- This topic has 20 replies, 8 voices, and was last updated 17 years, 3 months ago by
Jeremiah Johnson.
Jeremiah Johnson.-
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Adrian
I’ve missed a bunch of threads and pardon me if you’ve answered this already – but is the key element of difference in the temperature compenstion based on the US SAM -6.5c/KM assumed temperature difference in the first 10 Km ASL versus actual temperature.
So, is the Parrot’s post-processing recalculating the degrees/KM or is it just recalculating the base altitude?
If I understand your question correctly, the correction factor I’m talking about is caused by a constant temperature offset compared to the SAM assumptions, while still assuming the same temperature lapse rate of -6.5c/Km. So while the SAM assumes a temperature of 39F at 5400 feet and -6.5C/km as you go up from there, the proposed correction would start with a temperature equal to the ambient air temperature at 5400 feet, decreasing at -6.5 C/km from there.
So would the temperature explain the difference in the data recorded from my most recent flight? There is a 1500 foot difference between the accelerometer and the baro reading. I thought that it was caused from the baro not being calibrated to 4000 ft but now I realize it could have been the temperature. It was close to 90 degrees outside and inside of the foil lined av-bay probably much higher. However is this enough to cause almost a 20% decrease from the acc reading? How can I determine the max altitude achieved?
The difference is probably caused by a combination of the temperature effect I’m bringing up, and non-vertical flight caused by weathercocking, and possible calibration errors.
The black rock desert is at 3848 feet, according to Wikipedia, so the standard atmosphere temperature is 45 degrees F. The ambient air temperature at the launch site is 90 F (the internal av-bay temperature doesn’t matter if your altimeter reads pressure correctly at high temperature), so the delta from the standard atmosphere temperature is 45F. I’ll calculate that out a little later, but it should be somewhere around 7-8%.
That leaves about 12-13% that could be caused by a combination of calibration errors and weathercocking.
Jeffrey Joe Hinton
I love it when it gets to the gibberish level of geek. I fly, therefore, it is what the altimeter tells me.
mule
I love it when it gets to the gibberish level of geek. I fly, therefore, it is what the altimeter tells me.
You and me both!!! I’m a Geologist. Talk rocks and oil and I can keep up.
Adrian
When life hands you lemons, make lemonade. And when your wife hands you a sandwich baggie full of mint harvested from an overgrown AeroGarden, made mojitos. But don’t expect to be able to write clearly on a techical topic afterwards. 8)
mule
When life hands you lemons, make lemonade. And when your wife hands you a sandwich baggie full of mint harvested from an overgrown AeroGarden, made mojitos. But don’t expect to be able to write clearly on a techical topic afterwards. 8)
Hold on there Adrian. You sure that wasn’t something else that was harvested and tossed over the fence into your Aero Garden??? Did you have an intense feeling that you needed to watch a Cheech-n-Chong movie while munching on Pizza?????
So, I take it this has run it’s course now that Warren has wrangled us in??? Nothing left to do but remember the old days when we sat around the computer talking about altimeter temperature something or another 😆
Warren B. Musselman
I don’t know about “wrangling” anything. I fully support the idea of temperature compensation for altimeters but I and the other folks who have been involved in setting the altitude record rules have always been firm in the belief that it is the reported altitude by a completely stock, production altimeter that is the number we will accept. I, and others believe, that temperature compensation is something best done within the altimeter itself – not like there’s a dearth of the processing power necessary to perform the compensation. All commercial barometric altimeters go through a startup initialization to determine baro pressure at the pad (altitude 0) so it would seem a relatively small thing to add an appropriate temperature probe to measure ambient air temp within the e-bay that could be read at the same frequency as the baro sensor. Of course, this is a redesign issue for ALL altimeter manufacturers so I’m not holding my breath about it happening any time soon.
There are certainly things we haven’t thought of and are open to discussion on, however we will stick with the essence of Tripoli rules plus a few minor tweeks intended to either tweek things to NCR launch sites specifically or to make sure NCR members get recognized for their efforts where the Tripoli rules would not recognize them.
mule
This is a hobby with big toys….. There can’t be that much money made on making Altimeters, so I wouldn’t hold my breath either.
I wasn’t trying to be offensive, so I hope you didn’t take me that way….. You just happen to be the guy that knows the rules and reminds us free spirit types that we have to play by them….
I’m off like a herd or turtles asta
GAdrian
Hold on there Adrian. You sure that wasn’t something else that was harvested and tossed over the fence into your Aero Garden???
LOL. If they did, they broke into our kitchen. 🙂 The AeroGarden is a setup for your kitchen countertop. Kind of nice in the winter, but pretty redundant in the summer when sun lights up your garden for free.
All commercial barometric altimeters go through a startup initialization to determine baro pressure at the pad (altitude 0) so it would seem a relatively small thing to add an appropriate temperature probe to measure ambient air temp within the e-bay that could be read at the same frequency as the baro sensor.
You’re right that it would be relatively simple to sample the av-bay temperature, but that could give you worse results than assuming the standard atmosphere. The Av-bay temperature in Chris’s Cirrus Dart was 40F above ambient, and I think that’s probably pretty typical.
