Position estimation after GPS lost


I would like to create a process that after GPS lost to introduce position estimation from external sensors. I would like to test it using SITL before moving to HITL. As I understood PX4 uses mavros/global_position/global as gps data source (sensor_msgs/NavSatFix). As a test I would like to copy this msg and publish it with small increase in the loop to see if it follows the point, but I am not sure where should I send this msg in SITL?