Simultaneous operation of two over-the-horizon radars

Abstract—By exploiting the reflective and refractive nature of high-frequency (HF) radiowave propagation through the iono-sphere or the conducting sea surface, over-the-horizon radar (OTHR) systems perform wide-area surveillance at long range well beyond the limit of the horizon of conventional line-of-sight (LOS) radars. Improved characterizations of the targets can be achieved by using multiple OTHRs operating simultaneously as compared to a single OTHR operating alone. In this paper, we consider concurrent operations of two OTHR systems that occupy the same frequency band with different chirp waveforms. The objective is to respond to the advanced wide-area surveillance needs without reducing the wave repetitive frequency. For this purpose, a new cross-radar interference cancellation technique is developed and its effectiveness is verified through both analytical and simulation results. I

Radio frequency interference in radio astronomy

Radioastronomiassa mitataan heikkojen taivaallisten kohteiden säteilyä lähes koko ilmakehän suomassa taajuusikkunassa. Mitattava säteily on luoteeltaan heikkoa kohinaa, jonka mittaamiseen tarvitaan herkkiä radiovastaanottimia. Radioastronomisia mittauksia rajoittavat ilmakehän vaimennuksen lisäksi merkittävästi myös ihmisten tuottamat radiohäiriöt. Tämän diplomityön tarkoituksena on selvittää mitä nämä ihmisten aiheuttamat radiohäiriöt ovat, miten ne vaikuttavat radioastronomisiin mittauksiin ja mitä niiden vaikutusten minimoimiselle on tehtävissä. Radiohäiriömittaukset suoritettiin Metsähovin radio-observatoriossa kesällä 2010 usealla eri taajuuskaistalla. Näiden radiohäiriöiden mittaamiseen käytettiin spektrianalysaattoria ja parabolista antennia. Ulkoisten radiohäiriöiden lisäksi diplomityössä tutkittiin myös observatorion itsessään synnyttämää häiriösäteilyä. Radioympäristön vaikutusta radiovastaanottimien toimintaan tutkittiin usealla eri vastaanottimella, nauhoittamalla spektrianalysaattorilla radiovastaanottimen välitaajuuskaistan spektriä, kun radioteleskooppi asetettiin keilaamaan horisonttia matalalla elevaatiolla. Radioympäristön mittauksissa voimakkaimmiksi häiriötekijöiksi osoittautuivat digitaaliset televisiolähetykset, matkapuhelintaajuudet, ilmailun radioliikenne ja navigointi sekä radioamatöörilähetykset. Observatorion sisäisistä häiriöstä voimakkaimmat olivat peräisin mikroaaltouunista ja matkapuhelimista. Jotta myös tulevaisuudessa voidaan radioastronomisten mittausten luotettavuus taata jatkuvasti kasvavassa häiriöympäristössä, esitellään myös suunnittelu ehdotus uudesta radiohäiriömonitorista.Radio astronomy is a subfield of astronomy, which studies celestial objects with highly sensitive radio receivers and large radio telescopes. These radio astronomical measurements are performed in a wide range of frequencies which are limited by the radio window set by the attenuation in the Earth's atmosphere and the man-made radio frequency interferences (RFI). This thesis discusses, measures and identifies these man-made RFIs, their effect on these sensitive measurements and the methods to mitigate their effects on the observation. The external radio frequency environment in the Metsähovi Radio Observatory was measured in the summer of 2010 with an antenna and a spectrum analyzer in several frequency bands. As radio astronomical measurements are also vulnerable to the self-generated RFIs of the observatory, a few corresponding measurements were performed to determine the state of the internal radiation. Three radio receivers were tested to estimate the impact of external and internal RFIs on radio astronomical measurements. To identify the interference sources and their origin, a spectrum analyzer was connected to the intermediate frequency (IF) port of a radio receiver, while the radio telescope was set to sweep the horizon with a low elevation. The most significant external RFI sources discovered in this study consisted of Digital Video Broadcasts (DVB), GSM, UMTS (3G-network), aviation radio broadcasts and radio amateur activity. The strongest detected internal interference sources comprised cell phones and microwave ovens. This thesis also proposes a new radio frequency interference monitoring equipment to guarantee the safety of the radio astronomical observation in Metsähovi in the future

Adaptive Radio Frequency Interference Mitigation for Passive Bistatic Radar Using OFDM Waveform

High frequency passive bistatic radar (HFPBR) is a novel and promising technique in development. DRM broadcast exploiting orthogonal frequency division multiplexing (OFDM) technique supplies a good choice for the illuminator of HFPBR. HFPBR works in crowded short wave band. It faces severe radio frequency interference (RFI) problem. In this paper, a theoretical analysis of the range-domain correlation of RFI in OFDM-based HF radar is presented. A RFI mitigation method in the range domain is introduced. After the direct-path wave rejection, the interference subspace is constructed using the echo signals at the reserved range bins. Then RFI in the effective range bins is mitigated by the subspace projection, using the correlation among different range bins. The introduced algorithm is easy to perform in practice and the RFI mitigation performance is evaluated using the experimental data of DRM-based HFPBR

Towards coordinated site monitoring and common strategies for mitigation of Radio Frequency Interference at the Italian radio telescopes

We present a project to implement a national common strategy for the mitigation of the steadily deteriorating Radio Frequency Interference (RFI) situation at the Italian radio telescopes. The project involves the Medicina, Noto, and Sardinia dish antennas and comprised the definition of a coordinated plan for site monitoring as well as the implementation of state-of-the-art hardware and software tools for RFI mitigation. Coordinated monitoring of frequency bands up to 40 GHz has been performed by means of continuous observations and dedicated measurement campaigns with fixed stations and mobile laboratories. Measurements were executed on the frequency bands allocated to the radio astronomy and space research service for shared or exclusive use and on the wider ones employed by the current and under-development receivers at the telescopes. Results of the monitoring campaigns provide a reference scenario useful to evaluate the evolution of the interference situation at the telescopes sites and a case series to test and improve the hardware and software tools we conceived to counteract radio frequency interference. We developed a multi-purpose digital backend for high spectral and time resolution observations over large bandwidths. Observational results demonstrate that the spectrometer robustness and sensitivity enable the efficient detection and analysis of interfering signals in radio astronomical data. A prototype off-line software tool for interference detection and flagging has been also implemented. This package is capable to handle the huge amount of data delivered by the most modern instrumentation on board of the Italian radio telecsopes, like dense focal plane arrays, and its modularity easen the integration of new algorithms and the re-usability in different contexts or telescopes.Comment: 39 pages, 10 Figures and 7 Tables. INAF Technical Report n. 149 (2022). http://hdl.handle.net/20.500.12386/3208

Radar Technology

In this book “Radar Technology”, the chapters are divided into four main topic areas: Topic area 1: “Radar Systems” consists of chapters which treat whole radar systems, environment and target functional chain. Topic area 2: “Radar Applications” shows various applications of radar systems, including meteorological radars, ground penetrating radars and glaciology. Topic area 3: “Radar Functional Chain and Signal Processing” describes several aspects of the radar signal processing. From parameter extraction, target detection over tracking and classification technologies. Topic area 4: “Radar Subsystems and Components” consists of design technology of radar subsystem components like antenna design or waveform design