By each AIS channel there are 2250 AIS position updates possible.
That means that on both frequencies (161.975 & 162.025MHz) 4500 messages can be sent.
An AIS position report (Msg. 1/18) is send by a VDM-Message. This message has a length of 50 Bytes.
One Byte contains 10 Bits (8-Vit plus one start and stop bit)
If there would be a channel load of 100 percent, the calculation would be as followed:
From the beginning we know that one VDM has 500 Bit
So on a channel load of 100 percent there will be 37500 Bit / s.
The baud rate of 38400 is perfect then.
But what does that mean for a chart plotter which is only able
to handle a 4800 Baud rate (Bit/s)?
With this baud rate you only can receive 9,6 VDM messages and this is already a channel load of 12,8 %!
But now we come to the next problem because an AIS transceiver is not only transmitting VDM messages which it receives to the chart plotter. There are also GPS messages and some prop. strings (LED status, info about voltage, diagnose sentences, etc.)
Let´s have a look on VDM plus GPS sentences and take three of them which are standard:
- GPRMC: (RMC = recommended minimum sentence)
- GPVTG: Info about SOG and COG
- GPGGA: one of the main sentences, contains time, position including height and signal strength)
All of these three sentences will be transmitted every second and they have a length of ~2000 Bit.
Lets subtract the 2000 Bit / s from 4800 Bit/s and we have only 2800 available Bit/s.
And this is only 7,5% channel load which you will have very fast in some areas.
For example the highest meassured channel load from a pleasure boat was in
Rotterdam, Singapore, Shanghai with 40-50 percent!
In a near distance to waterways you can have 20-30 % very fast!
|Baudrate||GPS [ON/OFF]||Max. VDM/s||Max. channel load[%]|
If you are crossing and sailing in areas with waterways we really recommend using always the high baud rate of 38400 to not lost AIS position updates!
If you just drive within Non-High-Traffic zones 4800 Baud is ok when you switch off GPS sentences. For example the easyTRX2S can be set to „None“.
Example: easyTRX2-S settings within the Programming-Tool.