Novatel RTK In RTL mode

Hey, All.
Recently, I use novatel rtk for precise landing test.
I inject novatel message bestposb, bestvelb and psrdopb ( b indicates for binary mode) to pixhawk gps port, and change the params GPS_TYPE = 15 ( novatel mode ).The pixhawk works really well with RTK FIXED info in MP and the static position is really precise (+/- 5cm), but when I operate in RTL mode for precise landing, the result is terrible with error about 20-30cm away from home location.
I wonder anyone encounters these problems and any suggestions for improving landing result will be appreciated.

I experienced the similar issue, who can give us some hints? thanks in advance