various satellites involved (the tolerable errors when operating in X band are of the order of the part per million).

[0059] This solution is described, among others, in document M. Zonno, G. Krieger, M. Rodriguez-Cassola, J. Mittermayer, and A. Moreira: “A MirrorSAR-based singlepass dual-baseline SAR interferometer for the generation of very high quality DEMS”, Proc. Of EUSAR 2018, Aachen, 2018, document available on the web site: https: / / ieeexplorejeeeLgrQ / doQument / 8438215.

[0060] One of the first works on the use of a wired spatial system, called “Tethered”, for bi-static SAR interferometry is described in A. Moccia and S. Vetrella: “A tethered interferometric synthetic aperture radar (SAR) for a topographic mission” in IEEE transactions on Geoscience and Remote Sensing, VOL 30, no. 1 , pp. 103-109, January 1992, Doi: 10.1109 / 36.124220.

[0061] Among the various studies on Tethered satellites are mentioned the following documents:

[0062] - K. Uldall Kristiansen, P. Palmer, M. Roberts: “A Unification of Models of Tethered Satellites”, SIAM - Society for Industrial & Applied Mathematics -1 January 2011 - DOI: 10.1 137 / 090779887, aa document available on the website: https: / / core.ac.uk / outputs / 28964268,

[0063] - F. Zhang, H. Zhou, P. Huang and J. Guo: “Stable Spinning Deployment Control of a Triangle Tethered Formation System” in IEEE transactions on Cybernetics, 03 August 2021 , pp. 1 - 11 , Doi: 10.1109 / TCIB.2021 .3074981 , a document available on the website: hyps: / / ieeexplorejeee.org / documeni / 95056181

[0064] - K. D. Kumar and T. Yasaka: “Rotation Formation flying of three satellites using tethers” J. Spacecraft Rockets, vol. 41 , no. 6, pp. 973-985, Jun. 2004, document available oonn the website: https: / / www.researchgate.net / publication / 245438154 Rotation Formation Flying of Three Sate B ites Using Tethers , - P. Williams: “ Optimal deployment and offset control for a spinning flexible tethered formation” in Proc. AlAA Guid. NAVIG. Control Conf., Keystone, CO, USA, Aug. 2006, pp. 1-26. Document available at the website: https: / / www.researchgate.net / publication / 269254632 Optimal Deployment and

[0065] - Soon-Jo Chung, Danielle Adams, Alvar Saenz-Otero, Edmund Kong, David W. Miller, David Leisawitz, Enrico Lorenzini, Steve Sell: “SPHERES tethered formation flight testbed: Advancements in enabling NAS’s SPECS mission” Proc. SPIE 6268, advances in Stellar Interferometry, 62680B (27 June 2006); document available on the website: httpsjtfdoL^^

[0066] - B.S. Yu H. Wen, D P. Jin.: “Review of deployment technology for tethered satellite systems”, Acta Mechanica Sinica, 34, (4): 754-768(2018), document available on the website: htt£sV / c^

[0067] Noise Radar Technology (NRT) can be used in multi-static spatial systems; an analysis of Noise Radar Technology and its signal processing is available in the following documents:

[0068] - G. Galati, G. Pavan C. Wasserzier: “ Signal design and processing for noise radar", EURASIP J. ADV. Sign. Proc. (JASP) 2022, 52 (2022), document available on the websites httDs: / / doi.orq / 10.1186 / s 13634-022-00884-1 and https: / / asp- eyrasijyoumalsjser^

[0069] - G. Galati, G. Pavan C. Wasserzier: “Optimal Processing in Noise Radar: Implementation problems” Proc, of Signal Processing Symposium (SPSympo 2019), 17-19 Sept. 2019, Krakow, Poland, DOI: 10.1109 / SPS.2019.8882098, a document available on the website httesjtfieeexg^

[0070] Finally, an introductory analysis of SAR interferometry is presented in document M. A. Richards: “A Beginner’s Guide to Interferometric SAR Concepts and signal processing [AESS Tutorial IVJ’ in IEEE Aerospace and Electronic Systems Magazine, VOL 22, nnoo.. 9, pp. 5-29, Sept. 2007, Doi: 10.1109 / MAES.2007.4350281 , aa document available on httgsTAeeexp^

[0071] In the field of the present invention, the main problems of the present state of the art include: - the control of the Formation, which is cumbersome and has considerable operating limits for formations with independent orbits, such as those of the TanDEM-X described in the cited prior documents; for example, in order to avoid collisions, the distance between the satellites must be maintained greater than a threshold value; the optimum distance of a pair of satellites for interferometric SAR imaging is obtained only once or twice per orbit and so on; and

[0072] - the time synchronization and the consistency of the transmitters and receivers loaded on different satellites, difficult to obtain with the very stringent requirements placed by some applications; it should be noted that the MirrorSAR concept of the above mentioned above avoids the problems of synchronization and consistency, but it actually shifts them over the extreme accuracy of inter-satellite distances and on the transfer of the coherent signal in real time, from / to the different satellites, a problem that has complexities comparable to the state of the art (and possibly even greater). Therefore, MirrorSAR cannot be considered a fully satisfactory solution or even definitive solution to the problems described above.

[0073] SUMMARY OF THE INVENTION

[0074] The object of the invention has been achieved by a multistatic space system with interconnected satellites as defined in claim 1 .

[0075] The multi-static spatial system provides for the Formation of satellites deployed in a low earth orbit, and the Formation comprises at least three satellites logically and physically connected to each other by first cables. The first cables allow the exchange of signals and data between the satellites of the system. There is a main satellite or Hub in the system and at least two secondary satellites or Companions. The synchronization signals generated by the main satellite or Hub are also transmitted within the first cables, and said synchronization signals are shared among all the Formation satellites to ensure a common time and phase / frequency reference. The secondary satellites or Companions operate jointly and coherently thanks to their synchronization and to the geometry guaranteed by said first cables and by attitude control means which are able to control the mutual positions or the geometry of the Formation of satellites.

[0076] Therefore, the present invention is intended to represent a solution to the above-mentioned technical problems, with significant advantages in remote sensing of the Earth, in the surveillance of surface vehicles, in space sciences and in related fields.

[0077] The multi-static architecture adopted has significant operational advantages with improved performance, increased reliability and flexibility, frequent monitoring possibilities, high-resolution SAR imaging of large areas, creation of highly accurate digital terrain models, scene classification, real-time detection of moving objects (moving targets), passive detection and localization of radio-electrical sources and others.

[0078] BRIEF DESCRIPTION OF THE FIGURES

[0079] In the following description reference will be made to the drawings shown in the accompanying drawings, in which:

[0080] - Figure 1 shows a four-satellite embodiment of the present invention with star topology: Main elements and connections;

[0081] - Figure 2 shows a preferred embodiment of the Hub satellite of the system object of the present invention;

[0082] - Figure 3 is a closed-triangle embodiment with four satellites; and

[0083] - Figure 4 shows the general diagrams of some physical embodiments with three “Companion" satellites and the Hub satellite in the center: (a) Open triangle with axis of revolution parallel to that of the Earth, (b) Open triangle with axis of revolution oriented vertically toward the center of the Earth, (c) closed triangle with axis of revolution toward the center of the Earth.

