RT_INS_Pos_ECEF_1: Accuracy and INS ECEF X position
RT_INS_Pos_ECEF_1: 142, 4 (0x8E0420 + unit id) (0x384)
----
Byte 0: Validity
Byte 1: Accuracy X
Byte 2: Accuracy Y
Byte 3: Accuracy Z
Bytes 4-7: INS ECEF X position (m)
ECEF position resolution is m/100.
The validity byte is used for the data packets in this message *and* in RT_INS_Pos_ECEF_2.