Reflection Of Radio Waves Research Articles

Investigating Different Interpolation Methods for High-Accuracy VTEC Analysis in Ionospheric Research

The dynamic structure of the ionosphere and its changes play an important role in comprehending the natural cycle by linking earth sciences and space sciences. Ionosphere research includes a variety of fields like meteorology, radio wave reflection from the atmosphere, atmospheric anomaly detection, the impact on GNSS (Global Navigation Satellite Systems) signals, the exploration of earthquake precursors, and the formation of the northern lights. To gain further insight into this layer and to monitor variations in the total electron content (TEC), ionospheric maps are created using a variety of data sources, including satellite sensors, GNSS data, and ionosonde data. In these maps, data deficiencies are addressed by using interpolation methods. The objective of this study was to obtain high-accuracy VTEC (Vertical Total Electron Content) information to analyze TEC anomalies as precursors to earthquakes. We propose an innovative approach: employing alternative mathematical surfaces for VTEC calculations, leading to enhanced change analytical interpretation for anomaly detections. Within the scope of the application, the second-degree polynomial method, kriging (point and block model), the radial basis multiquadric, and the thin plate spline (TPS) methods were implemented as interpolation methods. During a 49-day period, the TEC values were computed at three different IGS stations, generating 1176 hourly grids for each interpolation model. As reference data, the ionospheric maps produced by the CODE (Center for Orbit Determination in Europe) Analysis Center were used. This study’s findings showed that, based on statistical values, the TPS model offered more accurate results than other methods. Additionally, it has been observed that the peak values in TEC calculations based on polynomial surfaces are eliminated in TPSs.

Design, fabrication and test of high temperature superconducting magnet for heat flux and radio blackout mitigation experiments in plasma wind tunnels

High heat flux and radio blackout are well-known challenges space vehicles have been facing during re-entry into a planet’s atmosphere since the early days of space-flight. Thermal protection systems have been developed to protect spacecraft and astronauts, however, they are often heavy and some have to be replaced after each mission. High temperatures in the compressed gas in the shock wave lead to partial ionization. The dense plasma can cause radio blackout, i.e. attenuation or reflection of radio waves thus blocking data-telemetry and communication with ground stations or satellites. One approach to solve both problems is to influence the plasma with magnetohydrodynamic effects using a strong magnet. In the framework of the European project MEESST (Magnetohydrodynamic Enhanced Entry System for Space Transportation) heat flux mitigation and radio blackout mitigation is investigated by means of modelling and ground experiments in plasma wind tunnels at the Institute of Space Systems (Stuttgart, Germany) and at the Von Karman Institute for Fluid Dynamics (Brussels, Belgium) using an HTS magnet. After a short introduction to the scientific background of the MEESST project, the boundary conditions for the design of the magnet and calculations of field distributions are presented. The pancake coils of the magnet were wound with a robotic winding system. Results from a preliminary test of the conduction-cooled magnet are presented.

Simultaneous observations of equatorial plasma bubbles with an all-sky airglow imager and a HF Doppler sounding system in Taiwan

High-Frequency Doppler (HFD) sounders at low-latitudes often detect characteristic oblique spreading Doppler traces in the spectrogram, known as Oblique Spread Structure (OSS). OSS has been expected to be generated by the dispersion of radio wave reflection due to equatorial plasma bubbles (EPBs). However, it has not yet been confirmed whether OSS is surely a manifestation of EPB by conducting simultaneous observations of EPB and OSS with different observational techniques. Additionally, it remains unclear what kinds of properties of EPB are reflected in the fine structure of OSS. In this study, we investigated three cases of OSSs and EPBs simultaneously observed by a HFD sounding system and an all-sky airglow imager in Taiwan. For the three cases presented here, the timing of OSS occurrence in the HFD data well coincided with that of the EPB appearance in the airglow data. The frequency shift of OSS is quantitatively explained assuming a radio wave reflection at 250–300 km altitudes. These results strongly indicate that OSS is formed by electron density variations at F-region altitudes accompanying EPB; thus, OSS is a manifestation of EPB in the HFD observations. Furthermore, it was suggested that the fine structure of OSS reflected the branching structure of EPB when the multiple branches of EPB reached the intermediate reflection point of the HFD observation. The detection of EPB occurrence and its fine structure using HFD observation enables monitoring of EPB regardless of weather conditions, which will contribute to monitoring the space weather impact of EPBs, for example, on GNSS navigation, in a wide area.Graphical

