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

[Bernard Davison]

Yep I agree that you would not use a pressure sensor for absolute height.
I don't see anyone in this discussion say otherwise.

But they can be easily integrated with I2C units which are available.
These could give a general reference which then hands over to another system 
that is more accurate such as the ultrasonic or laser units.

Cheers,
Bernie.

On 21/11/2009, at 12:27 AM, gisela.noci wrote:

> Hi Bernie.
> 
> Not bad, the 'bara' sensor. 2 problems though -  the range - only 800 to
> 1000mb, which will only give 2400 metes ASL. Certainly ok for fun flying,
> but when you are flying near the Alpine mountains...
> 
> Also, It gives 3 meters resolution, NOT accuracy; The accuracy is 1% at full
> scale pressure, ie, 1100mb. The number of 'counts' per millibar is 10.6, and
> the sensor count is 3600 (max pressure) - 400mb (min pressure) = 3200
> counts, for 1100mb-800mb = 300mb, which gives 3200 counts for 300mb, or 10.6
> counts per millibar (8meters). 1% of FS pressure = 1% of 3600 = 36counts.
> And 36 counts = (36/10.6) = 3.4mb = 27meters!!!!
> 
> 
> So, at sea level you can be out by 27meters alt, and still be within the
> sensors specified 'accuracy'
> One must be VERY careful when using the terms resolution and accuracy - they
> are NEVER interchangeable in my experience...
> 
> Alone, this one can never be trusted to land by...
> 
> Joe
> 
> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden
> ] On Behalf Of Bernard Davison
> Sent: Friday, November 20, 2009 12:31 PM
> To: address@hidden
> Subject: Re: [Paparazzi-devel] distance measurement for landing.
> 
> There are some I2C baro's that I've used that give about 3m accuracy.
> http://www.sensortechnics.com/index.php?fid=300&fpar=YToxOntzOjQ6InBjaWQiO3M
> 6MzoiMTczIjt9&isSSL=0&aps=0&blub=5c4acd01785490150591f3ec91b908bd&ge=0f03d3e
> 607784af0fb6f918ac0e74335
> 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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