2013 US Performance Cup Competition

[lurch 0]

Ow, tough crowd.
In my own defense though Robi, I get a lot of feedback from people who -want- descriptions in that level of detail. And there's just so much to describe.
But, duly noted. I'll work on it.
Hows this post?
-B

Live and learn... or die, and teach by example.

[lurch 0]

That's quite alright, Murf. Although Robi is inarguably correct, I am too wordy, if a page and a half, maybe two is too taxing for you, you're quite free to spend 2 years and about $20,000 in gear, travel, training, effort and experience to work all that out for yourself. I'm sure it will take less time than reading a couple of pages of -my- nonsense.
Have fun!
-B

Live and learn... or die, and teach by example.

[robibird 2]

Lurch,
Much better ! You see you can be shorter..
However there is lesson No2..
Your or others personal greatness has to be judged by others ( readers in this case ) not by yourself !! OK?!
This way you have to read message Murf62 wrote..
In other words, making the ''novel'' every time you enter in to DZ.com world won't help people to get better as rarely anyone reading it
...I am writing this w/o cherry and creme..

Robert Pecnik
[email protected]
www.phoenix-fly.com

[lurch 0]

Well Robi, I got one private "thank you" and two public smackings for that one post so I cut that mess in half for benefit and sanity of future victims- er, readers.

Still too long, really, but better.
-B

Live and learn... or die, and teach by example.

[crwper 7]

It does look like your first jump exceeded the model's acceleration limits:

Acceleration: http://imgur.com/KQFoGMZ

Interestingly, this seems to have had an effect on speed accuracy, but not position accuracy:

Speed accuracy: http://imgur.com/pZvZ9bw
Position accuracy: http://imgur.com/3Z0HSD0

Paralog bases its calculations on position data only, but I think I'll probably bump up the acceleration limit on the competition units to see if the velocity error is improved.

I've been giving some more thought to the article I posted, as well. The main concern for paragliders is to stay within certain flight levels, which are defined by barometric altitude. This doesn't apply to us, but I think a different consideration may.

FlySight logs raw velocity, but when it calculates tones, it adjusts velocity to account for the reduced air pressure at altitude. I'm not sure if Paralog currently accounts for these differences when comparing jumps at different locations, but it occurs to me that altitude could have a significant impact on the time and speed rounds. If we wanted to correct for this effect, it would be best to use barometric altitude/pressure rather than GPS altitude.

That said, your comments have me thinking that the error introduced by trying to measure pressure in a dynamic situation may be greater than the error introduced by using GPS altitude rather than barometric when adjusting velocities.

Michael

[lurch 0]

Ok perhaps this information could be of use to you then because I still haven't figured this out.

There is a tremendous amount of slop between alti-track and GPS. What I don't get is why the slop works as it does. My standard has been "reads a climb" at will as an arbitrary repeatable performance goal. So long as my speed is high enough in one direction or another, it works every time. But no two are alike.

Sometimes I get a clean sharp climb on GPS set to 2G, but nothing on the alti, planeouts to single digits. Sometimes I get both- On playback the altimeter rewinds dramatically, shows climbrates as high as -29 mph, regain of about 70-150 feet and the GPS shows a track that agrees with it give or take about 50 feet, maybe 100. (alti: says +120, GPS says +50, very small hump on the graph.) Sometimes I get a clean rewind on the altimeter but not on the GPS which may show a flatline 0mph planeout but no real climb.

Where is the majority of the error, here? GPS? Alti? I'd think, if anything, air pressure transient effects should damp out, muffle, true climb readings, increased pressure under wing surface. About the only significant variables I can think of are wrist angle and overall suit angle of attack. Aside from that, both devices experience the same conditions.
What else influences this? GPS isn't seeing it, erratic data rate? Number of satellites? How long it was on, on the ground? How can I tighten this up?
-B

Live and learn... or die, and teach by example.

[kallend 1,623]

Stop weaseling.

What you ACTUALLY wrote was:
"Meanwhile the lousy wingsuit pilot will be getting 9:1 and the good one light skinny guy in a way big suit 15:1"

...

The only sure way to survive a canopy collision is not to have one.

[mccordia 73]

Quote

Stop weaseling.

So you're saying being light/skinny and having a large wingsuit are of no influence?
Funny how everyone else seems to grasp the concept of humor and whats actually talked about. Yet you state incorrect things (regarding weight being no influence on glide, in real world situations) and just ignore it, trying to distract by putting your personal dislike center stage.