PRODUCTION TECHNOLOGY OF RADIO-TRANSPARENT CERAMIC MATERIALS

Ceramic radiotransparent materials are non-metallic materials that ensure the transmission of electromagnetic radiation in the radio frequency range of 105–1012 Hz with minimal dielectric losses in the operating temperature range (tgδ 10-2–10-5, ε < 10) and a low value of the radio wave reflection coefficient (P < 1 %). Ceramic radiotransparent materials are used to create rocket and aircraft fairings that protect antenna systems from external factors. For the synthesis of radiotransparent ceramic materials, it is expedient and promising to use such crystalline phases as celsian (BaAl2Si2O8) and slawsonite (SrAl2Si2O8) due to their low dielectric constant and dielectric loss tangent, as well as rather high thermal stability and melting point compared to other aluminosilicate compounds (mullite, spodumene, cordierite, etc.). The development of technology for obtaining radio-transparent ceramic antenna fairings and elements of protective structures of antenna systems of aviation facilities based on the BaO (SrO) – Al2O3 – SiO2 system is relevant. For the formation of complex profile ceramic radiotransparent materials slurry casting in porous gypsum molds is used. In order to compare the developed ceramic compositions, the physico-mechanical, dielectric and electrodynamic properties were studied under normal conditions and in the millimeter frequency range (26-38 GHz). As a result of the research, the following have been developed: a technology for the production of ceramic radiotransparent materials based on slawsonite by semi-dry pressing method, based on celsian-slawsonite by slip casting method and based on celsian or slawsonite by slip casting method. It has been established that the developed materials meet the technical requirements for radio-transparent ceramic materials and can be used to produce elements of protective structures of antenna equipment, such as nose fairings for aircraft.

Geophysical research of underground hydraulic structures, old Glubochinsky water tunnel case study

Research objective is to study the features of geophysical fields and parameters in the area of the artificial underground water tunnel and estimate the capabilities of geophysical methods of identifying hidden underground industrial facilities. Methods of research. Measurements of self-potential signals (SP) were carried out using nonpolarizing electrodes and an electrical exploration receiver. The magnetic exploration was carried out by a proton magnetometer. Electrotomography was carried out using multielectrode electrical survey equipment with a Schlumberger symmetrical scheme of surveys. The microseismic background was recorded using an autonomous seismic station. The georadar survey was carried out using the SIR-3000 ground penetrating radar and antennas with the frequencies of 100 MHz and 270 MHz. Results. Based on the survey results, graphs of magnetic and self-potential fields, electrotomographic and microseismic sections, and radarogram were built on the research profile. The research has shown different levels of the magnetic field and the SP in the area of volcanic rocks and dikes of gabbro-dolerites occurrence. A significant increase in microseismic noise was observed with an increase in the weathering crust thickness. A local decrease in the SP potential, abnormally high values of specific electrical resistivity and intensive reflections of radio waves from the tunnel top were recorded above the underground water tunnel. Conclusions. Qualitative and quantitative geophysical characteristics were obtained in the area of the old underground water tunnel. The research showed different levels of the magnetic field and spontaneous polarization potentials in the occurrence area of rocks of different composition. A significant increase in microseismic noise was recorded with an increase in the weathering crust thickness. Geophysical signs of the boundaries between geological structures and artificial underground objects are revealed. The exact position and depth of the old underground tunnel of Glubochinsky hydraulic structures was determined.

Accurate Phase Angle Measurement of Backscatter Signal under Noisy Environment

Backscatter communication is an emerging wireless access technology to realize ultra-low power terminals exploiting the modulated reflection of incident radio wave. This paper proposes a method to measure the phase angle of backscatter link using principal component analysis (PCA). The phase angle measurement of backscatter link at the receiver is essential to maximize the signal quality for subsequent demodulation and to measure the distance and the angle of arrival. The drawback of popular phase angle measurement with naive phase averaging and linear regression analysis is to produce erroneous phase angle, where the phase angle is close to $\pm\frac{\pi}{2}$ radian and the signal quality is poor. The advantage of the proposal is quantified with a computer simulation, a conducted experiment and radio propagation experiments.