In order to measure the air temperature even close to accurately, you need at a minimum 1) shade and 2) free air flow. An av-bay will give you 1 but not 2, and a fin-mounted sensor will give you 2 but not 1. (and I’m not holding my breath for altitude junkies to mount temp sensors on their fins :)). To measure the air temperature accurately, you also need to avoid radiant sources like putting the sensor above a hot parking lot. Official air temperature sensors are supposed to be mounted 2 meters above a grassy surface.
Because of the limitations of the standard atmosphere model, I’m afraid that altitudes read directly from an altimeter are inherently incompatible with accurate altitude readings, execpt when the air column happens to match the SAM temperature assumption. If Tripoli and NCR decide to include the effect of this temperature error, there are a couple of options that I think could work, but both require post-flight calculations:
1) Put the ambient air temperature and launch pad altitude on the form, report the altitude using a standard atmosphere model (including direct reporting from altimeters that do that), and then use a look-up table to correct for true altitude
2) Make all record applicants provide data from the nearest balloon sounding on the day of the record, and then either calculate the true altitude from a reported pressure reading, or adjust a reported altitude based on the balloon reference.#2 is easier than it sounds, given the convenient data availability at the University of Wyoming site, and it would eliminate weird results from air inversions, etc.
If NCR and Tripoli decide to continue to base altitude records on assumptions of the standard atmosphere, then I’ll start looking to see who has the highest waivers in the middle of winter in the northern parts of the country. And don’t bother trying to set records in July after I get done. 😉
Think we can get a 25k waiver for the North site or Atlas in January? I think I’ll be ready to go for the J record by then.
Warren B. Musselman
Isn’t there a way to adjust the SAM for ambient temperature? I would be very surprised if there weren’t.
As for waiver, the Atlas site will never exceed 12K due to the proximity of Hwy 14 and Briggsdale. The North Site is generally closed to us from December through March, although we have had exceptions to that. I’ve personally been pushing Joe to ask for an increase in our standing waiver to 25K at the North Site, but we’re more focused right now on getting the 35K windows graven into stone so to speak. 2006 we were able to get 30K windows a couple times, 2007 we had none. This year we have had 35K waivers given preliminary approval, but we haven’t activated them yet. Once the precedent is set, it becomes easier the next time around. I’m hoping after successfully getting 35K windows for an unscheduled launch this month and for Oktoberfest, we’ll be able to ask for 35K windows at all North Site launches in next years waiver agreement.
Good luck with that J flight in cold, dense air and especially with the jetstream usually right over us during the Winter months. Best time to fly it would be this month.
As a result of this thread, I’ve started a correspondence with the TRA BOD on this – first replies are that this was looked at some time back and the technical issues and both technical and potential political complexities of post-processing altimeter data based on ambient temperature or balloon temperature profiles for altitude records was too much and they decided to just stick with the altitude reported by the altimeters.
Warren
PS: One final thing, HPR altimeters have never been considered to be accurate OR precise. Given the limits of 8 and 10 bit A/D converters used in most common altimeters, you’re always going to have significant slop on the reported altitude. For instance, a Missileworks RRC2X uses an 8 bit A/D converter to read the baro sensor. All A/D’s consider their result to be +/- LSB, which in the case of a 25K rated RRC2X is 97.65′ more or less, meaning that the altitude readout can be as much as 195.6′ off from what the baro sensor reports. The 45K version is proportionately worse. Of course, variations in baro sensors can account for much more than that given their tolerances. These are not precision instruments and with few exceptions, most altimeters are lucky to be within a few hundred feet of actual AGL altitude. If you really want an accurate reading, a radar target and arrangements for a military radar track is your best bet. An alternative is a standard aircraft radar transponder that the FAA can track. (I believe that is what the CSXT project did). That is unless someone in the club has a surplus military millimeter-band radar sitting in their garage. GPS has the potential to get you within 20 meters or so of true altitude, but beware of high acceleration flights as most GPS’s shut down at 600mph or 60K feet – unlimited ones (mil grade) are very expensive.
Adrian
Isn’t there a way to adjust the SAM for ambient temperature? I would be very surprised if there weren’t.
Yes, that’s what I was proposing in method #1 for the post-flight adjustment. Denverdoc found a great website for more info:
http://mtp.jpl.nasa.gov/notes/altitu…udeScales.html
I’m hoping after successfully getting 35K windows for an unscheduled launch this month and for Oktoberfest, we’ll be able to ask for 35K windows at all North Site launches in next years waiver agreement.
That would be sweet. I think I’ll shoot for Oktoberfest for my next 38m bird.
As a result of this thread, I’ve started a correspondence with the TRA BOD on this – first replies are that this was looked at some time back and the technical issues and both technical and potential political complexities of post-processing altimeter data based on ambient temperature or balloon temperature profiles for altitude records was too much and they decided to just stick with the altitude reported by the altimeters.
Thanks for following up with that. I believe that we could make a good case for a simple post-flight correction based on ambient temperature, which is already part of the form. I’ll work on making something simple and easy to use.
Adrian
Jeffrey Joe Hinton
mule
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