An efficient imaging algorithm for GNSS-R bi-static SAR

Global Navigation Satellite System Reflectometry (GNSS-R) based Bi-static Synthetic Aperture Radar (BSAR) is becoming more and more important in remote sensing, given its low power, low mass, low cost, and real-time global coverage capability. Due to its complex configuration, the imaging for GNSS-R BSAR is usually based on the Back-Projection Algorithm (BPA), which is very time consuming. In this paper, an efficient and general imaging algorithm for GNSS-R BSAR is presented. A Two Step Range Cell Migration (TSRCM) correction is firstly applied. The first step roughly compensates the RCM and Doppler phase caused by the motion of the transmitter, which simplifies the SAR data into the quasi-mono-static case. The second step removes the residual RCM caused by the motion of the receiver using the modified frequency scaling algorithm. Then, a cubic phase perturbation operation is introduced to equalize the Doppler frequency modulation rate along the same range cell. Finally, azimuth phase compensation and geometric correction are completed to obtain the focused SAR image. A simulation and experiment are conducted to demonstrate the feasibility of the proposed algorithm, showing that the proposed algorithm is more efficient than the BPA, without causing significant degradation in imaging quality

Joint detection and localization of vessels at sea with a GNSS-Based multistatic radar

This paper addresses the exploitation of global navigation satellite systems as opportunistic sources for the joint detection and localization of vessels at sea in a passive multistatic radar system. A single receiver mounted on a proper platform (e.g., a moored buoy) can collect the signals emitted by multiple navigation satellites and reflected from ship targets of interest. This paper puts forward a single-stage approach to jointly detect and localize the ship targets by making use of long integration times (tens of seconds) and properly exploiting the spatial diversity offered by such a configuration. A proper strategy is defined to form a long-time and multistatic range and Doppler (RD) map, where the total target power can be reinforced with respect to, in turn, the case in which the RD map is obtained over a short dwell and the case in which a single transmitter is employed. The exploitation of both the long integration time and the multiple transmitters can greatly enhance the performance of the system, allowing counteracting the low-power budget provided by the considered sources representing the main bottleneck of this technology. Moreover, the proposed single-stage approach can reach superior detection performance than a conventional two-stage process where peripheral decisions are taken at each bistatic link and subsequently the localization is achieved by multilateration methods. Theoretical and simulated performance analysis is proposed and also validated by means of experimental results considering Galileo transmitters and different types of targets of opportunity in different scenarios. Obtained results prove the effectiveness of the proposed method to provide detection and localization of ship targets of interest

GNSS-based passive radar techniques for maritime surveillance

The improvement of maritime traffic safety and security is a subject of growing interest, since the traffic is constantly increasing. In fact, a large number of human activities take place in maritime domain, varying from cruise and trading ships up to vessels involved in nefarious activities such as piracy, human smuggling or terrorist actions. The systems based on Automatic Identification System (AIS) transponder cannot cope with non-cooperative or non-equipped vessels that instead can be detected, tracked and identified by means of radar system. In particular, passive bistatic radar (PBR) systems can perform these tasks without a dedicated transmitter, since they exploit illuminators of opportunity as transmitters. The lack of a dedicated transmitter makes such systems low cost and suitable to be employed in areas where active sensors cannot be placed such as, for example, marine protected areas. Innovative solutions based on terrestrial transmitters have been considered in order to increase maritime safety and security, but these kinds of sources cannot guarantee a global coverage, such as in open sea. To overcome this problem, the exploitation of global navigation satellites system (GNSS) as transmitters of opportunity is a prospective solution. The global, reliable and persistent nature of these sources makes them potentially able to guarantee the permanent monitoring of both coastal and open sea areas. To this aim, this thesis addresses the exploitation of Global Navigation Satellite Systems (GNSS) as transmitters of opportunity in passive bistatic radar (PBR) systems for maritime surveillance. The main limitation of this technology is the restricted power budget provided by navigation satellites, which makes it necessary to define innovative moving target detection techniques specifically tailored for the system under consideration. For this reason, this thesis puts forward long integration time techniques able to collect the signal energy over long time intervals (tens of seconds), allowing the retrieval of suitable levels of signal-to-disturbance ratios for detection purposes. The feasibility of this novel application is firstly investigated in a bistatic system configuration. A long integration time moving target detection technique working in bistatic range&Doppler plane is proposed and its effectiveness is proved against synthetic and experimental datasets. Subsequently the exploitation of multiple transmitters for the joint detection and localization of vessels at sea is also investigated. A single-stage approach to jointly detect and localize the ship targets by making use of long integration times (tens of seconds) and properly exploiting the spatial diversity offered by such a configuration is proposed. Furthermore, the potential of the system to extract information concerning the detected target characteristics for further target classification is assessed

