4Sardon E, Rius A, Zarraoa N. Estimation of transmitter and receiver differential biases and the ionospheric total electron content from global positioning system observations[J]. Radio science, 1994, 29(3) : 577～586.
5Sardon E, Zarraoa N. Estimation of total electron content using GPS data: How stable are the differential satellite and receiver instrumental biases [J]. Radio science , 1997, 32(5): 1899～1910.
6David S Coco, et al.. Variability of GPS satellite differential group delay biases[[J]. IEEE Trans on AES,1991, 27(6): 931～938.
7R Warnant. Reliability of the TEC computed using GPS measurements-the problem of hardware biases[J]. Acta Geod. Geoph. Hung., 1997, 32(3-4):451～459.
