## 4 Verification and Accuracy Analysis of Single-Frequency Occultation Processing  
### relTEC Correctness Verification  
In Section 2, the total relative electron content of the single-frequency ionosphere, relTEC, is measured by the difference between the pseudorange data of frequency B\({}_{1}\) observed from occultation and the carrier-phase observation data. The relTEC in the dual-frequency observations in this paper is the difference between the B\({}_{1}\) obtained from occultation observation data and the B\({}_{3}\) carrier-phase observation data and the correctness of relTEC is verified based on the relTEC change rate observed by dual-frequency observations. Because the excess phase Doppler solution (in Section 4.2) in the occultation inversion reflected in the TEC is the relTEC change rate, the relTEC change rate at different heights will better reflect the influence of single- and dual-frequency occultation processing on occultation products. Taking two periods of occultation time as an example, Figure 1 shows the calculation results of the relTEC change rate of the BDS single- and dual-frequency at 0-140 km altitudes of occultation tangency points, where the horizontal axis is the relTEC change rate in the unit mM/s, and the vertical axis is the tangent height in the unit km. Blue and pink dots represent the change rate of single-frequency and dual-frequency relTEC, respectively. The results show that the relTEC change rate between single- and dual-frequency occultation processing is in the same order of magnitude, and the two have good trend consistency. Compared with the pink dots (the dual-frequency relTEC change rate), the blue point (the single-frequency relTEC change rate) shows a slight divergence, which means that the relTEC change rate calculated by the measurement difference of the pseudorange and carrier phase of only one frequency still has residual ionospheric error, and it is also affected by the pseudorange noise. Even though the introduction of pseudorange observations also brings some noise, and there is a slight difference between the two at this stage, the overall trend of relTEC change rate is the same, and the effect is ideal. The relTEC change rate calculated by the differential solution of the pseudorange and carrier phase of the single-frequency and calculated by the differential solution of the carrier phase of the dual-frequency are basically consistent at different altitudes within 0-120 km. To better quantify the consistency of single-frequency and dual-frequency occultation processing results, first-order linear fitting should be done on all single-frequency relTEC change rates calculated by an occultation event, and then the fitted function could be compared with the function y = x. Figure 2 is the comparison diagram of the relTEC change rate function of between the BDS single-frequency observations and the BDS dual-frequency observations under two occultation events with different data volumes. Inthe figure, the horizontal axis is the reITEC change rate calculated by the dual-frequency carrier-phase observation combination, and the vertical axis is the reITEC change rate calculated by the single-frequency pseudorange-carrier phase combination. The blue dots represent the reITEC change rate of each single-frequency observation. The fitting function of the single-frequency reITEC change rate is represented by a red line, and the function y = x, representing the consistency of the two, is represented by a blue line. As the figure shows, the more data there is for an occultation event, the more consistent the results of the single-frequency occultation processing and the dual-frequency occultation processing. Moreover, the concentration is higher when the data volume is greater. The two occultation events show that the blue dots are distributed near the curve of the function y = x, and the red fitting curve is very close to the curve position and curve trend of the function y = x. The effect is highly ideal.