Posted: Thu Jul 24, 2008 12:02 pm Post subject: Accelerometers/Altimeters Part II
Quote:
The problem with computing altitude from acceleration is that it requires two integrations and as a result the error growth is proportional to the square of time. Small errors in the measurement grow into huge errors in the result.
-UhClem
Im sitting at work bored and was wondering how you do calculate altitude from acceleration? I suppose you integrate accel to get velocity then integrate velocity to get position?
My other question. Why do some altimeters then contain an accelerometer? Only for logging accel and velocity?
Can acceleration not be calculated from change in pressure with respect to time in a baro altimeter some way?
The problem with computing altitude from acceleration is that it requires two integrations and as a result the error growth is proportional to the square of time. Small errors in the measurement grow into huge errors in the result.
-UhClem
Im sitting at work bored and was wondering how you do calculate altitude from acceleration? I suppose you integrate accel to get velocity then integrate velocity to get position?
Of course.
This reminds me of when I was taking Engineering Dynamics. We were told not to memorize things like 1/2at2 but to derive them. A couple of integrations is all that is required.
Quote:
My other question. Why do some altimeters then contain an accelerometer? Only for logging accel and velocity?
Accelerometers have some advantages. It is easier and faster to detect launch using an accelerometer. Barometric detection must be done carefully to prevent minor pressure anomalies or noise from causing a false launch detect.
Mach transition also causes trouble for barometric based apogee detection. While it can be worked around, having an accelerometer handy simplifies life.
Staging pretty much requires some way of measuring acceleration. Even if it is a simple G-switch. Some altimeters have outputs to control staging and they use the acceleration measurement to control it.
Quote:
Can acceleration not be calculated from change in pressure with respect to time in a baro altimeter some way?
Certainly. But there are problems since it requires numerical differentiation. While integration tends to smooth out noise, differentiation does the opposite. Unless very powerful filtering is used, the result is usually more noise than signal. Even then the results are not nearly as good as an actual measurement of acceleration. The best filtering methods are "non-causal". Which simply means that they need data from the future as well as the past. This is fine for post flight analysis of recorded data but considerably less useful during flight.
Many recording barometric altimeters have sample rates so low as to be nearly useless for this type of post flight analysis. _________________ David Schultz
http://home.earthlink.net/~david.schultz
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