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Ben E

Using GPS for AIS timing

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A comment stream about GPS compasses veered off into how AIS timing works so I moved it over here:


What I find annoying is that AIS units all have their own GPS receivers. They do it to get the timing information for the radio protocols - they could accept external position and orientation data but very few of them do. In fact, there's no reason very accurate time couldn't be distributed via Ethernet and IEEE1558 so an AIS wouldn't need anything other than a VHF RF connection if that service was available.

Posted by: Mike at December 29, 2014 4:10 PM | Reply


Mike comments:

"AIS units all have their own GPS get the timing information for the radio protocols...."

That is a very interesting observation. I am not doubting your statement, but could you give a pointer to a more primary reference on that?

Posted by: Jim Hebert at December 31, 2014 10:51 AM | Reply


Hi Jim,

Navcen and Wikipedia have good info on the intricacies of AIS transmission slot timing:

Note that Class A AIS uses SOTDMA (Self-Organized Time Division Multiple Access) while current Class B units use CSTDMA (Carrier Sense TDMA) though the system is fully integrated. Apparently SOTDMA Class B AIS will be out soon, though I don't know how it will be different.

At any rate, as Mike said, they all involve precision timing based on GPS clock data.

Posted by: Ben at December 31, 2014 12:22 PM | Reply


Hi Ben--The timing of transmissions is certainly critical, but I am most interested in a primary source which explains how the timing is based on the clock data from a GPS receiver. I don't consider Wikipedia to be a primary source. Or the USA Coast Guard.

I did find a reference in the Wikipedia article to IEC standard 61993 and its Section 6.2. I was able to locate an electronic copy of the document, albeit a version dated 2001. (It seems that the IEC doesn't provide free electronic copies, but several host sites have them.) There I found this statement:

6.2 Internal GNSS receiver

6.2.1 UTC source


Since UTC is required for synchronisation purposes, an internal GNSS receiver shall be used to determine the UTC.

That's what I was looking for. A primary source that confirmed Mike's comment. Thanks for helping me find it.

Posted by: Jim Hebert at December 31, 2014 1:15 PM | Reply


CS (Carrier Sense) means that the sender detects that someone else is sending as well, and stops sending.

SO (Self Organized) is pretty unique to AIS, and one of the original AIS patents covered it. The trick is that once you have successfully sent a message the AIS also sends out the ID of the timeslot that it intends to use a few seconds from now.

With SO, once everyone hears every other party, the first one to claim a time slot in the next "group of slots" gets it, because every other sender knows that that sender already claimed it. Since there are usually plenty of slots, nobody cares who claims which slot.

Since you claim a slot that is 30s later or so, and many slots fit in one second, SOTDMA is even more absolute-time sensitive than CSTDMA.

SO works in AIS because there is no need for any sender to be "sender" for more than one slot every few seconds. In most applications some sender wants to send a lot of data but only less often. Normal VHF voice, for instance. With AIS every sender has only a little data to send.

I must say that SOTDMA by itself is already a work of brilliance, but shared CSTDMA with SOTMDA over a single network is pure genius.

It seems the inventor is not having an easy time:åkan_Lans

Posted by: Kees at December 31, 2014 1:27 PM | Reply


Re the source of timing reference, on some further study, and in Class-B AIS transponders of the carrier sense (CS) type, I found IEC specification 62287-1 applies. In that reference I found:

----------------------------- Synchronisation

Synchronisation shall be used to determine the nominal start of the CS time period (T0). Sync mode 1: AIS Stations other than Class B"CS" is received

If signals from other AIS stations complying with IEC 61993-2 or from Base Stations are received, the Class B CS shall synchronise its time-periods to their scheduled position reports.... Sync mode 2: no Station other than Class B"CS" is received

In the case of a population of Class B"CS" stations alone (in the absence of any other class of station that can be used as a synchronisation source) the Class B"CS" station shall determine the start of time-periods (T0) according to its internal timing.

From this reference, it seems that in a Class-B AIS with carrier sense--and I think this is the most common type--the internal GNSS receiver is not used for time reference if the device is receiving Class-A signals. The device aligns itself to the Class-A signals it is receiving.

Posted by: Anonymous at January 1, 2015 9:57 AM | Reply