Position Report for Vessels (FID 0) and Position Report for Aircraft (FID 3) are the only Message 25 payloads that include a 20-bit encrypted ITDMA Communications State. This allows STEDS units to maintain awareness of the planned activity of other STEDS users, thereby avoiding multiple STEDS transmissions during any one slot. Despite this self-organization among STEDS users, they will continue to yield to non-STEDS Class A traffic, who will “see” STEDS transmissions as strictly well-behaved transmissions on the VDL. The FID 0 and FID 3 position reports will be transmitted once every 30 seconds when the unit has registered a speed over ground (SOG) of less than 3 knots for over 3 minutes and once every 15 seconds when SOG is 3 knots or greater.
|
Parameter |
Number of bits |
Description |
Encryption Status |
|
Function ID |
5 |
3 = Aircraft position report |
AES encrypted |
|
Operating Mode |
1 |
0 = Autonomous 1 = Fixed |
AES encrypted |
|
Altitude |
12 |
Defined in Message 9 of ITU-R M.1371-5 |
AES encrypted |
|
Time stamp |
6 |
Defined in Message 9 of ITU-R M.1371-5 |
AES encrypted |
|
Longitude |
25 |
Longitude in 1/1000 min (±180 degrees, East = pos; West = neg 181 degrees = not available = default) |
AES encrypted |
|
Latitude |
24 |
Latitude in 1/1000 min (±90, degrees, North = pos; South = neg 91 degrees = not available = default) |
AES encrypted |
|
COG |
9 |
Course over ground (0-359°). (511 indicates not available = default). |
AES encrypted |
|
SOG |
10 |
Defined in Message 9 of ITU-R M.1371-5 |
AES encrypted |
|
Communication state selector flag |
1 |
0 = SOTDMA communications state 1 = ITDMA communications state |
AES encrypted |
|
Communications State |
19 |
Per Annex 2 to ITU-R M.1371-5 para 3.3.7.2.2 (SOTDMA) and 3.3.7.3.2 (ITDMA). |
AES encrypted |
|
Encryption checksum |
16 |
Cyclic Redundancy Check (CRC) 16- bit polynomial |
AES encrypted |
|
TOTAL |
128 |
128 encrypted bits |
|
