Schaer (1998) used 10 deg. 10 deg. corresponds to enough signal strength for reliable signal trecking. Also at higher elevations the troposphere influence decreases. Moreover 10-15 deg. provides close values for different mapping function (Schaer (1998) mentioned Klobuchar' MF and SLM those). 30 deg. are used when they want to exclude multipath and to decrese MF effects.

Schaer S. Mapping and predicting the Earth's ionosphere using the Global Positioning System: Ph.D. Dissertation. / S. Schaer; Astronomical Institute, University of Bern. - Berne, Switzerland, 1999. – 228 p.

Dear Ayomide Olabode,

At middle and low latitudes, the cut-off angle used depends mainly on the task to be solved.

For example, to map TEC variations with high spatial resolution, we use a cut-off angle of 70 degrees.

Nykiel, G.; Zanimonskiy, Y.M.; Yampolski, Y.M.; Figurski, M. Efficient Usage of Dense GNSS Networks in Central Europe for the Visualization and Investigation of Ionospheric TEC Variations. Sensors 2017, 17, 2298.

Best regards.

YZ

International time comparisons are performed at the National Physical Laboratory of Israel (INPL) regularly since 1996. In the framework of these measurements, the ionospheric and tropospheric delays were estimated during long time from 1996 until 2004. The results of long-term measurements of ionospheric delay, obtained at the INPL, were published in [Goldovsky N., Luria M., Ionospheric delay contribution to the uncertainty of time and frequency measurements by one-way satellite time transfer method, Measurement, 2004, 35 (4), pp. 353-362.d ]. The results of tropospheric delay evaluation at INPL, as well as comparisons between tropospheric and ionospheric delay were presented on the International Conference on Quality in 2004, Israel. These results you can see in the attached file.