Author Topic: Altitude, GPS vs Altimeter  (Read 2985 times)

Offline dbmccann

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Altitude, GPS vs Altimeter
« on: May 15, 2016, 02:21:32 PM »
I did a little playing with some KML files.   Interesting results.   The first number is what appears to be the altitude in meters (MSL)  from the GPS.   I converted it to feet, and subtracted 884-  the ground level at Potter, to get the AGL altitude.     The number after the AGL altitude is what my altimeter reported for AGL altitude.

Wondering which is more accurate.   I lean toward the GPS,  the data is fairly smooth going up. 
 

QnD I200ws

01162      2928 AGL  (2963 RRC2)

DarkStar K805G

02434      7101  AGL  (6850′ soaked RRC2)

Darkstar K1100T

02336      6780 AGL  (6947 MARSA)

screech J1026

02980      8892 AGL  (8578 MARSA)

Screech J510W

02977     8883 AGL  (8583 RRC3)

Screech K1127 A

03010        8991 AGL  (8723 RRC3)

Screech K1127 B

02970   8860 AGL  (8639 MARSA)

Villain J820LW A

02182       6274 AGL   (6198′ RRC3)

Villain J500 PJ

02046       5828 AGL   (6099′ MARSA)

Villain J820LW B

01995    5661 AGL (6033′ MARSA)

Offline dbmccann

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Re: Altitude, GPS vs Altimeter
« Reply #1 on: May 16, 2016, 04:06:07 PM »
For giggles- all the flights in question


Offline RKeller

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Re: Altitude, GPS vs Altimeter
« Reply #2 on: May 18, 2016, 02:59:23 PM »
This is awesome Dave! One downside to the eggfinder is it does not record data on the rocket. 1 second transfer rate never seems to be that accurate.
Riley Keller
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L1 25 Oct 2014 Estes MDRM CTI H133
L2 25 June 2015 Rocketry Warehouse DYOK, CTI L1030RL 14.7k ft mach 1.57

I'm addicted to building stuff, rockets are just a way to get high

Offline dbmccann

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Re: Altitude, GPS vs Altimeter
« Reply #3 on: May 18, 2016, 10:51:45 PM »
This is awesome Dave! One downside to the eggfinder is it does not record data on the rocket. 1 second transfer rate never seems to be that accurate.

Should be able to pull something of of the kml.   I just popped it open with textedit and went to work.  SO MANY LINES.   It's also how I deleted all the false points.

Offline Bob Krech

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Re: Altitude, GPS vs Altimeter
« Reply #4 on: May 19, 2016, 09:30:23 PM »
In theory a good barometric altimeter is more accurate below 30 kft than GPS which is why TRA has their altitude records set up that way.

GPS is very good a lat-lon (x-y) locating because there is a high probability that several satellites are located within 20 degrees of the horizon spread around 360 degrees.  To get accurate altitude (z) value there should be at least 1 satellite overhead, but this is not always the case so the z value can have an error that is 10x higher than the x-y values.

A good barometer has a digital barometer chips with 16 to 24 bit internal ADCs, however older altimeter use analog barometer chips with a separate 8 to 12 bit ADC.   The digital chip have 4"-6" resolution while analog altimeters have typically 1' to 10' resolution.  Noise typically reduces the resolution unless many readings are averaged together.  This is always done in the digital barometer and is not done with analog barometers so the accuracy of the digital units is far better than the analog units.

The errors due to non-standard temperature conditions are not compensated for in any barometric altimeters unless post flight processing is perform so while the relative values are very accurate and that's why you can use altimeter for highest altitude contests, the absolute accuracy is less accurate whenever the temperature differs from 59F, but currently no one does the post flight corrections.

If you measured the launch site temperature and used the standard temperature offset to correct the for the density difference versus temperature, you should be able to get better agreement.

As you are using the RRC2, RRC3 and MARSA, you should be able to use the manuals to figure how much resolution the altimeter have and by looking at the noise in the altimeter, figure out what accuracy you can expect.  If you can find out what the ground temperature was for the time of launch, you can correct the reported altitude for the temperature difference.

Bob

Offline dbmccann

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Re: Altitude, GPS vs Altimeter
« Reply #5 on: May 19, 2016, 11:41:25 PM »
Interesting info Bob, thanks.  I know some Handheld GPS units can ID sat info,  would likely be too cumbersome currently for our units to record that kind of thing. 

It's likely a situation of "close enough"   But it's interesting to know just where the inaccuracies are, and why.

Offline Bob Krech

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Re: Altitude, GPS vs Altimeter
« Reply #6 on: May 20, 2016, 12:04:07 AM »
Some of the GPS/inertial program in you phone will display the number of satellites used for the fix, their positions overhead and the standard deviations of the lat/lon position and the altitude.  You will observe the larger uncertainty in the altitude versus the lat/lon position unless there is at least one satellite overhead.

Bob