Which GPS? HDS-5m, CP300i, 551

[maby]

I have added an LGC4000 GPS to my 7m. A huge improvement on position error from the internal GPS.

I'm amazed you are seeing any positioning error - where do you have the plotter mounted. We have an HDS8m and the accuracy is usually amazing - when we are tied up on a pontoon, or moored to a buoy, the icon on the screen is exactly where the item is in real life. Can't remember the last time there was any detectable error.

 

[winsbury]

Something else about the Lowrances which they simply do not publicise at all is their gateway functionality. Each Lowrance plotter has one or more NMEA0183 ports, an NMEA2000 port and an ethernet port. All data received on any of those ports is gatewayed out onto the other two - we have two Lowrance plotters networked by NMEA2k with speed and depth instruments connected to the NMEA0183 port of one plotter and a VHF DSC connected to the 0183 of the other plotter. We also have a Raymarine autopilot with Raymarine control head on the NMEA2k network. Everything in the boat can see everything else! Each plotter just sits there quietly translating all the data it receives on any of its ports and sending it back out on the other ports.

Theres even the facility to turn on/ff various sentences per port which could be useful to prevent overloading some network segments with data.

 

[FWB]

I'm amazed you are seeing any positioning error - where do you have the plotter mounted. We have an HDS8m and the accuracy is usually amazing - when we are tied up on a pontoon, or moored to a buoy, the icon on the screen is exactly where the item is in real life. Can't remember the last time there was any detectable error.

Position error is one of the available display functions. It was around 30ft with the internal GPS and is now sometimes less than 11ft with the lgc4000.
I overlay the map with ground speed, time and battery voltage top left.
Bottom left I display position error, so that I don't get too carried away with the electronic chart position. Carried away onto the rocks maybe !
See http://continuouswave.com/ubb/Forum6/HTML/003037.html

 

[prv]

Funny you should say that, I was looking at the LVR series last night and there's some neat integration on the sets.

Pity they don't do a remote mic option though.

Pete

 

[winsbury]

Position error is one of the available display functions. It was around 30ft with the internal GPS and is now sometimes less than 11ft with the lgc4000.

I thought position error is an 'uncertainty figure' such as in a top hat triangulation - the more satellites you can 'see' the smaller this potential error gets so perhaps the antenna position isnt ideal. Bear in mind the indicated error is a maximum, you will be somewhere within the indicated error, ie quite possibly no where near 30ft away from the indicated position. For use in a sailing boat though I cant imagine ever having to need to be that precise.

 

[FWB]

Did you look at the link?
I got the lgc4000 because one day my chart showed me passing the wrong side of a buoy when in fact I wasn't. A buoy which I pass regularly. A 30ft lateral error could be serious in confined waters.
In any event displaying the EPE keeps me aware of the accuracy of the chart position.

The EPE varies all the time I should add.

 

[maby]

Position error is one of the available display functions. It was around 30ft with the internal GPS and is now sometimes less than 11ft with the lgc4000.
I overlay the map with ground speed, time and battery voltage top left.
Bottom left I display position error, so that I don't get too carried away with the electronic chart position. Carried away onto the rocks maybe !
See http://continuouswave.com/ubb/Forum6/HTML/003037.html

Mmm, not sure how much importance to attach to the displayed value - it is only an estimate calculated in the GPS receiver based on the configuration of the satellite constellation and other factors like signal strength and levels of interference. It cannot actually calculate positioning error without knowing accurately where it is - only estimate. Some chipset manufacturers use optimistic calculations and others use pessimistic calculations. I've seen a GPS receiver with the cross hairs dead centre on my exact position (by eye from my surroundings) but reporting high positioning errors. I suspect it's more a case of the LGC4000 being more optimistic than the chipset actually built into the plotter.

 

[winsbury]

Did you look at the link?
I got the lgc4000 because one day my chart showed me passing the wrong side of a buoy when in fact I wasn't. A buoy which I pass regularly. A 30ft lateral error could be serious in confined waters.
In any event displaying the EPE keeps me aware of the accuracy of the chart position.

The EPE varies all the time I should add.

