GPS position error

I am using Pixhawk AC3.3 and two ublox m8n gps (primary and secondary redundancy). I am currently facing two problems with gps.

  1. latitude and longitude for both gps mismatch nearly 4 to 5 meters difference in position
  2. whenever the SP in EKF4 spikes to 1 or more, gps switch from primary to secondary. But how it happens? when SP spikes to 1 or more??