Multiple GPS with different offsets

[dcmar]

Hello,

Is it possible to set different offsets for each of my GPS receivers which are located in the FWD and AFT of the ship respectively. I only see how to enter a single offset to a GPS in the 'Ships' menu using the 'Real Scale' ship icon.

Thanks
- D

[Hakan]

-D..
No, the result if you use both GPS as they are will be a ship icon jumping between the two positions.

If you really want to see e.g the bow's position you could read that GPS by a small program that reformat the GPS NMEA0183 position messages, like RMC or GGA, to a TTL message and send that to OCPN. Then the bow's position will appear as a ARPA target. The ARPA target looks like a filled circle and acts like an AIS target.
With a set of filters it may also be possible to use the plugin "NMEA converter" for the same purpose but then you've to filter out one position message from the "real" GPS.
So--: Doable but not simple.

It's maybe better to use a scaled ship icon and a compass heading and you'll get a correct directed ship icon and can see the ship's elongation when zoomed in enough. And can skip the bow GPS.

[stormalong]

If you are lucky your GPS is accurate within 10 meters. How big is your boat and why do you want to know where the bow is? If you are that close to an object you need eyes on it, not on the chart.

[dcmar]

Thanks Hakan. Looks like OCPN may not be for us but I will look into that.

[dcmar]

Storm - Using a Guassian function on >10,000 data points from each GPS receiver over the course of a day we can determine to within 1-2m usually the position of any part of the ship if the offsets to the GPS recievers are applied correctly. The ship is between 130-160m long for some idea. This is just for a project not actual navigation.

[Hakan]

dcmar..
Yes, OpenCPN as any other aid for navigation/plotter programs are meant for normal navigation and not for exact positioning of a work platform or similar I get the impression you are looking for.

But still; A satellite compass use a couple of GNSS receivers to calculate the position and heading. Since a ship, or what we're talking about, is not flexible but a rigid construction the movement of that body can be easily foreseen. Many satellite compasses can apart from position and heading also calculate and present the movement of the opposing part of the ship. If the satellite compass has a aft position the turning movements, angle and speed, of the bow can be presented. (The calculation is also used for the ROT, rate of turn, navigational message.)
That in combination with a navigation program can be what you're looking for?
Good luck
Håkan

[dcmar]

Hakan - Yes we have other tools for dynamic positioning and navigation which are completely separate. I'm not concerned about knowing the relative movement of the bow for example. (That's someone else's job Ha!)

The reason I am looking into navigation software like OCPN to use is simply for a nice way to graphically display the ship on a map/chart as well as make a recording of our track during transit. For example this may replace me overlaying our track and ship icon onto Google Earth because then I will not be reliant on consistent internet connection.

Secondary to the above, I also have a few instruments that append our coordinates along with each measurement. I would normally output NMEA data to them using serial connections. I haven't gotten that far to test yet, but it looks like this is available in the connections window and I will be able to filter the NMEA strings as needed.

Hope that makes things more clear.
-D

[mgrouch]

I think this functionality doesn’t really belong to OpenCPN. It should be built-in into data multiplexer software. SignalK has some setting for GPS antenna location on a ship but so far as far as I know only for one GPS antenna. Gpsd might have something for multiple gps too. I need to check

[mgrouch]

Also GPS sensor itself should have some kind of antenna phase center calibration, because it’s not that simple that it measures location of antenna. There are other factors

[stormalong]

Originally Posted by dcmar

Storm - Using a Guassian function on >10,000 data points from each GPS receiver over the course of a day we can determine to within 1-2m usually the position of any part of the ship if the offsets to the GPS recievers are applied correctly. The ship is between 130-160m long for some idea. This is just for a project not actual navigation.

The Gaussian function will only work for a stationary object. You are being very vague about what you are trying to accomplish.

"The reason I am looking into navigation software like OCPN to use is simply for a nice way to graphically display the ship on a map/chart as well as make a recording of our track during transit. For example this may replace me overlaying our track and ship icon onto Google Earth because then I will not be reliant on consistent internet connection."

OpenCPN can save a track of your movement and display it on a chart. It is a standard function. You can set track permanently set on by editing the config file.

It can also log your position, course, speed, etc and it could be downloaded for processing by other software. But still, why do you care about where the bow and stern are?

[dcmar]

Storm - I'm not trying to be vague. I think you are assuming too much. As far as my reply to you before, yes that is only useful when we are in dynamic positioning mode over a location. I'm not sure where you are getting the idea I need to keep track of the bow and stern from. That was an example Hakan made. Please read my other posts I try to make it clear what I need from OCPN. I don't need help with anything else thank you.

mgrouch - If I'm understanding you correctly yes, it would be nice if OCPN could act as a data multiplexer where it takes inputs from multiple GPS offsets and then applies a different offset to the output for another device. But as you rightly said this may not belong in OCPN, we have our own programs that could be expanded to handle this.

If the simple answer is I can only use one GPS offset at a time, then I can get it do do the minimum task I need(Make a ship graphic/track). Just may not be for us.
-D

[rgleason]

Originally Posted by mgrouch

Also GPS sensor itself should have some kind of antenna phase center calibration, because it’s not that simple that it measures location of antenna. There are other factors

This is a feature request in Tracker that you can vote for. Would like to get it into the Vetted Features List


FS#1102 - UI Option Ship Tab Boat Data- Draft, Speed, Clearance, Displacement

FS#920 - UI UNITS Setting measurement units


Why not VOTE for these they would be very useful, particularly if they were universally available to plugins too.