A. M. Padokhin

Ionospheric Radio Tomography Based on the GPS/GLONASS Navigation Systems

The specific features of the problem of ionospheric radio tomography based on the data from high-orbital navigation systems such as GPS and GLONASS are considered. An approach to solving this problem, which implies finding the solution with the minimum Sobolev’s norm (i.e., the smoothest solution) is proposed. The possibilities of the proposed approach are studied by numerical modeling with the use of real geometry of high-orbital satellite constellation and the layout of the receivers. The modeling shows that the proposed approach is very efficient in the regions with a sufficiently dense network of receivers. The resolution of possible radio tomographic systems over the territory of Russia is estimated. Examples of radiotomographic reconstructions of the ionosphere obtained from real high-orbital satellite data for the European region are presented.

Influence of GPS/GLONASS differential code biases on the determination accuracy of the absolute total electron content in the ionosphere

Systematic error arises when the total electron content (TEC) is estimated with the simultaneous use of phase and code GPS/GLONASS measurements. This is related to the different signal propagation times at L1 and L2 frequencies in the radio frequency path of the transmitting and receiving equipment, the so-called differential code biases. A differential code bias of 1 ns results in an error of ~2.9 TECU when TEC is determined. Differential code bias variations on a long time interval, which were obtained at the CODE laboratory, were analyzed. It has been found that the systematic variation in these biases and considerable seasonal variations apparently caused by the environmental state (temperature and humidity), which sometimes reach 20 TECU (in TEC units), are observed for several stations. The algorithm for determining differential code biases at an individual station and the results of correction for absolute slant TEC are also presented. Presented results show algorithm effectiveness for various geographical regions and solar activity.

Investigation of SBAS L1/L5 Signals and Their Application to the Ionospheric TEC Studies

With the development of satellite-based augmentation systems (SBAS), the dual-frequency L1/L5 observations from a number of geostationary satellites are now available. It provides the possibility of retrieving ionospheric total electron content (TEC) from these observations using the same approach as for dual-frequency GPS/GLONASS observations. In this letter, we study L1/L5 signals of American Wide Area Augmentation System (WAAS) and Indian GPS and geo-augmented navigation system (GAGAN) geostationary satellites observed with geodetic Global Navigation Satellite System (GNSS) receivers located at equatorial and midlatitudes and estimate corresponding geostationary TEC and errors of such estimations. One-hundred-second TEC RMS was found to reach up to 1.5 TEC unit (TECU) with typical values of 0.25-0.5 TECU, which is several times greater than for GPS/GLONASS observations. TEC RMS also manifests UT dynamics, which is specific for the satellite and not relevant to the signal paths. SBAS TEC was found to be in good agreement with the data of nearest ionosondes. We also conduct the wavelet analysis of geostationary TEC, providing typical periods of observed variations at different timescales and discuss the capabilities of SBAS TEC observations in connection with ionospheric effects of X1.7 solar flare on October 25, 2013.

Radiotomography and HF ray tracing of the artificially disturbed ionosphere above the Sura heating facility

We present the results of the radiotomographic imaging of the artificial ionospheric disturbances obtained in the recent experiments on the modification of the midlatitude ionosphere by powerful HF radiowaves carried out at the Sura heater. Radio transmissions from low orbital PARUS beacon satellites recorded at the specially installed network of three receiving sites were used for the remote sensing of the heated ionosphere. We discuss the possibility to generate acoustic-gravity waves (AGWs) with special regimes of ionospheric heating (with the square wave modulation of the effective radiated power at the frequency lower than or of the order of the Brunt-Vaisala frequency of the neutral atmosphere at ionospheric heights during several hours) and present radiotomographic images of the spatial structure of the disturbed volume of the ionosphere corresponding to the directivity pattern of the heater, as well as the spatial structure of the wave-like disturbances, which are possibly heating-induced AGWs, diverging from the heated area of the ionosphere. We also studied the HF propagation of the pumping wave through the reconstructed disturbed ionosphere above the Sura heater, showing the presence of heater-created, field-aligned irregularities that effectively serve as “artificial radio windows.”

Ionospheric TEC estimation with the signals of various geostationary navigational satellites

