Based on the available single frequency corrections (L1/E1 and L5/E5a) Ifree corrections can be formed on the airborne side.
Due to the combination of two code measurements, the Ifree solution leads to significantly higher multipath and noise.
The Ifree dual frequency combination removes the relevant ionospheric delay completely at both ground and airborne receivers.
The tropospheric error is not frequency dependent and [[sigma].sub.tropo] thus persists for all processing schemes (single frequency L1, single frequency L5, and dual frequency Ifree).
Another effect that will have to be considered is the interfrequency bias that affects the dual frequency Ifree processing.
Even though the mean VPL for GPS GAST D (green circles) is still lower compared to the dual constellation Ifree solution (violet line), we see that the maximum (around 3:00 in the plot) already extends almost as high.
Plot (1) in the same figure shows the influence of the increased Ifree sigmas in the dual constellation scenario.
In this figure the same split of VPL contributions as before is performed for GPS Ifree 30 s and L1100 s processing using the according models from Figure 5(d).
It compares the standard GAST D performance (based on the AAD-B model) and a GAST D curve based on our own new airborne model with three dual constellation curves, namely, single frequency L1/E1, single frequency L5/E5a, and Ifree dual frequency L1/E1 + L5/E5a.
Soon after that even the Ifree protection level is exceeded, making it the largest protection level of all modes in this figure.