Discrepancy between two GPSs

I have an RTK base with a hardware ublocks F9P sending RTCM correction packet to another two ublox F9P embedded in UAV. I observe that the difference between two gps rovers is about 10 meters sometimes, especially in altitude, even though both are fixed solution.

Why this phenomenon happens? It doesn’t always occurs , but I would like to understand how two fixed gps receiving the same RTCM have such a high discrepancy.

How high is EMI? Are you using USB 3.0 connections?

Are you using USB 3.0 connections?

The RTCM packets is encapsulated by UDP until reach the UAV, after that, the board computer get the udp payload (raw rtcm packets) and send via a simple UART (only tx, rx e gnd) connection into the uart entry ports of each zed-f9p.

How high is EMI?

I dont know the number, but the magnetometer is coupled to gnss and that phenomeon occurs in ground, with almost all electric systems off. Theres no “dynamic” variation of EM in that situation. If the EMI is high, can two gps 30 centimeters from each other have this kind of measure variarion?