But isn't it a lot more fun to sit around a campfire and argue about who's machine is the fastest?

 

Hmmm, some (very) basic errors regarding GPS by the original poster.

First, a GPS receiver needs 4 SVs for a 3D lock, not 3. Second, a receiver only uses more than 4 SVs if it's designed to calculate an over-determined solution. Using an OS doesn't increase accuracy by more than 15%, at best, so claiming 5 or more SVs are needed for "realistic readings" is inaccurate.

Also, SA (selective availability), not "c/a," is what causes uncorrected civilian GPS units to have lower accuracy than the system would otherwise provide. With SA off--and it is due to be turned off within the next couple of years--the (positional) accuracy of your typical consumer GPS increases enormously. However, speed computations aren't effected by SA nearly as much as position, so the MPH readings you get _today_ are about as good as you can expect (which is generally more accurate than your typical speedometer).

But, GPS only measures ground speed--regardless of incline--so it is inherently inaccurate on any slope. Drop a receiver from a 100-story building and it thinks it's going 0 MPH, right before impact.

Another:
"...the time required for a gps unit to calibrate and lock in is often not feasable for these kind of calculations..."

Not sure what this is supposed to mean, but at face value it is just plain wrong. Nearly all consumer GPS receivers are of the 12-channel variety, and lock very quickly. Plus, they constantly monitor "spare" SVs in case one of the SVs used in the solution drop out. Today's receivers can do all the calculations (and more) for all of this without breaking a sweat.


Still more:
"By the time you reach point "B" , you have nothing better than a computer generated estimate on how fast you went.....(since satellites are geostationary there could be a problem if the unit tries to latch on to a new satellite)"

I am forced to wonder how someone involved in _any_ kind of GPS project could not be aware that the SVs are not in a geo-stationary orbit. The "computer generated estimate" part, in the manner it was used, is just gibberish.

The bottom line is that the accuracy of ground speed readings from today's consumer GPS receivers is excellent, compared to speedos.


-Mike

 

I assume someone read the owners manual that came with the gps.

Since I do write code associated with GPS units and error correction....I am quite aware of how a gps works.....There is much to be said about how some companies handle error correction. (Though the error correction beams from above..your gps has firmware inside it....I might have written that code for all you know)

Yes there are some faults as to my original post. But for the most part....any layman, should be able to understand what I was trying to do there. which was a dispute over everyones quest for "how fast does my quad go"
Modern day gps systems are much more accurate than I have implied them to be.....and yes, most are of the 12 channel variety.

Your right about the point a-b comparison....It's not that general....But in terms that most people can understand... that comment alone turned some heads.

Your comment almost implies that you do not believe my credentials on the subject.....I would hope that the next time I have to submit a proposal to the government, that you will be by my side just to make sure that I don't tell any fibs.

My line of work requires the use of GPS systems for tracking and annalysis of certain objects. I run several tests a day trying to come up with valid answers for accuracy problems and the like.

You guys can sit there all day long with your manuals and hand held units... But do me a favor...go out in your backyard...set up a nice strait line.... and hammer through it on your atv or whatever...run for all I care.

and then do it again...in the same area...and again...again... My point is that it is very hard to duplicate the results. that is called margine of error....

To make a long story short...there are alot of you people out there that know everything that there is to know about everything....and some of you that have a ball trying to out talk or try to prove intelligence. Oh well.

When I commented on in the first post about a gps unit.....did I mention anywhere? about what kind I was using? .....because everyone just assumed which make and model I was talking about. Before all the "I know all" stuff began.... the second post should have read " Now what kind of gps are we talking about?"

That alone might have cleared a few things up.

I would still like to own a handheld unit...I just know for a fact through my testings that accuracy is definately an issue. We can argue that point all day long...

Sorry if I caused a stir for you all...but my wheeler runs right now..... I had to think of something to get some attention... I wish I had the right to send or post some of the results of my groups testings...It is quite useful... But maybe not for the average camper or hunter. Any of the latest gps's on the market will save your life if your lost.

 

Personally,
I eschew the use of any modern device when calculating speed, etc... The most reliable and accurate way to calculate MPH/HP/DISTANCE/grade of hill/ or whatever is by THE SEAT OF THE PANTS! I have used this method numerous times when tuning my LT 250 R and now have it tuned too perfection. I can do 85 MPH, utilizing 72.45 HP covering 20 feet up a 30 degree hill. Now I dont want to hear it. If this method was good enuff for my Great GranPappy, well then by gawd, its good enuff for me! Now take back that GPS, go to Wal-Mart (or any other of your favorite stores) and buy the Fruit of the Loom style boxers. I have found that the extra material in the rump area does not throw off said "seat of the pants" readings, while those boxers made in China tend to be inaccurate at speeds. Of course naked (my preferred method) works best. Try it...

RoostKing

Kidding of course!

 

sayIt_fmf,

Whoa!! Easy lad. Not worth losing it over this one.

I think it is safe to say the intention was not to beat up or offend. When any of us post something here for all to see, there will always be vastly differing opinions and comments. For the most part it is a learning experience. From the back and forth discussion we can choose the information we feel is accurate and learn from it. Whether it is GPS, clutches, wheels, or lift kits, there will always be points to differ on.

Keep in mind the contributors are young and old and from all walks of life. I sit on my *** in an office all day, as it sounds like you do. Tree Farmer seems to be well read. I think he contributes to the forum from the top of a large mountain somewhere in Tibet. RoostKing, well what can we say that he hasn't already said extremely well. We are all coming at this forum with different bases of knowledge. The thing we have in common is ATVs.

Kudos to RoostKing. I think he has very accurately expressed the attitude we should all take towards the discussions in this forum.

Keep it light and learn. Let's have fun.

DJ

 

My quads faster than yours.My quads better than yours.Now its about GPS units.I just dont understand what all the fuss is about.Have a nice day,Life is to short to stress....Bob

 

Hey guys, it's good to see a lively discussion in the forum besides the usual "my dad can beat up your dad" topics on who has the better quad.

Here is some information I have learned while developing GPS systems. The satellites do not orbit in a geo-synchronous orbit or else they would all be over the equator. You can get software that will link to your receiver, give you a sky view of your position and show you the different satellites as they move across the sky. The speed issue is a interesting matter. The relative error of your speed measurement is a function of speed. Most receivers will do a position calculation every second and every sample will have a new error. This means that you cannot rely on having a consistent error between two samples and the speed calculation will have some problems but this is dependant on the distance between the samples. If the distance is short ( 300 ft ) then the possibility of the vector being completely off and in the opposite direction is likely. Think of the case of being stationary and getting a sample that says that you are 300 ft North of you position and the next sample says that you are 300 ft south of your position (This is the stated error that the Military say S/A will give you). This means that without you moving, the receiver will say that between those samples you moved at 600 ft/s (409 miles per hour) this is greatly reduced by increasing your speed. The relative error drops off quickly. We saw that using differential corrected GPS compared to uncorrected GPS at 70 mph was about 2% error and at 20 mph was 15% error.

The reason that some receivers will only do a speed calculation using a two dimensional vector ( X and Y) because the error in the height( or Z ) can be much higher than the other two. But the receivers that use differential corrections will do a three dimensional vector. This means that you can get an actual speed going up or down hills.

So basicaly the hand held units without differential corrections will get you out of the bush but are a little off when trying to get the speed of slower moving quads.

Hope this helps

TDS on a 450es

 

This Is a little off the subject, I have a Eagle explorer II. I kind of like to play around with a DGPS beacon. Eagle offers one under $500, (thats alot of money that I could spend on my 250)does anyone know of something cheaper. I would appreciate any directions to look, thanks