Anna Mylnikova

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.

Determining the absolute total electron content from the single-frequency GPS/GLONASS data

In this study, we present a new approach enabling to estimate the absolute vertical and slant total electron content (TEC) of the ionosphere. The estimate is based on using single-frequency joint measurements of phase and group delay of a GPS/GLONASS signal from separate measuring stations. The vertical TEC calculated by singlefrequency measurements agrees, qualitatively and quantitatively, with similar estimates based on dual-frequency measurements. A typical value of the vertical TEC difference obtained through single-frequency and dual-frequency techniques, does not surpass ~1.5 TECU with the root-mean-square deviation (RMSD) being up to ~3 TECU for the stations that we selected.

Ionospheric Effects of Geomagnetic Storms on 26–30 September 2011 in the Different Longitudinal Sectors and Their Impact on the HF Radio Wave Propagation

Ionospheric storm is associated with the chain of events and phenomena in space environment, beginning at the Sun transmitted through the magnetosphere into the thermosphere-ionosphere system. The Earths ionosphere plays a key role in the space radio communication, radiolocation, navigation, and operation of the satellite navigation systems GLONASS/GPS. In this study, the parameters of the ionosphere-plasmasphere system during geomagnetic storms on 26–30 September 2011 were calculated using Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP).

Estimating the absolute total electron content, spatial gradients and time derivative from the GNSS data

Global navigation satellite systems have enabled to study the ionosphere in different regions of the world. The total electron content (TEC) of the Earth ionosphere can be determined from code and phase dual-frequency pseudorange measurements performed by receivers of GNSS signals. This technique is widely described in the literature (B. Hofmann-Wellenhof, H. Lichtenegger, J. Collins. New York: Springer-Verlag Wien, 389 p. 1998). To obtain the absolute TEC values, phase measurements are usually used, because they are weakly noised, and the ambiguity of the initial phase definition is eliminated with code ones. Thus, there occurs a systematic error termed differential code biases (DCBs). To determine absolute TEC accounting for DCBs from the data of a single GPS/GLONASS station as well as spatial gradients and time derivative, we have developed an algorithm. The algorithm includes estimating DCBs by using a simple model of measurements: equation where IV is the absolute vertical TEC value; Δ□(Δl) is the latitude (longitude) difference between the ionospheric point coordinate □ (l) and that of the station □ 0 (l 0 ); Δt is the difference between the measurement time t and the time t 0 , for which the calculation is performed; G □ =∂I V /∂□, G l =∂I V /∂l, G q_□ =∂2I V /∂□2, G q_l =∂2I V /∂l2 are linear and quadratic spatial TEC gradients; G t =∂I V /∂t and G q_t =∂2I V /∂t2 are the first and second time derivatives.

Systematic changing and variations of GPS/GLONASS differential code biases

Along with navigation and precise time applications, Global Navigation Satellite Systems (GNSS) are widely used nowadays to remotely sense the ionosphere in equatorial, mid-latitude and arctic regions. While estimating absolute TEC using the code and phase measurements simultaneously, a satellite and receiver dependent systematic error occurs. This error is associated with differential code biases (DCB) - the different, frequency dependent processing times of L1 and L2 signals in RF paths, both for satellites and receivers. Due to these DCBs, TEC, in some cases, can obtain even non-physical negative values. For example, a 1-ns DCB causes a ∼3 TECU error in TEC estimation.