AIS or Automatic Identification System is used by marine traffic to communicate their position to other vessels.
This communication is done via a digital VHF signal on +- 162mhz
This is not to be confused with a similar system DSC / Digital Selective Calling which also works on digital VHF signals.
Information that is transmitted includes GPS based lat/long, speed, heading, vessel name MMSI/Maritime Mobile Service Identity which is a unique identifying number for each vessel much like a phone number.
Upon receiving this information, an AIS receiver can calculate information such as bearing to the other vessel, closest point of approach, time to closet point of approach etc.
With AIS you can have a transmitter, a receiver or a transponder.
A receiver simply receives the data and may display this information on its own screen or pass it on to a MFD/chart plotter where it can be displayed. These are becoming more and more common on many VHF radio sets, most of which already have DSC.
An AIS transponder will receive and transmit information. These are common on most commercial vessels and if you visit the site ( or have the app) marinetraffic.com you can track millions of vessels worldwide. How does this work? Well there are multiple VHF receivers based around busy ports that take the VHF signal and then upload the data on the web. So not only can you track an AIS equipped vessel on a boat via VHF but you can also track it on the internet assuming there is coverage.
Pseudo transmitters. These mark items such as navigational hazards such as rocks. The signal usually comes from somewhere other than the position marked. Such as a VHF repeater station ( like the skytower in Auckland)
An AIS transmitter is a bit more of a rare beast. These transmit information but dont receive any.
Uses include emergency location devices and location tracking of fishing nets etc.
This is where my interest come in! I have been looking for a tracking device for awhile.
I wanted something can you transmit Lat/Long without setting off an emergency event and be rated to at least 100 meters depth.
The closest available product would be the Nautilus Lifeline which used both AIS and DSC. Its 130 meters rated and very small. BUT you can only active it on the surface and will only transmit on AIS in a distress situation.
The next closest product is the Matsutec HAB120 which is a fishing net AIS transmitter.Its a lot bigger than the Nautilus mainly because it was designed to be positively buoyant and have a fixed internal VHF antenna. While being waterproof it is not pressure proof.
I decide to get one to try it out.
While I sat in the comfort of my elevated sea view home, one of my fellow Coastguard skipper ( Kerry) took it out for a test.
Once thrown in the water the HAB120 lay on its side. Not the best as far as GPS receiving or AIS transmitting goes. Kerry then duct taped it to a datum marker ( vertical floating marker pole) just above the water line.
I was over 2 nnm ( 4kms) away and picked up the signal on my vessels AIS transponder. It was also about 2nm away from a AIS base station receiver so I could see it on marinetraffic.com. We didn't get to test it at greater distances than this. But I am also about 2nm away from said base station and both the HAB120 and my vessels AIS transponder have no issue reaching it.
The coastguard vessel motored away from the HAB120 but lost the signal at just over 2NM. Bearing in mind that the HAB120 was just above water level.
OK. so stage two was modifications!
I built a canister out of schedule 80 PVC and mounted the bared PCB and battery from the HAB120 into it. I removed the reed switch /magnet that is used to turn it on and off and fitted a toggle switch than must be turned on pre-dive. This also allows for a pre-dive check that it is working. I removed the small whip antenna and fashioned an external 1/2 have wave ( 0.9meter) antenna using stainless steel cable.The idea is that I can clip the free end of the cable to the top of an SMB and let the canister dangle below. This will allow me to deploy the transmitter at any depth I choose as long as I have a spool/reel long enough.
The HAB120 also has a "Mayday" button. AT this stage I have decided not to incorporate this in my build. It would mean an externally operated button on the canister which may easily get knocked during the dive. Anyway, the dive boat skipper can always to a Mayday call if the diver does not return to the boat at the allocated time and inform other vessels of the AIS system in place.
So why didnt I go with an EPIRB system?
EPIRB works differently. Once activated an EPIRB or PLB will receive GPS information then re-transmit this on 406mhz back to a satellite. This will activate a full on emergency response. This is fantastic and definitely the way to go..... if thats what you need. The AIS system wont set off a full on distress....even if the mayday button is pushed. Someone picking up your AIS distress must manually inform the authorities ( RCCNZ, MNZ Coastguard, police) about the situation.Interestingly, EPIRB's and PLB's are now available with AIS transmitters. So not only will they transmit your position back to RCCNZ ( or the equivalent authority) they will also transmit it to all AIS equipped vessels in range!
So what is the range of AIS? This depends on several things.
