Miles per hour is the relationship between distance traveled, and the time it takes to cover that distance. If either of these values is off the indicated MPH on the GPS unit will not be the actual MPH. The clocks in the GPS units are up to the task, but are the distance measurements accurate enough?

 

Well, I'm not sure if this verifies the actual "distance measurements" on the GPS, but I know that the one that we have (mid quality) is pretty accurate when it comes to tracking where you are, or where you have been. You could walk 10 miles through the woods, and it will lead you back to within 3 feet of where you left.

Like I said, I don't know what this proves, but if they are accurate in that sense, then they must be pretty accurate when it comes to their other functions.

Maybe somebody should physically measure out a distance, and check to see if their GPS measures correctly.

-Josh

 

Actually, the "clock" is not in the actual GPS unit itself, but are contained in each of the 24 satelites in orbit. Now, with that in mind, one needs to remember that these satelites are something like 10,700 miles above the earths surface. So, while timing a couple different clocks or watches here on earth, being even one second off, seems like nothing, translate that one second off to those 24 satelites that have to broadcast signal down the 10,000+ miles-that ONE SECOND now can make a HUGE difference in actual position placement. I.E.-at all identical times, you could be standing in the middle of your own driveway. But, with that one second variance of one of the clocks being off-translated over the distance from the surface of the earth, your GPS unit could tell you that you're now in the next county-or maybe in the next state for that matter.

In order for the GPS unit to work properly, you need at minimum 4 satelites in view to give proper lat,long and elevation-to provide a proper triangulation. Naturally, the more you can "see" with your unit, the more accurate your readings will be. Keep in mind though that the unit's original purpose was for the military, and until they lifted the SA band back in May of 2000, civilian signals were frequently "off" completely for safety sake of the military. With the band lifted, the units have become significantly more accurate, but occassionally they will "trip", confusing your GPS and telling you that you are out in the ocean somewhere for a second or two, before it gets back to the actual area you are at.

While this all seems "heavy" for just guaging MPH useage, the GPS is in fact one of the more accurate ways to judge MPH. I trust the GPS in my vehicles more than I trust the car's own speedometer. It varies from which rig I'm driving, sometimes as much as a 8mph difference. One car will say I'm doing 60mph, while the GPS says 68. Likewise, my Honda is just the opposite-it will say I'm doing 70mph, GPS says 65-66. Surprisingly enough, our Suburban is the most "accurate", being only 2mph off from the GPS. I've had a few of these "tested" with radar from the police, they'll verify always on the side of the GPS-though not "knowingly" of course. -I've been pulled over a few times going by the vehicles speedo, yet monitoring the GPS to see. When the cop pulls you over, he usually tells you how fast your going, so without knowing it if I don't tell him, the speed he gives me has always been at or within 1-2 mph + or - of the GPS. This has actually come in handy on one occassion, when I passed a semi, then was passed myself by another truck. The stater pulled me over, claiming I was doing 78 mph towing a trailer in a 70 zone. I showed him the GPS, with "Max Speed" feature, showing 75 mph, which was indeed 3 mph slower than what he had clocked "me" at. What had happened was as I pulled over, the next guy behind me passed me at 78, the radar couldn't differentiate. I got lucky in that he himself had a GPS unit, knew how accurate they were, and let me go with no ticket, but rather an apology, before he took off after that other guy in front of me.

I wouldn't test this yourself too often unless you know the police, as they will get aggrivated the more frequently you try it, but you might be amazed at how accurate they really are. There are other "factors" involved to throw off the units accuracy, such as atmospheric pressure, cloud cover, trees, other cover like that-same type of stuff that will effect normal radio waves-rain, precipitation, etc. However, considering what distances these satelites are off the earths surface, combined with all the outside factors, it is remarkable how accurate they can be.

The thing you need to remember for the best accuracy is that you need to "initialize" your unit the minute you take it out of the package. It is set from the factory for that specfic area, so when you turn it on, it will look for those satelites that it should see at the factory. You can only find up to 12 satelites at any given time, and the farther you move away from the factory, those 12 satelites will be different. The 12 satelites my GPS sees out here in Washington, are compltetely different than the 12 you would see back in Minnesota, or somewhere back east like that. Any time you move more than around 200 miles in any direction, you should "re-initialize" your GPS to update it on which satelites to look for. Once it knows what area it is in, it knows what to look for, and your information will be much more accurate as well.

Anyway, hope that's not too much stuff-I had a class in this last year for farming applications-really pretty cool stuff, but a lot to consider. Anyway, Best of Luck with that, hope that helps maybe a little,

Take care,

Mike

 

GPS is far more accurately than any mechanical speedomoter, even the ones that you buy from Wal-Mart. eTrex goes for less than $100.00 and it is a good investment.

 

GPS is by far the most accurate speedo you've ever seen, as long as your tracking 4+ sats...

We use GPS at work (construction company), and once you set a base station to fix the atmosphere problem, a GPS unit is within .25 inches of being accurate for up to 6 miles away.

The base station isn't needed for single GPS accuracy. You'll notice whenever you look at your GPS you have a +- #feet in accuracy. That happens because the radio transmitter in the sats all come from different angles. Each angle will have a different tempature the radio signal has to travel through to get to your GPS. (IE warmer in the south and colder in the north). Tempature has a direct effect on the speed of the transmission. Thats what accounts for the plus/minus accuracy in the GPS.

Now while you are traveling the +/- accuracy doesn't really change, or less than .25 inches per 6 miles, so you can safely say that a GPS is extremely accurate on distance.

The GPS unit also gets time from the sats, and yes they are extremely accurate as well.

I know form our rode building experience that GPS is an incredible tool, when harnessed correctly can give amazingly detailed and accurate figures to work with...

The only time a GPS loses accuracy: Loss of sat signal. Riding in the wooods can do this.

Just my $.02
Brad

 

GPS, yes best device I've played with for navigating, I use one in the airplane, and atv etc.
One big consideration about the atv useage, if your in a heavy forest with erratic or little sky view it can be less than 100 percent honest.
To check your speed, find a open area give it a run and your results will be great.
As with all toys, it takes time to get used to them, trust them, and really love em, once you have seen the light your hooked.
Good luck

 

This is somewhat off the subject, but since GPSs were mentioned, and you have one and have not discovered geocaching.com, take a look. A fun site.

 

To answer the question, Yes they are the most accurate way to measure speed available to the average person. Better than radar in my opinion.

Ok, fine, what makes GPS so accurate then? To start, it's true that miles per hour is a measurement of distance over time. But, GPS doesn't measure speed that way. Instead of measuring how far you've travelled in a given time (the way people seem to think it works) the GPS uses doppler shift (like radar). By calculating the change in frequency of the received signals from the satellites the GPS determines how fast you are going. The cool thing about this is the GPS gets it's timing from the Cesium clocks abord the satellites themselves since it would cost too much and make the GPS fairly huge the reference clock (cesium). This timing information is sent from the satellite to the GPS to calibrate it making sure it is always accurate. Since SA was turned off in May 2000 as cowboy said they are now much more accurate for reading lat/lon/alt than they used to be. Typically you can expect 15 meter accuracy for lat/lon and about 1.5 times that for altitude measurements. Speed accuracy is rated to be about .1 MPH.