Satellite AIS and class B transponders

[Bi111ion]

"Marine Traffic" web site uses Satellite AIS data, as well as surface stations.. It says that mainly it picks up class A as it is more powerful, has its own antenna and reports more frequently. Has anyone any evidence of to what extent S-AIS picks up class B transponders? I looked up Cheeki Rafiki, as I wondered if S-AIS had picked up its signal in which case last known position report would be a useful addition to EPRB and PLBs (showing at least a position when they still had power). The Marine Traffic web site http://www.marinetraffic.com/en/ais/details/ships/235052271/_:a59c59560592091093eb4a38c7eabcbb
says "more recent position available from S-AIS", but of course you have to pay.

Generally we use a 1/4 wave vertical dipole for AIS like VHF which does not radiate up very much. Just wondering if different antenna (or differently oriented antenna such as horizontal) would help S-AIS enough that class B would be detected. The satellites used are low earth (polar) orbit, with a period of about 90 mins similar to COSPAS-SARSAT. So a horizontal dipole with a north south orientation would be better. I wonder if that would still give enough signal strength. You could reorient it if there was low visibility.

I wonder if yachts generally switch off AIS to save power when in the open ocean in good visibility, as it is pretty rare to see another ship?

 

[Arida]

Coincidentally, I recently received the following press release from Software Radio Technolgy (SRT), who are responsible for the "guts" of many of the class B AIS solutions currently on the market:

PRESS RELEASE

23rd May 2014

REVOLUTIONARY NEW TECHNOLOGY ENABLES RELIABLE SATELLITE TRACKING OF CLASS B AND IDENTIFIER AIS TRANSCEIVERS

ABSEA™ technology is the result of a collaborative development program between SRT Marine Technology in the UK and exactEarth in Canada. For the first time Class B and Identifier type transceivers with ABSEA™ upgraded firmware can be tracked from space by the exactEarth satellite network.

As a VHF based technology, the tracking range of any AIS transceiver is line of sight between the transmitting and receiving antenna. This typically means that terrestrial ship to shore tracking range is up to 50 nautical miles. Satellite constellations with AIS receivers have been built to provide coverage beyond this range, but due to a host of technical issues inherent in the ITU defined AIS protocol, and low power VHF radio in general, can only reliably receive transmissions from Class A type AIS transceivers.

However, with the invention of ABSEA™ technology this now changes. Transmissions from Class B and Identifier type AIS transceivers with ABSEA™ technology can now be reliably received and fully decoded by the exactEarth AIS satellite network. This innovation opens the possibility of continuous tracking of vessels using Class B and Identifier type transceivers outside of terrestrial network range.

Simon Tucker, SRT CEO said: “AIS is a fantastic technology for which our customers are constantly finding new and innovative applications. Since launching the world’s first Class B in 2006, SRT’s role has been to provide our customers and the market with a full portfolio of high performance, customisable AIS technology and product platform solutions around which they can create their own unique market offer. Our exclusive partnership with exactEarth has enabled our R&D team to develop a truly amazing technology which extends the capability and utility of AIS thus opening yet new opportunities for our customers. This is another example of SRT supporting the market and its customers through leading edge technology development, something that has been a feature of SRT for the last 20 years and will continue to be so into the future.”

About SRT Marine Technology Limited.

SRT Marine Technology Limited is a wholly owned subsidiary of the UK based SRT plc group. SRT develops and provides AIS technology and product solutions for the global marine electronics industry for all geographic and application market segments: leisure, commercial and security.

For further information please visit: www.srt-marine.com

 

[Bi111ion]

That's interesting but I dont get it. Is it a different protocol and frequency? There are lots of protocols that could be used for position + MMSI transmission but I think one problem is that in crowded areas the satellite would be swamped by AIS as the data collision algorithm is based on not colliding with transmissions from nearby craft, but the sat could see over a much wider area.

If it is not AIS protocol you could not transmit it on the AIS frequency so maybe that is a partial answer to my question, but how would this new protocol fit in to the ITU frequency allocation and IMO regulations?

 

[Arida]

There is a technology white paper available to download here: http://main.exactearth.com/absea

But to be honest,it does not say very much, except that the Absea capable class B AIS units remain fully compatible with the class B AIS standard. There is reference to special signal processing at the satellite end and some small change to the software of the AIS unit.

Purely speculation, but one way to improve detection in the presence of noise would be to synchronise transmission to the GPS 1PPS signal in order that the satellite could derive a bit clock without having to lock on to the transmission preamble.

All kind of begs the question why not just deply lower power class A-compatible units - I understand that the intellectual property issues have now gone away and no reason the hardware shold be any more expensive if TX power is kept low and RX performance limits are eased.

