Yes, it would be an interesting contest where we might all learn something from others attempts - via the web postings and/or forums. A limited buy-in keeps it in the interest of all, and if CTI were interested in supporting this contest then we would be sure to get some national attention.

Basic SSS rules would still apply – buy in prior to flight, flight must be declared & witnessed, TRA or NAR approved contest altimeter, and only one attempt per motor class.

Perhaps NCR could "Go National" with our Single Shot Sweepstakes contest. This is an item we can bring up at the club’s annual meeting in January.

TRA or NAR approved contest altimeter

This could take along time...

Tony

What about GPS altitude for the higher ones?

Word is TRA records committee will not be accepting any altitude records over 25K without GPS including the full raw stream data capture. Baro or Accel derieved altitudes will not be accepted.

Warren

Word is TRA records committee will not be accepting any altitude records over 25K without GPS including the full raw stream data capture. Baro or Accel derieved altitudes will not be accepted.

Warren

That's true now (though I think the cutoff is 30 kft), but probably not for long. The committee member who was advocating that position has left the committee, and the remainder of the committee is less enamored with GPS because of the unreliability of acquiring/reacquiring GPS lockup before apogee. An accurate pressure reading plus NOAA balloon sounding data can combine for pretty good accuracy, even at 50 kft+.

I agree with you Adrian. I'm waiting for them to come up with certification standards for altimeters - when they do, I'm putting money down that yours returns more accurate altitudes.

Warren

Word is TRA records committee will not be accepting any altitude records over 25K without GPS including the full raw stream data capture. Baro or Accel derieved altitudes will not be accepted.

Warren

That's true now (though I think the cutoff is 30 kft), but probably not for long. The committee member who was advocating that position has left the committee, and the remainder of the committee is less enamored with GPS because of the unreliability of acquiring/reacquiring GPS lockup before apogee. An accurate pressure reading plus NOAA balloon sounding data can combine for pretty good accuracy, even at 50 kft+.

You still have this messy business with cross-referencing with NOAA balloon data. It's not like there are balloons floating around at every lat/lon at every altitude on every day. It becomes a hugely subjective exercise, which is ironic insomuch as the goal is to remove subjectivity.

On top of that, overlay the fact that altimeters can (and do) go out of tune, and it makes GPS look a lot more accurate. Anyone who has ever flown two of the same altimeter knows they may -- or may not -- come in close to one another.

I don't know how you solve this riddle, but "correcting" the data with balloon readings seems to have some inherent challenges.

Bottom line, if we are flying to those altitudes, we are more than likely carrying GPS these days - and that would seem to me to be the most straightforward way to go about discerning an altitude.

Agreed - on my 17k flight for example, my two baro altimeters (not the same model though) were 1200 feet off of each other (one read 17,002, one read 15,800). The GPS on board (Garmin Astro) showed 17250 feet as the peak, so that's what I'm considering to be the most accurate of the 3 (especially since it fairly closely agrees with one of the two altimeters).

We could go on for a while on this topic, but it should probably be moved to its own thread.

In the meantime, my position is that it's possible to use a test chamber to evaluate how accurately an altimeter measures pressure, and Tripoli should do that as part of a certification process. There are altimeter models that could meet a 1% scale factor requirement over a reasonable temperature range, and more models that would meet a 2% requirement. Altimeters that are worse than that shouldn't be accepted for records, in my opinion.

Weather conditions can add a lot more error than that (~10%+) when you convert pressure to altitude using the standard atmosphere (which is what all altimeters do for their beeped-out altitudes). Balloons really are floated twice per day at around 100 locations in North America. The closest one to NCR's sites and Hartsel is in Denver. The closest one to Black Rock is in Reno. I should do some research into how much error you get in pressure vs. altitude with that sort of distance. With two mouse clicks, anyone has access to the data from these, and the first 2 columns directly provide pressure vs. altitude.
http://weather.uwyo.edu/upperair/sounding.html
Just download that morning's table, look up the minimum pressure recorded by the altimeter and read off how high that was. It's really as simple as that. Tripoli takes weeks to months to approve records, and this step adds 5 minutes to cut the error rate in half or more. If you combine a 1% pressure accuracy requirement, and balloon sounding data, you'll get a final answer that's pretty accurate. I'd guess 2-3% total error, but I would need to back that up with the look into the variability of balloon sounding data with location.

Is GPS better than that? Sure, if you have a GPS in lock at apogee. I don't have personal experience with that, but from what I've heard, that can be pretty iffy.

How can you reconcile that data from over 100 miles away is accurate? Last week the Denver metro area got snow and at my house in Fort Collins we didn't get any. Do you mean to tell me that I should take data and apply it to a point 100 miles away that could have completely different weather conditions and expect it to apply apples to apples?

On my last L3 flight I had a few instruments in the e-bay - two of the altimeters were within 50' of each other, and those were within 115' of the GPS.

Edward

I just took a quick look at today's balloon sounding data for Denver and Grand Junction, and the 5 points I looked throughout the air column at had between 0 and 1.4% difference.

Temperature, dew point, and wind direction vary more from station to station than pressure vs. altitude. More comparisons are needed, though, in order to get a better feel for typical errors introduced by this method.

Update: Reno and Elko have about 1% difference also.
Reno and Oakland have about 0.5% to 1% difference.

Next I'll take a look at Reno and Medford Oregon just before the front went through at BALLS this year. That should be kind of a worst case for before/after the storm front.

That one was mostly 1% different and all points less than 1.5% different. So far, so good.

OK, i get that the balloons are sampling pressure at various altitueds. How is the altitude of the balloon when the pressure is sampled determined?

OK, i get that the balloons are sampling pressure at various altitueds. How is the altitude of the balloon when the pressure is sampled determined?

GPS or radio location. The other NCR Ken works for the company that makes those now, and IIRC the pressure sensor spec accuracy was about 0.1%

OK, i get that the balloons are sampling pressure at various altitueds. How is the altitude of the balloon when the pressure is sampled determined?

GPS or radio location. The other NCR Ken works for the company that makes those now, and IIRC the pressure sensor spec accuracy was about 0.1%

Which brings us back to where we started. If we use GPS to discern balloon altitude which is, in turn, used to discern pressure (which may be from a balloon that is 100 miles away) and is assuredly higher (or mayber lower?) than our rocket, aren't we predicating everything on GPS to begin with?

Here is where the rubber meets the road for me. The GPS lock of a dog collar or other GPS transmitter is almost assuredly not happening at apogee. So... it would appear to me that whatever the GPS says is a best-case scenario. There is no judgment, there is no stray weather pattern causing barometric aberation, there is no front moving through that may cause a variance in our readings.... "it is what it is", to coin an overworked saying.

On my GPS, elevation is the last thing that is determined - lat and lon are first. Is it thattaway with all GPS units? I don't know, hence my question.

I'll start a new thread as we are way off topic. We are also spending more time flying altimeter against altimeter instead of rocket against rocket.