Ionosphere induced spatial Imbert Fedorov shifts

The reflection and transmission of radio waves are two most common techniques used to study the ionosphere. Considering an electromagnetic wave with a finite diameter, however, displacements and deflections different from the predictions of geometric optics arise. In this paper, we investigate thru calculations one type of these shifts, the out-of-plane Imbert-Fedorov (IF) displacements, induced by the ionosphere on the reflected radio waves. We model the ionosphere as a simple collection of electrons moving in a uniform magnetic field and use the resulting dispersive expressions to calculate the corresponding IF shifts. Since the ionosphere is a dispersive medium, we consider three cases: (1) metallic, (2) dielectric and (3) epsilon-near-zero (ENZ) behavior of the layer. We determine that there exists non-zero differential spatial IF shifts for all the three cases and find that the maximum shift of up to 95 m occurs when the ionosphere acts like an ENZ medium. Finally, we show that these transverse shifts are measurable and discuss how these can be utilized as a potential tool to profile the ionosphere using reflected radio waves.

A Clutter Loss Model for Satellite Communication Systems

Buildings and vegetation in the proximity of a terrestrial base station induce a significant additional loss, typically referred to as “clutter loss”, which sums up to free space loss and atmospheric attenuation. Clutter loss is essentially due to the radiowave reflection and diffraction caused by buildings and vegetation, and tends to reduce the interference between terrestrial systems, such as upper 6 GHz (U6G), and satellite systems operating in the same frequency bands. In fact, for example, at low elevation angles, the clutter loss could reach some tens of dB in the U6G band. A novel clutter loss model in urban and suburban environments for frequencies up to 10 GHz is proposed. The model relies on the Monte Carlo simulation approach presented in Report ITU-R P.2402-0, but some limitations have been removed to extend its applicability to more complex scenarios and possibly increase its accuracy for U6G systems. An analytical approach is also proposed to model the clutter loss statistics obtained by properly fitting the obtained statistics for the cities of London and Melbourne. Finally, the proposed model is validated by comparing its results to those obtained by a commercial ray tracer.

Application of radio wave reflections from meteor showers falling into the near-Earth space for organizing communications in the subpolar regions of the Earth

Application of radio wave reflections from meteor showers falling into the near-Earth space for organizing communications in the subpolar regions of the Earth

Measuring setup for determining the parameters of materials that absorb radio waves

A measuring complex for determining the coefficients of reflection and transmission of electromagnetic radiation through radio-absorbing materials has been developed and manufactured. Measurements are made by the direct method by comparison with a reference material. When measuring the transmission of radio waves through an absorbing material, the standard is air, and when measuring the reflection of radio waves, a sheet of steel is used as the standard. When measuring the transmission of radio waves through an absorbing material, the standard is air, and when measuring the reflection of radio waves, a sheet of steel is used as the standard. The installation uses replaceable horn antennas and standard microwave generators. The distance necessary for carrying out measurements in the far field of antenna radiation was determined and the maximum sizes of the irradiation spots of the measured materials for various frequencies in the range from 1 to 20 GHz were calculated. A measurement setup that is efficient in terms of the “price-quality” criterion is made, easy to maintain and does not require computational work.

Physical effects of geocosmic storms. Part 3

A review of the results of observing the effects of the most interesting ionospheric and magnetic storms that took place in 2018–2019 is presented. Ionospheric and magnetic storms are considered as components of geocosmic storms. In accordance with the systemic paradigm of L.F. Chernogor, a geocosmic storm is a complex of processes caused by a solar storm, reduced to synergistically interacting storms in a magnetic field (magnetic storm), ionosphere (ionospheric storm), atmosphere (atmospheric storm) and electric field atmospheric-ionospheric-magnetospheric origin (electric storm). Using the analysis of specific magnetic and ionospheric storms as an example, it is shown that, in addition to general patterns, each storm has its own individual characteristics. The expediency of a comprehensive study of the manifestations of each new geocosmic storm has been confirmed. A wide variety of ionospheric and magnetic storms have been confirmed. A weak magnetic storm is not necessarily accompanied by a weak ionospheric storm, while a strong one is accompanied by a strong one. An ionospheric storm can be single-phase (positive or negative) or multi-phase, when the phases alternate. Storms can last from several hours to many days. Storms are accompanied by both aperiodic and quasi-periodic variations. The latter are caused by atmospheric gravity waves (periods 10–180 min) and infrasound (period less than 5 min). Of all the components of a geocosmic storm, except for academic ones, ionospheric storms are of the greatest practical interest. It is these storms that limit the potential characteristics of radio systems for various purposes (telecommunications, radio navigation, radar, remote radio sounding, radio astronomy). It is shown that ionospheric storms are accompanied by the appearance of multipath, broadening of the Doppler spectra (and even their collapse), significant aperiodic and quasi-periodic variations in the Doppler frequency shift, the amplitude of radio signals, and the trajectories of radio waves in the decameter range. In particular, the height of reflection of radio waves changed during ionospheric storms by 50–180 km. The electron concentration during moderate negative ionospheric storms decreased up to 2 times, and during moderate positive ionospheric storms it increased up to 3 times. Under the action of atmospheric gravity waves, the relative amplitude of quasi-periodic variations in the electron concentration reached 50%, and due to low-frequency infrasound, it did not exceed 0.7%.

