I just have to ask what is probably a dumb question, because somebody has now told me that GPS is not very accurate for determining your speed on the water. If not, why not? Does its relative accuracy have to do with how much distance you cover while taking the readings...or what? Is it likely to be more reliable at slow speeds than at fast speeds? And if GPS is not a reliable indicator, what would be a better way of determining your speed while under way? Don't the big ocean-racing cats use GPS to determine the speed at any given time?

I am under the assumption that your GPS is:
1. Only as accurate as the amount of satellites it is accessing to provide your readings with.
2. Only as accurate as the money you sink into it. I am probably wrong

I have used mine for Car Travel on the dashboard and it seems to maintain the same speed reading as my Speedometer and that is going 70-75mph on I-95. So I would say that it is accurate.
You can find a good lesson on GPS's at How Stuff Works. Com GPS info

Thanks, D.
What an awesome website! I love it!

But a little knowledge is a dangerous thing. Now I learned that a GPS is not as accurate as a DGPS.

I realize my original question is difficult to answer, because if you are on a boat, with no other way to verify your speed, how can you know whether the GPS is accurate or not. But I was hoping maybe somebody had gotten some independent verification of the validity of their GPS readings. D. was able to verify it against his car speedometer, but has anybody had corroborating verification when on a boat?

It all depends on how many satellites and in some cases towers you can connect to.

In most civilian applications, store bought GPS receivers are low power. Some of the better models include technology call WAAS. This allows the unit to lock on to land based signals as well as sat. based. Combining the two can be very accurate.

The only difference while using GPS in a marine environment I can see is there are very get overhead obstructions and if on the ocean, very few WAAS towers.

http://gps.faa.gov/Programs/WAAS/waas.htm

DGPS is a quantum leap in accuracy over GPS.
Unfortunately, to gain maximum accuracy from DGPS it requires either a portable base station set up for serveral hours or a permanent base station(s) mounted high enough for wide area use.
Many public entities are installing DGPS base stations for use by the public. We are installing one on the roof of City Hall here.
When the permanent base station is installed we will be able to use the "Holy Grail" of sateliite navigation - Real Time Kinematic Differential Global Positioning System. "Real Time Kinematic" means that the rover is continuously tracking and can take and store THREE dimensional readings every second if you want. I have seen a topographic map survey done by a guy on a four-wheeler just riding back and forth. He could do a weeks work for a survey crew with state of the art equpment in one hour.
The future goals of RTKDGPS include having base stations at every airport, harbor and large city.

Mary,

The accuracy of a GPS's measurement of velocity improves as speed increases. A boat ghosting along in light air will not have as accurate a reading as one flying along at 20+ knots. (Due to errors caused by atmospheric conditions, etc., you may even see a velocity readout when sitting still.)

I don't put a whole lot of faith in what the GPS shows for maximum speed since that calculation can be affected by the inherent errors. The average speed measurement is very accurate. The current speed measurement is accurate as well, as long as you don't pay attention to the occasional fluxuation.

how about one more opinion to really confuse you.

first, dgps is more accurate than gps without differential corrections (the d in dgps). with dgps accuracies of under 2 meters are possible with units that cost less than 300 dollars. for better accuracies there are two methods used. the first method takes multiple position readings over time at the same location and averages them together to find a more propable location (statistically speaking). for accuracies in the centimeter range on one of the inexpensive receivers this averaging can take many hours (days). other more expensive receivers actually use the two different frequency bands (gps signals are transmitted on two bands commonly referred to as l1 and l2) to calculate the error caused by satellite geometry and the path of the signal through the atmosphere (there are minute changes in the speed of the radio frequency based on density). remember, at the speed of light, we are talking about a very small amount of time for the signal to propagate from the satellite to the receiver. these receivers typically cost in the thousands of dollars (check out some of the trimble units used for surveying).

here is where the manufacturers get creative. the speed (or more accurately velocity since the gps also gives direction) displayed is calculated using two positions and the time it takes to get from one position to the other. you can think of the error in reported position as a circle that you are most likely located inside (statistics again). if the time used is very large, the speed reported will be less accurate for an instantaneous speed, but the average speed will be very accurate because the distance between the two points will be very large compared to the positional error of the two points. for example, i travel one mile in one minute. that is an average speed of 60 miles per hour (please excuse the change in units of minutes to hours), but what if i travel 5279 feet in 1 second and then travel the last foot in 59 seconds. the speed for the first second is almost 3600 miles per hour while the average over the minute is still 60 miles per hour. (back to the creative part) the manufacturers have algorithyms in the gps receiver that average the speed over some time (it varies)based on the distance traveled and the positional error calculated. so, if the distance traveled is greater, the instantaneous accuracy is better. if the distance traveled is less, the time used to calculate the speed will be greater (in some cases 1 second or more) and the instantaneous speed will be less accurate.

that said, on the typical cat where there is not too much pitching and rolling, and the speed is pretty constant, the standard gps using differential is usually about 99% accurate.

one more thing, someone mentioned that dgps requires special towers. while this is true, there is a new differential correction system called waas (wide area augmentation system). it was set up for pilots to use for instrument landings, but works really good for boaters too (this system is currently only in the united states). many inexpensive receivers have waas capabilities and can produce very accurate results (in the 2 meter range). waas is more accurate than the old differential system that uses towers and a separate receiver and has the added benefit of not requiring an extra antenna and receiver.

before the flaming begins, i would also like to add that this technology is changing about as fast as computers, so there may be other differential systems out there that i do not know about.

if you want more info on gps, go to http://joe.mehaffey.com this site has about everything or a link to everything relating to gps and how it works.

