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Old 27-05-2016, 16:33   #1
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GPS not updating at anchor

When I'm at anchor the nmea data from my GPS is not updating on the display (red boat doesn't move, dashboard position remains static). On checking the NMEA debug window, new GPS data is being received about once every second. It work fine when moving, and records a track.

This means I don't have an anchor alarm with the watch dog plugin.

It was working with ver. 4.0. What is peculiar is if I unplug the GPS from the computer and wait for the boat to go grey and then plug it back in, the boat hasn't moved and the position on dashboard doesn't alter.

But if I depower the GPS and then turn it back on, the red boat makes that little shift on the chart and the dashboard long and lat also updates. Maybe someone on the forum can make sense of this.

Thank you, jimthom
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Old 27-05-2016, 18:30   #2
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Re: GPS not updating at anchor

Jim...

Are you on Windows, linux, or Mac?

Thanks
Dave
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Old 27-05-2016, 18:40   #3
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Re: GPS not updating at anchor

Jim...

If you are at anchor, I would not expect the GPS position to change much. Maybe in the 4th digit of lat/lon, if your GPS receiver is sending 4 digits of precision. Or maybe you need a bigger anchor?

You probably will see some jitter in the COG value while at anchor, though.
COG as calculated by most GPS receivers is very inaccurate at low speeds.

Does this help?

Dave
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Old 27-05-2016, 18:51   #4
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Re: GPS not updating at anchor

If you are using degrees and decimal minutes I would expect to see the third digit past the decimal to wander while at anchor with an operating GPS - after all, a change of one digit represents about 6 feet in latitude, and less than that in longitude if you are off the equator. But, I'd expect that in the lat-long window in the bottom tool bar (unless you use the dashboard), not in the image of the boat on the screen. Unless you over zoom or have a long anchor rode and the wind changes, you might see no movement at all on the screen. Which is it?
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Old 27-05-2016, 18:52   #5
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Re: GPS not updating at anchor

I apologize. You answered that and I jumped the gun.
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Old 27-05-2016, 20:37   #6
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Re: GPS not updating at anchor

Thanks fellows; I'm using Windows 10, I have 120 feet of chain out, and my GPS displays decimal minutes to 3 decimal places using dashboard.

The first screen shot (Anchor 1) shows the chart and dashboard before I depowered the GPS. All of the tracks from swinging are there because I depowered the GPS several times before taking this screen shot, otherwise there would be no indication of the boat's movement. Where I am anchored the wind is constantly shifting. There is a track which shows my approach to the anchorage.

The second screen shot (Anchor 2) shows the chart immediately after I depowered the GPS and turned it back on. There is a track from the previous boat position to where it is now. I have been at this anchorage for 3 days, and there is about 12 hours between Anchor 1 and Anchor 2.

Cheers, Jim
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Old 27-05-2016, 21:09   #7
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Re: GPS not updating at anchor

jim...

Sorry, not following the problem.

Lets start from basics.
In the NMEA debug window, do you see changes in the GPS receiver position reported by RMC or GLL messages? Maybe you could post a screenshot showing this.

Dave
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Old 28-05-2016, 00:40   #8
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Re: GPS not updating at anchor

Thanks Dave. Here is a screen shot of my NMEA data stream. I don't know the difference between RMC and GLL.

Cheers, Jim
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Old 28-05-2016, 07:28   #9
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Re: GPS not updating at anchor

jim...

So, for at least this three second sample, there is no change in position reported by GPS receiver.

Look at the RMC lines. The lat and lon fields are constant.
"3348.0358" and "15116.9019".

You will need to watch these values for a longer time to see change. Of course, if you cycle the power on the GPS receiver, you may see a change, particularly if the receiver selects a different set of satellites to calculate the position. When this happens, by the way, it is a reminder of the essential limit of GPS positional accuracy. You are getting 3 digits of precision, implying around 6 feet of accuracy. But consumer-grade GPS is generally not repeatably accurate to 6 ft.

Dave
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Old 28-05-2016, 07:36   #10
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Re: GPS not updating at anchor

I have been told there exists GPS devices which stops renewing data when they detect no or small move only. This kind of "feature" have been build in by ask of the automotive industry.

Gerhard
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Old 28-05-2016, 07:45   #11
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Re: GPS not updating at anchor

Quote:
Originally Posted by CarCode View Post
I have been told there exists GPS devices which stops renewing data when they detect no or small move only. This kind of "feature" have been build in by ask of the automotive industry.

Gerhard
Nice feature indeed...

On the other hand from the first post:

Quote:
It was working with ver. 4.0.
Or maybe it's not the same GPS?
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Old 28-05-2016, 09:16   #12
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Re: GPS not updating at anchor

This does sound like a feature/bug in the GPS device, rather than an OpenCPN filter on the display. What make/model of GPS are you using?

John
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Old 28-05-2016, 17:46   #13
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Re: GPS not updating at anchor

Thank you for your help. I installed a program called VisualGPS and I think it is a faulty GPS receiver. It locks onto a particular long and lat and when I reset it, the long and lat will change a few thousandths of a minute every second about five times and then will again lock on to a specific long and lat. Yes, it did work with ver. 4.0 some time ago and the fault may have developed in the meantime. The signal strength of the satellite display on VisualGPS is also changing every few seconds and that doesn't lock.

The really odd thing is that it records a track.

I recorded the data stream from VisualGPS

Cheers, Jim
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Old 29-05-2016, 02:58   #14
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Re: GPS not updating at anchor

If you don't want to tell us the brand and type of your GPS receiver, I'm afraid nobody can really help you.

Gerhard
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Old 29-05-2016, 03:41   #15
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Re: GPS not updating at anchor

Sorry mate, I want to tell you, unfortunately I don't know. Purchased it in New Zealand about 6 years ago. It's the mushroom type mounted outside and unbranded. The specs were up-market when I bought it but its probably dated now.

From reading the data stream it looks like it could be the one you described for the auto market however once upon a time it did update small changes in position .
Cheers, Jim
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