[0084] The drawings are limited to a four-satellite configuration shown for descriptive and explanatory purposes, but other configurations remain within the scope of the present invention, such as for example that with only three satellites and the one with pyramid shape with six satellites of which the Hub at the center, three Companions on the equipotential plane (in other words, horizontal) and the remaining two Companions on the vertical line passing through the Hub.

[0085] The parts according to the present description have been shown in the drawings, where appropriate, with conventional symbols, showing only those specific details which are relevant to the understanding of the embodiments of the present invention, so as not to weight the description with details and / or elements of the general type, not specific to the present invention and / or not strictly relevant to it: these details and elements, in fact, will be immediately evident to those skilled in the art, with reference to the state of the art and to the description given below.

[0086] A DETAILED DESCRIPTION OF THE INVENTION

[0087] The solution according to the present invention will now be described, with the aid of the drawings.

[0088] The solution provides for a multistatic spatial system with satellites interconnected by non-conducting cables, provided with optical fibers for the transmission of signals and data and managed by mechanical devices suitable for using them to control the geometry of the system; this system is better defined in claim 1.

[0089] In space activities, the generic term “constellation" means a set of satellites that operate collectively with separations not necessarily defined. In the present description, however, the term “formation”" is used, since it refers to a number of satellites which fly in a given region with well-defined separations and positions, which are obtained thanks to a precise control of the mutual position of the satellites and of the geometry of the system. In particular, a “formation" is a structured constellation in which the satellites occupy precise mutual positions determined by a predetermined geometry. In the present invention the concept of Formation provides that the satellites are connected to each other by cables or tethers and, consequently, are mutually constrained.

[0090] In addition, the term “system" is also used in the present description to define the formation as a whole, when particular attention is paid to the functional aspects.

[0091] A fundamental feature of the present invention is multi-static operation, defined as that of a system in which some satellites transmit, others receive and others still transmit and receive.

[0092] This type of operation allows to realize systems of the MIMO (Multiple Input Multiple Output) type; naturally in the particular case of a single transmitter the system becomes a SIMO (Single Input Multiple Output) system. SIMO-type systems with a single transmitter and a single receiver define a particular case or a bi-static system.

[0093] Multi-static spatial radar systems can be divided into three categories or types:

[0094] (a) fully active, i.e. systems with all satellites transmitting and receiving;

[0095] (b) SIMO systems in which only one satellite transmits and all others receive; and

[0096] (c) of the mixed type, i.e. systems with an intermediate structure between (a) and (b).

[0097] Furthermore, a multi-static spatial system may consist of only receiving satellites using the emissions of sources outside the system itself (for example, the radar and the on-board radios of the mobile means and others) realizing a particular form of “passive radar” whose operation is distinct from that of conventional radar.

[0098] In order to obtain the necessary time and frequency / phase synchronization, a clock signal is distributed between all the satellites of the formation. Therefore, this clock signal is common to all transmitting and receiving parts of the system.

[0099] The transmission of the clock signal within the scope of the present invention takes place through optical channels, preferably through optical fibers. In the event of malfunctioning (or inadequate operation) of the optical fiber transmission, reserve transmission channels are provided with “Free Space Optics" technology, as, among others, described in the work of: Reza, M.; Amir, M.M. H.; Imran, M.; Pandey, G.; Camponeschi, F.; Maresca, S.; Scotti, F.; Serafino, G.; Malacarne, A.; Porzi, C.; et al. “Multi-Static Multi-Band Synthetic Aperture Radar (SAR) Constellation Based on Integrated Photonic Circuits”. Electronics 2022, 11 , 4151. https: / / doi.org / 10.3390 / electronicsl 1244151 .

[0100] The optical transmission mode requires a conversion of the signal from electric to optical (E-O) at the point of generation of the clock signal and an opposite conversion, i.e. from optical to electrical (O-E), from the user’s side.

[0101] In the state of the art, ultra-stable clocks suitable for space are available, and so are also the converters from electrical to optical and from optical to electrical, now widely diffused in the world of Telecommunications and applicable to other contexts among which that of radar, as documented, among others, In the work of: P. Ghelfi, F. Laghezza, F. Scotti, D. Onori and A. Bogoni, “ Photonics for Radars Operating on Multiple Coherent Bands", in Journal of Lightwave Technology, vol. 34, no. 2, pp. 500-507, 15 Jan.15, 2016, Doi: 10.1109 / JLT.2015.2482390. Among the ultra-stable clocks (generally not sufficient for the purposes of the present invention) are reported the so-called GPSDO (GPS Disciplined Oscillators), which combine a multi-channel receiver of the known GPS navigation system with an OCXO (Oven-Controlled Crystal Oscillator) or with a voltage- controlled rubidium oscillator, This is synchronized to the oscillator inside the GPS receiver.

[0102] Similarly, the transmission of radar signals within the system preferably uses optical channels having at their ends converters from microwaves to optics and from optics to microwaves, respectively.

[0103] A preferred mode of operation of this invention is synthetic aperture radar (SAR), in which a narrow antenna beam is synthesized due to the relative motion of the surface being filmed, or of the target, with respect to the radar antenna.

[0104] The bi-static and multi-static synthetic aperture radar systems have, by definition, transmitting and receiving antennas mounted on separate platforms, according to the above types (a), (b) or (c), where the type (b) has reduced costs and the potential to use the emerging technologies applicable to mini and micro satellites; these advantages must be carefully evaluated, at the design stage, with respect to the type (a).

[0105] A multi-static SAR architecture has significant operational advantages with improved performance, increased reliability and flexibility, frequent monitoring capability, improved resolution, wideband imaging, scene classification, single -pass cross-track interferometry (with multi-baseline operation that decreases ambiguity problems) and more.

[0106] The solution proposed herein overcomes the above-mentioned problems and those known to those skilled in the art in the field of multi-static satellite SAR systems.

[0107] The present invention therefore lies in the field of formations of more than two satellites (which in particular are equipped with the known Synthetic Aperture Radar) for remote sensing and for the surveillance of the surface of the Earth.

[0108] The particular technical solution of the invention is based on the physical and logical connections of the satellites which are constrained with special cables (also called tethers) which form a formation of tethered satellites, with specific characteristics, illustrated hereinafter with main reference to the typology (b).

[0109] Before the launch, each cable is wrapped in its own housing. The rigid part of the cable is preferably made of light and resistant material such as aromatic polyamide. The system is launched into orbit as a single load. Once the prescribed orbital position has been reached, a reaction wheel (or equivalent device) of the Hub satellite is activated and induces a revolution of the entire system around the chosen axis. Then, the winches and cables are released due to the centrifugal force which causes the Companion satellites to move away from the Hub satellite to the chosen distance according to the specific operating requirements.

[0110] The cable, flat or circular in cross-section, integrates optical fibers for signal transmission. Within the cables are then transmitted those synchronization signals which must be shared among all the satellites to ensure a common time and phase / frequency reference.

