Alain Bourdillon

Azimuth‐time‐intensity striations of quasiperiodic radar echoes from the midlatitude E region ionosphere

The Valensole high frequency (HF) radar in the south of France is an ionospheric Doppler sounder which can perform E region coherent backscatter measurements over an azimuthal sector of 86°, from 26° E to 58° W, with ∼2° angular resolution. This large azimuthal coverage is taken advantage of in order to study quasiperiodic (QP) echoes in the zonal direction using azimuth-time-intensity (ATI) analysis. ATI plots show sequential sloping striations of scatter reminiscent of those detected routinely in the range-time-intensity (RTI) plots of midlatitude radars which view the medium at a fixed azimuth about the meridian. It was found that ATI striation periods range from a few minutes to less than 30 min, whereas the striation slopes are systematically negative (motions westward) prior to local midnight, and turn positive (motions eastward) in the post-midnight hours. The zonal rates, dx/dt, computed from the striation slopes take values between ∼30 and 160 m/s. These are due to real motions of unstable plasma structures, most likely sporadic E patches that drift along with the neutral wind, that have zonal scale lengths of several tens of kilometers. The present observations imply that the mechanism responsible for QP echoes is independent of azimuth and can basically operate effectively in any direction in the horizontal plane.

Inversion of Backscatter Ionograms Optimization by using Simulated Annealing and Genetic Algorithms

The Over-The-Horizon Radars (OTHR) use the refraction of the waves on the ionosphere to reach the targets over the horizon. In order to know the real position of the targets the actual ionospheric characteristics must be estimated. So an inversion method to obtain them has been developed. This inversion method is capable to obtain the parameters of a model of the electronic density profile from some measurements realized with the radar, the backscatter ionograms. To optimize the method two algorithms has been introduced: the simulated annealing and the genetic algorithm.

Improvements in target ranging by HF radar using the European ionospheric model PRIME

Abstract Target ranging by HF radar can be done by correcting the group path of target echoes for the delay due to the propagation through the ionosphere. One possibility is to simulate the propagation by ray-tracing but in this case the final accuracy obtained in target registration depends mainly on the quality of the ionospheric model used to raytrace. In this study, the PRIME ionospheric model has been used to perform ray-tracing on a 950 km path, between Toulon and Lannion, for which oblique soundings data from the SCIPION sounder are available for December 1991. The ray-tracings were performed by using the vertical electron density profiles at five points along the path calculated by PRIME for monthly median and/or instantaneous conditions. Vertical electron density profiles from the IRI model have also been considered for comparison. The results of this investigation are presented and the accuracy in target ranging is discussed as obtained for the conditions of the simulations.

CONTINUOUS GPS NETWORK IN VIETNAM AND RESULTS OF STUDY ON THE TOTAL ELECTRON CONTENT IN THE SOUTH EAST ASIAN REGION

This study presents the continuous GPS network in Vietnam and the results of the study on time variations of the total electron content (TEC) in the South East Asian equatorial ionization anomaly (EIA) for the 2006-2013 period. In each year we observe a semiannual pattern of TEC diurnal variation in all the stations with its maximum amplitude at the equinox. In both hemispheres, the amplitude of the crest is larger in northern spring equinox than in autumn from 2006 to 2008 (descending phase of solar cycle) and smaller in spring than in autumn from 2009 to 2011 (increasing phase of solar cycle), from 2012-2013 (maximum period of solar cycle) the amplitudes of the crests in spring and autumn are equivalent. We also observe an asymmetry between the amplitude and the position of the two crests of ionization. There is a very high level of correlation between the amplitude of the TEC at the two crests and the sunspot number is approximately equal to 0.9. During the deep solar minimum 2008-2009, the amplitude of crests of ionization becomes small during several months in summer and winter (about 20 TECu). The results show that both crests move more significantly equatorward in winter than in other seasons and there is a tendency for both crests to appear earlier in winter and later in summer. In the solar minimum years, the amplitudes of the crests are minimal; the positions of the crests are nearer the magnetic equator and the crests appear earlier than in other years.

