Discussion and comparison of the mapping functions in radio frequencies

Some mapping functions in radio frequency are reviewed at the beginning of this paper. From the viewpoint of statistics and mathematics, the error sources of refractive delay computation are analyzed together with the ray tracing technique and radiosonde data application. The NMF model (Niell 1996) used globally distributed radiosonde network data and tabular coefficients in its continued fraction expression and, therefore, reached a high accuracy with wide latitude and meteorological parameter coverage (MacMillan and Ma 1997). The UNSW931 model (Yan and Ping 1995) introduced an improved continued fraction form, which was derived from the generator function method and had a good convergency at low elevation observations. The influences of ground meteorological records on the mapping functions are generally discussed. Comparisons among different mapping functions are performed together with radiosonde data. In order to fit the requirements of more accurate and lower elevation observations in future space technique applications, we suggest constructing a new mapping function, which will involve the main advantages of present models and improve or avoid inadequate expressions as far as possible