it’s a rule, use the higher number. From: David Abmayr Sent: Tuesday, July 14, 2015 8:54 AM To: Altus Metrum Subject: Re: [altusmetrum] TeleMega barometric altitude vs GPS Height
Thanks. The TeleMega reacquired shortly after boost and had 10 satellites at apogee. The GPS altitude trace looks good. The reason I ask it that the barometric altitude reads 9700 ft, but the GPS reported 10,240 ft. If that is accurate, it's my first flight above 10k (and a successful L3 to boot). I just want to know I have solid data to call it a 10k flight. Most of my flights have been with the TeleMetrum 1.2, and the GPS clearly doesn't get a good fix until well after apogee. My TeleMetrum 2.0 and TeleMega seem to work much better, with good fixes at apogee. David On Mon, Jul 13, 2015 at 11:33 PM, Keith Packard <[email protected]> wrote: Casey Barker <[email protected]> writes: > On the ascent, the barometric curve will be smooth and reasonably accurate, > save for the "Mach bumps" around the transsonic region, if you're going > that fast. The GPS is likely to lose at least a few satellites in its fix > at high speed, making GPS altitude inaccurate (or unreported) for most of > the ascent. If the static ports are in clean air, then this is true. If the static ports are too close to a transition, then all measurements above about .8 mach are likely invalid GPS also will probably lose lock under high acceleration (about 4g). Most of the time, it re-acquires within a few seconds of motor burn-out and happily reports accurate altitudes from there through apogee. Sometimes, it fails to re-acquire lock, and sometimes it reports having lock but continues to report invalid altitude data. > However, once it slows down, and assuming the GPS maintains or regains > lock, it will give you a more accurate absolute height at apogee. > Barometers can only infer altitude from a standard model of the atmosphere, > so they're subject to pressure deviations, wind, etc. A GPS fix with at > least 5 or so satellites will be quite accurate. Yup, within about 10m or so, independent of altitude, assuming it's working correctly. It's easy to tell -- if it looks like a rocket flight, it's working; if it's still reporting the pad altitude, it's not. -- -keith _______________________________________________ altusmetrum mailing list [email protected] http://lists.gag.com/mailman/listinfo/altusmetrum -------------------------------------------------------------------------------- _______________________________________________ altusmetrum mailing list [email protected] http://lists.gag.com/mailman/listinfo/altusmetrum
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