GPS speed while stopped?!

[Capn Mike]

While in our marina, or anchored, I notice my GPS speedometer registers .2 mph. It's not just mine: others of various brands seem to do the same.<br /><br />What's the answer? Rotation of the earth? Satellite movement? Everyone first guesses "river current," then is immediately embarrassed because that's obviously not the case.<br /><br />Sorry, this has probably been already answered ad nauseum , but I couldn't find it in a search.

 

[ThomWV]

Re: GPS speed while stopped?!

Its all in the filters.<br /><br />Ok, a quick primer. A GPS has no idea at all if you are moving or not. All a GPS knows is where it is at the moment. That said however, it is quite possible for a GPS to measure speed because the machine has memory. What the GPS does is remember where it is, and then about a second later it checks again to see where it is. If in the second reading it finds that it is not in the same spot that it was before it will calculate how fast it would have had to go to get where it is. That is how speed is determined by a GPS, by comparing where it was to where it is.<br /><br />Now, there is inherent position inaccuracy in every position location that any GPS gives. So although you haven't moved an inch your GPS may show you wondering around a bit just because your exact location at a specific moment in time is somewhat inaccurate. So if its reading from one location to another to determine speed and if the locations themselves vary because of inaccuracy then the GPS will come up with a calculation which shows it has moved, because it believes it has moved when in fact it really hasn't.<br /><br />Back in the days before President Clinton signed an executive order to make the Department of Defense stop degrading the GPS signals (the old Selective Availability) it wasn't uncommon to see a stationary GPS read that it was moving up to abuot 4 mph. That is because there was purposful inaccuracy of up to 100 meters tossed into the location. Back then you could erase your track plot and then just lay the GPS down and let it run for a couple of hours. When you came back you'd find a big black spot right in the middle of the screen that was the track of inaccurate locations. What you are seeing now is really the same thing, but on a much smaller scale. Most GPS units will filter that out for you so I'm suprised that you are still seeing it. My GPSMap-76 and GPSMap2006 will both read zero when we are standing still, but every now and then they will pop up and show us moving just for a moment.<br /><br />Thom

 

[Capn Mike]

Re: GPS speed while stopped?!

Thanks, Thom.....mine is a Navman, the others are Ratheon and Furuno. All are c-map chips, though I can't imagine why that would make a difference.<br /><br />I had a smaller-screen Navman until recently, and on the highest zoom-in setting, it stilled showed a chart. There were no speeds showing @ rest. This larger screen unit zooms down to 6(?)-meter grids (which is handy for my secret fishing spots!), so maybe that's the reason for the filters not filtering?! <br />Anyway, thanks for the explanation. I appreciate your expertise.

 

[kalan2000]

Re: GPS speed while stopped?!

What you're seeing may be the drift inherent in non-WAAS GPS units that are constantly seeking to refine their position. If the accuracy bubble or window of your unit is 30 meters, your unit's position would be drifting within that circle and the drift rate would be expressed as speed when stationary. As you introduce WAAS or differential GPS inputs to refine the unit's position, this drift accuracy bubble shrinks and the drift rate diminishes to near zero.

 

[18rabbit]

Re: GPS speed while stopped?!

Just so you will know, that GPS "speed" reading is the only way displacement hulls can effectively race ... while anchored ... and drinking beer.

 

[kd6nem]

Re: GPS speed while stopped?!

...unless it is BIG & painted gray. Of course having a reactor aboard helps even more....<br /><br />Our Navy has displacement hulls significantly faster than my planing hull!

 

[18rabbit]

Re: GPS speed while stopped?!

Speaking of big boats with unlimited power…<br /><br />I was reading about how cruise ships are changing the way they anchor. There are environmental concerns about repeatedly dropping big anchors on coral reefs. The ships use bow thrusters, stern thrusters, main screws, and navigation electronics. What happening is a big boat pull up, set something similar to an “anchor watch” on their nav equipment, and let go. The ship is left on computer controlled auto-pilot in ‘sit/stay’ or ‘park here’ mode without the need for any anchor. Screws turn, thrusters thrust, and that big boat stays right where it’s at. <br /><br />My first thought was GPS at each end of the boat, but how to correct for speed drift? Perhaps they use a proprietary beacon of some kind since cruise ships return to the same touristy ports.