RT_GPS_Pos_ECEF GPS XYZ accuracy and X position
RT_GPS_Pos_ECEF_1: 140, 4 (0x8C0420 + unit id) (0x304)
Byte 0: Validity
Byte 1: Accuracy X (m)
Byte 2: Accuracy Y (m)
Byte 3: Accuracy Z (m)
Bytes 4-7: ECEF X Position (m)
ECEF accuracy resolution is m/10.
ECEF position resolution is m/100.
The validity byte is used for the data packets in this message *and* in RT_GPS_Pos_ECEF_2.