One alternative would be to place the whole GNSS receiver outside and only run serial port and PPS signal from it (cat5 has just the right amount of pairs for R

One alternative would be to place the whole GNSS receiver outside and only run serial port and PPS signal from it (cat5 has just the right amount of pairs for RS-422 UART, RS-422 PPS and power. And is significantly cheaper than coax that is good enough for GNSS).