Perfectflite Stratologger Oddity

[Buckaroo]

Been flying Perfectflite MAWDs for several years. Recently upgraded one of my rockets with a new Stratologger. Flown for the first time on Sat, in a dual deploy setup, using the factory default settings. Nominal flight, with all events as expected. When I downloaded the data I noticed something odd. The ground elevation was reported as 64,000 feet (actually more than 64K, I just can't find the slip of paper I wrote it down on). Anyone seen this before? Any guesses on what the cause is? I am going to send an email to Perfectflite but I thought I'd see if folks here had any ideas. Thanks

 

[mikec]

The ground elevation was reported as 64,000 feet

Could you have been below sea level, which is possible at a low site with high atmospheric pressure?

Numbers around 65535 are likely small negative 16-bit integers.

 

[MWC]

Do you know the actual elevation of your launch site?

My guess is the ambient pressure conditions on your launch date at your actual launch site elevation equated to a below zero/sea-level reading... thus the negative elevation that resulted from this is being displayed as an unsigned word value (64000'ish) in the Capture program.

 

[Buckaroo]

I was flying with the SEARS club in Samson, AL which is about 200' above sea level. I'll have to dig a bit and see if I can find historical pressure data for Saturday.

 

[cerving]

Was it unusally cold or humid? Baro sensors are affected by that. There is a compensation for temperature in the sensor calculations, but humidity is generally not taken into account. Generally, humidity is relatively constant as long as you're not punching above the cloud cover. If you are, you need to read the GPS threads...

You didn't say what your reported apogee was. If it appears to be low by (65536 - your reported ground ASL), it's probably a signed integer underflow.

I was flying with the SEARS club in Samson, AL which is about 200' above sea level. I'll have to dig a bit and see if I can find historical pressure data for Saturday.

 

[duckhead]

Since this was your first flight and it stores 31 previous flights, could it be that you were inadvertently looking at one of the bench test 'flights' made by PerfectFlite before shipping it?

Doug

 

[Buckaroo]

Just to correct my original post, the ground elevation was reported as 65, 467 feet. Apogee altitude was reported as 2016 feet which is consistent with the sims and actual flight profile. It was not cold, or especially humid (although I spent the last 2 years in the Mojave Desert in CA so every day feels humid to me now). By the time I flew this flight it was pretty warm (80ish) and the rocket had been sitting out in the sun. The reported temp at liftoff was 108F which seems reasonable for inside the av bay. I did double check to make sure I had the right file. Finally I went back and looked at the data from my other two dual deploy flights (both using MAWDs). First flight reported a ground altitude of 20 feet. Second flight reported a ground altitude of -69 feet. The Stratologger flight was number 5 on the day, about 2 hours later. So all in all it looks like the unit was probably reading below sea level and just isn't able to process/display that value. Interesting stuff.

 

[BEC]

I had this problem with a Pnut in July of 2011. The solution was to download the current version of the interface software from the PerfectFlite site. Fixed.

Here is an excerpt from an email from PerfectFlite product support I received on 7/11/11:

If your actual field elevation is 190 feet above sea level, and the sea level barometric pressure on the day of your launch is 30.25"Hg (0.33" higher than standard), then the Ground reading would report approximately (190' - 330') or -140 feet. If you know what the actual field elevation is, you can also work backwards to determine the approximate sea level barometric pressure for the day of your launch to help you track weather conditions.

Unfortunately the formatting of that parameter on the display screen in your version of the download software didn't handle negatives properly -- the current version available on the website does. If you subtract 65,536 from the numbers you were getting (e.g. 65,400 - 65,536 = -136 feet) you will get the correct numbers. Or better yet download the current version of the software and it will display correctly...

 

[Zeus-cat]

There is a lot of variation in the ground altitude on my Stratologger. All 5 of these flights took place at the same place; yet they differ by nearly 500 feet. They took place roughly one month apart starting in December 2011 and running through May, 2012. The temperature and barometric pressure probably varied a lot too, but I don't have data on that. I think my field is about 800 feet, so if you were at 200 feet you could easily have had a ground altitude less than zero which appears to have been reported as a very high altitude.

551
994
794
1,034
894

 

[John Beans]

Yep, normal weather swings have magnitudes of hundreds of feet of pressure altitude. You can find graphs of pressure histories recorded from weather stations, then convert these to altitudes using calculators like https://www.srh.noaa.gov/epz/?n=wxcalc_pressurealtitude. For instance, Plano, Texas extremes in average pressure during 2012 have been from 29.77 to 30.16 inches of mercury. That translates to an apparent pressure altitude swing from 140 to -220 feet, or 360 feet total.
Tidbit of info.

 

[cerving]

But the ambient pressure doesn't tend to change very quickly, so as long as your rocket isn't sitting on the pad for a long time the reported ground ASL should be about the same. Even so, the altimeter should average real-time samples into the ground ASL reading every few seconds if it hasn't detected a flight in progress to compensate for this. I know that I have had flights sit on the pad for over 30 minutes, and I was glad that I put this into my software...

Yep, normal weather swings have magnitudes of hundreds of feet of pressure altitude. You can find graphs of pressure histories recorded from weather stations, then convert these to altitudes using calculators like https://www.srh.noaa.gov/epz/?n=wxcalc_pressurealtitude. For instance, Plano, Texas extremes in average pressure during 2012 have been from 29.77 to 30.16 inches of mercury. That translates to an apparent pressure altitude swing from 140 to -220 feet, or 360 feet total.
Tidbit of info.

 

[John Beans]

Cerving,
Yep, I think most altimeters update their "ground pressure" as they sit there, and you're right: it's not likely to be significant over a few minutes' time.