SailNet Community - Reply to Topic
Thread: Risk of GPS loss Reply to Thread
Send Trackbacks to (Separate multiple URLs with spaces) :
Post Icons
You may choose an icon for your message from the following list:

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

  Additional Options
Miscellaneous Options

Click here to view the posting rules you are bound to when clicking the
'Submit Reply' button below

  Topic Review (Newest First)
02-12-2007 08:05 AM
Pamlicotraveler The problem with Scott and Lydia's grounding seems to have been more fatigue and seasickness than any type of technology failure. And if you feel like you are going to lose your lunch I doubt a sextant would help that problem much. I think the lesson of their story is to keep planning and keeping abreast of your bearings when things seem to be going well. That way you can make decisions without too much required concentration when things aren't going well.

The same concept for me is to pencil in my GPS location on a paper chart at reguLr intervals, even though I have a chartplotter, and if I get sick (or the Chinese shoot down our GPS system ) I will have course to follow without having to do alot of thinking.
02-11-2007 09:23 PM
camaraderie Wind...sorry if my verbage wasn't specific enough about what I was complaining about. A large part og this thread has been about GPS and the need for Sextants and Coastal Piloting skills as backup. More recently we have those in favor of keeping Loran as well as a redundant backup on another thread.
My attitude remains that (in addition to keeping accurate paper charts and plotting as you go) GPS is all you need these days as long as you have multiple GPS's running and plenty of cheap backups w/ batteries.
This comes in for some criticism by others (damn luddites! ) but your posts which LUMP GPS and Chartplotters together (even though I know you know the difference) make it seem like there is DANGER in relying on nothing but GPS and your own eyes. The reality in my opinion is that there is danger in relying on anything ELSE other than GPS and your own eyes (which includes keeping a paper track and comparing reading from 2 GPS's to eliminate error possibility to the vanishing point.) I agree with you about comments on Celestial were for others on this thread. So if you say chartplotters or CP's instead of using GPS interchageably in you posts...we can talk about them without a problem all you want.
02-11-2007 08:50 PM
btrayfors mega98,

GPS signals are subject to error from a number of sources, including water vapor, though the error is usually small. However, there's an interesting tale on the DARPA website of a GPS signal apparently being significantly in error due to reflection of the signal off a landmass.

Here's part of that discussion: "Posted - 17 August 2004 14:15 DaveB - I've seen that satellite alignment problem in real life. My little handheld GPS was telling me that the EPE was 900 ft and the navigation display was pointing in a direction I didn't want to go. There were two satellites on the horizon and four directly overhead in a nice straight line. I found another problem last fall. I was approaching an island in my sailboat in heavy fog and suddenly the GPS was telling me to turn hard to the right. About the same time a cliff loomed out of the fog directly in front of me. I recognized the piece of the cliff I could see and groped my way in from there but the GPS would have lead me on to the beach. I don't know what kind of GPS interference you get off a cliff but I wish DARPA would rely less on GPS and concentrate on the harder problem of intelligent navigation. Rich"

Just goes to underscore the most fundamental rule of navigation: use all available means to fix your position, and don't rely 100% on any one source.

02-11-2007 08:49 PM
sailingdog A problem I've seen is that some people mistake the icon on the chartplotter screen for their position in the real world... it isn't. It is an electronic representation of a location of a physical boat on a probably inaccurate rendition or interpretation of the real world by a cartographer, often using data that is decades old. In many cases, it is good enough to prevent disaster, but there are places where the cartography and the real world don't even come close.


I've seen few situations where the GPS couldn't keep signal lock in torrential rain, and believe that the GPS signal, which uses just a 50 Watt transmitter IIRC, can be attenuated by water as is the case with most other radio signals. However, I get the feeling that fog would have to be exceptionally dense and high-reaching to attenuate the signal that much.

Of course, there are other factors involved... was his GPS being used inside the cabin with an integrated antenna? Was it a parallel channel, multiplexed receiver or a single channel receiver? Was it a shipboard unit with an external antenna? Were his batteries fresh? Etc...
02-11-2007 06:57 PM
mega98 My instructor for coastal navigation said he couldn't pick up a GPS signal once while in thick fog. Anyone else had this problem?
02-11-2007 06:07 PM
Originally Posted by camaraderie keep giving Chartplotter errors and calling them GPS errors.
Yes I was loose with the words GPS and chart plotter, and yet I imagined everyone would know I was speaking rather casually and not that I didn't know the difference.

