GPS radio interferometry of travelling ionospheric disturbances

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Abstract

This paper presents some results investigating the new possibilities of radio interferometry of Travelling Ionospheric Disturbances (TIDs) that are based on exploiting standard measurements of transionospheric radio signal characteristics and coordinate-time measurements using dual-frequency multichannel receivers of the Global Positioning System (GPS). A Statistical Angle-of-arrival and Doppler Method for GPS radio interferometry (SADM-GPS) is proposed for determining the characteristics of the TIDs dynamics by measuring variations of GPS phase derivatives with respect to time and spatial coordinates. These data are used to calculate corresponding values of the velocity vector, in view of a correction for satellite motions based on the current information available regarding the angular coordinates of the satellites. Subsequently, velocity and direction distributions are constructed and analyzed to verify the hypothesis of whether there is a predominant displacement. If it exists, then the pattern can be considered to be travelling, and the mean travel velocity can be determined from the velocity distribution. Through a computer simulation it was shown that multi-satellite GPS radio interferometry in conjunction with the SADM-GPS algorithm allows the detection and measurement of the velocity vector of TIDs in virtually the entire azimuthal range of possible TID propagation directions. The use of the proposed method is exemplified by an investigation of TIDs during the solar eclipse of 9 March 1997, using the GPS-radio interferometer GPSINT at Irkutsk.

Introduction

TIDs with a typical wavelength Λ ranging from a few hundred km to a thousand km are of great interest as a matter for scientific inquiry in ionosphere physics and as a factor that limits the accuracy of modern-day radio engineering systems used in navigation and radio interferometry; a rich variety of publications addressed the issue of TIDs (see a review of atmospheric gravity waves (AGW) and TIDs by Hocke and Schlegel, 1996).

A set of experimental data on the TIDs structure and dynamics was obtained using methods of transionospheric sounding by VHF signals from geostationary satellites in measurements of the Faraday rotation of the plane of polarization proportional to the value of a total electron contact (TEC) along the path between the receiver and the satellite—see reviews by Davies (1980) and by Hocke and Schlegel, 1996. A spaced-antenna reception scheme with a baseline, comparable with the wavelength Λ, was used in this case.

Further evidences of TIDs were obtained using radio interferometers relatively recently. TID characteristics are reconstructed based on measuring the phase difference of transionospheric radio signals (in VHF or UHF frequency ranges) from cosmic point radio sources or geostationary satellites at spaced (on the Earths surface) antennas. Such experiments utilize powerful modern radiotelescopes (Mercier, 1986, Mercier, 1996; Jacobson and Erickson, 1992, Jacobson and Erickson, 1993; Spoelstra and Kelder, 1984; Velthoven et al., 1990; Spoelstra, 1992), or special-purpose antenna arrays are devised (Webster and Lyon, 1974; Afraimovich et al., 1991; Carlos and Massey, 1994). In both cases these are sophisticated, expensive and unique projects, and much effort has to be directed toward implementing them; the first consideration being their time consuming nature.

The material disadvantage in the analysis of VHF signals from geostationary satellites is that satellites of this class are limited in number and are nonuniformly distributed in longitude. Another important limitation is a very much pronounced azimuthal dependence of the TIDs detection sensitivity at a transionospheric sounding (Georges and Hooke, 1970; Davis, 1973; Bertel et al. 1976; Afraimovich et al., 1992; Afraimovich et al., 1994; Mercier, 1996; Mercier and Jacobson, 1997; Beach et al., 1997; for more details, see 2 Methods of analyzing TID radio interferometry data, 5 Results of computer simulation. As a consequence the north-westward, northward or north-eastward directions of possible TIDs propagation were closed (depending on the relative distance in longitude between the satellites and the signal receivers) for geostationary satellites.

The new avenues for remote diagnostics of TIDs are based on exploiting standard measurements of transionospheric radio signal characteristics and coordinate-time measurements using dual-frequency multichannel receivers of the GPS (Dixon, 1991; Hofmann-Wellenhof et al., 1992; Melbourne et al., 1994; Klobuchar, 1997). Using these receivers almost at any site of the globe and at any time simultaneously at two coherently-connected frequencies f1 = 1575.42 MHz and f2 = 1227.6 MHz high-precision measurements are made of the group and phase delay between the receiver on the ground and the transmitters on the system satellites residing in the reception zone.

Until very recently, the main efforts of researchers were directed toward exploring the possibilities and developing methods of measuring and modeling the space-time distribution of TEC. Primarily, this was dictated by the necessity of a careful analysis and the need to develop methods of improving the GPS effectiveness.

A substantially less extensive literature is devoted to the use of data from the GPS in investigating ionospheric irregularities, in particular TIDs (Calais and Minster, 1995, Calais and Minster, 1996; Fitzgerald, 1997; Beach et al., 1997). However, the sensitivity of phase measurements in the GPS allows for detecting irregularities with amplitude of up to 1013 m−2, or 10−4 of a diurnal TEC variation.