Dịch chuyển vỏ Trái Đất theo số liệu GPS liên tục tại Việt Nam và khu vực Đông Nam Á

SUMMARY Crustal motion from the continuous GPS data in Vietnam and in the Southeast Asian region This paper presents an estimation of the velocity of the Earth’s crust in Vietnam and the Southeast Asian region, determined from the GPS data in nearly 5 years (4/2005-12/2009) at 3 sites in Vietnam (Hanoi, Hue and Ho Chi Minh city) plus more than 20 ones in Southeast Asia and other regions using GAMIT software. The horizontal velocity vectors in ITRF2005 at the considered Southeast Asian stations show that they drive to the south-eastward, but the KUNM, Hanoi and Hue (with velocity of about 32-35mm/yr) move faster than the Ho Chi Minh city, NTUS and BAKO (with the velocity of 22-26mm/yr) ; meanwhile the PIMO drives to the northwest with the velocity of 30 mm/yr. The fact that relative velocity vectors with respect to the Eurasian plate decrease from 9.7 mm/yr at Hanoi, 6.9 mm/yr at Hue to 4.0 mm/yr at Ho Chi Minh city, as well as the increase of the motion azimuths from 139°8 at Hanoi to 226°3 at HCM city show that the Indochina block rotates clockwise ; however, its northern part moves faster than the southern one. The increase of the azimuths of the relative velocities with respect to the Sundaland block from 201°6 at Hanoi, 221°3 at Hue to 254°4 at Ho Chi Minh City also implicates the clockwise rotation of the Indochina block. Significant differences of the relative velocities with respect to Sundaland at the stations in this block (1.4 mm/yr at BAKO, ~12 mm/yr at NTUS and HCM city, and 7.8 mm/yr at Hue) implicate that the Sundaland also undergoes a significant internal deformation.

Bước đầu nghiên cứu nồng độ điện tử tổng cộng và nhấp nháy điện ly sử dụng số liệu các trạm thu GPS liên tục ở Việt Nam

A preliminary study of the total electron content and ionospheric scintillation using the GPS continuous data in Vietnam ...

COST 296 MIERS: conclusion

The need for more reliable and efficient communications services, especially those involving ionospheric HF communications and navigational systems, imposes increasing demand for a better knowledge of the effects imposed by the Earth’s upper atmosphere and ways to mitigate disturbing effects. Temporal and spatial changes in the upper atmosphere act to limit and degrade the performance of terrestrial and Earth-space radio systems in many different ways and this is why mitigation activities must involve several topics like ionospheric monitoring and modeling, development of new hardware for communication systems and new propagation simulator, measurements and modeling of ionospheric Total Electron Content (TEC) and ionospheric scintillations, using in particular the Global Positioning System (GPS). The European ionospheric community has long been aware that cooperation research on an international basis is essential to deal with such complex issues. In particular, international cooperation is required for the collection of data, in both the real-time and in retrospective modes, the development and verification of new methods to improve the performance of both operational and future terrestrial and Earth-space communication systems and the exchange of expertise on space plasma effects on Global Navigation Satellite Systems (GNSS). In this context the COST 296 Action MIERS on the «Mitigation of Ionospheric Effects on Radio Systems» has made a significant impact in a number of areas.

Backscatter ionogram inversion validation with the help of a HF radio transmitter

Accuracy in target localization by over-the-horizon radar can be improved with a good knowledge of the propagation channel. An inversion method has been developed in order to provide an electron density profile of an equivalent ionosphere from backscatter ionograms. The method uses the measurements realized by the HF radar during elevation scans. The method has been validated on synthesized and real data by comparing the results with ionospheric forecasts and vertical ionosonde. The equivalent ionosphere determined by inversion will be used to convert group path into ground range and to realize coordinate registration of targets. In this paper, we present first experiments of radio frequency interference (RFI) source localization. RFI source localization is used to estimate the precision of our inversion method.

A SIMO System of Digital Transmission Through the Ionospheric Channel

This paper presents a SIMO system of trans horizon transmission through the ionospheric channel in the HF band (3- 30 MHz). The technical challenge is to increase significantly the data transfer rate if compared with standard modems (typically 4.8 kbits/s in a 3 kHz bandwidth). A multi channel receiving system is connected to an original array of collocated antennas : this device appears as polarization sensitive and, consequently, makes the separation of the incident multi paths efficient though there is no spatial diversity. A blind spatio temporal equalization, based on the Constant Modulus Algorithm (CMA), balances the distortions of the spectrum in an extended bandwidth of 9 kHz. An experimental radio link has been tested with a range of 1300 km. The corresponding results underline the improvement of the bit transfer rate which attains 30 kbits/s in a 9 kHz bandwidth resorting to a 16 QAM waveform.

Signal scintillations in the low latitudes and high latitudes regions

The prediction of ionospheric scintillation (PRIS) project that we present in this paper includes a measurement campaign of signal scintillations of satellite to earth links due to the propagation through ionosphere and the improvement and the development of models for estimating and forecasting the scintillations activity.