Here is an interesting summary of some common strengths and weaknesses of the GPS worth reviewing, particularly for those of us just getting back on the water.... See link
GPS Strengths/Weaknesses
- Thread starter Warren Milberg
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Charts off
Has anyone ever suspected that the charts are a little off. There are a couple of areas, ( south of Corpus Christi comes to mind ) where there is certainly an error in something. The GPS will show me on dry land when I am in fact in the middle of the channel. At first thought, the GPS is off. But my GPS tells me how accurate it is supposed to be at that time and location. So, here we are, center of the channel, GPS says it is within 10' of dead on accurate, but it shows me up on land. The kicker is, as soon as the chart scrolls to a new chart, it shows me in mid channel, where in fact I am. So, can that particular chart be a little bit off. There are a couple on the upper coast that are that way too, but nothing as much as the area s. of Corpus. It is very easy to tell, as when the chart scrolls to a new chart, the boundaries do not line up. Anyone else noticed this. Or is this something totally different, that I have not even thought of.
Has anyone ever suspected that the charts are a little off. There are a couple of areas, ( south of Corpus Christi comes to mind ) where there is certainly an error in something. The GPS will show me on dry land when I am in fact in the middle of the channel. At first thought, the GPS is off. But my GPS tells me how accurate it is supposed to be at that time and location. So, here we are, center of the channel, GPS says it is within 10' of dead on accurate, but it shows me up on land. The kicker is, as soon as the chart scrolls to a new chart, it shows me in mid channel, where in fact I am. So, can that particular chart be a little bit off. There are a couple on the upper coast that are that way too, but nothing as much as the area s. of Corpus. It is very easy to tell, as when the chart scrolls to a new chart, the boundaries do not line up. Anyone else noticed this. Or is this something totally different, that I have not even thought of.
GMP and paper charts
Way back when GPS was just a gleem in someones eye the map makers had to construct maps with optical levels and steel tapes. Needless to say some of them are not as right as their makers would have wished. What most folks don't know is the old maps are the new maps as NO ONE has mapped the world with GPS technology. Soooooo about the best you can hope for is 10 meter accuracy in your maps. The worst you can expect is 100 meter errors though. If you are seeing more than that you have experienced what most think can't happen (but does all the time), your GPS is not reading precisely due to "atmospheric anomalies". Seems the ionosphere actually does effect the GPS radio waves as they pass through it.
Way back when GPS was just a gleem in someones eye the map makers had to construct maps with optical levels and steel tapes. Needless to say some of them are not as right as their makers would have wished. What most folks don't know is the old maps are the new maps as NO ONE has mapped the world with GPS technology. Soooooo about the best you can hope for is 10 meter accuracy in your maps. The worst you can expect is 100 meter errors though. If you are seeing more than that you have experienced what most think can't happen (but does all the time), your GPS is not reading precisely due to "atmospheric anomalies". Seems the ionosphere actually does effect the GPS radio waves as they pass through it.
gps
as first a plot then a sailor- someone who has spent a good part of my live navigating one way or another - gps is a miracle. a miracle. period.
as first a plot then a sailor- someone who has spent a good part of my live navigating one way or another - gps is a miracle. a miracle. period.
Overlapping error
Some one correct me if i'm wrong. Polar orbiting satellites will move relative to each other such that the 3 or 4 signals used to locate your gps can make a smaller or larger polygon in space. The closer the satellites are to each other, the more error can be involved in your location. We assume that the error is very small and always the same, when sometimes it varies and becomes small but significant next to land.
Some one correct me if i'm wrong. Polar orbiting satellites will move relative to each other such that the 3 or 4 signals used to locate your gps can make a smaller or larger polygon in space. The closer the satellites are to each other, the more error can be involved in your location. We assume that the error is very small and always the same, when sometimes it varies and becomes small but significant next to land.
GPS nav is no different than clestial nav
The math is the same. When you have three "sights" that are in the same part of the sky your "error polygon" is pretty big in the direction perpendicular to the sights. When you have three satellites in the same part of the sky you get more error from your GPS unit for the same reasons. With that all said most GPS units today can track 12+ satellites so having "triangulation errors" is almost impossible. To check go to the satellite page on the unit and look at the "satellites above the horizon" view. If by some freak of mismanagement by the US Air Force they all cluster in one pert of the view you will see the position error go up. The Air Force has designed the placement of the satellites so this can't happen. So baring the terrorists having an anti-satellite capability and taking out a specific set of GPS (800 mile high orbits, yea right!) birds this is not something you have to worry about.
The math is the same. When you have three "sights" that are in the same part of the sky your "error polygon" is pretty big in the direction perpendicular to the sights. When you have three satellites in the same part of the sky you get more error from your GPS unit for the same reasons. With that all said most GPS units today can track 12+ satellites so having "triangulation errors" is almost impossible. To check go to the satellite page on the unit and look at the "satellites above the horizon" view. If by some freak of mismanagement by the US Air Force they all cluster in one pert of the view you will see the position error go up. The Air Force has designed the placement of the satellites so this can't happen. So baring the terrorists having an anti-satellite capability and taking out a specific set of GPS (800 mile high orbits, yea right!) birds this is not something you have to worry about.