Hi Mary,
GPS accuracy is also subject to the cpu power of the device and the quality of the software used, as well as the number of channels. High end GPS units have millimetre accuracy.
David

Ofcourse GPS can be far more accurate than the commercial apparatus are right now. Reason is ofcourse that the US military has made it inaccurate for commercial purposes as not to help the enemy highjacking their own positioning system. Ofcourse military GPS is corrected for the introduced inaccuracy.

Differential GPS was developed to overcome this.

But this is really not the main point of this post.

The most simple reason why GPS readings are not alwasy accurate is that a good GPS device measures the repositioning and speed relative to the earth. This is often not the same actual speed. Sea and Ocean current nearly always offset the speed reading. Sailing against the current and you're sailing faster than the GPS reading and sailing with the current and you're sailing slower than the reading. When relative to water ofcourse because your repostition speed is ofcourse exactly given by your GPS. And then ofcourse you have the little effects that the others are speaking of. Simple devices without the introduced inaccuracy surpressor may be offsetted as a result of that two, but I didn't know that any devices with one or the other technic for this were sold anymore.

Took my Garmin Legend skiing over the holidays and it showed the max speed of 264 mpg. I should be dead now.

Perhaps due to the mountains and the GPS was in my front coat pocket, there might be a bit of inaccuracy.

Spinakerjohn

If it's speed relative to the earth, It's true speed. As in true wind and apparent wind.

I'm no authority here on this, but to WHAT DECIMEL point are you talking about when you are talking about accuracy? Are car speedometers accurate? Sometimes, put different diameter tires on your truck, need to change the settings in the computer on board, if it's an OBDII vehicle.

For all practical purposes, within reason, a standard GPS seems pretty accurate, especially, as has been stated, when looking at average speed.

Again, to what decimal point do you want the accuracy?

Near my club we have a about 3 knots of current depending on the tide. These 3 knots can make my over-land-speed either 18 knots or 24 knots if I have a waterspeed of 21 knots depending on tbe directtion of the current. More than

Wouter

>Reason is ofcourse that the US military has made it inaccurate for commercial purposes

Selective Availability has been turned off for the past couple years, greatly improving the accuracy of civilian receivers and reducing the need for differential correction for most purposes.

>Sea and Ocean current nearly always offset the speed reading

When used for navigation, I'm not really concerned with how fast I'm going relative to the water. I want to know when I'm going to get to my destination. (If I'm surfing a standing wave in my kayak, my speed relative to the water's surface may be very high as I plane down its face, but I'm going nowhere downriver.)

We do have to consider current, however, if we're attempting a speed run from point A to point B. Certainly, if you have a favorable tide, or are riding the Gulf Stream, you will get there faster. (Just ask the sailors that make the Newport to Bermuda run.) For this reason, you would not want to try and make any claims about a boat's top speed based on runs that were made in conditions that involved ocean current.

>but to WHAT DECIMEL point are you talking about when you are talking about accuracy?

Todd,

Good point. I would follow with the question, For what purpose are we considering the viability of GPS?

If we're talking navigation, considering a device that can get you within a boat length of your next waypoint/destination, GPS is pretty darned amazing.

If we're talking about catsailors arming themselves with recreational GPS units to try and settle the score over which boat is faster, well, I think it's opening a bucket of worms

I agree that you can get an inaccurate max speed reading at times with a GPS, but overall I think they are very accurate. My favorite use is to download the tracks after sailing to see where I had been. (ExpertGPS software) Here's a track from last Friday morning in Key Largo. The winds got to be a bit much, but thanks to Mary and Ricks help when landing downwind on the beach, I didn't have any repairs to do when I got home.

Jack Hoying
Prindle 18
Fort Loramie, Ohio

actually, the inaccuracy you speak of is called selective availability was turned off on may 1, 2000, and has not been turned on since.

as far as the tides, i completely agree with you. gps measures ground speed, not water speed.

what software are you using? How did you get that image?

Jake,

See attachment for a sailing track uploaded to a PC using Chartview Planner software. On my Garmin 12, I have a choice of collecting data points for tracks by an automatic method (that uses resolution - i.e. logging points at a specified distance), or by defined time intervals. I usually set it to a specified time interval so that when I upload my tracks I can get a sense for speed by looking at the distance between data points. (Note in the upper right corner of the image that I still had the GPS on while I drove back to my house.)

Jake,

Take a look at www.topografix.com. They sell the ExpertGPS and I believe have a demo version that you can use for a short time.

Bill Moran
T4.9 #224

I missed the fact that it has been turned off in 2000. Thanks for the info.

Wouter