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

[gisela.noci]

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
> 
> 
> 
> 
> 
> 
> 
> 
> _______________________________________________
> Paparazzi-devel mailing list
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