[0111] The synchronization signals are generated in the main formation satellite, also referred to as Hub, converted to optical form, transmitted to the other secondary satellites of the formation and converted here into electrical form.

[0112] Moreover, in a preferred embodiment, the energy necessary for the operation of the satellites and their electronic devices is transmitted from the main satellite or Hub of the formation, provided with solar panels and batteries, to the secondary satellites called “Companions" thanks to a power laser (or to an LED) and to the optical fiber connection that connects the Hub with the Companions, according to the technique known as Power-over-Fiber. As described above, the secondary satellites called Companions are smaller and lighter than the Hub satellite.

[0113] With a diameter of the order of 300 micrometers, the estimated volume and mass of 1 km of cable (order of magnitude of the maximum foreseeable elongation for the applications of the present invention) are respectively of the order of 0.1 liters and 0.2 kg.

[0114] In summary, the solution describes the use of space systems with bi-static and multi-static synthetic aperture radar and the use in space of cables, also known as Tethers, suitable for signal transmission.

[0115] Thanks to its multi-static and distributed array characteristics with a flexible geometry, this solution (with its intrinsic adaptability) allows to realize, among other things: multi-static radar systems (with bi-static radar as a particular case), MIMO systems (multiple-input, multiple-output, with SIMO architecture as a particular case), space-distributed arrays, passive multi-material systems and more.

[0116] The transmission of the signals from the Companion satellites to the Hub satellite also allows On-Board Processing, i.e. the generation of the SAR images and of the surveillance data by means of a suitable on-board computer, then in “real time” mode without waiting for the transmission times of the raw data to a ground station and their subsequent processing. An advantage of On-Board Processing is the automatic analysis of the image quality and their re-acquisition if the quality falls below an acceptable minimum threshold. This analysis is facilitated by modem and increasingly widespread methods of Artificial Intelligence.

[0117] The solutions described herein allow an effective operation for a series of applications, including:

[0118] - high resolution radar images with wide coverage of the Earth’s surface,

[0119] - very high-resolution images of the Earth’s surface by radar tomography,

[0120] - elevation measurement with the creation of accurate digital elevation models (DEM) potentially throughout the Earth using multiple-base SAR interferometry,

[0121] - extraction of parameters and environmental data by radar polarimetry,

[0122] - active detection of moving targets on the Earth or sea surface, e

[0123] - passive detection and location of radio frequency sources such as, for example, navigation radar on board vessels (with the system in passive operation, i.e. in reception only mode).

[0124] Therefore, the present invention is intended to provide a solution to the above-mentioned technical problems, with significant advantages in remote sensing, surveillance, space sciences and other fields.

[0125] Since the optical fibers allow, in addition to communications, optical power transmission, the “Companion" satellites can be implemented with a considerable saving in power, mass and volume, avoiding solar panels with photovoltaic cells (and minimizing the batteries), when optical Power-over-Fiber (PoF) transmission from the Hub satellite is used, with a laser or light emitting diode (LED) as a transmitter positioned at one end of an optical fiber and a miniaturized photovoltaic cell as a receiver positioned at the other end.

[0126] The basic features of the invention are described below with reference - for brevity and clarity - to a particular embodiment in which the number of satellites is four as illustrated in the Figures, and in which of these four satellites one is the central or main satellite also called the Hub or Master of the Formation, and the remaining and smaller secondary satellites are the Companion satellites.

[0127] Naturally, the embodiment described is only one of the possible and conceivable embodiments; various other solutions can be obtained starting from the one described herein by varying some of the illustrated characteristics.

[0128] Among the many possible geometric configurations of the embodiment, with reference to those shown in Figure 4, the preferred one is that of Figure 4c “Earth Facing Closed Hub-and-Spoke” with the sides of the triangle preferably different from each other.

[0129] In this geometry, a rotation of the system, i.e. a revolution around the vertical axis, i.e. the line joining the center of mass of the system with the center of the Earth, continuously keeps the cables under traction. A ten-minute revolution period, equal to about one tenth of the orbital period in the Low Earth Orbit (LEO - typically the low orbit orbital period lasts about 100 minutes) with a 1000 m long cable corresponds to an acceleration of about 0.01 g. It should be noted that the accelerations of disturbance of the orbit are much below this order of magnitude. Therefore, this acceleration will generate a stable voltage of the cables which is in the order of 10N for a Companion satellite with a ground of the order of 100 kg. This acceleration is well compatible with a reduced cross-section of the rigid portion of the cable, the material of which has a resistance of between 2 and 3 GPa.

[0130] A possible material for the rigid part of the cable is the family of aromatic polyamides (trade name Kevlar™) whose typical parameters are the following: Density: 1.45 to 1.47 kg / dm3; tensile force: 3600 to 3900 MPa; elasticity: From 83 to 185 GPa with elongation between 4% and 2%.

[0131] As an example of embodiment of the above applications, the one of scattered and distributed arrays is considered. A preferred configuration when interferometry is the primary application is the Hub-and-Spoke configuration shown in Figures 4(b) and 4(c).

[0132] In the Hub-and-Spoke configuration with a given period of revolution Trevaround the vertical axis, for example by selecting Trev= 5 minutes (i.e. about one- twentieth of the orbital period in low orbit), a synthetic antenna can be formed with a coherent integration time of the same order as Trev. In this way, by implementing (time division) a very long synthetic opening, a very precise resolution in the three dimensions is obtained together with the suppression of the harmful effect of phase ambiguities in interferometric measurements.

[0133] A second example of preferred solutions is linked to the choice between continuous emission and pulsed emission. The traditional monostatic SAR suffers from the dilemma of the Pulse Repetition Frequency (PRF) according to which to obtain a wide strip (or swath) of the images it is necessary to use a low pulse repetition frequency (PRF) to avoid ambiguity in distance, worsening the crossrange resolution which is related to the Doppler frequency band.

[0134] Various “Wide Swath” techniques exist, and are documented in the technical-scientific literature, which seek to solve this problem, but accept a compromise in terms of imaging performance.

[0135] The multi-static nature of the present invention allows, instead, the implementation of the Wide Swath operation by creating a large number of antenna receiving beams within a wider transmission beam.

[0136] Moreover, this invention allows to solve said problem of ambiguities thanks to the continuous emission allowed by the separation of the receivers from the transmitters, which emit waves orthogonal to each other. In this context, a preferred route is the emission of pseudo-random waveforms according to Noise Radar Technology (NRT), explained, inter alia, in the document (referred to above): G. Galati, G. Pavan C. Wasserzier: “Signal design and processing for noise radaF.

[0137] The emission continues according to Noise Radar technology (NRT) increases, with the same peak power, the energy emitted on the target with respect to the conventional pulsed emission, thus ensuring a significant increase in the signal-to-noise ratio, and hence in the quality of the radar products with the same radar resources (or a reduction in the resources necessary for the same quality).