Physical bases of meteor registration methods and the instrument complex of the NKU Observatory

One of the actual directions of studying the physical characteristics of near–Earth space is the assessment of the concentration of meteoroids in it, and the time change of this parameter. In this regard, the principles of setting and implementing meteor observations at the observatory of the North Kazakhstan university named after Manash Kozybayev are considered in chronological order. Meteor observations were carried out on the basis of an integrated approach, including the use of meteor patrols, an All Sky camera, a system for recording radio wave reflections from a remote station from meteor tracks, and a camera based on an optical image intensifier.  Based on the analysis of the practical experience of using this equipment, the advantages and disadvantages of each of the methods of meteor registration are revealed. It is shown that the use of the latest image brightness amplifiers and wide–angle cameras with the highest possible sensor sensitivity is the most progressive for their optical observations. The closest attention is paid to the analysis of the experience of implementing the registration of plasma meteor tracks in the radio range. The method is based on the selection of reflected (scattered) musical–speech signals of a remote radio station having a specific pulse structure. Comparison of the obtained results on the frequency of meteor phenomena with the data of known observation points allows us to speak about the effective operation of the newly created meteor monitoring system in the radio range. In the final part of the work, ways to improve the efficiency of recording meteor phenomena in the optical range based on the use of the latest radiation receivers are considered.

Hostile UAV Detection and Neutralization Using a UAV System

The technologies of Unmanned Aerial Vehicles (UAVs) have seen extremely rapid development in recent years. UAV technologies are being developed much faster than the means of their legislation. There have been many means of UAV detection and neutralization proposed in recent research; nonetheless, all of them have serious disadvantages. The essential problems in the detection of UAVs is the small size of UAVs, weak radio wave reflection, weak radio signal, and sound emitting. The main problem of conventional UAV countermeasures is the short detection and neutralization range. The authors propose the concept of the airborne counter-UAV platform (consisting of several vehicles) with radar. We use a low-cost marine radar with a high resolution 2 m wide antenna, embedded into the wing. Radar scanning is implemented by changing the heading of the aircraft. For the countermeasures, the authors suggest using a small rotorcraft UAV carried by a bigger fixed-wing one. A mathematical model that allows the calculation of the coordinates of the detected drone while scanning the environment in a moving UAV with radar was created. Furthermore, the results of integrated radar performance with a detected drone and the results of successful neutralization experiments of different UAVs were achieved.

