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

[gisela.noci]

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