DIFFERING LAT LONG FROM SAME GPS

alisdair4

alisdair4

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midnightdrifter.net
So, today finally attached new Garmin 152. Luckily, the previous GPS on MD was also a Garmin, and they haven't changed the plugs. This saved lots of swearing and chasing down wires.

However, the GPS gives one read-out, the Radar gives a position which is slightly different. Any ideas why? (Radar is Foruno SL70; GPS is outputting in NMEA. Both are set to WGS 84).
 
SL 70, thats a raytheon isnt it?, not a furuno.....

On a previous boat had 2 70 series (One radar, one radar/plotter) linked up to a Garmin 152, but I cant remember that the devices displayed any difference in the positions....

However I would suspect that this is something to do with the way that the different sets interpet the WGS84 survey info in reference to the position data from the GPS signal,,,,,

Different surveys give slightly different positions, ie osgb aint the same as wgs84, so you have to convert the position, and my understanding is that different devices convert the data in different ways and give very slightly different results on the chart.....do you have a chart in the SL70?? (Or is that the pure radar verusu radar plotter... cant remember.)

Anyways,,, must be something to do with the survery and the devices interpetations.

/forums/images/graemlins/smile.gif
 
Are they displaying the position to different number of decimal places, maybe one is rounding up or down? I find this with my Icom 505 VHF and Lowrance plotter.
 
Maybe you have a very large nav station and the two instrunents are really that far apart? I have noticed this dimensional problem on my own yacht as the ice in the G&T is often significantly melted by the time it has been brought aft to me by m'steward.
 
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Maybe you have a very large nav station and the two instrunents are really that far apart? I have noticed this dimensional problem on my own yacht as the ice in the G&T is often significantly melted by the time it has been brought aft to me by m'steward.

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I too have noticed dimensional problems, especially with the G&T, however my problems have not been loss of ice, more so the loss of G&T, I suspect m'steward as the main culprit.
 
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I thought that I had a similar problem until I realised that one device was displaying in minutes and seconds and the other in decimals!

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Much the most likely answer.

The NMEA feed from the GPS will give the lat and long to the datum set in the gps itself. The radar will simply display the numbers in whatever format it is set to use
 
... and don't rely on GPS if you're on the Thames close to Greenwich. The GPS meridian 0-00-00 is actually 105 metres to the east of the real meridian.

The earth is a bit squishy apparently and it's the closest fit they could get.
 
Hadn't thought of that. But the boat is laid up in the yard, so there are no variables...!
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Forget about timing and different antennas as these are red herrings as you clearly said you have ONE GPS sending out the info.

I think you should go with the majority IE equipment can be set up to display position information in one of two ways. Either degrees minutes and decimal points of minutes or degrees minutes and seconds. You seem to have your equipment set to both so this will result in the radar showing a slightly different position to the other equipment.

You will have to refer to your manual to see how to set to the alternative option.
 
I too have a Garmin 152 and a Garmin gps 76(which has an internal aerial) and this set only has a 12v feed,no nmea.The 2 sets give me different lat./longs and speed,usually about 1 knt out of 6knts!!Drives you mad don't it.
 
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... and don't rely on GPS if you're on the Thames close to Greenwich. The GPS meridian 0-00-00 is actually 105 metres to the east of the real meridian.

The earth is a bit squishy apparently and it's the closest fit they could get.

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Wrong way round Raedweld - the meridian is in its correct place, its the Observatory that has moved! /forums/images/graemlins/grin.gif

To be serious, what has happened is that we have moved from the old OSGB datum (and others) to the ETRS50 datum which at the moment (changing due to continental drift) is the same as the WGS 84 datum. Your charts have been re-drawn to the WGS datum - in some cases this was a simple conversion because the errors werent large but in other cases like the north of scotland, the opportunity was taken to redraw the charts which in some cases were several hundred meters out.

So if you use your GPS and a modern chart, you will end up where the meridian now runs which is not through the observatory any longer. Someone at UKHO once told me that it ran through a football field but I think that was an embellishment.

Everything changes - just like the way that GMT has become UT or even (god forbid) zulu.
 
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However, the GPS gives one read-out, the Radar gives a position which is slightly different. Any ideas why? (Radar is Foruno SL70; GPS is outputting in NMEA. Both are set to WGS 84).

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The question that no one seems to have addressed is, what is the difference in metres? If it is only 2 or 3 metres, then you don't really have any problem. If it is say 1,000 metres . . . /forums/images/graemlins/frown.gif

One final question, does your SL70 give a DGPS and not a straight GPS reading. If it is a 'differential' system then this would be the one to use.

Methinks you need to do accurate measurements to see which of your systems is giving the better and more accurate measurement. /forums/images/graemlins/confused.gif

Take a reading from the SL70 and note it down. Now, how far do you have to move away from your boat with the GPS152 unit to obtain exactly the same reading assuming it is to landward or use a tender if to seaward? You might have to take its own power supply for this test.

Of course, the Garmin is running DGPS under the guise of Wide Area Augmentation System (WAAS). This might be the more accurate system /forums/images/graemlins/smile.gif
 
Methinks...

[ QUOTE ]
Methinks you need to do accurate measurements to see which of your systems is giving the better and more accurate measurement. /forums/images/graemlins/confused.gif

Take a reading from the SL70 and note it down. Now, how far do you have to move away from your boat with the GPS152 unit to obtain exactly the same reading assuming it is to landward or use a tender if to seaward? You might have to take its own power supply for this test.

Of course, the Garmin is running DGPS under the guise of Wide Area Augmentation System (WAAS). This might be the more accurate system

[/ QUOTE ]Methinks you haven't much of a clue about this, lenseman. You should note that the GPS152 is the only GPS the poster has, and he's feeding an output from it to his radar. You should also note that the SL70 radar has no intrinsic GPS processing capability, it can only display a position provided to it by an external GPS receiver.

As for WAAS, it might be the more accurate system - if the poster's in North America, where WAAS operates. In Europe, the similar EGNOS system is still not fully operational.
 
Well, is it that one is min/sec and the other, min/dec.min?
If not, could you give us an indication of the amount of difference? Did you change the antenna location when you installed the new GPS? I wonder if the SL70 doesn't have antenna offset in the setup page? Intriguing.
 
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