Physical effects of geocosmic storms

A review of the results of observing the effects of the most interesting ionospheric and magnetic storms that took place in 2018–2019 is presented. Ionospheric and magnetic storms are considered as components of geocosmic storms. In accordance with the systemic paradigm of L.F. Chernogor, a geocosmic storm is a complex of processes caused by a solar storm, reduced to synergistically interacting storms in a magnetic field (magnetic storm), ionosphere (ionospheric storm), atmosphere (atmospheric storm) and electric field atmospheric-ionospheric-magnetospheric origin (electric storm). Using the analysis of specific magnetic and ionospheric storms as an example, it is shown that, in addition to general patterns, each storm has its own individual characteristics. The expediency of a comprehensive study of the manifestations of each new geocosmic storm has been confirmed. A wide variety of ionospheric and magnetic storms have been confirmed. A weak magnetic storm is not necessarily accompanied by a weak ionospheric storm, while a strong one is accompanied by a strong one. An ionospheric storm can be single-phase (positive or negative) or multi-phase, when the phases alternate. Storms can last from several hours to many days. Storms are accompanied by both aperiodic and quasi-periodic variations. The latter are caused by atmospheric gravity waves (periods ~10–180 min) and infrasound (period less than 5 min). Of all the components of a geocosmic storm, except for academic ones, ionospheric storms are of the greatest practical interest. It is these storms that limit the potential characteristics of radio systems for various purposes (telecommunications, radio navigation, radar, remote radio sounding, radio astronomy). It is shown that ionospheric storms are accompanied by the appearance of multipath, broadening of the Doppler spectra (and even their collapse), significant aperiodic and quasi-periodic variations in the Doppler frequency shift, the amplitude of radio signals, and the trajectories of radio waves in the decameter range. In particular, the height of reflection of radio waves changed during ionospheric storms by 50–180 km. The electron concentration during moderate negative ionospheric storms decreased up to 2 times, and during moderate positive ionospheric storms it increased up to 3 times. Under the action of atmospheric gravity waves, the relative amplitude of quasi-periodic variations in the electron concentration reached 50%, and due to low-frequency infrasound, it did not exceed 0.7%.

Single-point positioning of ground-based sources of radio emission during reflection of an ionospheric radio wave from an inclined layer of the ionosphere

Single-point positioning of ground-based sources of radio emission during reflection of an ionospheric radio wave from an inclined layer of the ionosphere

РАДІОЛОКАЦІЙНЕ ВИМІРЮВАННЯ КУТІВ МІСЦЯ ЦІЛЕЙ МЕТОДОМ ROOT-MUSIC З УРАХУВАННЯМ СФЕРИЧНОСТІ ФАЗОВИХ ФРОНТІВ РАДІОХВИЛЬ

Subject and Purpose. The paper is an effectiveness study of the root-MUSIC technique as applied to elevation angle measurements of low-altitude radar targets over the sea, specifically with account of the radio waves’ phase front sphericity. The purpose is to find out whether and how much can the measurement accuracy be improved through account of the phase front sphericity, as compared with the classical approach assuming a plane phase front. Methods and Methodology. The work proceeds from computer simulation, considering a variety of sea roughness levels, and covers a range of radio wave reflection conditions from a nearly specular return to strong diff use reflection from the sea surface. The simulation involves a wide range of target distances within the far-field region with respect to the receive array. Results. A new approach has been suggested for taking into account the phase-front sphericity of the radio waves arriving from the target. The level of errors of elevation angle measurements has been estimated and compared with such shown by the conventional root-MUSIC method which assumes a plane phase front for the waves reflected from the target. The comparison concerns a wide range of sea roughness levels and target separations. Conclusion. It has been established that the proposed version of the root-MUSIC technique which takes wave front sphericity into account can significantly reduce the errors in elevation angle measurements for low-altitude targets.

Day after day variations of arrival angles and polarisation parameters of Q bursts recorded at Antarctic station “Akademik Vernadsky”

We present variations in the source bearings and polarization of the horizontal magnetic field of Q bursts recorded at Antarctic station “Akademik Vernadsky” (65.2458° S, 64.2572° W). We analyzed the parameters of the “polarization ellipse” inscribed in the initial impulse of the Q-burst, which is formed by a direct wave propagating from lightning to the observer along a short path in the Earth-ionosphere cavity. Three days, from March 31 to April 2, 2019, a significant part of the Q burst flux was stably observed in a narrow northerly azimuth sector associated with the direction to lightnings in South America. During this period, regular deviations of the measured azimuths from the northern direction were observed up to 8–10°, to the west when the morning terminator passed and to the east at the evening terminator. Duration of these deviations was about 4 h. The azimuth deviations were accompanied by characteristic oscillations in the “ellipticity” value of the direct wave field, N-shaped during the passage of the evening terminator, and a mirror-symmetric И-shaped oscillation on the morning terminator. We found that, on average, the polarization of the direct wave field is close to linear when the paths from the sources to the observer were on the day side, while at night side its “ellipticity” increases to 0.1. The observed azimuth deviations were qualitatively interpreted using the concept of geometric optics of the reflection of radio waves from the day-night non-uniformity in the Earth-ionosphere cavity. The measurement period was close to the day of the vernal equinox, when the propagation path from the sources and the lines of the solar terminator run almost parallel and radio waves are incident on the day-night interface at “sliding angles”, which enhances their “reflection".

