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Project: AIS
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WHAT IT IS: AIS, short for Automatic Identification System, is the world-wide implementation of SOLAS's requirement that ships over 300 tons (certain exceptions apply) broadcast over radio information about who they are and what they're doing. This information includes their position, course, speed, and direction and rate of turning. If you have the proper receiver and display, you'll be able to see where nearby ships are and where they're going, if they pose any danger to you, and what you might need to do if that's the case. The transmitted information also includes the name and MMSI number of the ship, so if you've identified a potential problem, you can call them on the radio by name (or MMSI if you've got a DSC-equipped VHF) and stand a much better chance of getting a response ... no more "Eastbound ship 10 miles north of Point Fubar, this is the sailboat 5 miles east of you". If the ship does respond to your call, you'll be able to negotiate passing arrangements. If they can't see you, you'll also be able to give them a relative bearing and distance to your location to help them find you. (For all the technical information you can stand on AIS, plus a bit more, check out the US Coast Guard's info page at http://www.navcen.uscg.gov/enav/ais/ .) If you've got a computer onboard with a spare COM port, it's fairly easy to add AIS capability to your set of watch-keeping tools. What follows is a "core dump" of my thoughts and experiences installing AIS on Daydreamer. I should also say here that low cost was one of the main considerations in my decision-making. Be aware that AIS is not a direct substitute for radar. AIS will show ships but it won't show, in particular, fishing vessels or military ships or, of course, other yachts and small boats. If there are any AIS transmissions within radio range, you may be able to hear a random "blurt" on your VHF radio on channels 87 and 88 ... it'll sound like a very, very brief burst of static. Now, you won't be able to use this signal from your VHF for anything, but at least you'll know that there might be somebody out there transmitting an AIS signal. The transmission is in the form of a code called GMSK ... and each GMSK transmission must first be decoded into an NMEA sentence. Unfortunately, the AIS NMEA sentence itself looks like gibberish, so it too needs further decoding to yield the info you're really seeking. The three main elements of an AIS receiver installation include:
RECEIVERS: The reason the signal you hear on your standard VHF is of no use is that your VHF radio has taken the signal and "chopped and channeled" it to make it audible and legible, as you would want for any audio signal you wanted to hear. This signal processing screws up the GMSK and makes it impossible to decode. (It appears that a general purpose receiver might work well to receive AIS's GMSK signal but rather than get diverted by that possibility just now, I'll just append my thoughts on this possibility as a note at the end of the page.) I suspect that the day will soon come when manufacturers of VHF radios will include AIS as an added feature. Until then, for most of us the easiest and lowest-cost way to get AIS will be with a dedicated receiver connected to a computer-based display. There is one dedicated yachtie-type receiver that has an integral display about which more later. Most yachtie-type receivers will receive the GMSK transmission, decode it into an NMEA sentence, and pass that sentence on to a computer for further processing and display. Ships transmit AIS signals on 2 channels (162 MHz ± 75Khz) alternately but it's the same info on each channel, so if you can receive 1 channel clearly, you're good to go ... it may just take a bit longer to get some info such as the name of the ship. The professional AIS receivers get both channels simultaneously. There are yachtie-type receivers that do this too (at a price of course, maybe $100 - 200 more), but most work only one channel at a time. Ideally, a yachtie-type receiver would have a number of characteristics. First would be the ability to receive and pass on NMEA sentences from a GPS because the AIS display software needs to know your location as well as where the ships are. This way, the AIS software would only tie up one COM port. If the software needs separate AIS and GPS input, either because the software itself is brain-damaged or because the receiver can't relay GPS data, you'll use up two COM ports! Second, if price is a concern, it'll probably be a single channel receiver. In that case. it should have the ability to receive on either channel even if only one at a time and should be able to switch from one channel to the other, if needed. Third, it'd be real nice if there were some indication that AIS signals are being received and that NMEA sentences are being passed to the computer's COM port. I'm aware of 3 reasonable-to-low cost dedicated receivers. One is a unit made in Europe, the most expensive of the three and probably the most difficult for me to purchase, so I didn't consider it further. The pictures I've seen show what looks like very good build quality. The second is marketed by NASA (no relation to the US's National Aeronautical and Space Admin). And the third is the SR 121, made in China and sold in the US (and, most likely, elsewhere). The NASA unit's main advantage to me