Garmin Foretrex 101 accuracy?

[Sessholvlaru]

My buddy and I have been doing some running in the large local pond, and up until today I was the speed leader with my 8S Genesis. My best two runs were 73.5 mph and then 75.5 mph, both given from the Garmin 101. I saw no reason to question this as my setup is capable of such speeds. My buddies 6S SuperVee was clocking in around 60-ish, and these speeds seemed accurate in relation to one another. However...

Today my buddy was running his SuperVee 27R with his hottest setup to date and the boat was smoking fast. It "looked" very close to what my Genesis was running, and after the run the Garmin says...75.5 mph. Video is below.

Now I know these boats were close, but this is suspect. We had a situation before with this GPS where two of my runs with a different boat were identical speed-wise, which I thought was strange.

I know this GPS isn't exactly designed for this, but I'm not sure what is going on. I can't help but wonder how accurate it is, how "liberally" it is dishing out these numbers and why identical speeds have been seen in different runs. Is 75.5 an average or something?

My buddy has a nice Tracker radar gun which we didn't really get a chance to use before our hands were frozen, so we could not try and verify anything with it.

What do you guys think?

[megalops]

Toss it in your car and make a few "Runs". Thats how I check mine.

[megalops]

I have seen some Instances with my Garmin foreRunner 305 that it is wrong but when its wrong its seems to be way off. For Instance my HM runs some where between 53-60 mph depending on prop/setup. So far on one occasion it was showing a top speed of 27 mph after a couple of passes.

I took the same Garmin down to our club pond the next weekend and it was back to normal reading. I then borrowed another Garmin (totally different model) from a club member for the second run and it came back perfect.

I guess bottom line IMO they can Intermittenly have false readings and usually when they do they are pretty significant.

After looking at the video i think your friend hit the nail on the head when he says "theres not enough room to open it up." A tip is have plenty of room to hold WOT so the GPS has time to take more location samples/speed calcs. Also crashes sometime leave false readings but usually error on the high side.

[smokestackin]

Look close??? I thought I heard someone in the video say they didn't have the record anymore

[keithbradley]

Not accurate.

I've GPSed boats, cars, ect more times than I could estimate and one thing I can tell you for sure is that if you crash, you have to disregard the GPS reading. I've had readings in excess of 200mph after crashing in a boat. Keep this in mind when using GPS and bring it in after a couple passes before you crash.

Another thing is if you lose signal for a second, it will throw the reading off. I have a bridge I drive under when driving home from work and the GPS will blip for a second and then return to normal, throwing off the "max" speed. Ive had runs in my boats where I take off the hatch and the GPS reading is just too good to be true. I've gotten pretty good at judging speed, and I can tell the difference between 50, 60, 70, 80, ect. When I pull a boat out of the water and it reads 10mph fast I just throw that reading out.

Here's a run where the GPS read in the mid 70's, and I can tell just by looking at it that it wasn't that fast.

[keithbradley]

I have seen some Instances with my Garmin foreRunner 305 that it is wrong but when its wrong its seems to be way off. For Instance my HM runs some where between 53-60 mph depending on prop/setup. So far on one occasion it was showing a top speed of 27 mph after a couple of passes.

I took the same Garmin down to our club pond the next weekend and it was back to normal reading. I then borrowed another Garmin (totally different model) from a club member for the second run and it came back perfect.

I guess bottom line IMO they can Intermittenly have false readings and usually when they do they are pretty significant.

After looking at the video i think your friend hit the nail on the head when he says "theres not enough room to open it up." A tip is have plenty of room to hold WOT so the GPS has time to take more location samples/speed calcs. Also crashes sometime leave false readings but usually error on the high side.

I use the 305 for running but the refresh rate is too slow for speed run use. I tried it in a 70 mph 1/10 scale car and it was reading 20's on somewhat long passes.lol
I ran the car on a short road where I probably got it close to full speed and it said 17mph. If I take it in my real car it will reach accurate mph, but it takes a while. If I just get up to speed and stop quickly, it will read low.

[megalops]

I use the 305 for running but the refresh rate is too slow for speed run use. I tried it in a 70 mph 1/10 scale car and it was reading 20's on somewhat long passes.lol
I ran the car on a short road where I probably got it close to full speed and it said 17mph. If I take it in my real car it will reach accurate mph, but it takes a while. If I just get up to speed and stop quickly, it will read low.

Yeah the 305 is my wifes running aparatus. One of our club members has/had a 305 and the same issues. Its sample rate is too slow for sure.

[Rumdog]

Keith is spot on here.
anyhow, that is a damn fast SV27!

[Sessholvlaru]

Thanks for all of the great tips.

I agree that the boat in that video does not look like it is doing 70, although based on my experience flying fast R/C planes I do believe that my buddies boat is cruising at somewhere around 70. I do however prefer concrete data over outlandish claims (I've heard my share). I totally agree that a crash could throw it off, but neither of my Genesis runs (73.5 and then 75.5) had any sort of crash.

Also, we did test that GPS in a car and it was accurate, although I'm not sure how quickly it refreshes. Below is the video of my Genesis during the 73.5 run (best passes at 1:15 and 1:55).

I guess the real litmus test will be checking the GPS against the Tracker radar gun. I also have an audio program that analyzes the Doppler effect to calculate speed. It worked well for my speed plane (~200 mph) but I am not sure how well it will work with boats due to water related noise.

[Sessholvlaru]

And just for giggles here is a fast pass where it did crash.

[Gary]

GPS reading will always be very accurate if you stay into the run as long as possible. My Eagle Tree readings are almost always identical day to day and i always stay into the run untill the boat has leveled out in speed. You can see it on the play back. Also the rpm tells me the speed aswell. I know at 43,000 the boat is always at 92-93 mph.

[keithbradley]

GPS reading will always be very accurate if you stay into the run as long as possible. My Eagle Tree readings are almost always identical day to day and i always stay into the run untill the boat has leveled out in speed. You can see it on the play back. Also the rpm tells me the speed aswell. I know at 43,000 the boat is always at 92-93 mph.

If you get a HIGH reading from intermittent communication, lengthening the run will do nothing in terms of an accurate "max speed" reading.

[m4a1usr]

Testing a handheld GPS unit in your car to verify its accuracy will do nothing other than inflate your ego. Your car does not accelerate nor deccelerate, nor turn in a 30 foot radius at 40 mph like your boat will. As well, a boat does not have any suspension nor does it ride on a static media for vertical stability. Lastly your GPS measurements in a boat are likely to be 2 to 3 minutes in duration. Most of us travel in enough traffic you cant even go from zero to 70 mph in 3 minutes.

Google "GPS inaccuracy" and take the time to read most of the first page hits. The one thing we seek in using our hand held GPS units is actually what its worse at accomplishing.

John

[Gary]

If you get a HIGH reading from intermittent communication, lengthening the run will do nothing in terms of an accurate "max speed" reading.

If your readings are wrong or inaccurate most of the time then something should be wrong.
If you accelerate up to 100mph in 2 seconds a GPS reading will not even be close , at all. It never had enough time to calculate an accurate reading..... this is only an example. I have seen this time and time again on not enough time to get a measurement that is fairly accurate. I know for a fact at a standing 2 mile top speed car event that when I go through the "traps" at 180mph and still accelerating my gps is within 1-2 mph of the actual measured 2- mile run. I still believe a good GPS will give some pretty good results..........nothing to bank on but pretty good non the less. Just my 2 - cents and only my personal experience.