Single-frequency navigation receivers can now measure the number of electrons in the ionosphere with ten times greater accuracy.

The results of the study, supported by a grant from the Russian Science Foundation (RSF), have been published in the journal Sensors.

Located at altitudes between 60 and 1000 kilometers above the Earth's surface is the ionosphere — a region of the atmosphere that contains a large number of free charge carriers, electrons, and ions. As a result, radio waves passing through it are refracted and deviate from their original straight-line propagation, causing additional delays in the transmission of navigation signals, which can subsequently lead to a deterioration in the performance of navigation systems. The concentration of charged particles in the ionosphere is not constant — it depends on various factors (such as solar activity, energetic particles from solar wind, and atmospheric conditions) and can vary significantly from day to day.

Therefore, to account for the influence of the ionosphere on the functioning of navigation satellite systems, it is essential to continuously monitor the changes occurring within it. Typically, devices capable of receiving navigation system signals at two or more frequencies are used for this purpose. While these receivers allow for high-precision assessments of the concentration of charged particles in the ionosphere, they are expensive and require additional methods for accounting for inter-channel delays of navigation signals, not only within their receiving systems but also in the transmitting systems of navigation satellites.

This has led scientists to seek alternative methods for observing the ionosphere using signals at just one frequency. Single-frequency observation methods are employed in navigation systems such as GPS, GLONASS, Galileo, and BeiDou, but they are generally less accurate compared to dual-frequency methods. However, recently, the European system Galileo and the Chinese BeiDou have adopted a new method for coding radio signals, known as AltBOC, which has positively impacted single-frequency measurements.

Researchers from the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences (Irkutsk) and Lomonosov Moscow State University (Moscow) compared the quality of single-frequency observations of the ionosphere using the new AltBOC signal coding method with traditional BPSK and QPSK coding methods previously used in navigation systems Galileo and BeiDou, which are still in use in GPS and GLONASS.

“Initially, a radio wave is a standard sine wave that carries no information. To encode data using it, certain parameters of the wave must be altered, such as amplitude, frequency, or phase. Different navigation systems employ various coding methods. Consequently, the properties of the received signals differ, and it is necessary to identify those that will be optimal for single-frequency ionospheric measurements,” explains project leader Yuri Yasyukevich, Doctor of Physical and Mathematical Sciences and Deputy Director for Research at the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences.

For the study, the authors utilized data from high-precision receivers that capture signals from various global navigation satellite systems, including GPS, GLONASS, Galileo, and BeiDou. Such data is provided by the International GNSS Service (IGS) as well as numerous organizations in Russia and around the world. The researchers selected the most modern receivers with the maximum set of features for measuring signal characteristics.

Different combinations of ranging and phase measurements enable the evaluation of total electron content — the number of electrons along the satellite-receiver line. Based on this data, researchers modeled various scenarios, such as what would happen if signals with different characteristics were recorded. As a result, the researchers assessed the signal-to-noise ratio and the quality of ionospheric measurements for standard coding systems and the new AltBOC.

The study revealed that single-frequency methods for estimating electron concentration in the ionosphere using AltBOC coding yield data quality comparable to that of multi-frequency measurements. In contrast, traditionally used coding schemes such as BPSK and QPSK in single-frequency mode produce results that are an order of magnitude worse due to high noise levels in ranging measurements. This significant improvement in ionospheric monitoring quality with the use of AltBOC coding is attributed to its ability to substantially reduce noise levels in ranging measurements due to a broader signal bandwidth. Thus, the results indicate that the AltBOC coding method could also be beneficial for the domestic navigation system GLONASS.

“In the future, we need to investigate whether the new coding system has any drawbacks under specific conditions, in other words, when it is better to use older coding types. Understanding this is crucial since the primary task of GLONASS is to ensure precise navigation, which is influenced in complex ways by both the coding method and measurement conditions, such as urban development,” summarizes Artem Padokhin, a candidate of physical and mathematical sciences and associate professor at Lomonosov Moscow State University, who is the main executor of the project supported by the RSF grant.