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

[Bernard Davison]

There are some I2C baro's that I've used that give about 3m accuracy.
http://www.sensortechnics.com/index.php?fid=300&fpar=YToxOntzOjQ6InBjaWQiO3M6MzoiMTczIjt9&isSSL=0&aps=0&blub=5c4acd01785490150591f3ec91b908bd&ge=0f03d3e607784af0fb6f918ac0e74335
CSDX range that is if the link doesn't work.
They survive 100G accelerations and whistles from supersonic flight just fine. 
I've even had them survive 3000G+ impacts.
So will survive anything we can put it through on one of our planes.

Cheers,
Bernie.

On 10/11/2009, at 7:31 AM, gisela.noci 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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