GPS Tips for AIS Class B Transponders and AIS Receivers

Posted by Doug Miller on 9/26/2011 to Products & Technology

UPDATE Jan 2012: The AMEC CAMINO-101 now supports using an external GPS so we now offer the product without a GPS antenna. For more info see our blog post here.

Many folks looking at buying an AIS transponder have questions about the requirement for a GPS antenna and ask if they can use an existing GPS device to supply GPS data to the transponder. Unfortunately (or fortunately depending on how you look at it) all Class B AIS transponders must use their own onboard GPS system and are not able to use an external GPS source. So you won't be able to use your chartplotter to feed GPS data to the transponder. Instead you must install a GPS antenna and insure it is attached to the transponder in order for it to work correctly. Some of our transponders e.g. the AMEC CAMINO-101, include a GPS antenna with the transponder so that means you'll get the right antenna. The AMEC GPS antenna also works with other Class B transponders such as the Comar CSB200 or the Vesper Marine AISWatchMate 850 (which has its own internal GPS antenna but some customers prefer to use an external GPS antenna).

Now for a useful tip: when it comes to installing a GPS antenna, we have found that most modern GPS antennas work well under the deck of a fiberglass boat. This means you can often install the antenna behind a storage locker or in some other convenient location and not have to run new cable and drill new holes in your deck to install the new antenna. The best advice we can give is try the antenna out in different locations on the boat and see how it works. Make sure there are no metal fittings directly above the area where you locate the GPS antenna. Most Class B transponders include a GPS diagnostic tool to show satellite reception performance so use that as a guide for getting the best results.

Other tips to keep in mind:

These transponders use a powered GPS antenna with special Low Noise Amplifier. The antenna interfaces with the transponder using 3.3 volts DC. You cannot use a GPS receiver or a non-conforming antenna that does not match the required specifications or one that uses a 5 volt power source. Using the wrong antenna will damage both the transponder and the antenna.
Do not cut the GPS antenna cable. Since this is a powered antenna, if you cut the TNC connector off and attach a new connector and get it wrong you will potentially damage both the antenna and the GPS antenna and void your warranty.
Our AIS transponders use a TNC connector for interfacing with the antenna with the transponder. Don't use an antenna with a BNC connector and try and adapt it for use with a transponder. Chances are good it won't work and / or will cause damage.
Keep in mind that the transponder has the GPS circuitry built into the transponder itself. The GPS connection is for an antenna only. There is no GPS built into the antenna - only an amplified antenna.
If you have questions, contact us and we can make sure you chose the right antenna for your transponder.

Do AIS Receivers Need a GPS?

On a related note, we often get asked what type of GPS is required for an AIS receiver. The answer is: no GPS is required for correct operation of an AIS receiver. An AIS receiver simply decodes AIS transponder transmissions and converts these into NMEA data sentences which are output via the receiver's data port(s). It is then up to the chartplotter or computer navigation program to make sense of these sentences. This is where the GPS comes in. The chartplotter or computer program needs to establish your position using a GPS and then uses the AIS sentences from the AIS receiver to calculate the distance and bearing to other AIS-equipped vessels. So it is the chartplotter or computer program that requires a GPS. The AIS receiver does not require a GPS signal to work correctly. That said, some AIS receivers, including the Smart Radio SR161 and the Comar AIS-MULTI, include a mini-multiplexer that allows users to feed GPS data in on input wires at 4800 baud and the AIS receiver then multiplexes the GPS data with the AIS data and outputs a single data stream at 38400 baud by default with both AIS and GPS sentences. This is a great solution for folks who want to have a single data connection into their computer with both AIS and GPS data.

Once again, if you have questions on how this all works see our online documentation and FAQs in our support area or give us a call.

Happy cruising!

Doug Miller

Milltech Marine