AIS via Satellite

[Ian_Edwards]

Great responce from KompetentKrew.
I sail the west coast of Scotland, and sometimes ask the coastguard if they see Eynhallow on AIS. Sometimes they can't. Out of range, in a steep sided Loch, or what ever. Satallite reception will solve that problem.
Since AIS could be very important in an emergency, and if I'm going to renew old equipment, before it breaks, rather than waiting for it to fail. I like to buy to most upto date tech I can afford.

 

[[2574]]

Ok, I looked it up, and as far as I can ascertain the difficulty in receiving AIS by satellite has never been the transmission power, but the clutter - lots of signals in high density areas, and also the large number of ships within the satellite footprint interfering with each other.

Consequently two new radio channels have been allocated to message 27, which is transmitted less frequently - only every 3 minutes, provided the ship is not within range of a base-station. It is assumed that base stations will be operating in areas of high traffic density (near me markers delimitating a traffic separation scheme are shown on AIS, for example) and that it's not necessary for satellites to also capture these AIS transmissions, Therefore the countdown timer for transmitting message 27 will be reset back to 3 minutes whenever a base station is heard (message 4 is reserved for base stations), and thus ships do not transmit message 27 when they're in range of base stations.

AIS is transmitted at 9600 baud, which means that a single bit is sent in just over 0.1 milliseconds; a normal AIS message is nominally 256 bits, or about 25 milliseconds, in length. Hence there are 2250 AIS transmission slots in a minute - these slots must presumably be coordinated by GPS time (accurate to 3 nanoseconds), to ensure that all ships transmit in their own AIS slot.

However, AIS messages from a ship on the nearest edge of its footprint arrive at the satellite slightly sooner than those from a ship at the far edge of its footprint. For a satellite orbiting 1000km above the earth (the highest orbit in use for this) the difference is about 9 milliseconds - the time it takes for 87 bits to be transmitted. Hence message 27 is shorter by 96 bits (to allow a bit of extra wiggle room) to ensure that the delayed message from a distant ship does not stomp on a message received from a nearer ship using the next time slot.

Ships which are nearby and can hear each other's AIS will never choose the same time slot, and thus never stomp on each other's transmissions, but when they're far apart they can choose the same time slot and cause this problem. But the shorter message 27 ensures that interference can't also happen when they're transmitting in adjacent slots.

In order to make message 27 shorter the timestamp is omitted - "The receiving system is expected to provide the time stamp when this message is received."

It's taken me about 3 hours to figure this out from the slightly abstruse documentation and summarise it. I think I got it all right, but I'm not sure. I welcome any corrections.

Sources:

• https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-M.2169-2009-PDF-E.pdf
• Long-range AIS message analysis based on the TianTuo-3 micro satellite

Excellent, thank you.

 

[[2574]]

Great responce from KompetentKrew.
I sail the west coast of Scotland, and sometimes ask the coastguard if they see Eynhallow on AIS. Sometimes they can't. Out of range, in a steep sided Loch, or what ever. Satallite reception will solve that problem.
Since AIS could be very important in an emergency, and if I'm going to renew old equipment, before it breaks, rather than waiting for it to fail. I like to buy to most upto date tech I can afford.

Agree, I do the same, though I confess I haven’t yet found a quiet anchorage in which to read and understand the capabilities of the DY B+ AIS that I fitted over winter.

 

[PaulRainbow]

SOTDMA AIS (B+) doesn't really offer much to sailing yachts. Yes, more transmit power, but does it matter that you're seen a few more miles away ? It will reserve a transmission slot, that might be useful in a really crowded area. Another benefit of SOTDMA is that it will transmit more frequently as the vessel speed increases (as class A does), but modest sailing yachts will never reach those speeds.

IMO, if fitting AIS for the first time, SOTDMA is worth considering for the relatively small additional cost, but if you have an operational CSTDMA AIS i wouldn't bother changing just for the sake of it.

 

[Roberto]

Great responce from KompetentKrew.
I sail the west coast of Scotland, and sometimes ask the coastguard if they see Eynhallow on AIS. Sometimes they can't. Out of range, in a steep sided Loch, or what ever. Satallite reception will solve that problem.
Since AIS could be very important in an emergency, and if I'm going to renew old equipment, before it breaks, rather than waiting for it to fail. I like to buy to most upto date tech I can afford.