[0138] Moreover, the continuous emission according to Noise Radar Technology avoids limitations on the pulse repetition frequency (PRF) values and allows to operate with smaller antennas and wider Doppler bands. By calculating the correlation between transmitted and received waveforms in the ranges of interest only of the Range-Azimuth plane by the Range Filters Bank, a technique explained in the document (referred to above): G. Galati, G. Pavan C. Wasserzier: “Optimal Processing in Noise Radar: Implementation Problems".

[0139] From the hardware point of view, the receive-only Companion satellites can be implemented with considerable power, mass and volume savings, also avoiding solar cells, when the Power-over-Fiber (PoF) optical transmission described above is used.

[0140] Of course, it is possible to provide embodiments in which all the Formation satellites are equal to each other for dimensions and equipment. Alternatively, it is possible to define embodiments in which some Formation satellites are different for example simplified.

[0141] The system of the present invention is described in Figure 1 as a whole with reference to a simple and understandable preferred embodiment in which the Formation consists of four satellites connected together by cables, or “tethers".

[0142] A main satellite, designated by the reference numeral 1 , represents the Hub satellite of the system (i.e., the Formation of satellites). The formation also includes secondary satellites, called Companion satellites, which are indicated in the Figures by the references 11 , 12 and 13. The secondary satellites or Companions are satellites all equal to each other.

[0143] All Companion satellites of the Formation are connected to the Hub satellite by means of cables indicated with the references 8, 9 and 10. These cables 8, 9 and 10 connect, both physically and logically, the Hub satellite 1 with the Companions 11 , 12 and 13.

[0144] The Hub satellite 1 is provided with three motorized winches indicated with the references 2, 3 and 4 connected by the aforesaid cables 8, 9 and 10 with respective winches indicated with the references 5, 6 and 7 present respectively in the Companion satellites 11 , 12 and 13.

[0145] Figure 1 shows, as anticipated, a four-satellite embodiment of the present invention with a star topology showing the main elements that form the Formation and the connections between the various elements.

[0146] The Hub satellite 1 is placed at the center of the Formation and the three Companion satellites 11 , 12 and 13 are arranged in the vertices of a triangle.

[0147] Figure 2 shows a preferred embodiment of the Hub main satellite 1 of the multi-static system in one embodiment of the present invention.

[0148] For the sake of clarity, Figure 2 also shows the elements described above and indicated with the references 2 to 13 and already described with reference to Figure 1 .

[0149] The main elements of the radar of the Hub satellite 1 are a transmitter 17, an antenna 18 and finally a receiver 19.

[0150] The output of the receiver 19 is converted and digitized into a converter and digitizer block 20. The output of the converter and digitizer 20 is processed in a preprocessing block 21 which performs the functions of pre-processing and conditioning the data necessary for transmission to the Earth.

[0151] Data and signal transmission to Earth is performed in a communication subsystem designated as block 22, the output of which is sent to a data link antenna 23, for transmission of information to a ground receiver station.

[0152] The operation of the radar is managed by a radar controller 24, which interfaces with all the radar blocks, i.e. the transmitter 17, the antenna 18 and the receiver 19. The radar controller 24 generates the radar waveforms and receives from a clock generation unit 25 the time and phase references to be distributed among said radar units.

[0153] In various embodiments the radar controller 24 performs in real time the processing necessary for the creation of the radar images and verifies the image quality, ordering the repetition of the acquisition of the radar echoes in case of quality lower than a predetermined threshold.

[0154] The output of the clock generating block 25 is sent to an electronic-optical converter 26. The signal thus converted in output from block 26 is then sent by a transmitter 27 and by means of a bidirectional optical fiber connection 28 is transmitted to the three secondary Companion satellites 11 , 12 and 13 belonging to the Formation. The optical fiber connection 28 comprises the optical fibers contained in the first connection cables 8, 9 and 10. The Companion satellites 11 , 12 and 13 receive radar echoes and / or other signals emitted in the selected frequency band (also known as the range) and send them to the Hub satellite 1. Although the spatial SAR systems can operate in different frequency ranges normally allocated in the microwave region, for the constellations of small satellites the X band, around 9.6 GHz, is very often preferred, it has a considerable bandwidth available according to international rules and therefore allows an excellent spatial resolution along the aiming line, i.e. at a distance.

[0155] The optical signals originating in the Companion satellites 11 , 12 and 13 are sent through the bidirectional optical fiber connection 28 to the Hub satellite 1 and converted from optical to electrical by a converter block 29. and finally transferred to the converter and digitizer block 20 for conversion to digital format.

[0156] From the converter and digitizer block 20 the set of radar signals received from the Hub 1 , by the receiver 19 and from the Companions 11 , 12 and 13 is sent to the pre-processing block 21 where these signals are pre-processed and the conditioned data are sent to the communication subsystem 22. and from it to the data link antenna 23.

[0157] As far as the electrical energy of the system is concerned, the primary source is that of the solar panels (photovoltaic) 30 arranged on the main Hub satellite 1 , whose output is connected to a power subsystem 31. The supply subsystem 31 deals with the distribution of the energy to the various users in the Hub satellite 1 itself, as well as with the distribution to the Companion satellites 11 , 12 and 13. The energy is processed by power conditioning in a block 32 and, in one embodiment, sent via an optical power source 33, the output of which is sent to an optical fiber by an optical transmitter 34. Finally, to be supplied by the power subsystems to the Companion satellites 11 , 12 and 13.

[0158] Reference 35 indicates a Telemetry, Tracking and Command (TT&C) subsystem, which is a standard component of artificial satellites, well known to operators in the field.

[0159] A block 36, on the other hand, represents the orbital and attitude control system. The orbital and attitude control system 36 not only uses the conventional optical sensors of the celestial bodies and operates on conventional actuators, such as propulsors, reaction wheels, control gyroscopes, but also has laser telemeters for measuring the distance of the Companion satellites.

[0160] Moreover, the control system 36 acts on the motors (not shown) of the winches 2, 3 and 4 present on the Hub 1 , in order to control the geometry of the Formation (i.e. of the system) and, where required, its speed of rotation (i.e. the angular speed of its revolution motion).

[0161] As said in Figure 2, a preferred embodiment of the Hub satellite of the system of the present invention is illustrated.

[0162] The preferred embodiment of the other Companion satellites is similar to that of the Hub satellite 1 of Figure 2, but with the possible elimination of one or more functional blocks, which can be easily obtained from the description as a result of the specific functionalities of the Companions which, as already illustrated, in one embodiment of the invention they can be passive, and thus without the radar signal transmission block.

[0163] The receiver 19 supplies a microwave-optical converter which in turn supplies an optical transmitter, from which the optical signal is transferred via the bidirectional optical fiber connection 28 to the Hub satellite 1 .

[0164] The presence of two winches, one for each Hub and Companion side, allows, when the cable is not fully unwound as it happens in some applications of the present invention, to suitably distribute the cable mass between the pairs of satellites without modifying the geometry of the system. In this way it is possible to vary the moment of inertia of the system with respect to the vertical axis passing through the center of gravity (in practice, with respect to the Hub) and consequently, to adjust the speed of revolution (spin) without modifying the geometry of the formation, and therefore, optimize how radar images are filmed on specific portions of the Earth’s surface.

