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How to Choose an AIS Transponder

Posted by Doug Miller on 4/8/2020 to Products & Technology
How to Choose an AIS Transponder

NOTE: This article was updated in October 2000 to include the latest AIS solutions.

One of the most common questions we get from customers is:
“How do I decide which AIS transponder is right for me?” 

After all, there are many different models and virtually every major marine electronics manufacturer makes an AIS transponder plus you have a number of AIS specialty manufacturers (such as AMEC, em-trak and Vesper) who make many different models.

This article is mainly targeted at recreational boaters looking to add AIS to their vessel although much of this information can also be used by commercial vessel operators. Note, if your boat is required to use an AIS transponder for regulatory reasons, you will likely need a Class A AIS Transponder although in some cases a Class B AIS transponder can be used. For more information on AIS regulations in the United States, check out our US AIS Rule Change Information Page.

There are dozens of models of AIS transponders to choose from and prices range from less than $500 to well over $1,000 for a Class B AIS transponder. Typically, there are two major factors that go into the price differences.

First, the manufacturer brand plays a role in pricing. You will typically pay a bit more for mainstream brand products from Garmin, Raymarine, Simrad, and Furuno. Vesper Marine is a well-known AIS brand and often their products tend to be premium priced.

The second factor that impacts prices are integration features beyond the basic AIS transmit and receive capability. By this we mean what else is included with the AIS transponder in the way of additional functionality (such as an integrated AIS/VHF antenna splitter) or additional interfacing options such as Wi-Fi support. In this article, we’ll attempt to define what those major differences are and how to choose the right model for you. 

Start with the basics

As a starting point, all legitimate AIS transponders (also known as AIS transceivers) do exactly the same thing. All are put through the same internationally recognized test suites and are certified by various national and international testing agencies. All AIS transponders receive all types of AIS transmissions from other nearby AIS devices and they all – once programmed – transmit your own vessel’s information. When we say “legitimate AIS transponders” we mean devices that have a Grant of Equipment Authorization by the Federal Communications Commission (FCC) and ideally are on the US Coast Guard Approved Equipment List in the USA or the equivalent agencies in other countries. Be aware that there are non-certified AIS devices sold on Amazon, eBay and other online markets that cannot be legally used in North America. All of the products we sell on the Milltech Marine website are FCC authorized and US Coast Guard approved.

All of our AIS transponders support NMEA 0183 and NMEA 2000 for interfacing with chartplotters and USB and/or Wi-Fi for interfacing with navigation programs on a Windows or Mac computer. It is also important to note that AIS information output by all transponders conforms to NMEA 0183 and NMEA 2000 standards so any AIS transponder can be used with any modern chartplotter. In other words, if you have an all Garmin network on your boat, for example, you can go with the Garmin AIS 800 Class B Transponder if you want to maintain a single brand or you can go with any other AIS transponder from any other brand. Either way, the AIS data output by the transponder you choose is the same. Another point worth noting is all AIS transponders have some method for putting the device into “silent mode” which means it turns off the transmission of AIS data but the device continues to function as an AIS receiver. This is usually done via a physical toggle switch and / or via a software setting. Finally, all AIS transponders must also have their own GPS system embedded within the unit and as a result, all units also need a GPS antenna which is typically supplied in some form with the transponder. Some units include an embedded GPS antenna in the case as well as an antenna port for connection to an optional external antenna. Some include a marine-grade mushroom-style external GPS antenna and some include a portable GPS antenna. All AIS transponders have the ability to output GPS NMEA data in addition to AIS data.

Finally, all AIS transponders can be interfaced with DSC-capable VHF radios (to send GPS data) or DSC radios with AIS display capabilities (to send GPS and AIS data) such as the Standard Horizon Matrix GX2000 VHF Radio & AIS Display or the Icom M506 VHF Marine Transceiver. The new Vesper Cortex is a unique product that combines a Class B 5-watt SOTDMA AIS transponder with a multi station VHF radio system, all controlled in a single hub.

Four key questions for choosing the right AIS for you

So with that base of information, there are four questions I typically ask a potential customer to help zero in on a recommendation. Note, that we offer a free AIS Transponder Comparison Guide that lists all of our AIS transponders with a side-by-side comparison of features and pricing.

Here are the questions and the pointers for solutions that address each need.

