Posted by Doug Miller on 6/2/2011 to Products & Technology

Having sold hundreds of Class B AIS transponders to customer all over the world, one of the most common questions we get from customers is “I’ve set up my new transponder but no one can see me”. In virtually every case the transponder is working fine so here are some things to keep in mind when testing a Class B transponder:

All Class B AIS transponders transmit at 2 watts vs. the 12.5 watts that Class A transponders typically use. This difference in power has a huge impact on the transmission range of each transponder.
The transmission range for a Class B transponder is typically 5-7 miles in perfect conditions. “Perfect conditions” means using a good VHF antenna, placed high and clear of other antennas with good cables and connectors. This limited range is by design in order to not clutter the AIS system with lots of recreational traffic.
While a Class B transponder will only transmit out to vessels 5-7 miles away, the AIS receiver in the transponder will typically see Class A vessels that are 20-30 miles away or even more in excellent conditions.
If you are using a dedicated AIS/VHF antenna for your transponder, be sure that is it placed at least 6 feet away from other VHF antennas or vertical metal objects and ideally install the antenna on a different vertical plane from other VHF antennas. In our testing, mounting two VHF antennas beside each other typically reduces the range for both antennas by 50-70%.
Testing the transmission range for a vessel with a Class B transponder in a marina will not usually work. Given the low transmission power of the transponder and the interference from other metal antennas and masts from nearby boats can often mean that your transponder can’t be seen even by other vessels on the same dock. Try testing out in the open water before concluding that the transponder isn’t working.
Probably the most common call we get is that users can’t see themselves on online AIS sites such as http://www.marinetraffic.com/. There is usually a good reason for this. MarineTraffic is a wonderful system which uses AIS data received by hundreds of volunteer-operated AIS receivers around the world. Since the system is not formally organized but instead relies on anyone who decides to upload data to MarineTraffic, there is no guarantee that there is a receiver near your vessel. Also, given all the other considerations mentioned above (e.g. the receiver would need to be within 5 miles of your position and if you are in a marina, it is almost guaranteed that you will not be seen) there is a low probability that you will see yourself on MarineTraffic. However, if you travel on the open water in an area with known networked AIS receivers, you will usually see yourself appear on MarineTraffic sporadically. In my experience, I’ll see my vessel in Puget Sound about a third of the time cruising in the Seattle area with lots of gaps in my track as I move between AIS receiving stations.
It is probably not a good idea to call a large commercial or military vessel or the Coast Guard and ask them if they see you on their AIS display unless you are in an emergency situation. These are busy folks and often don’t respond to these types of requests. Try calling a recreational vessel or a friend with an AIS receiver and conduct your testing with them. But keep in mind, just because you can see them, they won’t necessarily see you for the reasons above and especially if the other vessel has a Class A transponder.
Related to the previous point, some larger vessels internationally filter out Class B transmissions, especially in busy areas, in order to not clutter their AIS displays. So it is possible the reason someone can’t see you is they don’t normally see Class B vessels.
Also related to these points, some vessels have older software or AIS displays that are not fully compatible with Class B transponders. In some of these cases, you might show up on their display as an MMSI number without a vessel name. This is usually due to the receiving device not knowing how to process the Message 24 static data from Class B transponders.
In some other cases, a receiving station can see you but only your MMSI number is displayed. If the receiving station is using modern equipment, this often happens when you are close to the maximum range for your transponder (i.e. 5-7 miles away) or there are interference issues from other transmissions or antennas. The reason for this is that position reports are sent out as single NMEA sentences whereas static data reports with your vessel name and other information is sent out over two NMEA sentence broadcasts. If there is an issue with either of the static data packets then the whole static data message is discarded. The receiving vessel won’t have the opportunity to process that static information packet from you for another 6 minutes and the same interference issues might exist then as well. Since the position reports are single sentence packets which are sent more often they are more likely to make it “through the noise”.
Class B users should also keep in mind that Class B transponders do not broadcast position updates as often as Class A commercial transponders. As with Class A transponders, the full static information transmission is broadcast every 6 minutes; however the position update will only be sent out every 3 minutes if the vessel is moving slower than 2 knots or moored. To add to this, if the receiving party is using a single channel receiver such as the Smart Radio SR161, then in perfect conditions, the receiver will get your full static information every 12 minutes and your position information every 6 minutes if you are moored.
If you GPS antenna is not setup correctly, your transponder will see other vessels fine but you will not be sending out your position. All transponders need a good GPS fix before it will send out any type of transmission. The color and state of the LEDs on the front panel of the transponder will let you know if the unit is in transmission mode or not. Check your manual for details.

Bottom line - in order to get the best performance from your transponder keep the following guidelines in mind:

Read the manual for your transponder and follow the directions for installation.
Do not cut the GPS antenna cable. Cutting the GPS cable and attaching your own connector will void the warranty on both the GPS antenna and your transponder and in most cases will damage your transponder. This is really important. Don’t do it.
In many cases we have found that mounting the GPS antenna under a fiberglass deck away from metal objects works just fine.
VHF antenna placement, antenna quality and cable / connector quality are by far the most important factors in a transponder installation. Either use a good quality antenna placed as high as possible and as far away as possible from other vertical metal objects or use one of our AIS transponder-capable antenna splitters for sharing your best VHF antenna.
Double check all antenna connections and ensure there is no corrosion on antenna connectors.
Before testing transmission range, ensure that the setup is receiving AIS targets. If you are in an area where you would expect to see AIS targets, a good setup will typically see Class A vessels that are 30 or more miles away. At the very least, look at the LEDs on the transponder and make sure there is activity on the receive channel LEDs. If you are not receiving targets in a target-rich area, you definitely won’t be transmitting anything useful.
Before testing, make sure the LEDs on the unit are in the correct state. Check your manual for information on the meaning for each LED and color. Most units must show a GREEN LED before the unit will beginning transmitting.
Do your testing on open water, not in the marina. Really – we mean it. Class B transponders just do not have enough power to punch through all the metal clutter in a typical marina.
Test your range with another vessel or receiving station you are familiar with and ensure they are within a 5 mile line-of-sight range and they have equipment capable of seeing Class B transponders.

Class B AIS is a great system for close-quarters collision avoidance but is not suitable for long-range vessel tracking.

If you are really looking for a solution for tracking vessels from greater distances, then consider a Class A transponder. The Comar CSA-300 Class A transponder is the lowest priced unit in the market and works very well for long-range tracking.

Finally, always remember, just because another vessel might see you on their AIS display, always use common sense when operating around other vessels. There is no substitute for keeping a good watch and using all available information to avoid collisions.

Be safe!

Doug Miller