Automatic Identification System (AIS) technology allows vessels to transmit and receive identification and dynamic data between each other over dedicated VHF frequencies. AIS has a range of about 50 nautical miles, with time slicing allowing around 200 vessels to operate simultaneously in an area by taking turns transmitting data in time slots. When multiple vessels are in close proximity, momentary blanks can occur on displays as time slicing results in simultaneous transmissions.

1. VTS
Station
OVERVIEW OF AIS
TECHNOLOGY

2. Automatic Identification System does the transmission on two
dedicated frequencies :
Chanel A =
Chanel B =
These frequencies
belong to the VHF
Bandwidth.
Thus, they conduct a line-of-sight transmission. Height of
Antenna will become very much relevant in this case.
Range of operation will be around 50 Nautical Miles, 20
NM being the average.
VTS
Station

3. VTS
Station
Each AIS equipment is designed to handle about 200 other
equipment simultaneously, which are operating in its vicinity.
Upon finding other transmissions, these AIS equipment have an
electronic capability to form a loop between them and conduct a
TIME SLICING in regard to transmission and reception.

4. VTS
Station
Each AIS equipment does transmission of its DATA for a specific
‘TIME SLICE’ and then switches into reception mode. Then, the
second AIS does its transmission; and after that the third, and so on
and so forth. This is TIME SLICING.
During reception mode, the AIS equipment does ‘scanning mode’
reception on both Chanel A and Chanel B.
The two channels are meant to provide for redundancy.
TIME SLICING

5. VTS
Station
In case, some other passing-by ship reports to you that “possibly your
ship’s AIS is not working” because they are not able to see your ship
on their AIS, DO NOT get worried and jump into trouble-shooting.
Such momentary ‘blanking’ are regular feature if the Time Slicing has
accidently made two or more vessels to be in Transmission mode
simultaneously. Give your AIS equipment some time, and they should
be able to receive your vessel on their AIS.
As a matter of fact, when there is more traffic in a
given area, such ‘blanking’ will be more common.

6. USCG AIS Overview
(source: Wikipedia)

7. Working of the bridge mounting part.

8. AIS receives feed from Autopilot and
the GPS and transmits it to other
vessels.

9. It receives data from other AIS , and
does the mathematical processing to
calculate the RANGE, BEARING, TCPA,
CPA and other results. This is then
put on display on the AIS screen, the
RADAR and ECDIS
The AIS paint and the RADAR paint of same target
may not exactly match on RADAR screen.

10. If the “VIRTUAL” BUOYS are
identified by the display device, such
as the RADAR and/or ECDIS, then
they are accordingly showed.

11. TYPE OF AIS DATA
Fixed data
1) Vessel name
2) Call Sign, IMO Number, Port of Registry, MMSI
3) LOA, Beam , Height
4) Type of vessel
Manually-fed data
1) Draught
2) Number of crew
3) Destination port
4) ETA
5) Navigational Status (at anchor/CBD/RAM/NUC/Fishing/Underway)
6) Safety/Urgency/ Distress/Routine message
Dynamic data
1) Heading
2) Position
3) Speed
4) Course
5) Rate of Turn
Calculated Data
1) Range
2) Bearing
3) TCPA
4) CPA

12. The VTS AIS unit does the
transmission of the position of
the “virtual” navigation buoys
VTSSTATION
Where are the ‘virtual’
navigation buoys located ?
‘Virtual’ Navigation Buoys have no connection
with the bandwidth (S-Band, X-band) of the
Radar in use.
If your radar is not showing the virtual buoys ,
try after switching the feature “MAP OFF” to
“MAP ON”.
Otherwise, it is likely that your equipment
software does not support virtual buoys.