Dynamic processes in the magnetic field and in the ionosphere during the 30 August–2 September 2019 geospace storm: influence on high frequency radio wave characteristics

Abstract. The concept that geospace storms are comprised of synergistically coupled magnetic storms, ionospheric storms, atmospheric storms, and storms in the electric field originating in the magnetosphere, the ionosphere, and the atmosphere (i.e., electrical storms) was validated a few decades ago. Geospace storm studies require the employment of multiple-method approaches to the Sun–interplanetary medium–magnetosphere–ionosphere–atmosphere–Earth system. This study provides general analysis of the 30 August–2 September 2019 geospace storm, the analysis of disturbances in the geomagnetic field and in the ionosphere, as well as the influence of the ionospheric storm on the characteristics of high frequency (HF) radio waves over the People's Republic of China. The main results of the study are as follows. The energy and power of the geospace storm have been estimated to be 1.5×1015 J and 1.5×1010 W, and thus, this storm is weak. The energy and power of the magnetic storm have been estimated to be 1.5×1015 J and 9×109 W, i.e., this storm is moderate, and a characteristic feature of this storm is the duration of the main phase of up to 2 d. The recovery phase also was lengthy and was no less than 2 d. On 31 August and 1 September 2019, the variations in the H and D components attained 60–70 nT, while the Z-component variations did not exceed 20 nT. On 31 August and 1 September 2019, the level of fluctuations in the geomagnetic field in the 100–1000 s period range increased from 0.2–0.3 to 2–4 nT, while the energy of the oscillations showed a maximum in the 300–400 to 700–900 s period range. During the geospace storm, a moderately to strongly negative ionospheric storm manifested itself by the reduction in the ionospheric F-region electron density by a factor of 1.4 to 2.4 times on 31 August and 1 September 2019, compared to the its values on the reference day. Appreciable disturbances were also observed to occur in the ionospheric E region and possibly in the Es layer. In the course of the ionospheric storm, the altitude of reflection of radio waves could sharply increase from ∼150 to ∼300–310 km. The atmospheric gravity waves generated within the geospace storm modulated the ionospheric electron density; for the ∼30 min period oscillation, the amplitude of the electron density disturbances could attain ∼40 %, while it did not exceed 6 % for the ∼15 min period. At the same time, the height of reflection of the radio waves varied quasi-periodically with a 20–30 km amplitude. The results obtained have made a contribution to the understanding of the geospace storm physics, to developing theoretical and empirical models of geospace storms, to the acquisition of detailed understanding of the adverse effects that geospace storms have on radio wave propagation, and to applying that knowledge to effective forecasting of these adverse influences.

Algorithm of Low-Flying Target Tracking in Monopulse Radar Stations Based on a Unscented Kalman Filter

Using the model of multipath propagation of radio waves and the method of unscented Kalman filtering, a nonlinear adaptive algorithm for tracking low-flying targets in a monopulse radar station has been developed. This algorithm, in contrast to the extended Kalman filter, makes it possible to compensate for significant autocorrelated interference in measurements of the elevation angle of low-flying targets, arising from reflections of radio waves from the underlying surface. The results of simulation of the proposed algorithm are presented.

Особенности патентного анализа радиолокационных систем различного назначения с использованием ресурсов патентного ведомства США

The patent documentation is one of the main information sources for forecasting the development of various technical (technological) areas. This paper examines the features of radar systems development with the use of the US Patent and Trademark Office resources. In this study, the data concerning the dynamics of USA invention patents registration for ten-year period (2010-2019) have been found for G01S13 (“Radar systems”) group of the International Patent Classification. The results obtained confirm the significant progress achieved in the development of the systems using the reflection or reradiation of radio waves in specific practical applications. In particular, the inventions intended for using in space (“space”), airspace (“air”), as well as in ground and surface (“surface”) systems. It is shown that during the last ten years the systems using reflection of radio waves (“primary radar systems”) and radar systems, specially adapted for specific applications, have demonstrated the best dynamics in their development. As a result of the data analysis, it is concluded that the radar systems, specially adapted for such specific applications as anti-collision purposes, radar systems for such specific applications, and primary radar systems, should be considered as the most promising in the next two to three years.