GPS speed accuracy

[John_S]

Re: GPS speed accuracy

Don't let the deliberate position error fool you into thinking it introduces a speed error in a GPS.

The amount of position error is constant from one position fix to the next, so the measured distance traveled negates the position error and the speed calculation is far more accurate than the position calculation.

Position Error (PE) is not constant. Just power your gps on and stay stationary. As it acquires more birds the PE will drop. If you get WAAS, PE will drop. Staying in place, over time the PE changes as some birds come closer and others move away. Or you can drive down the road and see PE change. I would say it is generally constant, one sample to the next.

I do find the discussion interesting, because everyone generally understands gps is the most acurate speed indicator, but actual specs are not in any typical manuals I have seen. I suspect, like position error, it depends on the number of birds and WAAS, and a number of other variables, and is not linear to give such as easy to understand number, such as a percent.

 

[redfury]

Re: GPS speed accuracy

As far as the paddle wheel on the river discussion goes, it's telling you how fast you are travelling across the SURFACE of the water. If the surface of the water was a solid structure, a boat at anchor would be moving either forwards or backwards ( depending on position pointing upstream or downstream ) at the fixed rate in which the body of water is moving. A fish in the water that isn't swimming with or against the current is going to be moving downstream.

Now, as far as GPS goes, it's probably a more accurate way to make comparisons with different vehicles due to the calibration being the same across all platforms. However, it isn't always accurate. I was driving to work the other day and happened to notice that I was doing 155. No, not Km/h...MPH! My max speed shows that I've hit 166 MPH! I yelled at a guy that borrowed my truck for doing 125 in it. I had to apologize because I figured the GPS was accurate and would tell me if someone was abusing my truck ( it's a V6 Dakota...I'd be lucky to hit 100mph with it. ).

 

[John_S]

Re: GPS speed accuracy

Redfury,

I can't say that I have had that happen, but I have been beamed to a different location for a minute or two, and then back!

 

[NSBCraig]

Re: GPS speed accuracy

If your traveling at a speed of 50 mph heading into a current going 10 mph.

Gps will say 40 mph.

Old school speedo says 50 mph.

How fast are you going 40 mph.

Turn around and run 50 mph.

Gps says 60 mph.

Speedo says 50mph.

How fast are you going 60 mph.

Both feel like 50 mph except for wind, oh man not another variable.

 

[dingbat]

Re: GPS speed accuracy

Silvertip,

There is no question in my mind that GPS is more accurate, but the absence of specs is not a good arguement.

I've yet to see a good unit that does not include the accuracy of the velocity in their specifications. The advertised accuracy of velocity remains the same with or without WAAS correction so apparently the velocity isn't affected by the positioning accuracy.

 

[Nandy]

Re: GPS speed accuracy

Staying in place, over time the PE changes as some birds come closer and others move away.

Err, the birds (satelites) are not going anywhere, the are "stationary". The reason your gps will wander while stationary is due to atmospheric conditions.

 

[tx1961whaler]

Re: GPS speed accuracy

Err, the birds (satelites) are not going anywhere, the are "stationary". The reason your gps will wander while stationary is due to atmospheric conditions.

Well, no. The GPS satellites are in "medium earth orbit" and are not geosynchronous or geostationary.

There's actually a very extensive, well sourced Wikipedia article on GPS:
http://en.wikipedia.org/wiki/Gps

 

[grego]

Re: GPS speed accuracy

my gps sayes I am going 11mph while I am tied to dock.

 

[jhebert]

Re: GPS speed accuracy

Perhaps you are tied to a floating dock.

 

[John_S]

Re: GPS speed accuracy

Dingbat and others are correct. While I didn't find a spec in Eagle/Lowrance, Humminbird, or Garmin auto, a Garmin chartplotter did. 0.05 m/sec on velocity, with or without WAAS. And that is damn accurate.

I believe the WAAS are geo-synced and positioned over equator.

 

[sschefer]

Re: GPS speed accuracy

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

 

[pvanv]

Re: GPS speed accuracy

sschefer is 100% correct.

GPS receivers get only two pieces of data: Position and Time of Day. Period.

By sampling POS and TOD at set intervals, the local gps receiver software can compute speed OVER GROUND (or over the bottom, if you prefer); not necessarily speed through the water. If you are in a current, the speed through the water can be dramatically different.

Speed through the water will be most accurate with a local knot log meter. Ultrasonic units are cool, but paddlewheels are also good if they are correctly calibrated. Racing Sailboats use knot meters (in addition to complex wind instruments) for this very reason.

The GPS speed display can be way off at very slow speeds, especially with older receivers, because they don't update very frequently. WAAS and Differential receivers, and faster, (more simultaneous channels) receivers help a lot. My old 1994 Magellan cannot reliably display any speeds below 4 knots.