AIS Transponders and GPS
Conditional Filtering


More and more boat owners decide to install an AIS class B transponder. To be able to follow the movements of other vessels is great, but "being seen" by others is even better.

Today many chart plotters and software products support AIS, but unexpected problems can be encountered when a transponder is installed, due to conflicting and unwanted NMEA 0183 traffic



AIS transponders have a built-in dedicated GPS. An external aerial is used, but the GPS electronics are an integral part of the AIS transponder hardware.

Besides standard AIS sentences with identification, position and navigation data of other vessels, the NMEA 0183 output of the transponder also includes GPS NMEA sentences from the built-in GPS. In addition, proprietry NMEA sentences are often included, with transponder status information.

All this data is transmitted to the connected computer or chart plotter. This can cause problems, if the chart plotter already has its own GPS. Position data from 2 different GPS receivers is never exactly the same. Small differences in lat/lon cause constant adjustment of the plotted position of the own vessel in the electronic chart. Not only is this disturbing, it can also cause substantial  extra overhead which has a negative effect on the performance of the equipment. The proprietry status sentences are not recognized by the chart plotter and also contribute to unnecessary overhead.   

The solution is to filter out the unwanted NMEA sentences.  However, the GPS sentences can be put to good use as backup for the main GPS. It would therefore be ideal if filtering out the AIS GPS data only takes place if the main GPS is active. If the main GPS ceases to provide position data, filtering the AIS GPS sentences should stop.

To make the above possible, the filter-capability of Brookhouse multiplexers has  been refined and a new feature "Conditional filtering" has been implemented.

Filtering in Brookhouse multiplexers is script-driven, i.e. a set of simple instructions in text-format is uploaded to the multiplexer from a computer and stored in internal flash memory.

For example, the script for filtering out all GPS sentences from the AIS output unconditionally would look like this:


"Wipe" all sentences starting with $GP, received from multiplexer port 4 (AIS transponder). 


To make this filter script conditional on the presence of NMEA data received from another input port, it is modified as follows:


The first line tests if the GPS sentence RMC is received from the main GPS on input port 1. The second line "wipes" all sentences from input 4, but only if RMC is received. If RMC has been absent for 10 seconds, the wipe is not executed. 

The following script has the same effect, but in addition, the line with *W,4,AIVDO filters out all VDO sentences (own vessel report) unconditionally. VDO is often unused by chart plotters and software and can generate unnecessary overhead. The line with *W,4,P???? removes all proprietry AIS sentences, which are not recognized by chart plotter or software.


If the optional Seatalk-NMEA conversion has been installed in the multiplexer, the above script can detect if the Seatalk GPS is active. If the Seatalk GPS stops, the RMC sentence is not generated and the multiplexer includes the GPS sentences from the AIS transponder in the output data stream.

This mechanism is ideally suited for providing a backup for the Seatalk GPS in installations with Raymarine C- and E-series chart plotters.

Besides Raymarine C/E series, there are various other chart plotters from other manufacturers where the same applies.

There are many other applications for conditional filtering. Multiple "Conditional Wipe" instructions are possible.