is that it is available in Australia where I am currently located ... but for about $AU 400 ... from an outfit called "Software On Board". I think it's also sold in US under the SITEX brand. It was the first consumer-level receiver on the market and made some pretty good headway because of that but it has a number of drawbacks. Build quality looks a bit dodgy; plastic case, no LEDs to show if it's receiving or even working, power plug doesn't latch, it doesn't pass on all GPS NMEA sentences nor does it correctly handle some long AIS sentences. It can apparently switch between AIS channels but the mechanism for this change is unclear to me. NASA also sells a standalone receiver and integral display that costs $US 700 or more. I haven't seen it but the pictures I've seen show a small lo-res B&W LCD screen display. I was going to go computer-based anyway so didn't examine the stand-alone unit closely but it may be a good choice where power limitations preclude running a computer full time. The SR (Smart Radio) 121 is available in the US for $US 200 plus $US 25 S&H to Australia from Miltech Marine. Build quality looks good, black-crackle-finish metal case that's non-magnetic (aluminum?), latching power cord, red LED flashes when receiving AIS transmission, green LED flashes when transmitting any NMEA sentence (GPS and AIS), it relays all GPS NMEA sentences and correctly handles long AIS NMEA sentences. Documentation indicates it automatically switches to the alternative frequency if the frequency it's on shows interference. I bought the SR 121. No regrets so far (the above comments are, therefore, not unbiased, but they do accurately reflect my decision-making process). ANTENNAS: You'll need a vertically-polarized antenna tuned to VHF frequencies for this service. And like your VHF radio; the higher the antenna, the better the range; the higher the gain, the better the range; the longer the antenna, as long as it's tuned, the better the range. I want to suggest here that the longer the range, the better. Picking up a ship that's 5 miles away and starting to turn into you is not good. Conversely, dealing with a potential collision with a ship that's 20 miles away requires only a small course change.
There are 2 main alternatives; 1) a dedicated antenna for the AIS receiver or 2) share your current VHF antenna.
Easiest is a separate rubber-ducky type antenna, but you'll be limited in range to, maybe, 5 miles or so ($25?). A dedicated 3 db gain whip up on the masthead will give you about 25 miles but it'll interfere with your regular VHF antenna, if it's up there, and vice-versa ($150?). Or you could mount a dedicated whip on your stern rail or spreader for intermediate performance ($150?). You can also build an excellent J-pole antenna out of a bit of TV twinlead and small PVC pipe ($10?). Simply cut some 300 ohm TV twinlead to about 5 feet long. Strip the insulation off one end and solder the two leads together, this becomes the base of the antenna. Measure 53.65 inches to the other end and cut off excess wire. Separate the shield and conductor from the end of your antenna lead coax. Solder the center conductor to one of the twinlead wires about 1.4 inches from the base of the antenna. Solder the shield to the other twinlead wire at the same 1.4 inch distance from the base. Cut out and remove about 1/4 inch of the twinlead wire that the shield is soldered to about 15.3 inches above the base of the antenna. Done! For protection and rigidity, you can stick the antenna into a bit of small diameter PVC pipe. If you decide to share the antenna with your VHF, you'll have to arrange some way to disconnect the AIS receiver from the antenna circuit when you are transmitting on your VHF. Also, if you do this, you'll probably reduce your VHF antenna performance by 3 to 6 db because of impedance mismatch and signal split. Manual switching is always an option but one that's fraught with peril as you only need to forget once. You can buy relays ($150 - $200) that senses transmittion on VHF and disconnects the AIS receiver during the transmission -- probably the simplest way. I built a relay box that does the trick. I mounted a micro switch inside the VHF mike hanger that's open when the mike is hung up. The micro switch controls a DPDT relay that has both VHF and AIS sharing the masthead antenna as long as the mike is hung up. When I pull the mike out of the hanger, the micro switch closes and powers the relay coil which disconnects the AIS from the masthead antenna and connects it instead to an alternate lower-performance antenna - a 1/4 wave whip, rubber ducky, or the J-Pole. After all, you still want to see the AIS status of the ship you're talking to on your VHF. UPDATE: I installed a new DSC VHF radio in the nav station w/ a wireless remote mike just prior to the winter 2008 cruising season and got rid of the not-as-waterproof-as-advertised Standard VHF in the cockpit. This freed up a VHF antenna, so it's now dedicate to the AIS receiver. DISPLAY: AIS-display software seems to fall into two general categories. One type is radar-like, in that it'll show your position in the center of several range rings and AIS-transmitting ship icons spotted around the screen showing relative motion. Range and bearing are usually available using cursor and a mouse click on a ship icon will usually give AIS data (Ship Name, speed, course, turning or not, destination etc.) Potential collision/narrow CPA usually cause an alarm.
