## 1 Introduction  
Currently, the global navigation satellite systems (GNSS) enable the study of the ionosphere at any spot on the globe. Such studies are based on dual-frequency phase and pseudorange measurements of the total electron content (TEC) [1]. Since the phase measurements draw a lower level of noise compared to pseudorange measurements, most studies use the phase measurements to determine TEC. There are classical papers [2, 3], as well as many up-to-date ones (for example, see [4] and references therein). [PERSON] and [PERSON] [2], for the first time, suggested mapping the TEC. They compared, mapped and measured total ionospheric electron content by using global positioning system (GPS) and beacon satellite observation and revealed that the difference between them is less than 1 TECU. Pioneering papers by [PERSON] and [PERSON] [3] showed the efficiency of dual-frequency phase measurements to investigate ionospheric responses to the strong ground displacement associated with earthquakes. [PERSON] et al. [4] presented a number of new results when studying the ionospheric irregularities caused by solar flares, solar eclipses, solar terminators, earthquakes, magnetic storms, rocket launches and tropical cyclones based on GPS/GLObal Navigation Satellite System (GLONASS) data. The studies showed the efficiency of dual-frequency phase measurements. A comprehensive study by [PERSON] et al. [5] showed a high potential of TEC phase measurements to monitor natural hazards. [PERSON] and [PERSON] [6] implemented 4D GNSS radio tomography based on phase measurements. [PERSON] et al. [7] suggested a new approach to use L1/L5 phase measurements from satellite-based augmentation system. However, when using phase data, there is an ambiguity of phase measurements. For some applied problems, we need not relative, but absolute measurements. [PERSON] and [PERSON] [8] evaluated the ionospheric effects on low frequency radio astronomy measurements. [PERSON] and [PERSON] [9] suggested an approach based on absolute TEC data for ionospheric correction of radio telescopes and radio interferometers. [PERSON] et al. [10] used absolute GNSS TEC data to correct radar data. Pseudorange measurements are thought to be absolute, however, they are noisy. For this reason, one often uses pseudorange and phase measurements jointly to eliminate the phase ambiguity. Herewith, there is a bias related to a different time of the signal passage through the channels of a satellite and a receiver. In literature, this error is referred to as differential code biases (DCBs). This error is known to vary systematically, and it may have irregular variations [11]. [PERSON] et al. [12] showed some cases of DCB annual variations caused by receiver/antenna environment conditions. Instability in DCB can be caused by change in grounding [13]. A change in DCB can lead to significant apparent TEC variations [14]. There are some procedures to estimate the DCBs [15; 16]. [PERSON] et al. [15] suggested an efficient algorithm to estimate DCB for a network, when one of receiver DCBs is already known. [PERSON] et al. [16] published MatLab code for DCB calculation. They reported an agreement with the global ionospheric map (GIM) data with a mean difference of less than 0.7 ns (\(\sim\)2 TECU) and an RMS of less than 0.4 ns (\(\sim\)1 TECU). [PERSON] et al. [17] analyzed triple-frequency combinations and proved that the real satellite DCBs are time-varying. [PERSON] et al. [18] mitigated variations in DCB by separating DCB from ambiguity. As a rule, the DCB estimates are a by-product of estimating the model parameters of TEC measurements. Having the DCB estimated, one can correct the slant TEC series and then use these TEC absolute values in applications. An important goal is to obtain absolute characteristics reflecting ionospheric conditions. A good way is to have an electron density profile that an incoherent scatter radar or an ionosonde can provide [19; 20; 21; 22]. The cost of such equipment is very high, particularly that of radars.