Yes I did, as far as I know WAAS only provides correction to about 7-8m which is adequate for the air transport industry its intended for. This is because the signal is sent every few seconds during which time the psudo-random GPS error may have been varied. This means that even if you see a WAAS corrected EPR of less than about 25-30 feet its probably the GPS receiver being over optimistic.

Personally I think I prefer to stick to using my eyes and depth sounder for navigating at distances of less than 50 feet or so.

 

[FWB]

I was thinking more of low visibility.
Anyway changing the subject a bit to AIS..... The ro-ro ferry Norman Trader is moored up the Fal with its AIS transmitting.
It was interesting to note that as I passed the AIS delta symbol on the chart, there was still 400 ft of ship left to pass. The AIS antenna being on the bridge.
Made me think.

 

[prv]

Did you look at the link?
I got the lgc4000 because one day my chart showed me passing the wrong side of a buoy when in fact I wasn't. A buoy which I pass regularly. A 30ft lateral error could be serious in confined waters.

But the buoy was there, so you didn't need the GPS to know exactly where you were.

Relying on a plotter to that sort of resolution seems to me to be making an unreasonable demand on the accuracy of the whole system, including the surveying of the charts, the placing of the buoy, hell, even the scope on its mooring chain.

Pete

 

[prv]

It was interesting to note that as I passed the AIS delta symbol on the chart, there was still 400 ft of ship left to pass. The AIS antenna being on the bridge.
Made me think.

Good!

Use your eyes in close quarters, not a video game powered by signals from space

Pete

 

[FWB]

Yes but one foggy day I shall be glad that I'm aware of the limitations of my kit.

 

[winsbury]

Yes but one foggy day I shall be glad that I'm aware of the limitations of my kit.

Indeed this is essential - even the thickness of a pencil line on a paper chart can lead to errors, its always best to use technology to augment your senses and not replace them entirely.

You observation with AIS is very interesting as I hadnt appreciated that aspect - clearly with a 1/3 mile long tanker or cargo ship the same issue applies - I guess the lesson is to ensure you set the proximity alarms to something sensible given the size of shipping likely to be encountered the waters you sail.

 

[maby]

Indeed this is essential - even the thickness of a pencil line on a paper chart can lead to errors, its always best to use technology to augment your senses and not replace them entirely.

You observation with AIS is very interesting as I hadnt appreciated that aspect - clearly with a 1/3 mile long tanker or cargo ship the same issue applies - I guess the lesson is to ensure you set the proximity alarms to something sensible given the size of shipping likely to be encountered the waters you sail.

The proximity alarms on our Lowrance go off if both we and another vessel are in the North Sea simultaneously!

 

[winsbury]

N2K power doesnt work as I thought

I just took delivery of a Lowrance LVR880 and hooked it up to the HDS with NMEA 2000. Seems like a nice radio at first glance but surprised to find that inspite of power being hooked up on the NMEA network neither the plotter nor the radio powered on. I cant find any setting to get them to take power from the network so can only assume these draw too much power so I still have to have separate cabling for power - very unimpressed. Even when the plotter and radio are hooked up to power, the N2K network does turn work unless its ALSO got a live power drop fitted, damn thats yet another switch on the panel I'm going to have to find !

 

[prv]

the N2K network does turn work unless its ALSO got a live power drop fitted, damn thats yet another switch on the panel I'm going to have to find !

Why would you want to switch the N2K on and off separately? Just run it off the plotter supply.

Pete

 

[winsbury]

Why would you want to switch the N2K on and off separately? Just run it off the plotter supply.

Pete

You're right, I wasnt thinking.

I still dont understand why the plotter wont get its power from the N2K cable , after all its a very low power consumption compared to the radio.

 

[maby]

I just took delivery of a Lowrance LVR880 and hooked it up to the HDS with NMEA 2000. Seems like a nice radio at first glance but surprised to find that inspite of power being hooked up on the NMEA network neither the plotter nor the radio powered on. I cant find any setting to get them to take power from the network so can only assume these draw too much power so I still have to have separate cabling for power - very unimpressed. Even when the plotter and radio are hooked up to power, the N2K network does turn work unless its ALSO got a live power drop fitted, damn thats yet another switch on the panel I'm going to have to find !

The NMEA2k power is only intended for low consumption devices - primarily display heads - saves running two wires up to the binnacle.