NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: GPS Accuracy Now.
From: William Trayfors
Date: 2000 May 04, 12:18 PM
From: William Trayfors
Date: 2000 May 04, 12:18 PM
It's indeed a wonderful thing that DoD has been instructed to "turn off" selective availability, and that civilians now will have access to the same GPS signals the military has enjoyed. This action is long overdue. However, one predictable consequence of enhanced GPS system "accuracy" is that some of our fellow mariners, and maybe even an aviator or two, are gonna have GPS-assisted groundings! This will happen because word will get around that GPS is now an extremely accurate positioning system and some people will use it blindly. There are several reasons to think twice about how GPS is used: 1. Even if GPS were 100% accurate 100% of the time, there are a lot of charts in common use which are referenced to a different datum than WGS-84; differences can be LARGE, e.g., some charts of the Virgin Islands have islands "misplaced" by 1/4 mile or more. I have personal knowledge of a custom 70' sloop which was lost on Neckar Island after successfully completing a round-the-world voyage with a professional crew; this yacht ran aground just 1/4 mile north of the indicated GPS position and, guess what....the datum error at that location is 1/4 mile North/South! 2. GPS isn't available 100% of the time. If system availability and accuracy are, say, 99% reliable that means that during the year it will not be available or will not be accurate during some 5,256 minutes or 87.6 hours!! 3. There are numerous other sources of GPS error -- both technical and user related -- which can render GPS readings misleading for the mariner. For most practical navigation uses, I believe the turning off of SA will/should have little impact (except, perhaps, GPS-indicated speed will be more accurate). I will continue to believe that GPS readings should be treated as just one source of positioning information to be compared with all other available sources at all times. Yes, GPS is the best thing since sliced bread. But, unless you use it intelligently it can cut your finger off! Bill S/V Born Free Tortola, BVI At 07:20 AM 5/3/00 -0400, you wrote: >Philip, I agree with your thought that one mile accuracy is adequate for >off-shore passagemaking, but I can only imagine that from your comment of >accuracy of 30 meters or less being unnecessary, that you don't fish. > >Will Martin > > > >At 12:57 AM 5/3/00 +0200, you wrote: >>For maritime users in small craft a GPS accuracy of 100 metres is more than >>sufficient. Provided you can find the fairway or safe water buoy at the >>approaches to ones destination or the entrance to the harbour, it is far >safer >>to navigate by sight (i.e. pilotage) rather than by 'head-down' reference >to the >>GPS. >> >>Out at sea an accuracy of one mile is quite sufficient. >> >>There are occasions when a greater accuracy would be useful such as an anchor >>position in a crowded anchorage. There is no real need to know the boat's >>position to an accuracy of 30 metres or less. >> >>Philip Ouvry >> __________________________________ Bill TrayforsThe Washington Decision Support Group, Inc. Specialists in Advanced Information & Communications Technologies 2401 South Lynn Street, Arlington, VA 22202 Office (703) 838-8784 Tech Support (703) 573-WDSG FAX (703) 838-0019