Edistysaskeleita keilaavissa ja vähäisen sironnan antenneissa viestintään ja kartoitukseen

Defence is held on 23.4.2021 11:00 – 14:00 Remote: https://aalto.zoom.us/j/68339893260Communication networks are being developed to handle the increased wireless data traffic that the existing systems cannot handle. Fifth-generation mobile networks (5G) are adopting millimeter-wave (mm-wave) frequencies for higher spectral efficiency and wider spectral bands in order to increase network capacity. Additionally, the number of network antennas will increase exponentially, since the propagation at mm-waves suffers from intrinsic atmospheric attenuation. Furthermore, the mm-wave antennas are required to be highly efficient, and beam-steering capabilities are also necessary to focus capacity where it is needed. The first part of this thesis discusses phased array designs for the mm-wave base-station applications and tools for the analysis and optimization of the antennas. The presented antennas operate in Ka- and E-bands, and they combine low-loss, waveguide-based power division networks and antenna elements with phase shifters that are integrated on a printed circuit board (PCB). The resulting proposals are antennas with high efficiency, where the majority of the losses are caused by the used phase shifters. The performances of both antennas (e.g., their beam-steering capabilities) have been validated with measurements. Furthermore, the antenna diagnostic results based on the holography data are presented, along with optimization methods that allow performance enhancement in terms of higher antenna gain and lower side lobes. Because antenna development can be a time-consuming and costly process, utilizing the same antenna in multiple different scenarios is desirable. The second part of the thesis explains how the previously presented Ka-band antennas, which were initially developed for communications, can be used in imaging applications. A frequency-diverse imaging method is explained, in which a computational algorithm is used to reconstruct the image from the observation data. A theoretical evaluation and experimental test results are presented. The proposed method has been used successfully to reconstruct an image of the scene locating a conducting sphere, and future research paths are discussed. Modern radar applications may require co-locating multiple antennas together, especially if the area or the volume reserved for the antennas is limited. Therefore, electrically invisible or transparent antennas that do not affect the performance of the co-existing antennas are required. The third and final part of the thesis focuses on this topic and describes the design steps required to realize a low-scattering antenna, i.e., an inductively loaded, chopped dipole that is transparent at a higher frequency than where it operates. It is experimentally demonstrated how the radar cross section of the designed antenna is reduced at the higher frequency 15 dB, while the radiation efficiency of the dipole decreases 0.4 dB at its operation frequency due to the inductive loading.Tulevaisuuden viestintäverkkoja kehitetään vastaamaan alati kasvavan dataliikenteen asettamia vaatimuksia. Viidennen sukupolven matkapuhelinverkot (5G) hyödyntävät millimetri-aaltotaajuuksia (mm-aalto) verkon kapasiteetin parantamiseksi, mm. lisäämällä käytettävissä olevaa taajuuskapasiteettia. Lisäksi verkon rakentamiseen tarvittavien antennien määrä tulee kasvamaan