With the development of receiver equipment and GNSS and SBAS constellations, the coherent dual-frequency L-band transmissions are now available from a number of geostationary satellites. These signals can be used for ionospheric total electron content (TEC) estimation. The quality of these data, i.e., the level of noise in such TEC estimation is of great interest and importance. We present results of comparisons of noise patterns in TEC estimation using signals of geostationary satellites of augmentation systems such as the Indian GAGAN, the European EGNOS and the American WAAS, as well as the signals from the Chinese Beidou navigation system. We used data from two receiving sites in the European part of Russia and the USA, which are equipped with JAVAD Delta receivers. We found out that the noise level in TEC estimation based on geostationary satellites of the Beidou system is one order smaller than that for SBAS and corresponds to those of GPS/GLONASS at the same elevation angles. Typically, the TEC RMS was about 0.05 TECU for GPS/GLONASS satellites at elevation range 5–15°, 0.06 TECU for Beidou geostationary satellites at elevation range 15–25°, 0.6 TECU for GAGAN at elevation range 15–25°, 0.7 TECU for WAAS at elevation 45°, and 5 TECU for EGNOS at elevation 20°. We also discuss the capabilities of geostationary TEC observations in connection with the recent G4 geomagnetic storm of March 2015 using six IGS MGEX stations in the American, Southeast Asian and Australian sectors. We demonstrate the hemispheric asymmetry in the ionospheric TEC response during this storm.

Sura Heating Facility Transmissions to the CASSIOPE/e-POP Satellite

Throughout a night-time pass of the CASSIOPE satellite at an altitude of about 1300 km above the Sura Heating Facility, transmission of O-mode radiation from Sura to the ePOP Radio Receiver Instrument on CASSIOPE was maintained. Also during this pass, continuous VHF/UHF transmission from the ePOP CERTO radio beacon to three coordinated ground receivers in the Sura vicinity was achieved. Tomography of the VHF/UHF received wave data based on total electron content permitted the two-dimensional distribution of ionospheric ambient electron plasma frequency fpe to be determined in the latitude-altitude space between Sura and CASSIOPE. foF2 values about 0.1 MHz above the Sura pump frequency of 4.3 MHz were measured by the tomography. We examine the question of whether the observations can be explained on the basis of classic propagation in a smooth ionosphere. Tracing of rays from Sura towards CASSIOPE orbital locations finds most rays reflected away from the topside by the patchy ionospheric structure in bottomside fpe. It is concluded that O-mode ducting in under-dense field-aligned irregularities is responsible for maintaining the transionospheric transmission across the 2-min pass. O-to-Z mode “radio-window” conversion in the F-region bottomside is not required to explain these data.

Solar flare forcing on ionization of upper atmosphere. comparative study of several major X-class events of 23rd and 24th solar cycles

By analyzing the GNSS (Global Navigational Satellite Systems) signals recorded at the IGS (International GNSS Service) network, we compare the effects of ionization of the upper atmosphere by a series of intense X-class solar flares during the 23rd and 24th solar cycles. We develop the methods for estimating the geo-effectiveness of solar flares from the GNSS data and suggest using the rate of change of the ionospheric total electron content averaged over all the receiving stations located on the sunlit side of the Earth reduced to the solar zenith angle during the flare as the characteristic of the flare’s geo-effectiveness.

Earthquake Prediction Research Using Radio Tomography of the Ionosphere

Under development since its invention in 1990 as an ancillary application of ionospheric radio-tomography (RT), a new earthquake (EQ) prediction system is being evaluated. It has already been deployed along the United States West Coast, from Vancouver in Canada to San Diego in Southern California, and is currently undergoing Beta testing. This Chapter addresses RT–EQ prediction concepts, the underlying RT theory, evolution and implementation, and a few examples of the Beta test system’s performance. This work is an investigation of EQ precursors, which we hope will lead to an operational system. The current system provides a foundation and the tools to study ionospheric effects linked to conditions in the Earth’s crust prior to major earthquakes. Progress toward a fully operational system will require several more years of data acquisition and analysis.

Ionospheric perturbation indices based on the low- and high-orbiting satellite radio tomography data

Methods are suggested for constructing ionospheric perturbation indices (IPIs) based on empirical radio tomographic (RT) electron density distributions taking into account spatiotemporal resolution, and coverage of low- and high-orbiting (LO and HO) RT data. The LORT-based IPIs are calculated as spatial root mean square values of electron density derivatives indicating the presence of local ionospheric structures on a spatial scale of dozens of kilometers. The HORT-based IPIs are based on the statistical characteristics, i.e., means and deviations from the means, of electron density or vertical TEC distributions with different normalizations and subsequent spatial averaging, which take into account the geomagnetic activity and seasonal behavior of ionospheric plasma. Various schemes of IPIs construction are considered, correlations between IPIs and geomagnetic Kp index are analyzed, the indices most sensitive to geomagnetic activity are identified, and additional modifications of computational algorithms enhancing this sensitivity are suggested.

HF ray tracing of the artificially disturbed ionosphere above Sura heating facility

We present the results of high-frequency (HF) ray tracing of a pumping wave in an artificially disturbed ionosphere above the Sura heating facility applying parameters that are reconstructed using the radiotomography (RT) approach with the signals of the Parus and CASSIOPE beacon satellites. We also discuss the possibility of generating atmospheric gravity waves (AGWs) with special regimes of ionospheric heating and present the examples of such structures in radiotomographic reconstructions.