VMA430 + external antenna + 1pps

Hi, I 'd like to make the VMA430 (GPS) delivering 1PPS and using one external antenna, connected to the SMA connector. (regular active patch antenna )

The module is supposed to work with embeded ceramic antenna or external antenna, but i can’t find anything related to selecting the source.

from my tests, it looks like the signal out from the external antenna is not used (one other GPS module from another manufacturer actually does in the same configuration).

the 1PPS signal is generated in a non continuous way, making me think the GPS signal comes from the integrated antenna. Indeed the module is currently inside a steel case, with bad reception condition, whereas the external antenna is located outside with very good reception conditions.

please advise , thanks in advance