 

[Bi111ion]

Yes I read the uninformative white paper too. I agree it looks like a tweak of the protocol within the existing spec. I am still wondering if just reorienting the antenna will work ok. After all you can reach an amateur satellite on 2m with a 3W handheld transceiver just by angling the antenna correctly. If it works it is a very cheap alternative to other tracking solutions if you don't already have eg an HF radio.

If we tried this, will we actually see our position show up on any AIS tracking website without paying a subscription?

 

[snooks]

One thing I remember from my VHF operators licence is that VHF works on line of sight. Now on ground that's to the horizon and a bit, but if it didn't hit an obstruction, for example land, as long as the two antenna are in line, and there is nothing obstructing the line from one to the other then transmission and reception are possible. To this end the instructor said that NASA transmit at 1W to the space shuffle.

Fact or fiction. I'd like to know, as it all sounds about right to me.

 

[Bi111ion]

Yes you are right. Line of sight to satellite but the radiation pattern looks like this http://content.westmarine.com/wm-img/westadvisor/articles/VHF-Antennas-2.jpg

For a yacht vhf aerial it s fairly broad and works with a reasonable heal angle but will not get much energy radiated in the upward direction.

Radio Hams use hand held VHF radios, even with small aerials to talk through satellite repeaters, but you have to orient the antenna perpendicular to a line to the satellite.

(edit)
That said my EPIRB has a vertical whip and that does not seem to make sense as the SARSAT gets a fix on you by looking at the Doppler shift at its closest approach. I think that when it is floating the sea acts as a ground plane very close to the antenna, and this changes the shape of the radiation pattern with a bit more power going upward.

 

[BrianH]

Yes you are right. Line of sight to satellite but the radiation pattern looks like this http://content.westmarine.com/wm-img/westadvisor/articles/VHF-Antennas-2.jpg

For a yacht vhf aerial it s fairly broad and works with a reasonable heal angle but will not get much energy radiated in the upward direction.

But obviously enough as there are now a number of satellites in orbit successfully testing for AIS reception. Even the Class B 2W transmissions seem to be viable for reception. As exactEarth is involved with the ABSEA technology development and they charge for their satellite services this is unlikely to be a cheap option.

 

[BrianH]

Yes you are right. Line of sight to satellite but the radiation pattern looks like this http://content.westmarine.com/wm-img/westadvisor/articles/VHF-Antennas-2.jpg

Slight Fred Drift here - sorry Bill - but I was concerned at the spasmodic loss of my AIS target in both Marinetraffic and Vesselfinder reporting sites since I replaced my receiver with transponder a month ago. I have been the entire time in my marina berth in northern Italy and for the early period appeared to have solid target reporting showing on both site maps of the area. Then recently it was disappearing immediately or shown as a long period since the last report while others in the same marina were consistently displayed. I suspected an antenna connection problem, although I was still receiving targets directly.

Then I noticed a consistency of the site displays - it was synchronised with tidal rhythm. At low tide (we have ca. 1m range in the northern Adriatic) the pushpit-mounted 1/2 wave antenna was sinking behind the fixed pier to which I was moored stern-to and, additionally, behind the dish antenna there with which I get my television signal.

When I was appearing to get a consistent report on the two sites after installation it just happened to be my check was at high tide and the past week at low tide. "Line of sight" indeed, although I suspect that dish antenna had most to do with it.

 

[Bi111ion]

When I was appearing to get a consistent report on the two sites after installation it just happened to be my check was at high tide and the past week at low tide. "Line of sight" indeed, although I suspect that dish antenna had most to do with it.

Yes line of sight certainly matters. I mounted my AIS antenna on the mizzen mast and both Marine Traffic and Inverness Marina spotted me on the way to Inverness from well down the Caledonian Canal, which rather surprised me given the hills. On a ketch its a nice option - the main mast has the VHF antenna and the mizzen antenna is also a backup for that.

 

[BrianH]

Yes line of sight certainly matters. I mounted my AIS antenna on the mizzen mast and both Marine Traffic and Inverness Marina spotted me on the way to Inverness from well down the Caledonian Canal, which rather surprised me given the hills. On a ketch its a nice option - the main mast has the VHF antenna and the mizzen antenna is also a backup for that.

It is clearly advantageous to have an alternative higher mounting to site the AIS antenna - I am surprised how easily the radiation pattern from my pushpit mounting position is disturbed. However, at sea where it matters, there should be no problem, it is no reason to change to a splitter on the radio as I have no other alternative such as your mizzen mast.

As for line of sight, as you found, that is not quite the case. Below is a screen shot from OziExplorer with my boat the large red arrow, of a target (at almost 30 knots probably one of the many fast ferries there) tracked in 2008 from behind the island of Premuda in Croatia, using an early AIS receiver, then a photo of the island to show the topography, taken a few minutes later of another commercial ferry that was transmitting no AIS signal. It does illustrate how AIS can complement radar, which would not have registered that first target ... although it would the second, which AIS did not