First, the strength of the transmitter. This is usually 0.5-2 watt.
The quality and the height of the antenna above sea level. Also consider sea height!
The quality and height of the receivers antenna.
What is in the way.
I have found that vessel to vessel, you can get 10NM reliably. With the advantage of about 80 meters of height at home, I have picked up vessels 30NM away.Nautilus Lifeline claim 34NM for their unit. I am sure its true.....but we need to ask under which conditions did this happen? Given that its only a 1 watt AIS transmitter and a less than ideal antenna transmitting at water level.... I 'd suggest 34NM is probably a one off and definitely not something to expect in real world conditions.
The HAB 180 is rated at 2watt transmit power. I will do some further testing over the next month or so . But I would think 2NM should be a reliable range. Obviously in a distress situation a EPIRB or PLB is a much better choice with a range to the shy and back.
EDIT: HAB180
Battery life- 240 hours-Untested
Transmission interval- 3 minutes ( Faster when traveling at speed)
UPDATE:
I just got hold of another Chinese AIS buoy. This is the SH-998 from Shunghang.
This has a couple of key benefits over the HAB120.
1st is that you can adjust the reporting time from 10 seconds to 4 minutes. Obviously the more frequent the reporting the shorter the battery life, but I have bench tested it at 10 seconds and it has last several days which is more than plenty for my requirements. Although when set to 10 seconds the reporting time seemed to be more like 30 seconds when I tested it with my boats AIS transceiver. I will look into this further at a later date.
The second benefit is that the PCB is smaller. At 56mm wide it should fit into most common dive light canisters or scheduled 80 PVC tube without boring the ID out as I had to do with the HAB120 PCB.
The battery is 8.4v max vs 12.6v but capacity is quoted as 3000mah vs 2000mah (On the battery vs sales material) and battery life is 15days vs 10 days.
Another benefit here is that I can easily swap out the 8.4v LiPo (?) pack and replace it with 2x 18650 pack of known genuine 3400ma cells. I dount think I can do that with the HAB120 due to space restrictions.
Both units are listed as 8w transmitters, although I have seen a 15w version called the SH-098 that runs 12.6volt (2000mah) and uses an external aerial.
UPDATE:13/07/2018
After a slight leak in my canister the SH998 has a transmission problem. The range is greatly reduced so I assume its an issue with the VHF section.
I have now sourced a different AIS buoy again. This is the ONWA KS33-NTX
There are several KS-33 models
There is the KS-33R which is a dedicated MOB AIS unit. Its nice and small but issues with it include only sending a MOB distress signal, non rechargeable non replaceable battery ( easy solved with custom case), limited power of 2 watt.
KS-33NT is the same as KS-33NTX except it has a small whip antenna built into the casing, while the X has an external 1/2 wave 900mm+- antenna. Both units may come in different battery voltage and VHF power specifications.
Once again the KS-33NT and X are programable for MMSI vessel name and reporting interval. The program cable is a simple serial programmer and I was able to modify the one I got with the SH998.
One of the great features of this is you can set it to report every 20 seconds up to every 3 minutes but every time it reports it repeats 4 times each time 1 second apart. This means if it was in the trough of a wave at the first instance, it may be on the crest for one of the others.
I have not done a in water test with it yet but I have tested it from my house which has an altitude of 80 meters and I picked it up in a vessel from 11.7nm away ( 22kms). Now, the signal wasn't consistent but given that it traveled all that was over land and sea, I am quite impressed. Not only that, but it gets constantly picked up by a marinetraffic.com AIS base station 30km's away with Rangitoto directly in the way ( I am guessing their antenna has some height).
Battery life is quoted at 360 hours (15 days) but this would be based on 3 minute interval polling.
I have ran it at 20 second polling and got over a week.
I have been in touch with a fellow diver today about this. He has been speaking with Nautilus who make the small DSC/AIS unit. Apparently they may be developing some firmware that will transmit a non distress signal at regular intervals instead of the MOB distress signal it now sends. Given its size it will be a great tool. Just dont go believing their quote range of 34 miles ( statute or nautical). At 1 watt power and such a short antenna it may have happened once with elevated antenna in perfect conditions but it sure as shit wont happen in real world situations at sea level. I'd be very surprised if it managed to get anywhere close to 1/10 of that. But 3.4 miles ( 2.9NM or 3.9km) is still pretty good range (if that what it can do). I'd love for them to send me one for testing to prove me wrong!
