Marine Traffic 1 vessel AIS Satellite tracking

[Mariano]

(Sorry, hit the send button before ending)
The reason some signals get lost on bussy harbors is that class A Ais have transmit priority over class B and every time they transmit data they also reserve a slot for the next transmission.

Mariano

Fair winds

[Paul Elliott]

Originally Posted by Mariano

The point that most are missing is that ALL AIS TRANSPONDERS (A and B) ARE VHF, so once out of reach fron a land based station, your vessel is invisible to the tracking service provider. They are not (and never meant to be) a satcom device like an EPIRB

Actually, it *is* visible to the satellites used for AIS tracking. The AIS system wasn't designed for this, but to the surprise of many people it turns out that the transponder signals can be picked up by satellite receivers. The signals are a bit marginal, and Class A does better than Class B (due to power and timeslot contention).

One of the issues is that a satellite may be picking up AIS traffic across multiple AIS synchronization domains, so there may be multiple transmissions in the same timeslot. The satellites may use Doppler shift to resolve some of these overlap issues, but I don't know for sure.

Here's one company's satellite-AIS system: https://www.orbcomm.com/en/networks/satellite-ais

I think MarineTraffic.com uses Orbcomm.

[bcn]

About the four Norwegian AIS spotting satellites:

https://www.utias-sfl.net/?p=2713

[Mariano]

Those are different AIS systems, not class A ir B AIS, it's called S-AIS (sat-Ais) , mostly used by shipinng lanes fue to its cost. If VHF can be reached by a satellite (a civil one, not military) then why don't the SAR teams all over the world use them?

[Paul Elliott]

Here are examples of Marinetraffic's satellite AIS coverage. Most of the coastal targets are of course from land-based receivers, but (with a very few exceptions) anything more than 100 miles offshore has been received by satellite. The two images show: All AIS traffic, and Class-B traffic. This doesn't tell us much about the reporting rate and the changes of receiving any particular vessel's transmission, but it is interesting. I like how we can see the Class-B boats leaving the west coast of NA and SA, heading to Hawaii and the south Pacific. We've got major jump-off points in southern California, Puerta Vallarta, and Ecuador.

[Paul Elliott]

Originally Posted by Mariano

Those are different AIS systems, not class A ir B AIS, it's called S-AIS (sat-Ais) , mostly used by shipinng lanes fue to its cost. If VHF can be reached by a satellite (a civil one, not military) then why don't the SAR teams all over the world use them?

The systems I am describing use satellites to receive regular Class-A and Class-B AIS signals from standard AIS transponders. I've used this service to track friends' sailboats that were equipped with regular Class-B transponders. As I mentioned previously, it doesn't work all that reliably with Class-B, but it works well enough to be useful.

I've been following the development of this capability since the early research phases. Of course VHF signals can reach a non-military satellite. Hams have been doing this for years, sometimes with very low power (similar to that of AIS transponders). And for that matter, the older EPIRBS used a low-power 121.5 MHz (VHF) signal that was received by satellites.

[bcn]

Originally Posted by Mariano

Those are different AIS systems, not class A ir B AIS, it's called S-AIS (sat-Ais) , mostly used by shipinng lanes fue to its cost. If VHF can be reached by a satellite (a civil one, not military) then why don't the SAR teams all over the world use them?

No, these satellites are tracking the standard AIS signals.

And the military are using those quite a lot. Or tracking furtive fishing (which then turn their AIS off...)

[DavefromNZ]

Hi Paul. I also follow with interest AIS developments. I was sponsored by exactEarth with Sat AIS coverage when I sailed around New Zealand. I originally tried the spitter with VHF aerial and detection was poor. However coverage was excellent with a dedicated and correctly tuned aerial. The gaps in coverage were only because of more limited satellite coverage down south. But more AIS satellites are being launched.

There was a big discussion on these forums on aerial location. Must not be next to the VHF aerial so I put it on the top spreader. Yes against some advice but I have received ships up to 80nm away.

The other fantastic development is personal AIS rescue beacons that all my crew must wear offshore.