Originally Posted by camaraderie
The GPS was not the problem. (Nor does it seem the chartplotter was wrong in this case.) What was wrong was the crew was either not capable of operating the chartplotter or was not plotting on paper charts from her GPS or simply was unable to navigate due to illness and made a grave error. (Or all three).
It is a shame but Celestial wouldn't have helped her here...and even if she had piloting skills the keys are rather featureless and she was in no condition to figure anything out anyway.
The real error here is an error of judgement and not one of navigational discrepancies. We all make errors of judgement...thankfully no lives were lost.
We all hope Skip and Lydia come out of this OK.
I didn't say celestial would have helped, I never even mentioned celestial. I simply said there are risks associated with relying on GPS/chart plotting and listed a bunch of common causes of failure, and then in this previous post said that I thought this incident fit a few of the things I had listed. I never said this incident had anything to do with GPS being inaccurate, that there were any kind of descrepancies, or anything of the sort. This isn't the first time this kind of thing has happened, people end up on the rocks all the time while using a chart plotter, for all kinds of reasons. I don't know why we can't talk about causes of failure while using GPS without everyone assuming you are saying GPS sucks, or that you shouldn't use it, or that you are suggesting people use celestial or whatever ... nobody said that. We should be able to talk about common problems with GPS/chart plotting in the same way we can talk about ways that a diesel engine can fail without everyone jumping to the conclusion that you are suggesting nobody ever rely on a diesel engine. I never suggested anyone not use GPS or chart plotting, I use it too, I have a bunch of receivers, hell I carry a GPS receiver with me everywhere and a compass, and I've written software to decode GPS strings too, I love GPS. But there is a lot of room for failure between a working system of GPS satellites and a total comprehension of context and full situational awareness, a lot of things can go wrong, I just listed some.
02-11-2007 04:56 PM
camaraderie keep giving Chartplotter errors and calling them GPS errors. The GPS was not the problem. (Nor does it seem the chartplotter was wrong in this case.) What was wrong was the crew was either not capable of operating the chartplotter or was not plotting on paper charts from her GPS or simply was unable to navigate due to illness and made a grave error. (Or all three).
It is a shame but Celestial wouldn't have helped her here...and even if she had piloting skills the keys are rather featureless and she was in no condition to figure anything out anyway.
The real error here is an error of judgement and not one of navigational discrepancies. We all make errors of judgement...thankfully no lives were lost.
We all hope Skip and Lydia come out of this OK.
02-11-2007 04:05 PM
Goodnewsboy A competent navigator is capable of using all available means to fix position. It is the use of two or more unrelated methods that often reveals an error derived from one or the other.
02-11-2007 02:04 PM
wind_magic Was reading the log entries of the s/v FLYING PIG captained by Skip Gundlach (a contributor here) and it reminded me of this GPS thread here on Sailnet, so I thought I would stir the pot a little. Following is his recent entry into the log, a reflection on the recent grounding of the s/v FLYING PIG in the Keys of Florida.

Originally Posted by Skip Gundlach
So, having discarded the Key West option, Lydia wanted me to get some sleep, as the boat was sailing along at a comfortable (for me) 5.6 under triple reef, again still far from anything. So, she took over and I went to the aft berth where the motion, while substantial, was easy and thus was of no issue, and I slept soundly. What I couldn't have known was that she was very uncomfortable, nearly seasick, and rather than standing watch, was on the saloon sole, popping up every few minutes, looking around, trying to make sense of the chartplotter which - since she'd not been monitoring it, and making range adjustments to look ahead and also in detail at where we were headed by zooming in along the intended route - she really couldn't comprehend, worsened by her physical state.

