Protrack 2 Logs

[ThrustVectored 4]

Hello there,

I have been waiting patiently for the last 3 days waiting to finally write this post.. I guess Dropzone.com had some terrible experience with spammers, however I used this time to go through the question I am about to write.

I think I should start with a few details upfront.


I am a 'new' Skydiver, I received my licence at the end of 2016 and finally broke the 100 jumps in august 2017.

I bought myself a L&B Protrack 2 as a ditter of my choice for freefly as it is a requirement here in Germany to start training, according to the national skydivers manual, since body positions will make it hard to constantly look at my altimeter, however I still peek a few times on it just to make sure I have my proper height awareness.

The skydiving season ended with the last weekend in october and with 291 jumps completed so far, I thought it was a great time to take a look at my logs, stored on my ProTrack 2.

I hooked it up to my computer via USB cable and looked what was on the device. Here is a little shout out to L&B for making this quite reasonable since every jump is stored in a seperate .txt file. Brilliant move :)

There is also a Help.txt file that explains the stored data within a .txt file.

So this is what happens:

The ProTrack 2 measures the air pressure on the ground and sets the measured value as 0m. Once I exit the aircraft the device starts recording the pressure values up to 3600 points, which equals 15 minutes, this means it saves 4 samples per second during freefall.

( 15 min= 60s * 15 = 900 | 3600 / 900 = 4 )

According to the Help file, all pressure values are saved as Deca-Pascal (dPa, which could be a typo, since dPa is Deci-Pascal while Deca-Pascal is shortend as daPa. There are several typos, but this one is a crucial one, however I am not judging, I probably made a few typos in this post :P )

So instead of having 1013.25 hPa I have 10132.5 d(a)Pa, one digit moved to the left.

The ProTrack also records the temperature on ground level in °Celsius, in my example it is 268 (???), which is also odd, I believe it means 26.8°C which is more likely.


For my specific jump, I have 864 pressure samples going up from 6166 d(a)Pa to 10064 d(a)Pa, which was my groundlevel, 0 metre.
864 samples equal 3,6 mi (216s), from which the first 55 seconds where freefall. Within my logged data there is also my exit altitude, 3910m and the deployment altitude of 840m, I will come back to those values later.

Out of curiousity I punched the 864 samples in an excel file to figure out my graph. It was quite easy and the visualisation made sense.

However, the next step for me was to understand and convert the pressure into height information. For this I used the barometric formula.

On the german Wikipedia page I found it already solving for height and so I put in the numbers ( see the attached image "formula.png")
I changed up the temperature in the picture from 288.15 K to 299.95K (26.8°C) , ph to the measured 864 samples and 1013.25hPa to 10064 d(a)Pa.

0,0065K/m is the temperature gradient to up to 11km , so for every meter the temperature cools down 0,0065 K. 5.255 seems like a magic number, but in the wikipedia article it is explained that for the stated temperature gradient the exponent Mg/Ra usually converts to this number.

And this is where my actual question begins:

I let excel convert my pressure values into height data which works well ( see attached image "height_graph.png).

However, it says that my jump started at the height of 4107m which is almost off by 200m while my ProTrack 2 says I jumped out at 3910m. I tried to compensate this by adding a -200 in my formula in excel, however this does not check out.

Why is it like this? It certainly has nothing to do with TAS (True Air Speed) or SAS (Skydiver Air Speed) as those are meant for interpretation of the relative speed.

Iam sitting here, scratching my head, trying to come up with a logical answer to my question, maybe one of you has an explanation for this.
Digital pressure sensors are far too precise in comparision to the aneroid barometers.

Thanks for reading so far, this has turned into a rather long post :)

Blue Skies!

Faras

[sundevil777 93]

You are I think assuming a lapse rate which is constant. Perhaps, or even maybe likely, the protrack is using a lapse rate which isn't quite linear.

If you have the software from L&B, you don't have to use excel to crunch the numbers.

People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am

[ThrustVectored 4]

L&B recommends using Paralog in order to evaluate the logged data. They used to have their own software called Jumptrack, however they stopped the development.

My point is, I would rather figure this out instead of buying an already available solution. The graphical display is a nice feature, but what counts for me is how this is calculated :)

By looking at my graph from my first post I can see why the ProTrack 2 says that I deployed at 840m. This is where the graph is drastically changing, which makes sense as this is not an immediate stop but rather a fast deacceleration