Garmin Instinct GPS watch speed readings?

[Caribsurf]

anyone have the Garmin Instinct watch?  when I wing foil I use the windsurfing app and  it has been showing really high max speed readings which I know are not true..yesterday's max speed was 28 knots 32mph. no way that's accurate.

[B-Walnut]

Hey Caribsurf,

I recently got the watch and run it in kitesurf mode. Slightly different readouts than windsurf mode but I get the same glitch. I can't see it on the garmin website, but when I load up my session on my phone the graph shows what has happened.

Here you see the speed spike right before it goes to zero. I assume this is my body ragdolling across the water and accelerating the watch for a split second giving the false reading. A bit unfortunate that they track split second speed spikes, I'd like something that shows top speed held for 1-2 seconds for better accuracy. Other than that though, love the watch, especially the ability to track miles on my sails.

[Caribsurf]

Thanks B-W.  It is odd. Later today after my original post i had a 55 minute foil session and my watch reading  seemed accurate.  I like your explanation of the watch registering top speed as result of a fast crash. Fortunately I was on foil the entire session and never wiped out either session.  Maybe just a hiccup with the watch

[PonoBill]

GPS speed is always inaccurate in short burst since it's purely time/distance with an error band that can go either way. No modern GPS is useful for timing. Creaky old GPS handhelds that used Doppler for speed measurement are the gold standard for speed tests, you might be able to find one on ebay for a few thousand bucks since no one makes a GPS Doppler timer anymore and the old ones are slowly dying despite the elaborate care anyone who has one lavishes on it. It's like Blackwing pencils which used to eBay for 40 bucks each for an unused one until someone bought the old factory and started making them again. Now they're a mere $2.50 each for suckers like me who love them.

[LaPerouseBay]

anyone have the Garmin Instinct watch?  when I wing foil I use the windsurfing app and  it has been showing really high max speed readings which I know are not true..yesterday's max speed was 28 knots 32mph. no way that's accurate.

Mine works great, it's very dependable and durable.
I use it to add heartrate to my videos.  Great watch. 
 
You can go thru this list and improve the GPS.

https://support.garmin.com/en-US/?faq=xQvHXbfaT27Zr4hxZDvjv5

The drone guys use this site to check the availability of satellites.

https://www.uavforecast.com/

Back when I used a Garmin for telemetry, I would mount it to the top of my hat.  It works. 

Swinging arms can compromise accuracy.  Your wing may also block a signal. 

[LaPerouseBay]

Here you see the speed spike right before it goes to zero. I assume this is my body ragdolling across the water and accelerating the watch for a split second giving the false reading. A bit unfortunate that they track split second speed spikes, I'd like something that shows top speed held for 1-2 seconds for better accuracy. Other than that though, love the watch, especially the ability to track miles on my sails.

The graph you posted looks great for what we ask of them.  Spikes like that occasionally slip thru.
They are far more common the more you zoom in on the data. 
I've always been fine with Garmin data, because downwinding doesn't have violent starts and stops. 

That's actually not a split second speed spike.  Garmin has a 1Hz limit.  That spike is just a wild error in the math.   

The Seabreeze forum windsurf racers engineered amazing GPS systems to track acceleration back in the day - it was all custom software and sensors.   

I use Gopro's GPS now, because of the 18 or 10Hz sample rate.  It's far more accurate than my Instinct.
It's also easier to embed in video.

[LaPerouseBay]

GPS speed is always inaccurate in short burst since it's purely time/distance with an error band that can go either way. No modern GPS is useful for timing.

The racecar guys have been all over GPS for years.  This tiny unit is 25Hz, Racebox has another system at 50Hz.  The drone guys go up to 100Hz.

https://www.racebox.pro/products/racebox-mini-s

[PonoBill]

Yup, GPS speed is great for average speed, but location precision for high-quality GPS units is +/- 6 feet. Way better than it used to be, but there are still spike errors and high sample rates don't fix the issue, they make it worse at the same time they are making the average speed more accurate by providing more samples to average over short periods. But if you suddenly move 12 feet (the worst case error) in .1 microseconds your GPS will think you're hauling ass if you're standing still  Most good systems don't report instantaneous errors, even what looks like instantaneous speed isn't. It's an average taken over some number of samples with a smoothing algorithm applied. For a high sample rate, like 10KHz, a system would have about 10 samples per microsecond. More than enough to discard spikes and get a reasonable average even if the screen updates at 1KHz (they don't, it's more like 200hz, though shitty ones might update at a low enough rate to get flicker from LED lights or the backlight). In other words, the errors are there, but nobody cares. And yes, of course, the errors can compound, but usually they don't. Some racing bodies used to require Doppler GPS measurement for record attempts. But after a quick google search I couldn't find any that do anymore. Doppler directly measures speed instead of calculating it. But it's like preferring vinyl records to CDs or MP3, Laserdisk to MP4. Analog vs. Digital. 

[Velasco]

I use the bicycle mode - your track is in color (based on your relative speed).  I still get those high speed spikes, but most of the time, I get pretty decent speed charts.

[PonoBill]

If you have some trouble sleeping this paper--Evaluation of the performance of GNSS-based velocity estimation algorithms--is guaranteed to conk you out quickly: https://link.springer.com/article/10.1186/s43020-022-00080-4?fromPaywallRec=false

[LaPerouseBay]

For a high sample rate, like 10KHz, a system would have about 10 samples per microsecond.

10,000Hz sample rate isn't in my world.  Nor are 10th's of a millionth of a second. 

I'm a simple person.  10Hz means 10 data points per second, each one one a speed, on a graph, per unit time.

The 25Hz RaceBox I linked includes an accelerometer and a G force sensor.  When they fold that into their model, they can get location within 4"
The Seabreeze forums windsurf slalom guys were engineering their own system 15 years ago, and it worked.  Accurate stops and starts.  On the ocean.   

As for location, it doesn't matter to me if my line is off by 6 feet.  I have tested it for location and it's damn accurate.
We were experimenting with locating paddlers in our group if someone was missing.  We were using Google Maps "share my location."  It's free and it works. 
I downloaded my Garmin telemetry into Google Earth and located my position on my downwind line.  It was easily within 6 feet on the few points I checked.
 
All I need are speed numbers, in real time. 
Here are videos using the same model, with imported 1Hz and 18Hz sample rates.
The graph has 10 seconds on the x axis, speed on the y axis.
 
Note the Garmin on my head. That was 2013.  Gopro and Garmin telemetry imported into a model I use now.  That's 1Hz.   

The other video is Gopro 8 telemetry at 18Hz.  Note all the data points in the same 10 seconds? 
More data, more accurate graphs if you drop a point (or 10).   

It's not rocket science. 

https://youtu.be/1Sk-Zi3fh90?si=sQJYOJZ07x1BXq6V

https://youtu.be/rZ5OMt3NzbM?si=C73hFJPbrazeMsE-

[LaPerouseBay]

Here's one with the Instinct on my wrist. 

I had turned off the GPS on the Gopro8 to save battery.
I like to record full runs at 4k/30fps, the 8 could only do 43 minutes.

The Gopro 11 battery records 1:15. at 10Hz.

GPS is tough to add to anything.  Gopro eliminated it entirely in the 12. 
All the pro's are probably using separate sensors now.

The dropped 1Hz data points are easy to see on my simple graph line. 
Pretty amazing what those little watches can do. 

https://youtu.be/dRruxRkw5Y4?si=4nEs-MzuvJTIaJaV