View Full Version : Speedo Calibration
Professor
03-02-2005, 05:04 PM
I know there are probably many methods but I have been using my Forerunner to check my boat speed, and I am curious how you guys are calibrating your speedos.
ski_king
03-02-2005, 05:06 PM
I know there are probably many methods but I have been using my Forerunner to check my boat speed, and I am curious how you guys are calibrating your speedos.
Timing thru the slalom course.
Or better GPS if you have it.
boatwake
03-02-2005, 05:12 PM
Portable GPS!
rodltg2
03-02-2005, 05:40 PM
fork out the dough and buy pp.
Professor
03-02-2005, 05:57 PM
Okay, thanks guys and I believe my Forerunner (201) does a pretty good job but the specifications quote 0.1mph accuracy. It seems as if the error (accuracy) will be scenario dependent and in a typical situation may be closer to 0.4 mph. Any ideas?
AirJunky
03-02-2005, 06:12 PM
Portable GPS.
It's interesting to do it opposite directions & see the difference due to the travel of the satellite. I've never seen more than 1/2 mph or so though.
sfitzgerald351
03-02-2005, 06:27 PM
Just remember that with GPS the speed is usually "smoothed" because the receiver has to calculate the speed from distance/time from the different points it takes in time. Because accuracy is on the order of 15m simply doing the math between every two points would result in speeds jumping all around. So it does some sort of moving average of a bunch of points. So that means that you'll only get decent accuracy after you've been at the same speed for a minute or so (maybe more?). Garmin claims about +/- 0.1 mph in accuracy though I'm not sure I really believe that based on my experiences.
Professor
03-02-2005, 06:43 PM
Thanks that helps. Now, if the error is close to a mile per hour then don't you think slalom skiing can really be affected?
Cloaked
03-02-2005, 08:28 PM
A slow, clean pass at 38 off in a tournament. It does make a difference. :steering: :)
BriEOD
03-02-2005, 09:12 PM
Just remember that with GPS the speed is usually "smoothed" because the receiver has to calculate the speed from distance/time from the different points it takes in time. Because accuracy is on the order of 15m simply doing the math between every two points would result in speeds jumping all around. So it does some sort of moving average of a bunch of points. So that means that you'll only get decent accuracy after you've been at the same speed for a minute or so (maybe more?). Garmin claims about +/- 0.1 mph in accuracy though I'm not sure I really believe that based on my experiences.
The Rockwell model that the military uses has a continous mode and it's pretty darn dead on. I don't know the stats off the top of my head but it does some neat stuff. But, it's also big a and bulky. I have a little Garmin Rhino I keep on the MC to calibrate.
Leroy
03-03-2005, 12:03 AM
A lot of the accuracy depends on the number of GPS sats the receiver is hitting. I think most need to "hit" 4 for best accuracy. With that I think they are as good as the military allows them to be. I need to get one of these to play with.....
sfitzgerald351
03-03-2005, 12:09 AM
Since they turned off the 'scrambling' a few years ago GPS has been a lot more accurate even with the little handhelds. You need a minimum of 3 to get a position, 4 for accuracy, and 6 to really know what you're doing. But without some sort of differential correction you're still looking at a repeatable position of +/- 50ft. The good news is that speed only relies on relative position and is much more accurate. Which is why I think they feel they can claim +/- 0.1 mph. Though I'm still not sure I'm ready to believe it. But it'll definitely get you within 0.5mph and who can read the speedo more accurately anyway?
BriEOD
03-03-2005, 07:41 AM
Yes, I agree I was glad when they stopped encrypting the military satelites. I can't tell you what a pain in the arse it is the run a classified tape through a tape reader and enter a code to get the GPS to run encrypted.
It was my understanding (I've actually had quite a bit of training with GPS) that you only need 3 satelites for 2D location. You need 4 satellites for 3D (flying) location. Obviously the more satelites you have the more accurate you'll be. Depending on the abilitiy of your GPS you can put them in different modes: continous, fixed, standy, etc. Their are pros and cons to each mode, one being power consumption.
The Army uses the Rockwell GPS and integrates it with their communication system CINGARS. When they key their radio it also sends a their location back to the Operations Center. Pretty cool I think.
Here is a shot of what the military uses. It's the Rockwell PLGR. This one is a second generation. It is green the first generation was khaki. The second generation primarily addresses power consumption. It can run on 8 AA's or one big lithium. It also takes a Ni-Cad memory battery. It's probably about half the size of a lap top. It has something like 36 menu screens and is not the most user friendly piece of equipment.
Wow-With that title-I'm glad this is not a swimsuit thread!
SteveO
03-03-2005, 09:16 AM
Wow-With that title-I'm glad this is not a swimsuit thread!
Tex, The thought of you in a speedo almost made me spit my coffee on my screen.
BriEOD
03-03-2005, 09:22 AM
Wow-With that title-I'm glad this is not a swimsuit thread!
Dude, you've got "issues."
sfitzgerald351
03-03-2005, 11:03 AM
It was my understanding (I've actually had quite a bit of training with GPS) that you only need 3 satelites for 2D location. You need 4 satellites for 3D (flying) location. Obviously the more satelites you have the more accurate you'll be.
That's my understanding as well. But it's true that going from 3 to 4 satellites increases your positional accuracy quite a bit because there are more reference points to triangulate.
east tx skier
03-03-2005, 11:57 AM
Between a handheld garmin gps and running baselines in perfect pass, it's close enough for me. After I run it through the course to check the baselines, I check it with the gps. It's usually pretty close (although I don't know about .1 mph either). Scott, maybe that's just referring to the readout capabilities of the unit. ;)
BriEOD
03-03-2005, 12:21 PM
That's my understanding as well. But it's true that going from 3 to 4 satellites increases your positional accuracy quite a bit because there are more reference points to triangulate.
Without a doubt Fitz.
Dude, you've got "issues."
And???-It's not like I was hoping it was a swimsuit thread.
Steve-I'm glad you almost spit coffee. I would be concerned if you didn't.