The problem with barometric sensing is that you either need to know pressure at mean sea level (MSL) to use your barometer to give you altitude or you need to know altitude to be able to read your local pressure (from the sensor) and calculate pressure at MSL. You can't get both unknown values for MSL and altitude simultaneously from a local pressure reading.liz wrote:It's actually pretty accurate in terms of barometric sensing; we did a lot of testing by embedding one in a foam football and dropping it off the roof of Pi Towers (seriously). I believe Marc has the resulting graphs somewhere!
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#!/usr/bin/python import math import sys import argparse if __name__ == '__main__': parser = argparse.ArgumentParser() parser.add_argument('-a', '--altitude') parser.add_argument('-p', '--pressure') parser.add_argument('-m', '--msl') args = parser.parse_args() if args.altitude is None: args.altitude = 112.2 # Value from my GPS for my house if args.pressure is None: print "Pressure value required" sys.exit(40) if args.msl is None: args.msl = 1013.25 args.pressure = float(args.pressure) args.altitude = float(args.altitude) args.msl = float(args.msl) print "Alt:", args.altitude, "Pressure:", args.pressure, "MSL:", args.msl print "Sealevel:",args.pressure/pow(1-(args.altitude/44330.0),5.255) print "Alt:",(44330.0*(1-pow(args.pressure/args.msl,1/5.255)))
GPS is bobbins viz accuracy wrt altitude - get your home altitude from Ordnance Survey data instead. The better cycling GPS devices include barometers for this reason.DougieLawson wrote:
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args.altitude = 112.2 # Value from my GPS for my house
But you have to re-calibrate them every time the weather changes, which could by every minute when something like Storm Desmond rolls through. GPS may be off by a few metres but it's consistently inaccurate (the Z is the least accurate value from my old domestic GPS). Barometric has variable inaccuracy - which is why planes use flight levels based on a standard setting of 1013.15hPa.gregeric wrote:The better cycling GPS devices include barometers for this reason.
So it's relying on a known datum because barometric is variably inaccurate.gregeric wrote:I don't know if the commercial GPS units do it, but the android app ipBike recalibrates the barometric altitude every time it sees a known waypoint eg your house as you set off.
Commercial GPS update rate will be too slow I think (about a second per measurement?). It also has issues with heights and speeds over a certain altitude (to stop it being used for rocket guidance). But perhaps worthwhile adding for completeness.jahboater wrote:The last GPS I had with a barometer would (by default) auto calibrate the barometric altitude from the GPS! It was sensitive to very small changes in height compared to the GPS though.
Modern 60+ channel GPS sets receiving both GPS and Glonass, with differential corrections, are fine for altitude (for hill walking anyway).
Yes indeed, once per second - I agree it sounds unsuitable for a catapult.Commercial GPS update rate will be too slow I think (about a second per measurement?). It also has issues with heights and speeds over a certain altitude (to stop it being used for rocket guidance). But perhaps worthwhile adding for completeness.
-165 dBm sensitivity, 10 Hz updates, 66 channels
5V friendly design and only 20mA current draw
Breadboard friendly + two mounting holes
PPS output on fix
We have received reports that it works up to ~32Km altitude (the GPS theoretically does not have a limit until 40Km)
Internal patch antenna + u.FL connector for external active antenna
Fix status LED
Won't work. An object launched from a catapult and following a parabolic trajectory experiences no acceleration during flight. As far as it is concerned it is travelling in a straight line at constant speed.What about just measuring 3D acceleration. With a little integration over time you get the flight path relative to the catapult.
I don't think you are right. Newton says there is a force (gravity) acting, which, assuming a denser-than-air object, results in acceleration towards the ground. Otherwise an object released with no vertical velocity would float. What is missing is the opposing force we all experience from the ground we stand on. In the case of an object on the ground, the downwards force exerted by gravity and the upwards force of the ground on the object are equal and opposite, and therefore cancel out, so no acceleration relative to the ground.Heater wrote:
Newton says it won't work. Not during flight anyway. Ignoring air resistance for a minute, the projectile will experience no accelerations during it's flight. It is basically in orbit around the Earth. Any accelerometers it is carrying will read zero in all directions. As they will if carried on the space station in it's orbit.