Looking back, she should have gotten me, despite how tired I was, or how much she wanted me to get some rest. My practice with the chartplotter would have revealed our course taking us dangerously close to the reefs on which we eventually came to grief, and we could have pinched up, rather than doing our broad reach, or, even, simply tacked off in the opposite direction, to take us away from where we were.
This all reminded me of the list of GPS threats I had written earlier in this thread. Always on the look out for opportunities to look brilliant even at the risk of seeming like a jackass I just had to dredge it back up. I've highlighted the things in the list below that I think were related to this incident. I think this incident perfectly highlights the kinds of "problems" with GPS that get people and their boats into serious trouble. This problem of zooming the chart in and not being able to judge the distance to hazards is I think one of the biggest problems with chart plotter use. That kind of thing doesn't happen with paper charts because the scale is always much more obvious because you simply can't zoom a paper chart unless you have a magnifying glass in your hand. Of course I'm not saying we should always use paper charts (an absurd thought), I'm just saying it is something to pay close attention to when using these gee whiz chart plotters.

Originally Posted by wind_magic
I think the biggest threats w/ GPS in sort of descending order are ...

Main threats ...
  1. Over dependence - thinking GPS IS navigation, it isn't. It is just a really precise fix. Not cultivating alternative means of getting a fix because "GPS is so reliable".
  2. Electrical - total dependence on wired GPS chart plotters that need a complex functioning electrical system to support them.
  3. Battery - lack of batteries for the hand held GPS.
  4. Mis-identification - blindly doing what the GPS says against common sense, spelling the name of the landmark wrong, fat fingering the coordinates when you type them in, etc, and ending up in trouble.
  5. Losing handheld - over the side, buried in dirty laundry, forgotten on shore, etc.
  6. Breaking plotter - screen stops working, corrosion, etc.
  7. Wrong charts - having the wrong charts for the area, or fat finger the setup of the plotter, screwing up the plotter configuration, etc.
  8. Fatigue - try making sense of a GPS when you haven't slept for 48 hours, then try paper charts, much easier.
  9. General user errors - random acts of stupidity, using GPS headings as compass headings, etc.
  10. Zoom errors - Having the chart zoomed in or out to a point where it looks like what you are seeing but is actually zoomed wrong so all the hazards are on the wrong scale and you are actually looking at the chart completely wrong.
  11. Swirling - looking at a chart plotter instead of looking at your destination and feeling the wind on your face, can lead to "swirling" in a spiral around your target as the wind acts against your heading instead of standing in the cockpit and feeling the wind and seeing the obvious.
  12. Theft - ooh, pretty gadget, think I will take it home with me ...
  13. Crapware - increasing reliance on less rigorously tested and developed software, poorly written chart plotter software, software development by technology companies that don't actually know anything about how the device gets used on a boat, etc.
  14. Sizzle - buying the prettiest thing instead of the best thing, or buying the "very best top of the line" and setting out to sea under the assumption that it is going to do everything you need because "it's the best".
  15. Expense - spending so much on one gadget that you can't afford to have backup plans, can't afford paper charts to fall back on, etc.
  16. Discontinued - having older (even just 3 years old) plotter that you can no longer get charts for because the company stopped supporting it, what if that happens at an inconvienent time for you ?
  17. Inexperience - buying the latest gee-whiz but not having any idea how to actually use it, or having to go to the manual during a critical time to figure something out, relying on statistics and important facts that were calculated using math and methods that you do not understand or have the background for, etc.

Secondary threats ...
  1. Wrong datum
  2. Overly precise - just because your GPS is correct within 10 feet doesn't mean your chart is.
  3. Nothern exposure - high latitudes increase error.
  4. Manufacturing/design problems - a few GPS devices have simply been wrong.
  5. Exotic errors - electrical mischief, software glitches, acts of God, etc.
  6. Localized GPS blackouts - off the coast of Yemen for example, jamming ?
  7. Sensitivity - it used to be that all GPS units had big antennas and picked up a good signal, but many now have such small antennas that there are conditions under which you cannot get a fix even standing out under a full sky.

Never gonna happen threats ...
  1. U.S. Military turning the system off, introducing error, etc.
  2. Chinese people blowing up GPS satellites
  3. Tooth faerie cooking the satellites and enjoying them with soy sauce, etc.
02-02-2007 09:26 AM
Originally Posted by xort
The Chinese Are Coming! The Chinese Are Coming!
They're already here. "MADE IN CHINA" is printed on lots of stuff on America's store shelves. It's as close as they want to get.

This thread has more than 10 replies. Click here to review the whole thread.

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

For the best viewing experience please update your browser to Google Chrome