[0165] Figure 3 shows an embodiment of the system according to the present invention, which embodiment is preferable when, depending on the particular mode of operation of the system, extremely precise control of its geometry is required (see also Figure 4). In particular, in this case each Companion satellite 11 , 12, 13 is connected, in addition to the central Hub satellite, also to two other Companion satellites. In this way, the Formation is even more stable and it is possible to better control the relative distances between the various satellites of the Formation 1 , 11 , 12, 13.

[0166] The additions of this embodiment to the system shown in Figure 1 are the second cables 14, 15 and 16 and the corresponding motorized winches, indicated with references 5A and 5B, 6A and 6B, 7A and 7B. They allow a very precise control of the overall geometry of the system (triangular in this particular case) by the same methods as the blocks 2, 3, 4 and 36 previously illustrated.

[0167] The optical fiber connection 28 also comprises the optical fibers contained in the second connecting cables 14, 15 and 16.

[0168] As explained above, Figure 3 shows a closed-triangle embodiment with four satellites. The main elements and their respective connections are shown in this figure.

[0169] Figure 4 shows some physical configurations of the invention for various operative contexts and for different cost / effectiveness ratios.

[0170] The drawings are limited to an exemplary configuration with four satellites including a Hub, but other configurations (not shown) remain within the scope of the present invention as the simplest one with three satellites and as the most complex (already mentioned) pyramidal configuration with, for example, six or seven satellites including the Hub in the center.

[0171] In particular, Figure 4 shows some embodiments with three Companion satellites and the Hub satellite in the center:

[0172] (a) Open triangle (star topology) with axis of revolution parallel to that of the earth,

[0173] (b) Open triangle with axis of revolution toward the center of the earth, and

[0174] (c) closed triangle with axis of revolution toward the center of the earth.

[0175] The features of a preferred embodiment of the present invention are described below with reference to the Figures.

[0176] With reference to Figure 1 , the invention relates to a space system consisting of a main satellite, the Hub, which transmits radar signals to the Earth’s surface, and a number (for example, three, but two as a minimum) of satellites, called Companions, they receive radar echoes by implementing a synthetic aperture multistatic radar. These echoes are sent to the Hub (see Fig. 2) for transmission to Earth stations where they are processed for the purposes of the particular mission. The Hub satellite is tethered to the Companion satellites and the cables house optical fibers for signal and data transmission, including synchronization cables required for the operation of the multi-static radar system. The synchronization signals are generated, such as time and phase / frequency references, on board the Hub.

[0177] The cables also have the function of controlling, with their length in space, the geometry of the entire system (see Figures 2 and 4). Of course, the length of the cables can be varied operationally to meet the purpose of particular missions, including SAR interferometry, SAR tomography, moving object detection and more. The variation in the length of the cables is carried out by the winches and by the motors described above.

[0178] The overall system is maintained dynamically in mechanical equilibrium (in addition to conventional means such as propulsors, reaction wheels, control gyroscopes and others) by a revolution motion of the Formation about an axis which may be vertical or horizontal (or other), As shown in Figure 4, according to the requirements of the mission. The position and attitude of the Formation with respect to the Earth is controlled by said means (i.e. cables and winches).

[0179] In one embodiment, the system is also provided with a Free Space Optics (FSO) subsystem for transferring radar signals and related data, together with auxiliary signals, to other visible space vehicles, including systems of the same type that fly over different orbital positions. By using the latter as transponders, said subsystem enables the relevant information to be sent rapidly to Earth stations that are momentarily beyond the horizon, i.e. not in visibility. Moreover, it allows communication between the Companion satellites and the Hub in the event of malfunctioning of the cable transmission described above.

[0180] The multi-static nature of the invention allows an additional, passive, operating mode in which radar transmission is absent and reception is dedicated to radio-electric signals emitted from surface sources such as ships.

[0181] The purpose of said mode is to detect and locate surface mobile (and more generally, radio-electric) objects (targets) by means of multilayer, i.e. by using arrival time measurements and, where possible and convenient, also Doppler frequency measurements.

[0182] The Companion satellites can be implemented with reduced mass, size and cost by avoiding solar panels when, in one embodiment of the invention, they are transmitted the energy required by the Hub via the fiber optic connection (Power- over-Fiber technology).

[0183] Depending on the specific mission requirements and technical / economic compromises, various embodiments of the present invention may be provided.

[0184] As already described, the solution provides, as a radar architecture, a multistatic spatial radar system, or Formation of satellites, consisting of at least three satellites logically and physically connected by means of suitable cables, of which at least one satellite (Hub) such that, when activated by the radar controller, it transmits radar signals, And at least two other satellites (called Companion) such that they receive radar echoes and / or other signals emitted in the selected frequency band. Said Formation satellites operate jointly and coherently to generate and collect signals which allow to reconstruct the two-dimensional and multidimensional images of a selected portion of the surface of the Earth (terrestrial and / or marine) and of possible artifacts, and finally to detect and locate moving objects.

[0185] The multi-static spatial radar system comprises at least three satellites connected by cables, of which at least one satellite transmits and receives radar signals and the other satellites transmit and receive or receive only.

[0186] The at least three mutually connected satellites operate jointly and coherently to generate and collect radar signals which make it possible to detect and locate moving objects, such as for example the various moving vehicles on the Earth or on the sea surface.

[0187] The system allows to enhance imaging and detection performance by processing radar echoes according to the principle of synthetic aperture radar (SAR) whose imaging and detection performance is enhanced using SAR interferometry.

[0188] In more complex solutions, imaging and detection performance is also enhanced by SAR polarimetry and tomography.

[0189] In the preferred embodiment, the Formation satellites are connected to each other by non-conductive cables which control the inter-satellite distance, and hence the positions of the satellites with respect to a defined point preferably, the center of mass, or center of gravity, of said system or Formation.

[0190] In said embodiment the satellites are connected to each other by optical fiber cables which allow the transfer of radar signals and auxiliary signals and data, including those necessary for the time and phase / frequency synchronization between said Formation satellites. In one embodiment, said cables also allow the transmission of the power from the Hub satellite to the Companion satellites.

[0191] In the preferred embodiment, the satellites are connected to each other by cables which integrate optical fibers. Preferably the optical fibers are placed in the core of the cable. The cables are preferably constructed in the form of a sock around said optical fiber core, said socks being made of flexible materials and with excellent tensile strength, with a controlled elongation such that the optical fibers undergo a zero (or, more precisely, negligible) stress, and remain at constant length.

[0192] In various embodiments, the satellite connecting cables have a circular cross-section with the optical fibers at the center and the resistant material, preferably of the aromatic polyamide type, is arranged in the form of a sock around the rest of the cable. In other embodiments, the satellite connecting cables have a flat shape which facilitates winding of the cable.

[0193] The solution described herein provides that all the satellites are connected by means of said cables to a central satellite, or Hub satellite, which is topologically the center of a star structure. In greater detail, the satellites are connected in pairs by said cables by implementing, topologically, a “mesh” structure, which is reduced in a triangle when the number of satellites is three and to a tetrahedron when this number is four.

