读取CORS数据，重采样成30秒，计算电离层总电子含量-Read CORS data, resampling into 30 seconds, calculate the total electron content of the ionosphere

Estimate tec (total electron content) by least-sqaure adjustment using GPS dual-frequency code observables. Ionosphere is approximated as the single- layer model at the height of 350km with local or global tec model. Satellite/receiver dcbs (diffe