Trust Your Eyes, Not the GPS

[David M]

GPS Lat and Long positions are wrong periodically. While tied the dock at my slip, normally the GPS position is spot on. Occasionally though I see a hiccup in the system that puts the boats position on rare occasion a good distance away....far enough where if you were in a narrow channel, it could put you aground.. Other times I have seen the boats position make radical jumps. You cannot assume your GPS Lat and Long position is always correct.

I think most errors we see though are not inaccuracies in the GPS system but mapping and software errors external to the GPS system.

Also, Satnav (NAVSAT) and GPS are two different things. The Satnav system was developed in the early eighties before the GPS system. The Satnav system only gave periodic fixes. Satnav is no longer operational.

Also, integrated shipboard navigation systems will not steer a ship around shoals, rocks etc. Its up the the watch officer or the master to lay in the tracklines that go around such things. These system will communicate with the autopilot to follow track lines. Its of course the responsibility of the watch officer to constantly monitor the system to make sure it is operating correctly.

[John A]

Wasn't there a model of TV antenna that created interference with the GPS signal? Seems I read that once.

[GordMay]

Originally Posted by John A

Wasn't there a model of TV antenna that created interference with the GPS signal? Seems I read that once.

Yes.
According to the Coast Guard (2002), the television antennas suspected of causing interference to GPS receivers were:

* TDP (Tandy) Electronics MINI STATE Electronic Amplified UHFNHF TV antenna Model No. 5MS740

* Radio Shack Corp. Long Range Amplified Omni Directional TV Antenna, Model No. 1501624

* TDP Electronics Model No. 5MS921 (looks like Radio Shack 15-1624)

* Shakespeare Corporation Sea Watch Model No. 2050

* TDP Electronics Model No. 5MS 750

* Thompson RCA (not confirmed, but TDP sells to them)

[forsailbyowner]

Our top speed on recent leg (according to GPS) was 183 Kts. Between eyes and binocs depth sounder radar and gps, there's usually enough redundancy to point out when one system is acting goofy.

[s/v Beth]

183 knots! I want what you got! And what kind of sails did you have up???
I like what you said, and how you said it-EYES, Binocs, then electronics. And those eyes should be trained in running fixes, and the binocs have a compass in them. I don't think a lead sounder is even out of line...

[clausont]

Originally Posted by John A

There is no one to ask in this section of the country! Remember the story of the couple and their baby who were stranded in the snow several years ago? 200 miles between towns is not uncommon. Ranch houses are sometimes ten miles from the main road. School age children live in town with aunts and uncles during the school year. Several Indian Reservations dot the area. Yes, everyone has cable TV, Internet, Cell phones, indoor plumbing(most), etc.

GPS never gives the safest route, only the shortest.

This is so true. Almost every year, some form of this same situation happens.
The Jim and Jennifer Stolpa story is a well known one here where they tried to take a shortcut across Cedar pass in the snow.
Another very notable situation was the James Kim situation here that ended tragically James Kim: 1971-2006 - CNET News .
A couple of others were a motorhome in the 35' - 40' range (I forget the exact size) "disappeared" for two weeks in the same general area of Oregon as James Kim. They took a rural road that has few or no people on it at that time of year. Stuck in the snow for two weeks and even with a massive SAR effort, were not found until one of the occupants walked out of the woods to a highway.
Others in our area alone have been people who have gotten lost on logging roads. We have had two in the last several months that had fatal endings.
One a couple of years ago that I was personally involved in where a lady got lost in her car and called 911. She had suffered a stroke. Despite hundreds of searchers, helicopters and everything you can imagine, it was far too late before she was found by friends of her family.

[windships]

Absolutey right! The Cruise ship had a GPS plugged into their chartplotter. The external antenna connection had corroded and thus gave intermittent readings. Then the chartplotter switched to Dead Reckoning on last course and speed. It changed the sorce reading to "DR" but no officer noticed it. For many hours the ship continued dead on track, which should have made anyone suspicious. Meanwhile the current pushed the ship over 10 miles (from memory) off track and straight onto the shoals. Therein lies a few lessons. 1.Check your antenna connection regularly. 2. Don't rely on GPS and chartplotters. 3. Always be suspicious if you are always dead on track. 4. Doulblecheck your position with alternate means......

[Albro359]

Absolutely right...if you rely on only one source of nav data, then you are no sailor....anything can break, malfunction at any time...

Having said that, I have used GPS for more than 10 years and NEVER had a position that was out so far as to be dangerous.

I think theres a lot of confusion here over GPS accuracy and CHART accuracy..the lat / long is correct but the chart is wrong is what I'm reading from most of you

When you're that close to danger / land, you should be using additional inputs ...eyes , radar, depth sounder...they are ALL aids....use em !

See you out there

Alan

[Roaring Girl]

Hmm - I actually meant to ask themselves if they could possibly be in the right place!

We went all the 25 nM from Isola Capraia to Elba this year and had a strange 'hole' in the middle where none of the 3 GPS on board could get a signal. We were very keen to keep east of a specific headland and still cross the small shipping lane at right angles and the vis wasn't fantastic so it was a minor pain, and made us laugh a lot. But actually we wouldn't normally bother to turn it on for a trip like that.

[Tim B]

Technology has improved a lot in the last 10 years. A recent GPS with good signal will typically give your distance to within a boat-length (yes, sometimes better, sometimes worse). However, at the end of the day, the position is only useful if you relate it to something (which probably isn't so accurate as Alan said). It's then down to the user to make the small adjustments to keep the vessel safe.

How confident would you feel if you let the SatNav drive a car for you? Not so much? Well, it's a good job that it's the driver that's taking the decisions then.

Tim B.

[bastonjock]

I was sailing on the Uks east coast and i decided to go for a gap in the sand banks,the GPS showed that i was at the gap but my eyes told me different,i could see from the surface water pattern that i was not where the GPS indicated,either that or the sand bank had moved,which is also a possibility.

I use GPS for a fairly close guide to where i am.

[s/v Beth]

What being expressed in the last couple of posts is how we really do sailing...Look at the Gps for an opinion, look at the water and features around us, consult a good chart, and then make a seasoned guess by comparing all the information in front of us. That type of reasoning makes us good navigators. I only go aground occasionally