[0194] Various embodiments provide that the satellites are connected to each other by optical fiber integrating cables - preferably in the core of the cable - providing optical connections for the transmission of signals and data between pairs of satellites.

[0195] In different embodiments the satellites are connected by cables which integrate optical fibers which allow Power-over-Fiber transmission from one central satellite to the other Companion satellites. For this purpose, the central Hub satellite contains an electric-optical power converter implemented by a laser or by an LED (Light Emitting Diode) with which the energy generated by the photovoltaic panels and / or stored in the batteries is sent via optical fiber to the remaining Companion satellites.

[0196] In particular, each satellite other than the Hub (i.e., the core of the system) receives the energy from the Hub via optical fiber and this energy supplies an optical-electric power converter from which the electric energy necessary for the various users of the satellite is sent.

[0197] As illustrated, the cables are connected to the satellites by means of winches, thus allowing a system controller to vary the inter-satellite distances to control the geometry of said system by varying the deployment of the cables by means of motors connected to the winches.

[0198] In the proposed solutions, the maintenance of the cable tension is facilitated by the revolution around an axis, at a suitable angular speed, of the entire system and, in configurations with satellites not all lying on the equipotential plane (an example of these is the one with six or seven satellites illustrated above), from the vertical gravity gradient.

[0199] Moreover, the maintenance of the tension of the cables is favored, when necessary, by the use of (preferably ionic) propulsors which help to control the geometry of the system.

[0200] In some embodiments the system is provided with a free space optical transmission subsystem which allows the transfer of radar signals and of the related auxiliary data and signals between the satellites even in the event of malfunction or absence of said optical fiber connections for the transfer of signals and data.

[0201] In particular, the optical transmission subsystem in free space also allows the transfer of radar signals and of the relative data and auxiliary signals between the system itself and other space vehicles, including systems of the same type on different orbital positions, to quickly reach Earth stations not currently visible.

[0202] In the solutions proposed here at least one of the satellites of the system, called Hub, collects all the relevant signals from the other satellites, called Companions, and transmits such signals, including its own radar signals, by radio or optical connection, to one or more earth stations where said signals are collected and processed to obtain the information necessary to reconstruct the images of a selected part of the earth surface as well as to detect and locate moving objects.

[0203] In various embodiments the system has a MIMO radar (Multiple-Input- Multiple-Output) operation to improve geometric resolution and detection of moving objects.

[0204] In other embodiments the system has a radar SIMO (Single-Input-Multiple- Output) operation. In this case, a satellite transmits radar signals over a wide antenna beam. Moreover, the remaining satellites receive the radar echoes by means of narrow contiguous antenna beams suitably synthesized (e.g. by means of the known Digital Beam Forming technique). The beam elevation width is N times less than the above width of the transmit antenna beam, thereby increasing the extent of the image strip on Earth by N times, where N is an integer greater than the unit.

[0205] Preferably, in the embodiment providing for radar transmission of pseudorandom signals, the echoes are treated with the appropriate filter adapted according to Noise Radar Technology.

[0206] The system described herein can use laser telemeters on board satellites for precise measurement of inter-satellite distances, thus ensuring precise control of the geometry of the system in real time. This functionality is particularly necessary for radar tomography / interferometry.

[0207] In various embodiments each satellite has a precise attitude control which is added to the position control.

[0208] The solution described herein provides that the platform (also known as a bus) of each satellite is mechanically arranged so that its winch is positioned as close as possible to the center of mass (center of gravity) of said satellite, thus allowing the attitude control of the satellite to operate with the minimum energy consumption.

[0209] In various embodiments, the system is able to control the positions of the satellites with respect to the center of mass of the system by means of propulsors, reaction wheels and gyroscopic devices, as well as to control the angular velocity and the phase of the revolution motion (rotation of the entire system about its axis). These controls are used to optimize the geometry of the system where and when certain defined portions of the Earth surface are to be acquired for imaging and surveillance.

[0210] In the system proposed herein, the control of the angular velocity and of the phase of revolution (rotation about the vertical axis) of the entire system is obtained by dynamically varying the length of the cables using for this purpose the motors connected to the winches.

[0211] In some embodiments, it is possible to provide at least two receive-only mini or micro Companion satellites having a simplified bus without solar panels and receiving energy from the optical fiber connection to a larger satellite, such as for example the Hub satellite.

[0212] In embodiments with small receive-only Companion satellites they are connected to the system via cables such that the entire system can be launched as a single body with the cables fully wound; after the launch, when the operating orbit is reached, the entire system assumes the design geometry with the cable deployment.

[0213] In various embodiments each Companion satellite is connected to the Hub by means of optical fiber connections; at the two ends of each fiber there are, respectively, a microwave-optical converter and an optical-microwave converter. Said converters allow the transfer of the radar signals received from the Companion to the Hub where they are collected to be transferred to the users.

[0214] In various embodiments, a main radar satellite, called Hub, contains a clock whose signal, after an electric to optical conversion, is sent via optical channel to the Companion satellites. In the Companion satellites, the clock signal, after an optical-to-electrical conversion, generates the time and phase / frequency reference.

[0215] In particular, in the solutions proposed herein, the delay introduced by the optical fibers is measured by reflectometry with sub-picosecond accuracy and compensated by signal combination.

[0216] In some embodiments the system “exploits” the radio-electric emissions of the means on the surface of the Earth (mainly ships) to locate them even if said means do not cooperate (for example in the case of smugglers or traffickers).

[0217] To this end, the system, operating in passive radar mode (i.e. without transmission), carries out the Multilateration. In this mode of operation, the system receives the signals emitted by external radio sources positioned on the Earth surface (or near it) and locates these sources by measuring the arrival times of the Earth signals to the radar antennas, of which it calculates the differences (known as Time Differences of Arrival, TDoA) with respect to one of said times taken as reference. In the system just described the measurement of the differential Doppler frequencies (Frequency Differences of Arrival - FDoA ) is used in addition to the Time Differences of Arrival (TDoA), and in synergy with it, to locate said radio sources. In some embodiments the sources to be located are the radars, particularly those of navigation on board ships or, in general, of mobile means. For this purpose, the main satellite or Hub transmits said radio-electric signals emitted from surface sources to Earth stations where said signals are processed by said Multilateration techniques (analysis of the differences in arrival times to the different Companion and / or analysis of Doppler frequency differences) to reconstruct the traffic information of the Earth’s surface (position and speed of the mobile means) and on the individual means, also non-cooperating.

[0218] The above description of embodiments of the invention is capable of showing the invention from the conceptual point of view so that other subjects, using the prior art, can modify and / or adapt such specific embodiments in various applications without further research and without departing from the inventive concept. It is therefore understood that such adaptations and modifications will be considered equivalent to the specific embodiments, and that the means and materials for carrying out the various functions described heretofore may be of various nature without thereby departing from the scope of the invention.

[0219] Finally, it is understood that the expressions or terminology used are for purposes of description and, for this reason, are not limiting.

[0220] Naturally, the principle of the invention remaining the same, the details of construction and the embodiments may vary widely with respect to what has been described and illustrated purely by way of example, without thereby departing from the scope of the present invention.