  1. Do you need wireless support? A lot of customers not only want to see AIS targets on their chartplotter or computer, but they also want to see AIS targets and get collision alarms on their mobile device such as an iPad or an Android device. If you are interested in this then you will want to select a transponder with built-in Wi-Fi support. Any of the units we sell that are identified as having Wi-Fi support will output AIS and GPS data over Wi-Fi to popular marine navigation mobile apps such as Navionics (iOS and Android), iNavX (iOS only), Time Zero iBoat (iOS) or OpenCPN (Android, Mac, PC). However, you might want to select an AIS device that can also gateway and multiplex other marine data on your NMEA networks (e.g. depth, wind, heading). For those devices, the additional instrument data is combined with the AIS and GPS data and all of the information is output to the navigation app. For more information, be sure to see our article titled AIS Transponders with NMEA Multiplexing: Why You Should Care.
  2. Will you use a dedicated VHF antenna for AIS or use a VHF/AIS Antenna Splitter? All AIS transponders must be connected to a VHF antenna in order to send and receive AIS information. To accomplish this, you can either connect the AIS device to a dedicated VHF antenna (preferably tuned to 162 MHz) or you can use a specially-designed antenna splitter to safely share your existing VHF radio antenna with your marine VHF radio and your AIS transponder. If you are planning on using a dedicated antenna, then the process is pretty straightforward – select an antenna, find an antenna location free of interference from other vertical metal, install the antenna, lay the cable and plug it into your AIS device. Or maybe consider an antenna splitter. AIS transponder capable antenna splitters work really well, are fully automatic and the technology from all the major manufacturers has advanced to the point where the loss from sharing the antenna is negligible due to amplification on the receive path. Because of this, a majority of our customers with sailboats go with an AIS solution coupled with an antenna splitter so that they can take advantage of the VHF antenna that is installed at the top of the sailboat mast – which is the best possible place for a radio antenna. For power boats that already have two VHF antennas, a splitter may make most sense as well. We sell some AIS transponders with integrated splitters as well as models that can be coupled with separate AIS/VHF splitters. For more information on antenna considerations see our blog article: VHF Antenna: The most important factor for getting AIS to work well.
  3. Transmit power – 2-watt or 5-watt? There are two types of Class B AIS transponders. The original 2-watt system uses Carrier Sense Time Division Multiple Access (CSTDMA) protocol for managing transmissions and works fine for many recreational scenarios. The nominal transmit range is 5-7 miles but this can vary quite a bit depending on how good your VHF/AIS antenna setup is. Note that this system transmits less often and is a “polite” system that waits its turn to transmit, giving preference to Self-Organizing Time Division Multiple Access (SOTDMA) protocol traffic – which is what Class A commercial AIS transponders use.
    We now also have 5-watt AIS transponders which, as you can imagine, transmits farther – typically 10-15 miles. These transponders use the same SOTDMA system for managing transmission slots as the commercial Class A systems, so they transmit on a more regular schedule and more often. These units tend to be more popular with faster boats and / or smaller commercial vessels. These SOTDMA units also have the ability to transmit AIS message 27 which is a short packet optimized for reception by AIS satellites. Therefore, if you are crossing an ocean or going to remote waters you might consider one of these systems so that friends and family can track you via an AIS satellite tracking service.
  4. Do you need other features such as an integrated display or enhanced alarms? Finally, there are a range of value-add features that may be appealing to you. For example, some transponders have their own display and can act as fully self-contained AIS devices. These units show AIS targets in relation to your position and feature the ability to set up alarms directly on the AIS device. On the subject of alarms, some units such as the Vesper smartAIS range support sophisticated alarm profiles as well as anchor watch alarms integrated with AIS collision alarms. Note that typically AIS collision alarms are handled by the display device (chartplotter, computer or mobile app) so having an AIS device that also does its own alarms may not be essential. Also, as mentioned earlier, the Vesper Cortex is a unique product that combines an AIS transponder with VHF radio and remote monitoring as well as handsets for VHF calls, screens for AIS collision avoidance, anchor alarm and more.

Those are the major considerations for selecting an AIS transponder but other considerations such as warranty or price may impact your preference.

Some recommendations

In conclusion, here are some feature-driven recommendations based on popularity and feedback from customers.

Note that we stand behind all the transponders we sell and the recommendations above are there to give you some ideas on where to start. Again, I recommend you download, print out and go through our AIS Transponder Comparison Guide for a more detailed comparison of features and pricing. Also be sure to look at the second page which has explanations in plain English for each of the areas of comparison. Finally, be sure to look at the customer reviews for each product you are considering. All reviews are written by real customers who have bought and used the product.

We also welcome feedback below. If you have questions, feel free to call us at 1-206-299-2217 or contact us online.

Safe boating!

Doug Miller

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