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FlipperBizkut

Speed Accuracy

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Can anyone comment on the accuracy of the speed measurements provided by the Alti-2 Atlas altimeter?

I know that for speed skydiving, a couple of ProTracks are used, but I don't have even one ProTrack to compare results with. Also, my Atlas is hand mounted instead of being mounted on my rig's laterals, but my body position doesn't change throughout the fast part of the descent, so I wonder what effect the different placement might have.

Thanks for any insight!
Blues

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Hi!

Barometric speed measurement devices (SMDs) are very inaccurate in general. The current best practice is the use of GPS SMDs. There are formal proposals now to replace the ProTrack with FlySight for formal competitions.

Most barometric SMDs use a Bosch sensor and similar firmware. Whether you use a ProTrack, a ColoAlti/SonoAlti, or an Ares II you're getting the same readings, the same data across all of them. I expect the Alti-2 Atlas to have the same sensor.

We conducted tests (25 jumps) comparing Ares II, ProTrack, ColorAlti, and FlySight, all worn by the pilot during the same speed skydiving jump. The barometric altimeters were all sensitive to wobbles and tumbles, erroneously reporting altitude spikes.

So - if you're looking for a good altimeter -- Alti-2 is the way to go. It'll be as accurate as anything else on the market at a similar or higher price point.

If you're looking for accurate speed measurements you'll be better off with a FlySight.

Cheers!

pr3d
Eugenio, home: Bay Area Skydiving, CA USA

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Thanks for the info! I went out last weekend with the intention of going as fast as I could, and I got a reading of 272mph. My friend that I was with said that he was in a full stand and said that I was gone. Said I was super fast, and that he couldn't keep up with me. I felt my helmet squishing against my head, which I thought was novel, and otherwise, just basically knew I was hauling ass. Whether it was 272mph fast or not, well, that's what I'm interested in finding out.

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Good job!

The fast change in air pressure causes the barometric inaccuracies. We’ve seen readings of 300+ mi/h on a barometric SMD that result in 230-250 mi/h on GPS. The higher the change in pressure, the greater the inaccuracy.

Try borrowing a FlySight from a friend. Ensure it’s active, then put it anywhere in your body (back of the helmet and gaffer tape works well — we used this for tracking competitions with Bionic Avionics’ blessing) and measure again.

Compare the results vs barometric, let us know.

For speed competitions, by the way, current rules state that you measure the mean vertical speed within a window. V = 0.5*(Vend+Vstart) for current purposes.

Cheers and let us know!
Eugenio, home: Bay Area Skydiving, CA USA

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