When the map makers work they set up their
theodolites at a few locations and mark that position with great accuracy. all of the other positions are secondary. So Grove point and Howell point may be exactly where the chart shows them but everything else is a close approximation. Don't despair, you can still depend on GPS to tell you if you are entering the Potomac river and heading to Washington or the Patapsco river and heading for Baltimore.
theodolites at a few locations and mark that position with great accuracy. all of the other positions are secondary. So Grove point and Howell point may be exactly where the chart shows them but everything else is a close approximation. Don't despair, you can still depend on GPS to tell you if you are entering the Potomac river and heading to Washington or the Patapsco river and heading for Baltimore.
Right on Ross
The error is 100 meters MAX. Barring blind navigation in the fog in a tight channel at night with no nav aids, you will be able to PILOT the boat when within 100 meter of the shore. The error we complain about GPS giving us is not what I'd call unsettling. If you are using your GPS to navigate like that then you don't understand navigation and need to learn how to PILOT a boat. Last time I went out and went to a new location I used the GPS to set a course until I could ID a nav aid. Then you can turn your GPS off and PILOT the boat to the gunkhole. Most of the time I just "nav aid hop" to the destination and use GPS for SOG and confirmation of current set and drift. On another note, the USCG sent out a local notice to mariners about GPS signals being tested for interference on the Chesapeake Bay and that during the day on weekdays this spring GPS would be experiencing "unknown" errors. Seems the US Air Force is testing the GPS system due to problems encountered during the war on terror and is trying to find a solution. or as we have learned in the special operations community: Special Operations Imperative #5 develop multiple options So GPS, Pilotage, DR, local knowledge..... should all be used when and where needed and no one tool should ever be used alone.
The error is 100 meters MAX. Barring blind navigation in the fog in a tight channel at night with no nav aids, you will be able to PILOT the boat when within 100 meter of the shore. The error we complain about GPS giving us is not what I'd call unsettling. If you are using your GPS to navigate like that then you don't understand navigation and need to learn how to PILOT a boat. Last time I went out and went to a new location I used the GPS to set a course until I could ID a nav aid. Then you can turn your GPS off and PILOT the boat to the gunkhole. Most of the time I just "nav aid hop" to the destination and use GPS for SOG and confirmation of current set and drift. On another note, the USCG sent out a local notice to mariners about GPS signals being tested for interference on the Chesapeake Bay and that during the day on weekdays this spring GPS would be experiencing "unknown" errors. Seems the US Air Force is testing the GPS system due to problems encountered during the war on terror and is trying to find a solution. or as we have learned in the special operations community: Special Operations Imperative #5 develop multiple options So GPS, Pilotage, DR, local knowledge..... should all be used when and where needed and no one tool should ever be used alone.
GPS Accuracy
Having just completed some 27,000 Nautical miles cruising the Pacific and Indian Oceans, my experience is that the relative accuracy of my onboard GPS systems is good to excellent - in my case usually better than one metre. With one important proviso - that the relevant charts have been updated to the default datum - usually WGS84. In countries like Australia and New Zealand, I have confidently navigated via GPS to very close tolerances - including navigating bays and rivers at night in heavy rain - using radar and depth sounder in support. Where problems arise is when the original paper charts are themselves not accurate - for example in French Polynesia, where some of the main islands, including Tahiti, have an inbuilt inaccuracy of up to 15 meters - enough to put you on a reef if you were to rely totally on your electronic aids. A case of user beware!! I operate two independent GPS systems on my 14 metre ketch - running the MaxSea program on a laptop computer using C-Map charts as well as a brand name plotter/display unit driving my auto pilot, using Navionics charts. I also carry a "spare" laptop plugin GPS and another hand held portable GPS which are not connected to ships power - the idea being to use them as backup in case of loss of ships power or a lightning strike. (A fellow yachtie here in Thailand has just suffered a lightning strike, which took out all his electonics, including his GPS navigation system - something to think about).
Having just completed some 27,000 Nautical miles cruising the Pacific and Indian Oceans, my experience is that the relative accuracy of my onboard GPS systems is good to excellent - in my case usually better than one metre. With one important proviso - that the relevant charts have been updated to the default datum - usually WGS84. In countries like Australia and New Zealand, I have confidently navigated via GPS to very close tolerances - including navigating bays and rivers at night in heavy rain - using radar and depth sounder in support. Where problems arise is when the original paper charts are themselves not accurate - for example in French Polynesia, where some of the main islands, including Tahiti, have an inbuilt inaccuracy of up to 15 meters - enough to put you on a reef if you were to rely totally on your electronic aids. A case of user beware!! I operate two independent GPS systems on my 14 metre ketch - running the MaxSea program on a laptop computer using C-Map charts as well as a brand name plotter/display unit driving my auto pilot, using Navionics charts. I also carry a "spare" laptop plugin GPS and another hand held portable GPS which are not connected to ships power - the idea being to use them as backup in case of loss of ships power or a lightning strike. (A fellow yachtie here in Thailand has just suffered a lightning strike, which took out all his electonics, including his GPS navigation system - something to think about).
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