The alternative is a chart-based display, as you are used to with Nobeltech, Maxsea etc. but adding the ships' locations and projected true motions and turns. Again, AIS data is usually available by mouse click, range and bearing by cursor, and any potential collision/narrow CPA alarmed.
It seems that most of the commercial navigation packages (Nobeltech et. al.) are now supporting AIS. It's certainly an expensive option if you have to buy a new version just to get AIS capability. And you never know until it's running if it'll do what you want. As a case in point consider a version of Maxsea V.10 I've seen, which receives the NMEA sentence perfectly well but doesn't seem to be able to translate that into anything worthwhile on the display. There are some other high-priced AIS packages that appear to be priced for professional use, as in many 100s of dollars. I really haven't checked the high-priced spread any further since shareware/freeware seems like it'll do it for me.
Which get us to low cost/shareware/public domain software. I've more-or-less checked out the following; ShipPlotter, SOB (Software on Board), SeaClearII, YachtAIS, and AISTrack.
SOB and SeaClearII are both chart-based displays. SOB costs $50 (but may be free with the purchase of a NASA receiver) but requires the purchase of a whole new set of CMap charts. So that's strike 1,2, and 3, big costs for new charts. The interface also looks really clunky and non-intuitive. I decided to pass on this one. SeaClearII is freeware, uses a variety of raster chart formats including Maptech and Australian raster charts, is straightforward to use once you've puzzled thru the documentation (which is generally complete but awkward). A copy is included with the purchase of the SR 121. Interestingly, it can also use charts you have scanned as a bmp. This raises an intriguing possibility for developing your own AIS raster charts from paper charts where good, detailed raster charts aren't commercially available (e.g. Indonesia). It might also be possible to use a screen-grab bmp of a vector chart of that same area; please let me know about your results if you give this a shot. YachtAIS and AISTrack are both radar-type displays. AISTrack looks like it's still in beta and has a couple of display problems but it's worst feature is that it requires separate COM ports for AIS and GPS NMEA sentences. YachtAIS, on the other hand, seems to be very well done - particularly the higher priced version, YachtAIS_Pro. I really like everything about it except for its price - $US80. Not too bad for a one-off purchase but I'm running two computers (essentially mirrored backups as far as navigation is concerned) plus a backup laptop ... $240 is more than I want to go. ShipPlotter does both chart-based and radar-like displays. It's the only choice for general purpose receivers as far as I know but I wasn't able to get a demo version to function properly with my ICOM PCR1000. On the other hand, I didn't try real hard. I found it used a lot of computer resources, would significantly bog down my low-powered computer, did not play well with other software, and the chart handling function was very clunky. I'm using SeaClearII so far - it does what's required. But I'm thinking real hard about trying to develop something similar to YachtAIS on my own but with a different collision/CPA display. If I do, I'll put it into public domain here ... stay tuned. UPDATE: I tried out MaxSea V11 in 2008 and found that it does an acceptable job displaying AIS targets. While MaxSea's AIS data is not as easily accessed as in SeaClearII, the convenience of having only one display for both navigation and AIS more than offsets this slight deficiency. General Purpose Receiver: If you have a general purpose receiver with a "discriminator out" plug (sometimes called "pactor" or "packet"), the signal going to this plug hasn't been processed and should present a good GMSK signal. If you have a general purpose receiver that does not have a discriminator outlet, it may be possible to go into the guts of the circuit boards and extract one ... there is a website that has a listing of receivers where this mod is easily done, but you'd have to google for it 'cause I didn't keep the URL. As far as I know, the only computer program that will process the GMSK signal (via your computer's soundcard) is ShipPlotter. It'll also convert the NMEA sentence it has extracted into a display that is meaningful, so you don't need any additional software. Footnote: Occasionally, as we approached medium-sized towns along the Australian coast, we'd see some AIS targets moving very, very fast in a straight line. This was quite mystifying until we figured out that we were probably seeing airplanes taking off from the local airport! |