Again..try and understand what's written. And also try and see how 15:1 glide ratios might be a 'slight' exaggeration on the actual real world phenomenon a light frame and big wingsuit, together or as seperate factors being of WAY bigger influence on your results in glide/hangtime than skills.

Again, not saying skills dont come into it. But with just low weight, and a big suit you can already get there 3/4th of the way when it comes to good hangtime results.

The only weaseling on display is your own inability to look past your personal dislike for a person, and try and actually mingle in the actual discussion at hand.

JC
FlyLikeBrick
I'm an Athlete?

[crwper 7]

lurch

What else influences this? GPS isn't seeing it, erratic data rate? Number of satellites? How long it was on, on the ground? How can I tighten this up?

The two GPS "warm-ups" allow the FlySight to download orbital data from the GPS satellites. Each warm-up addresses a different kind of orbital data:

Almanac: Takes 12.5 minutes to download. Tells the FlySight roughly where all the satellites will be in the sky and gives basic ionospheric correction information. This allows the GPS receiver to get a fix faster and also helps improve the accuracy of the measurements. Almanac data is valid for about 6 months, but I recommend doing the 15-minute warm-up at the start of each weekend as a matter of habit.

Ephemeris: Takes 30 seconds to download. Tells the FlySight much more precisely where visible satellites will be. This means that when you get out of the plane, the FlySight knows precisely where it's expecting the satellites to be, and it can quickly separate their signals from the background noise. Bear in mind that GPS signals are well below the level of thermal noise, so this kind of information is critical in getting a quick fix. Ephemeris data is valid for about 2 hours, which is why it's important to do the 1-minute warm-up every time while waiting for the plane.

Without these two pieces of information, what you'll usually see is that the FlySight won't get a fix until you're under canopy (a much less dynamic situation), or it will get a fix in freefall, but only using a handful of satellites. It's important that the two warm-ups be completed, but leaving the FlySight on for longer than the recommended time won't usually improve things--either the FlySight has the most recent almanac and ephemeris, or it doesn't.

There are two things I look for to determine if the FlySight's data is solid: number of satellites used in the fix and the error estimates (horizontal position, vertical position and speed). Usually, in the skydiving environment, the number of satellites is not a limiting factor. If the warm-up has been completed properly, you should see 8-9 satellites once you're in freefall--more than enough to compute a good fix. Things become more complex on BASE jumps, where half the satellites might suddenly disappear behind the mountain on exit.

The error estimates use "circular error probable". Of course, the FlySight doesn't know where it actually is, so it estimates error based on the spread of results it gets from different satellites. Usually, this gives a pretty good estimate of the position error, but it's important to remember that there are some cases--e.g., if the signal is bouncing off a valley wall--where the estimated error can be strangely low. In the skydiving environment we usually have near-optimal conditions, so the error estimates are generally good.

Let's take the jump data I posted earlier as an example. It's not in the plots, but the number of satellites in freefall sits around 8-9, so you're in good shape there. Looking at the errors, you can see that things improve rapidly once you leave the plane. It takes about 30 seconds for things to really settle out, but the errors are mostly decreasing during this time, which leads me to believe the GPS is just building confidence in data which is generally good.

The exception on those plots is the sharp spike in velocity error where you plane out. On paper, it looks like it goes from about 0.3 m/s up to about 1.6 m/s, but I wouldn't treat these figures as absolutes. The spike is only about 3 seconds wide, so it's a very dynamic event. I would take any velocities reported during that time with a grain of salt.

However, if we used the "2 g" or "4 g" settings instead, we might find that this peak is significantly reduced. In that case, I would be more inclined to say that the GPS recevier's internal model has a pretty good handle on things, and the data is likely accurate within the stated errors.

Assuming that the number of satellites, position error and velocity error all look good, I would trust the GPS data over the barometric data.

I'd be very interested to see the raw pressure readings which the Alti-Track is using to make its measurement. When I was developing FlySight, I experimented with pressure data, but found that it was incredibly noisy. I'm curious to know what the altimeter sees, for example, when you suddenly plane out. It could be that a subtle change in your wrist position puts the Alti-Track in a low-pressure region, and that smoothing filters turn that into a slow climb.

Ultimately, I think what gives me the most confidence in the GPS data is that it reports several values which help us determine its reliability. Without access to the raw pressure data from the Alti-Track, it's hard to say whether or not we should trust its results except in a very stable situation. Maybe it's time to start experimenting with pressure sensors again. :-)

Michael