Recent GPS inaccuracy (in the Caribbean)?

[NotCook]

Has anyone else been encountering GPS positioning to be off by about 100 feet the past several months? I can only speak for here in the Caribbean, but I used to be able to depend on accuracy to put me in the center of a channel. Recently, however, at least since about this past July, I have consistently seen it showing us around 100’ from where we are. Thypcally, Luke this past Tuesday, as we were entering the channel to Renners Cove on St. Thomas, as we were clearly in the center of the channel, it showed us west of and outside of the channel. We have been reported consistently by AIS as about 100’ from where we actually are located at the dock, and we observe other vessels on AIS reported similar distances from their actual location.

I know this is splitting hairs, but we have become accustomed to having GPS locations within 6-12’.

Anyone else encountering this?

 

[Brian D]

A long shot here. Several years ago I was installing a (large) satellite antenna on a ship. After the installation was complete, the auto tracking system of the satellite dish would cause the dish to move and point in a different direction. Spent a lot of time trying to figure this one out. Apparently the tracking system was picking up a reflection (bounced signal from the satellite) off a building near the ship. Once the ship moved the dish orientated itself correctly.

So I am wondering if for some reason the GPS signal is getting delayed slightly, enough to skew the delay by 100 ft. Like I said, a long shot.

Or, maybe Elliot Carver is messing with the GPS signals again.

 

[heritage]

You say that your gps position is off. Is that based on your position on a chart plotter? If so, has your chart plotter map been updated recently? Could be a faulty chart update. Try this,, while in your slip, find your boat on google earth and note its coordinates and see if they match your gps coordinates.

 

[Don S/V ILLusion]

the GPS seemed precise between Antigua and St Lucia when we made that trip in October.

 

[David in Sandusky]

Not all charts have been upgraded to GPS accuracy. Are others experiencing the same error at your location?

 

[Benny17441]

Yes the charts can be off in a chart plotter but the position by coordinates should be accurate to 30' or better. I have had experience with the military degrading the signal in certain areas while conducting tests and they may have something going on.

 

[jssailem]

We can thank Reagan for the availability of the GPS system. It was developed for the Military...

President Reagan ordered that GPS should be made freely available for civilian use once it was sufficiently developed.
​

GPS trust must consider that the number of satellite signals received influences the accuracy of the unit up to 100 meters if fewer than 7 are received.
And:

The version of GPS available for civilian use was downgraded using what was termed Selective Availability, SA. The full accuracy was reserved for military use. President Bill Clinton ordered that Selective Availability should be turned off at midnight on 1st May 2000. The USA protected their advantage because they could deny service to potential adversaries on a regional basis.
​

Most likely your receiver was not registering more than 5 satellites. This can be a problem with the receiver as well as limited transmitted signal strength.

 

[CapitalistSailor]

NotCook, to clarify your question, are you having problems with GPS not being accurate in locations where it was accurate before, or are you visiting an area that is new to you or your GPS? Some areas in the Caribbean and Bahamas appear to have accurate charts when looking at the terrain, but the GPS location is consistently off. I have read that many of these charts are based on surveys done in the 1800's, or certainly pre-GPS, so their locations relative to the rest of the globe are not accurate by today's standards, but as good as could be done at the time.

Your location is listed as the US Virgin Islands, so I'm guessing the charts are (geo)located pretty accurately, especially given the area's military importance in the 1940's, but maybe not. You have plenty of rocks and reefs there so small errors count.

As an aside, I sail on the Great Lakes and even there some of the depth surveys are 50 or 100 years old, done from a government survey (row)boat or ship's tender with a lead line, although the land is generally in the right place in terms of latitude and longitude. Even on Lake Erie and Lake Huron, which have large populations on the US side, and a fair amount of commercial traffic on both sides, new shoals and obstructions are occasionally found (hard ones, not just sandbars), two of which I know of that large boats have been going close by for years. And we have plenty of rocks or other obstructions marked on the chart with "position approximate".

 

[Benny17441]

I think Jssailem may have hit the nail in the head; your unit may not be receiving all satellites needed for a more accurate positioning. Antennas and cables can degrade with time or be accidentally damaged.

 

[Rick486]

Global warming.....

 

[jssailem]

Oh no. That GW really gets to affect everything.

 

[SG]

Notcook:
There are a bunch of suggestions above which have been made, including questions. For clarification, could you confirm:

• Does the Lat Long coordinates of your position vary when you are in exactly the same place? (i.e., do the signals appear dithered?)
• If you go to your GPS receiver, can you determine how many satellites it's receiving signals from ? Does that seem to correlate with the times that your location is erroneous?
• Are there places where your observed position on your chart plotter "exactly" agrees with your reported GPS position?
• If you take the coordinates on the GPS reported position in Lat-Long and "plot" them on the chart, do they agree?

P.S. NotCook, you might confirm speculation as to the origin of that nom de web ;^)))

 

[Tom J]

I just read in BoatUS magazine that the US Coast Guard has discontinued Differential GPS. All ground stations will be phased out over the next three years. Could this affect your GPS in your area?

 

[RoyS]

What you describe happened to me once in Quincy, MA. Lasted only one day. At the time I was acquainted with a scientist very familiar with GPS systems. He suspected that there was a short term disturbance in the ionosphere. He claimed that was the reason planes are not allowed to land by GPS alone. Probably unlikely that your persistent problem is caused by the ionosphere and perhaps we all put too much faith in our GPS system.

 

[Tom J]

What you describe happened to me once in Quincy, MA. Lasted only one day. At the time I was acquainted with a scientist very familiar with GPS systems. He suspected that there was a short term disturbance in the ionosphere. He claimed that was the reason planes are not allowed to land by GPS alone. Probably unlikely that your persistent problem is caused by the ionosphere and perhaps we all put too much faith in our GPS system.

Amen to that. We probably put too much faith in all our electronics. Every summer, I sail between Boston and Cape Cod, and very often, the fluxgate compass goes crazy at Minot Light, just off Cohasset, so I have learned not to use the auto-pilot around there. It might have something to do with all the granite ledges out there.

 

[jssailem]

I suspect it has something to do with a change in the magnetic flow.
Dioritic rock in the Dedham Granite rock prevalent in the Boston Area will exhibit Magnetic Anomalies. A USGS study states the area "includes dioritic rock near Scituate and Cohasset". The concentration may be enough to cause your FluxGate Compass to "want to go home to Mama"... while transiting the area.

 

[CapitalistSailor]

The upper Great Lakes have several areas high in iron ore, including magnetite, which is the black kind and is magnetic. Under Lake Huron there are deposits that could easily upset a compass, and the chart is labeled as such. But I haven't experienced large errors personally, just intermittent weirdness on the instruments. I imagine Lake Superior has a similar problem, as there are large, active ore mines there, and we often pass the freighters coming down the lakes to the mills, and going back.

 

[SG]

What do local magnetic anamolies have do with GPS errors?

 

[jssailem]

Ok sorry I saw that @TomY was experiencing issues on his cruises and the FluxGate compass was going wacky in a specific area. Just happens that they’re is a large deposit of magnetic rock in the area that has been identified to cause Magnetic amomalies in the area.

The issues with the GPS is different and was commented earlier based on NOAA and NASA data.

 

[SG]

I think you meant Tom J. And not Tom Young ;^)))

I remember years ago following a boat through a big draw bridge. The chart warned of a magnetic anomaly as you approached the bridge. Sure enough, the boat ahead veered suddenly, but the helmsman forced the wheel back and then disengaged the autopilot. My wife commented something to the effect that: "boys shouldn't be playing with their toys in tight places."