However, the problem here is not with the technology of obtaining data but with the development of methods of using them. The necessity of further developing these methods is particularly critical at present because progress in the field of technology and telecommunications has made it possible to receive, via INTERNET, measurements from a far-flung worldwide International GPS Service for Geodynamics (IGS) network (Klobuchar, 1997) numbering 167 GPS receivers according to INTERNET data as of the end of 1997.

Currently there also exist other, quite different-purpose GPS arrays, both permanent and temporary; data obtained from them can be used in GPS radio interferometry of TIDs (EOS, 1996; Klobuchar, 1997; Shen et al., 1996). Furthermore, the possibility exists of using, for research purposes, signals from the Russian GLObal NAvigational Satellite System (GLONASS) which, in its parameters and capabilities is very similar to GPS. Hybrid dual-frequency phase-code receivers have been and are being developed, capable of receiving signals from both systems simultaneously (data received via INTERNET—http:⧹⧹www.ashtech.com). A side benefit of the GLONASS system is an increased (compared with GPS) inclination of satellite orbits (for GLONASS—64.8°; for GPS—55°; Hofmann-Wellenhof et al., 1992). This allows a larger (compare with GPS) portion of the sky to be kept under observation, especially in polar and mid-latitude regions.

This paper is devoted to the analysis of the new possibilities of the GPS radio interferometry of TIDs. Section 2gives a brief outline of methods of analyzing TIDs radio interferometry data. Section 3is devoted to the description of Statistical Angle-of-arrival and Doppler Method for GPS radio interferometry (SADM-GPS) proposed by the authors. The proposed method has been tested for validity through a computer simulation of GPS radio interferometry of TIDs (4 Computer simulation of GPS TEC measurements, 5 Results of computer simulation. The use of the proposed method is exemplified by an investigation of TIDs during the solar eclipse of 9 March 1997, using the GPS-radio interferometer GPSINT and Irkutsk (Section 6.

Section snippets

Methods of analyzing TID radio interferometry data

We consider some methods of reconstructing TID characteristics, which were used in classical radio interferometry and can be useful when processing GPS data. These methods are used following the required procedure of band filtering separating the signal phase variations caused by the influence of TIDs, from slow variations as a consequence of the movement of the radio source and diurnal variations of the TEC.

Let us consider an imaginary interferometer that consists of three receivers, installed

SADM modification for GPS radio interferometry

Signals from GPS satellites can be used following a classical radio interferometry scheme at a network of GPS receivers located at the desired distance from each other, similar to Carlos and Massey, 1994 radio interferometer. The high stability of reference generators on satellites and in dual-frequency receivers (no worse than 10−10–10−12; Hofmann-Wellenhof et al., 1992), and also the high level of technology of processing of phase measurements in GPS permit the recording system and the

Computer simulation of GPS TEC measurements

The validity of the proposed algorithm is difficult to evaluate without an appropriate computer simulation of GPS radio interferometry of TIDs for experimental conditions similar to real situations. The method and results of such a simulation are discussed in 4 Computer simulation of GPS TEC measurements, 5 Results of computer simulation. In this paper the central objectives of a simulation was to investigate the azimuthal dependence of TEC response for multi satellite GPS radio interferometry

Pretreatment of data and testing of SADM-GPS algorithm for model of single travelling wave

In this section we shall describe the order of the principal steps in the data treatment which is common to the series I0,A(t), I0,B(t), I0,C(t), θS(t), and αS(t) obtained through a simulation (Section 4, as well as to a processing of the same type of experimental data series (Section 6. Following other authors (Calais and Minster, 1995, Calais and Minster, 1996), we give the values of vertical TEC I(t), calculated from oblique TEC I0(t) measured at different angles θSIA(t)=I0,A(t)sinθS(t)

Fig. 4

TIDs parameters during solar eclipse of 9 March 1997

Radio probing of the ionosphere during solar eclipses provides important information about atmospheric processes in a wide height range. The literature on the subject is quite extensive (see, e.g., a thorough review by Cohen, 1984). Regular ionospheric effects of solar eclipses imply a decrease in TEC down to 25%, an increase in F-layer minimum height and effective reflection heights, and a decrease in density in the F-layer maximum, which is characteristic for the nightside ionosphere (Cohen,

Conclusion

A Statistical Angle-of-arrival and Doppler Method for GPS radio interferometry (SADM-GPS) is proposed for determining the characteristics of the TIDs dynamics by measuring variations of GPS phase derivatives with respect to time and spatial coordinates. These data are used to calculate corresponding values of the velocity vector, in view of a correction for satellite motion based on current information available regarding the angular coordinates of the satellites. Subsequently, velocity and

Unknown BIBs

Afraimovich et al., 1997b, Ichinose and Ogawa, 1958

Acknowledgements

We are indebted to Drs V. T. Zalutsky, V. V. Koshelev, K. G. Levi, V. V. Chernukhov and A. V. Lukhnev for help in organizing the experiment. The authors thank V. G. Mikhalkovsky for his assistance in preparing the English version of the manuscript. A great contribution was made by two referees. This work was supported by the Russian Foundation for Fundamental Research under Grants 96-05-64162 and 97-02-96060.

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