Doug Kelch wrote:

This summer in late August I was cruising up the Tred Avon river in a light fog with a good 75 yards visibility and light winds.

There is a marker that sits right on a Longitude line on the chart.

I was running down the same longitude line and was planing to turn at the mark.

I saw the dreaded shore first and finally found the the marker 100 yards off the longitude line. The land was actually on the longitude line as displayed by the GPS.

The chart is 3 yrs old. Would anyone know what might have been going on? Could the GPS be that far off? The day before it seemed ok.

The trip a few weeks earlier it was dead on.

Thanks

Doug Kelch

"Seas the Day"

Doug, I have some professional knowledge here that might help. Mark Dvorscak was right, you have to be sure your datums match. A professional cartographer or navigator (I'm not) and/or specialized software can convert a measurement from one datum to another, but it's not exactly pleasant for mere mortals. I think the Nautical Almanac has the procedure, or a reference to the procedure. Many new map editions show locations of selected things in GPS coordinates, to finesse the problem. The GPS coordinates are usually printed in a distinct color. Datum differences are enough to put a ship aground, versus in the channel. As to your problem this last summer, it was known by most everyone in the technical GPS community that a GPS satellite had a significant problem last summer and was putting out bad data for a number of days. Since the GPS system itself does not put out integrity data in the GPS downlink, there was no way for your receiver to know if the data it was getting was correct or corrupted. I couldn't say if on that particular day you were getting the bad data from that satellite, but it sounds like it. The important point to remember is, (Danger, Will Robinson!) the GPS SYSTEM DOES NOT HAVE INTEGRITY DATA. Do not use it as your primary and/or sole navigation system when your life is on the line. Integrated integrity data will come, but not for several years. The only way to get integrity data now is to use the Coast Guard's differential GPS system, or the FAA's WAAS system, and have special and expensive receivers that integrate this data with the GPS system. And those systems are not without their own unique problems. As to several people's comments that from time to time the DoD has deliberately corrupted the GPS signal, this is not true. Since selective availability was turned off last year by Presidential decree, the only signal corruptions have been due to temporary technical issues, not deliberate actions. In fact, GPS still works over Afghanistan right now, although it gets jammed from time to time, but not by us. That's another point. GPS is susceptible to intentional or unintentional jamming, which corrupts and/or degrades your navigation solution. Unless you carefully monitor the dilution of precision metric, you won't notice the corruption until you run up on the rocks. And in the future, when ultra-wideband devices become more prevalent, so will unintentional and widespread jamming. While the DoD has not committed to not turning off or corrupting GPS signals, in practice they don't. And they won't until 20 years from now, when some nation or extra-national group that hates us buys some cruise missiles from a country that sells advanced weaponry indiscrimminately, and launches these missiles against our shores. Until that sad day, rest assured that GPS will be there for you. And when that day comes, hopefully us engineers will have figured out how to keep GPS available for aircraft and ships at sea and other valid uses (especially safety of life uses), but deny it to everything else. (We're working on it!) So remember, keep up your coastal and dead reckoning and celestial navigation skills, because GPS is not a complete substitute. Besides, any idiot can follow a GPS box, but it takes a skilled sailor to safely navigate. And hopefully, the thrill and pride and sense of accomplishment you feel after bringing your ship and passengers safely into port, will more than outweigh the stress and dread and gnawing uncertainty you feel when navigating in poor conditions. Regards, John Fleming M-17: "Star Cross'd"