Re: [Paparazzi-devel] distance measurement for landing.

[antoine drouin]

this is done with a baro and a 10 bits ADC

http://poinix.org/video/booz2_vert_ctl_v0.2_perturb.mp4

On Mon, Nov 9, 2009 at 9:31 PM, gisela.noci
<address@hidden> wrote:
> Baro_altitude sensors can certainly measure to that kind of accuracy. The
> difficulty arises in digitizing the output ( via an A/D convertor) and
> ensuring that the noise on the signal is very low. Some typical sensors give
> a 0-4v output for a pressure variation from 1013millibar (close to sea
> level) to around 200millibar ( a difference of 813mb, 81kPa, about
> 8000meters alt change). This means to achieve 14cm resolution, you need
> 8000m/0.014m = 57000 bits, or increments. This implies at least a 16bit A/D,
> and then remember that each bit is 4v/57000 = 70uV !!!   Couple this with
> the sensor output variation over temp ( not insignificant!) and the noise on
> the signal, you will be VERY lucky to get 1meter accuracy, and that at a
> specific temp.
>
> Ask any real pilot - land with your eyes, not your altimeter!!!
>
> Ground ranging (Radio-Altimeter, radar, ultrasound, etc is the only way to
> go.
>
> Joe
>
> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden
> ] On Behalf Of Elden Crom
> Sent: Monday, November 09, 2009 7:57 PM
> To: address@hidden
> Subject: RE: [Paparazzi-devel] distance measurement for landing.
>
>
> I was considering the pressure/altitude route
>
> http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6776&Prod
> uct_Name=WS_HowHigh_Altimeter_Feet_&_Meter_(New_Version)
>
> But I think this may require one on the ground station as well to deal with
> a cold front coming in and changing the barometric pressure.
>
> Supposedly, this thing can measure altitude accurately to 14cm.
> Does anybody believe that? (I have on one order to see if it's actually that
> accurate)
> I wonder how to mount it such that it is not affected by wind speed.
>
>
>
>
> -----Original Message-----
> From: Dirk-Willem van Gulik [mailto:address@hidden
> Sent: Fri 11/6/2009 1:04 PM
> To: address@hidden
> Subject: Re: [Paparazzi-devel] distance measurement for landing.
>
> Roman Krashanitsa wrote:
>
>> The http://www.maxbotix.com/ founder did some comparison for performance
>> of his sensors with Sharp sensors you are linking to. As far as I
>> remember, there are some plots in the FAQ section and in "Preformance
>> Data" section that might be useful for you.
>
> I found that even very simple DIY shop ultrasonic meters; like below:
>
> http://www.sella.co.nz/general/building-renovation/tools/other-tools/503tx7/
>
> which can be had for 5 to 10 euro's at the local DIY market - are easily
> hacked and not that unreliable.
>
>
> Thanks,
>
> Dw
>
>
>
>
>
>
>
>
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