[0221] Where the constructional characteristics and techniques mentioned in the subsequent claims are followed by reference signs or numerals, such reference signs have been introduced with the sole aim of increasing the intelligibility of the claims and, accordingly, they have no limiting effect on the interpretation of each element which has been identified, by way of example and / or merely explanatory, by such reference signs.

Claims

CLAIMS1 ) A multistatic space system provides a Formation of cooperative satellites (1 , 11 , 12, 13), deployed in a low Earth orbit, wherein said Formation comprises at least three satellites (1 , 11 , 12, 13) logically and physically connected to one another by means of first cables (8, 9 10), wherein said first cables (8, 9, 10) allow the exchange of signals and data between the satellites in the system, wherein there is a main or Hub satellite (1 ) and at least two secondary or Companion satellites (11 , 12, 13), wherein also the synchronization signals generated by the main or Hub satellite (1 ) are transmitted inside said first cables (8, 9 10), wherein said synchronization signals are shared among all satellites (1 , 11 , 12, 13) in the Formation to ensure a common time and phase / frequency reference, wherein said secondary or Companion satellites (11 , 12, 13) jointly and coherently operate by virtue of the synchronization thereof and the geometry of the entire system, wherein said geometry is optimized before and during the operation of the system as a function of the mission of the system itself by varying the length of the deployed portions of said first cables (8, 9, 10) in addition to the use of the position and attitude control means, wherein said position and attitude control means allow for real-time control of reciprocal and absolute positions, with respect to the reference integral to the Earth, of the Formations of satellites (1 ,11 ,12,13), wherein said position and attitude control means also allow high-precision geometric control involving connecting the secondary or Companion satellites (11 , 12, 13) of the Formation to each other by means of second cables (14, 15, 16), wherein said second cables (14, 15, 16) comprise unwound portions and wound portions, wherein said first cables (8, 9, 10) and said second cables (14, 15, 16) create a mesh structure which implements a connection redundancy and a particularly accurate control of the geometry and of the distribution of the masses of the Formation, including the masses of the unwound and wound portions of the first cables (8, 9, 10) and second cables (14, 15, 16), wherein said means of position and attitude control comprise ionic motors and exploit centrifugal force to keep said unwound portions of said first cables (8, 9, 10) and said unwound portions of said second cables (14, 15, 16) taut.2) The multistatic space system according to claim 1 , wherein said Formation satellites (1 , 11 , 12, 13) receive signals emitted from radioelectric sourcesaboard mobile means on the surface of the Earth and wherein said main satellite or Hub (1 ) transmits said radioelectric signals to the Earth stations where said signals are processed by multilateration techniques to reconstruct information about the traffic of the surfaced of the Earth and individual means, including non-cooperating means, for surveillance of said mobile means.3) The multistatic space system according to claim 1 , wherein said satellites of the Formation (1 , 11 , 12 ,13) transmit and receive signals operating in Synthetic Aperture Radar mode, and wherein said at least one main or Hub satellite (1 ) comprises a transmitter (17) and an antenna (18) which transmits radar signals, and wherein at least one secondary or Companion satellite (11 , 12, 13) receives signals consisting of radar echoes and sends them to said main or Hub satellite (1 ) by means of said first cables (8, 9, 10), wherein said signals received from said at least one secondary or Companion satellite (11 , 12, 13) are used to remotely sense the surface of the Earth and monitor moving objects on said surface.4) The multistatic space system according to claim 3, wherein in addition to the transmitter (17), the main or Hub satellite (1 ) comprises a receiver (19) which receives radar signals, and the other secondary or Companion satellites (11 , 12, 13) comprise functional blocks for receiving and / or transmitting and receiving radar signals.5) The multistatic space system according to claim 4, wherein said secondary or Companion satellites (11 , 12, 13) receive said radar echoes and send them by means of said first cables (8, 9, 10) to the main or Hub satellite (1 ) which in turn transmits them to Earth stations where they are processed in order to reconstruct the multidimensional images of a preselected portion of the surface of the Earth and to detect and locate moving objects.6) The multistatic space system according to any one of claims 3 to 5, wherein the synthetic aperture radar employs the SAR interferometry or SAR polarimetry technique, or both, to enhance the performance of the multidimensional imaging and of sensing the multiple attributes of the surface of the Earth by the system itself.7) The multistatic space system according to any one of the preceding claims, wherein said system is driven by a motion of revolution around an axispassing through the center of gravity thereof, and wherein said first cables (8, 9, 10) operate in traction by virtue of the centrifugal force consequent to said motion of revolution, and therefore they control the inter-satellite distance, and therefore the positions of the satellites (1 , 11 , 12 13) in the Formation with respect to a defined point as the center of gravity of the aforesaid system, wherein the length of the unwound portion of said cables also controls the angular moment of the system relative to the revolution axis.8) The multistatic space system according to any one of the preceding claims, wherein said first cables (8, 9, 10) integrate optical fibers in the core and are built with flexible materials which are resistant to traction with a controlled lengthening such that the inner optical fibers, not subject to traction force, undergo null or negligible stress and the optical fiber transmission channels allow the transfer of auxiliary signals and data, including those required for the time and phase / frequency synchronization between said satellites (1 , 11 , 12, 13) in the Formation.9) The multistatic space system according to claim 7 or 8, wherein the first cables (8, 9, 10) for connecting the satellites (1 , 11 , 12, 13) in the Formation have circular, ellipsoidal or flat section with the optical fibers in the middle, and the resistant material is arranged as a sock around the rest of the cable.10) The multistatic space system according to claim 9, wherein the resistant material is selected from the family of aromatic polyamides.11 ) The multistatic space system according to any one of the preceding claims, wherein all the secondary or Companion satellites (11 , 12, 13) in the Formation are connected by means of said first cables (8, 9, 10) to said main or Hub satellite (1 ), obtaining a “star”-shaped structure in which, topologically, said main or Hub satellite (1 ) is the star center.12) The multistatic space system according to any one of the preceding claims, wherein the secondary or Companion satellites (11 , 12, 13) in the Formation are connected to one another by pairs of second cables (14, 15, 16), creating a mesh structure which implements a connection redundancy and a particularly accurate control of the geometry and distribution of the masses of the Formation.13) The multistatic space system according to any one of preceding claims, wherein the main or Hub satellite (1 ) comprises a radar system which, in addition to the transmitter (17), to the receiver (19) and to the antenna (18), also has photovoltaic solar panels (30) which provide to the same system the power required for the operation thereof.14) The multistatic space system according to claim 13, wherein said main or Hub satellite (1 ) in the Formation is provided with solar panels and batteries, wherein the satellites (1 , 11 , 12, 13) in the Formation are connected by first cables (8, 9, 10) and second cables (14, 15, 16) which integrate optical fibers and allow the (Power-over-Fiber) power transmission from the main or Hub satellite (1 ) to the other secondary or Companion satellites (11 , 12, 13) in the Formation, and wherein said main satellite or Hub (1 ) of the Formation provided with solar panels and batteries transfers the energy to the secondary or Companion satellites (11 , 12, 13) by means of said first cables (8, 9, 10), according