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 shall be transmitted once every 30 seconds when the unit has registered a speed over ground (SOG) of less than 3 knots for more than 3 minutes and once every 15 seconds when SOG is 3 knots or greater.
|
Parameter |
Number of bits |
Description |
Encryption Status |
|
Function ID |
5 |
0 = Position report |
AES encrypted |
|
Operating Mode |
1 |
0 = Autonomous 1 = Fixed |
AES encrypted |
|
Navigational Status |
4 |
0 = under way using engine 1 = at anchor 2 = not under command 3 = restricted maneuverability 4 = constrained by her draught 5 = moored 6 = aground 7 = engaged in fishing 8 = under way sailing 9 = reserved for future amendment of navigational status for ships carrying DG, HS, or MP, or IMO hazard or pollutant category C, high speed craft (HSC), 10 = reserved for future amendment of navigational status for ships carrying dangerous goods (DG), harmful substances (HS) or marine pollutants (MP), or IMO hazard or pollutant category A, wing in ground (WIG); 11 = power driven vessel towing astern (regional use), 12 = power-driven vessel pushing ahead or towing alongside (regional use); 13 = reserved for future use, 14 = AIS-SART (active), MOB-AIS, EPIRB-AIS 15 = undefined = default (also used by AIS-SART, MOB-AIS and EPIRBAIS under test) |
AES encrypted |
|
Parameter |
Number of bits |
Description |
Encryption Status |
|
Hour of position |
5 |
0-23 |
AES encrypted |
|
Minute of position |
6 |
0-59 |
AES encrypted |
|
Second of position |
6 |
0-59 |
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 |
7 |
Speed over ground in knots (0-126 knots) with 127 = not available (default) and 126 = 126 knots or higher. |
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 |
|
