Weird AIS information from Garmin GPSMap

[donradcliffe]

This week's delivery was an older boat with a Garmin GPSmap and an AIS transponder.

The transponder seemed to be working well, and I had no problems locating the boat on Marine Traffic.

The targets showed up on the Garmin chart in about the right position. The target range and bearing was believable, as well as the static data.

However, the target speed was wildly off on the Garmin display. For example, a towing vessel was listed with a SOG of 98 knots, the CPA was listed at 0 ft, and the TCPA was 84:mm:ss. The same CPA and TCPA was given for all targets regardless of whether they were getting closer or further away.

Rebooting the electronics gave the same CPA and TCPA, but the SOG of the
towing vessel was only 89 knots.

Has anyone else run into this issue, and what might be the cause?

[wholybee]

I have seen it before, more than once. It was caused by the vessel in question transmitting missing information. Like, if it wasn't transmitting a speed at all, then the chart plotter plotted a crazy high speed, and all the other calculations for CPA were nonsense. By not transmitting a speed, I don't mean a zero, I mean the actual field in the data was missing.

If you are seeing it will ALL ships, then something else is wrong. Maybe if there is some sort of NMEA1803 to NMEA2000 converter it is dropping some data and causing the issue.