to the Power-over-Fiber technique through a power emission device such as a laser and an optical fiber connection (28) which connects the main or Hub satellite (1 ) with the secondary or Companion satellites (11 , 12, 13).15) The multistatic space system according to any one of claims 13-14, wherein the main or Hub satellite (1 ) comprises a converter and digitizer block (20), which output is processed in a pre-processing block (21 ) which executes the preprocessing and conditioning functions of the data required for the transmission thereof to the Earth stations and comprises a communication subsystem (22) and a data connection antenna (23) for transmitting information to the Earth stations.16) The multistatic space system according to any one of claims 13 to 15, wherein the main or Hub satellite (1 ) comprises a radar controller (24) which interfaces with the transmitter (17), the antenna (18) and the receiver (19) and generates the waveforms of the radar to be transmitted, and a clock generation unit (25) which generates the time and phase / frequency references of the Formation.17) The multistatic space system according to any one of claims 13 to 16, wherein the main or Hub satellite (1 ) comprises an optical-electronic converter (26) which receives the signal output from said clock generation unit (25) and comprises a transmitter (27) which, by means of a bidirectional optical fiber connection (28),transmits said clock signal to the secondary or Companion satellites (11 , 12 and 13) belonging to the Formation.18) The multistatic space system according to claim 17, wherein the main or Hub satellite (1 ) comprises an electrical-optical power converter block (29) which receives, by means of said bidirectional optical fiber connection (28), the optical signals originating in the secondary or Companion satellites (11 , 12, 13) and converts them from optical into electric to transmit them to the converter and digitizer block (20) for the conversion into digital format.19) The multistatic space system according to any one of claims 13 to 18, wherein the main or Hub satellite (1 ) comprises a feeding subsystem (31 ) adapted to the distribution of energy to the different utilities in the main or Hub satellite (1 ) and to the distribution to the secondary or Companion satellites (11 , 12, 13), and wherein the main or Hub satellite (1 ) comprises a power conditioning block (32) for processing the energy and an optical power source (33) and an optical transmitter (34) for the transmission to said bidirectional optical fiber connection (28) to be made available to the power subsystems of the secondary or Companion satellites (11 , 12, 13).20) The multistatic space system according to any one of the preceding claims, wherein the main or Hub satellite (1 ) comprises a system and the relative orbital and attitude control means (36), wherein the control of the angular speed and phase around the axis of revolution of the whole system is obtained by dynamically varying the length of the first cables (8, 9, 10) using the motors connected to the winches (2, 3, 4).21 ) The multistatic space system according to claim 20, wherein said orbital and attitude control means comprise thrusters, reaction wheels, control moment gyros and laser range finders to measure the distance of the satellites.22) The multistatic space system according to any one of the preceding claims 8-21 , wherein the delay introduced by the optical fibers (28) is measured by reflectometry with sub-picosecond accuracy and compensated at signal combination level.23) The multistatic space system according to any one of the preceding claims 20 to 22, wherein the main or Hub satellite (1 ) comprises winches (2, 3, 4)for deploying said first cables (8, 9, 10) and the orbital control system (36) acts on the motors of the winches (2, 3, 4) in order to control the geometry of the Formation and the speed thereof around the axis of revolution and vary the inter-satellite distances by modifying the length of the unwound part of the cables and the portion of the residual wound part to control the moment of inertia with respect to said axis.24) The multistatic space system according to claim 23, wherein each secondary or Companion satellite (11 , 12, 13) comprises a winch (5, 6, 7) for deploying said first cables (8, 9, 10) and the control system (36) acts on the motors of the winches (5, 6, 7) in order to control the geometry of the Formation and phase and revolution speed around the axis by varying the inter-satellite distances and modifying the length of the unwound part of the cables and the portion of the residual wound part to control the moment of inertia with respect to said axis.25) The multistatic space system according to claim 24 when it depends on one of claims 12 to 22, wherein each secondary or Companion satellite (11 , 12, 13) comprises two winches (5A, 5B, 6A, 6B, 7A, 7B) for deploying said second cables (14, 15, 16) and the orbital control system (36) acts on the motors of the winches (5A, 5B, 6A, 6B, 7A, 7B) in order to control the geometry of the Formation and the revolution speed thereof around the axis and vary the inter-satellite distances by modifying the length of the unwound part of the cables and the portion of the residual wound part to control the moment of inertia with respect to said axis.26) The multistatic space system according to any one of the preceding claims 7 to 25, wherein the main or Hub satellite (1 ) comprises a free-space optics transmission sub-system which makes it possible to transfer the radar signals and the related ancillary data and signals among said satellites in the event of malfunctioning or absence of the optical fiber connections for the transfer of signals and data.27) The multistatic space system according to any one of the preceding claims 7 to 26, wherein the main or Hub satellite (1 ) comprises a free-space optics transmission sub-system which allows the transfer of the radar signals and the related ancillary data and signals between the system itself and other spacecraft, including systems of the same type on different orbital positions, to quickly reach Earth stations which are temporarily not visible.28) The multistatic space system according to any one of preceding claims 7 to 27, wherein the main or Hub satellite (1 ) collects all the relevant signals from the other satellites and transmits such signals, including its own radar signals, via radio or optical connection, to one or more Earth stations where the aforesaid radar signals are collected and processed to obtain the information required to reconstruct the images of a chosen part of the surface of the Earth as well as to detect and locate moving objects.29) The multistatic space system according to any one of the preceding claims, wherein the main or Hub satellite (1 ) comprises a radar controller (24) which operates according to a “MIMO radar (Multiple-Input-Multiple-Output)” type operation to enhance the geometric resolution and the detection of moving objects.30) The multistatic space system according to any one of the preceding claims, wherein the main or Hub satellite (1 ) comprises a radar controller (24) which operates according to a “SIMO radar (Single-Input-Multiple-Output)” type operation, wherein one satellite transmits radar signals on an antenna beam which is wide in elevation and the remaining satellites receive the radar echoes via contiguous synthesized narrow beams.31 ) The multistatic space system according to any one of the preceding claims, wherein the main or Hub satellite (1 ) comprises a radar controller (24) which executes, in real time, the processing required to create radar images and verifies the quality of the image, ordering the repetition of the acquisition of the radar echoes in the event of quality below a preset threshold.32) The multistatic space system according to any one of the preceding claims, wherein the main or Hub satellite (1 ) comprises a radar controller (24) which continuously generates and emits pseudorandom signals which echoes are treated with the pertaining matched filter according to the Noise Radar Technology.33) The multistatic space system according to any one of the preceding claims, wherein said secondary or Companion satellites (11 , 12, 13) are receiving- only mini or micro satellites having a simplified platform (also known as a bus) with no solar panels.34) The multistatic space system according to any one of preceding claims 13 to 33, wherein said secondary or Companion satellites (11 , 12, 13) comprise anOptical-Microwave converter for the transfer of signals received from the main or Hub satellite, and said main or Hub satellite (1) comprises a Microwave-Optical converter.

Images