Two connections to two different GPS

[mgrouch]

Hello,

I have 2 GPS and I create two connections from OpenCPN to them with different priority. When I do that
I notice that course and speed over the ground jumps from one correct value to another value with wrong direction.
Location seems always fine.

What could be the issue?
Antennas for GPS are located 3m apart.

Thanks,
—MG

[rgleason]

Slightly different positions from the two GPS could create this situation. Try it with each GPS only to determine which appears more accurate and consistent, set that to ppriority 0.

Then maybe try picking a lower priority number for the gps that is less accurate like "5" ? I have no idea if this will help, but it is worth a try.

https://opencpn.org/wiki/dokuwiki/do...psd_connection

[mgrouch]

Does OpenCPN calculate COG and SOG? If it does shouldn’t it use only one GPS source in each equation to calculate it? Two different GPS sources might have antennas in different locations and report different coordinates. If OpenCPN uses coordinates from 2 different sources for one calculation of SOG or COG then it looks like a bug to me.
Thanks

[Hakan]

Originally Posted by mgrouch

Does OpenCPN calculate COG and SOG?

No. We receive that from a GPS.

If you want to put in two positions both will be used and if they differ the ship will jump. Use the filter and prioritization methods in Options->Connections to sort out what's inappropriate.

[rgleason]

Hakan,

Are you suggesting that he could filter out by ignoring sentences with COG and SOG such as Recommended Minimum Navigation (RMC and RMB) from the second GPS? (RMB does not have COG / SOG)
https://opencpn.org/wiki/dokuwiki/do...and_speed_data

or perhaps

Under Options > Ships > Own Ship the bottom has a check for "Calculate COG & SOG from position values with a selection for time between updates?

I don't know if this option then disregards RMC and RMB, but I expect it does.

[Hakan]

If you would like have two GPS connected for redundancy and both are transmitting the same NMEA messages, e.g. RMC, GLL, GSV, the connection's prioritization take care of that as long you give them difference prio.
In the case the two position sources have different messages, like a AIS and a GPS you've to filter out some messages from the low priority source to avoid "jumping".

For example my AIS is newer than the GPS and I assume a more accurate position. So, to have a redundant devise I also use the dated GPS but filter to achieve RMC and GSV only. The latter to have the GPS compass icon to show staples. (GSV is normally not received from a AIS.)

[mgrouch]

GPS position of the ship doesn’t jump. It’s the speed vector which jumps sometimes off almost 180 degrees in direction. Might be SignalK does it.
I think I multiplex two GPS sources both via SignalK into one NMEA stream for OpenCPN. I do not think it would jump
if all it did it would show values reported by GPS. There seems to be some calculations done for speed based on position. Do you know by any chance if SignalK would do that?

Thanks

[cagney]

@mgrouch

Do you have the setting Options->Ships->Calculate Values->"Calculate COG and SOG from position changes" activated ?
If so, turn it off, as this is an "emergency" setting if COG & SOG is not available through NMEA .

Thomas