eksponentiaalisesti, sillä mm-aalloilla ilmakehän vaimennus kasvaa. Mm-aaltoantennien on myös oltava energiatehokkaita ja niiden täytyy tukea sähköistä antennikeilan kääntöä, jotta kapasiteettia voidaan kohdistaa sinne, missä sitä tarvitaan. Tämän väitöskirjan ensimmäisessä osassa esitellään tukiasemasovelluksiin soveltuvia mm-aalloilla toimivia vaiheistettuja antenniryhmiä ja käsitellään työkaluja kyseisten antennien analysointiin sekä optimointiin. Antennit toimivat Ka- ja E-kaistoilla, ja niiden uutuusarvona on piirilevypohjaisten vaiheensiirtimien integroiminen matalahäviöiseen, aaltoputkiin pohjautuvaan antennirakenteeseen. Näillä antenneilla on korkea hyötysuhde, ja suurin osa niiden häviöistä johtuu käytetyistä vaiheensiirtimistä. Antennien suorituskyky, esim. keilankääntö, on todennettu mittauksin. Samoihin antenneihin liittyen esitellään myös holografiatietoihin perustuvat antennidiagnostiikkatulokset yhdessä optimointimenetelmien kanssa. Menetelmät mahdollistavat antennien tarkemman analysoimisen ja suorituskyvyn parantamisen suuremman vahvistuksen ja matalampien sivukeilatasojen muodossa. Koska antennien kehitystyö voi olla aikaa vievää ja kallista, on suositeltavaa käyttää samoja antenneja useissa eri sovelluksissa. Väitöskirjan toisessa osassa selitetään, miten alun perin viestintään kehitettyjä, edellä mainittuja Ka-kaistan antenneja voidaan käyttää kuvantamisessa. Tätä varten on työssä kehitetty antennikeilojen taajuusdiversiteettiä hyödyntävä kuvantamismenetelmä, jossa laskennallisesti muodostetaan kuva havainnoitavasta avaruudesta mittadatan perusteella. Työssä esitellään menetelmään liittyvät teoreettiset ja kokeelliset tulokset sekä keskustellaan mahdollisista uusista tutkimuspoluista. Väitöskirjan kolmas osa keskittyy sähköisesti läpinäkyviin/näkymättömiin antenneihin. Nykyaikaiset tutkasovellukset saattavat edellyttää useiden antennien keskinäistä yhteensopivuutta ja sijoittamista rinnakkain, varsinkin jos antenneille varattu alue on rajallinen. Sähköinen läpinäkyvyys tarkoittaa väitöskirjan kontekstissa vähäistä sirontaa, ja tälläiset antennit eivät vaikuta muiden läheisten antennien suorituskykyyn. Työssä esitellään suunnitteluperusteet vähäisen sironnan antennille ja lopputuloksena on induktiivisesti kuormitettu dipoliantenni, joka on läpinäkyvä korkeammilla taajuuksilla, kuin millä se itse toimii. Suunnitellun dipolin tutkapoikkipinta-ala pienenee korkeammilla taajuuksilla 15 dB verrattuna referenssirakenteeseen, ja samanaikaisesti antennin säteilyhyötysuhde heikkenee 0.4 dB sen toimintataajuudella

Point spread function analysis for GNSS-based multistatic SAR

This letter presents an analysis of the multistatic point-spread function (MPSF) for passive synthetic aperture radar (SAR) with navigation satellites as opportunity transmitters and a stationary receiver. It is shown that a noncoherent combination of bistatic SAR images, obtained by multiple, spatially separated satellites, can yield multistatic imagery that may be essentially improved in terms of resolution when compared with a single bistatic SAR image. The MPSF is derived analytically, for an arbitrary number of bistatic acquisitions and for any bistatic topologies. Analytical results are confirmed using both simulated and experimental data. The obtained result could be applied for the analysis of spatial resolution in multistatic real time radar, thus enabling the adaptive selection of the more suitable opportunity transmitters