Does the CG constantly receive AIS messages relayed by AIS satellites back to earth?
From what I gathered reading here and there, AIS satellites are owned by private companies which mainly sell data to shipping companies; they say they can provide data to MRCCs to help with SAR, but I suspect their data is not constantly automatically available to the CG. I think someone in an area without AIS coverage should first initiate a distress by other means, then possibly the CG would ask the private company to assist with AIS data for the unit in distress or nearby vessels.
IMHO if you ask the CG "Routine message - can you see me in AIS satellite" they would not bother checking.
Anyone knows how this s-AIS data participates in MRCCs routine activities?

 

[Daverw]

Great responce from KompetentKrew.
I sail the west coast of Scotland, and sometimes ask the coastguard if they see Eynhallow on AIS. Sometimes they can't. Out of range, in a steep sided Loch, or what ever. Satallite reception will solve that problem.
Since AIS could be very important in an emergency, and if I'm going to renew old equipment, before it breaks, rather than waiting for it to fail. I like to buy to most upto date tech I can afford.

I’m not sure it will change much for you, I would be surprised if CG would look at AIS from satellite. Marine traffic and likes seem to get people confused over what they are trying to do, it’s a commercial information service nothing more imho

 

[Hermit]

AIS is not part of GMDSS so, at best, is supporting information for SAR.

 

[Iliade]

In the sport aviation world there are a large number of competing standards for electronic conspicuity. As a result, a number of the tracking websites now accept a feed from any number of devices/protcols, which enables, say, a FLARM equipped aircraft to 'see' an ADS equipped aircraft in almost real time. Real enough to be aware of any collision risk. e.g. Livetrack24.com or the Open Glider Network (OGN.) Thus the XC_Guide app has my GSM based Livetracker, InReach and SPOT tracks appearing alongside my ADS track in more or less real time, depending on each devices configuration and capabilities.

I wonder if any such thing exists for AIS data? (Maybe Livetrack24 has this capability, I don't know?)

 

[AntarcticPilot]

It also depends on the transmitter power. Class A has a higher transmitter power than class B.
There is also a new class B+, which has a higher transmitter power, but the only one I could see on the market now is a Digital Yachts box, which fits between the VHF and the antenna.
But I guess it's only a matter of time before it becomes an option on VHF hand sets.

Emtrak do class B+.

 

[Refueler]

I’m not sure huge benefit upgrading to B+ Unless in very crowded area or fast vessel

Two immediate benefits :

1. Greater transmit power - giving greater distance received ...
2. Cycle time of transmissions is significantly less - meaning your AIS is sending out info more frequently.

But yes - for most the upgrade is not that important. I sail Baltics and decided that Class B (2W) was sufficient. But if I was to venture further afield - then B+ might be considered. Soon anyway - Class B alone will be not available probably. All will migrate to B+

 

[PaulRainbow]

Two immediate benefits :

1. Greater transmit power - giving greater distance received ...
2. Cycle time of transmissions is significantly less - meaning your AIS is sending out info more frequently.

But yes - for most the upgrade is not that important. I sail Baltics and decided that Class B (2W) was sufficient. But if I was to venture further afield - then B+ might be considered. Soon anyway - Class B alone will be not available probably. All will migrate to B+

Point 2 is not entirely accurate, data is sent out exactly the same, except when SOG exceeds 14kts

 

[Refueler]

Just to be clear .... agreed - I generalised and omitted the speed adjustment part ............ so :

"The New Class B+ Technology :
The new Class B+, often referred to as “Class B SOTDMA” or “Class B 5W”, has been defined to bridge the gap between Class A and Class B transponders, offering some clear advantages for some types of vessels and applications. Class B+ uses the same SOTDMA technology as Class A and therefore has the same priority when it comes to reserving a time slot, guaranteeing that it will always be able to transmit, even in busy AIS congested waters. For fast moving vessels this is important as a missed transmission can result in a vessel moving a long distance before it next manages to send a transmission. Another feature that the new Class B+ technology it has taken from Class A, is the increased and automatic changing of transmission rates depending upon speed. Unlike Class A, the update rate is unaffected by whether the vessel is manoeuvring, but as the vessels speed increases, the number of transmissions increases so that other vessels get a clearer and more up to date view of where the boat is. For slow moving vessels the increased update rates of Class B+ are not so important, but a fast power boat travelling at say 23 knots, will move 360 metres in 30 seconds, which is the update rate of a normal Class B transponder. On a Class B+ vessel travelling at 23 knots or more, the update rate is 5 seconds, so (using the above example) only 60 metres would be moved between updates. Finally, Class B+ transponders have a higher power transmission 5 Watts instead of 2 Watts and this not only increases the range over which the vessel’s transmission will be received, assuming good antenna height and performance, but it also significantly improves the AIS Satellite reception, enabling global tracking. "

Comparison table :