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
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