[ale] WAY [OT] but geeky - how do I calibrate GPS barometric altimeter

[Ron Frazier (ALE)]

Hi Scott,

Regarding your weather station.  I'd consider exchanging the unit.  The 
temperature sensors are usually pretty good.  Although you didn't 
mention the brand or price, more pricey usually means better sensors.  
If you have two units side by side, they should read within 1- 2 degrees 
of each other.  From my prior research on consumer weather stations, I 
think any humidity sensor under $ 300 will only give about + / - 5% 
accuracy.  Anything under $ 30, you can probably forget that sensor.  It 
may be way off.

Regarding the "atomic clock" signal, make sure you've initialized the 
unit to search for the atomic clock signal.  Then, leave it very still.  
Even if it's receiving, it may take several minutes to synchronize.  
Make sure there are no large metal appliances or building parts blocking 
it toward the West.  The signal comes from Colorado.  Usually, in the 
daytime, you cannot receive the signal due to ionospheric interference.  
However, I have 2 wall clocks, a weather station, and a wrist watch that 
all read this signal.  They usually synchronize at between 2 and 5 AM.  
There are very few days when they miss.  Most units have an indicator 
that tells if it synchronized or not.

If you cannot get yours to work, I'd exchange it or take it back, if you 
can.

Sincerely,

Ron

On 2/26/2013 7:45 PM, Scott Plante wrote:
> Derek,
>   I'm pretty sure Ron realized that--that's why he mentioned getting 
> it from online sources for his locality. The problem was they all 
> reported different values.
>
> Separately,
>
>   A family member gave me a fancy weather station / atomic clock 
> (well, it supposedly reads the atomic clock radio signal) with an 
> outdoor remote unit this Christmas. It tells the indoor/outdoor 
> temperature, pressure, and other stuff. Testing it out, I place the 
> remote unit right next to the indoor unit and they consistently 
> reported values 10 points off! It also never read the clock signal. Oh 
> well, so much for technology.
>
> Scott
>
> ------------------------------------------------------------------------
> *From: *"Derek Atkins" <warlord at MIT.EDU>
> *To: *"Atlanta Linux Enthusiasts" <ale at ale.org>
> *Sent: *Tuesday, February 26, 2013 11:17:53 AM
> *Subject: *Re: [ale] WAY [OT] but geeky - how do I calibrate 
> GPS        barometric        altimeter
>
> Ron,
>
> ...
>
> The issue is that the barometric pressure is constantly changing.
> Pilots use barometric altimeters, and the rule is that they need to
> reset them every 100 miles or every hour.  An airport will report the
> current altimeter setting every hour, but some automated systems will
> report it every minute, and yes, it can change rapidly if you have
> strong storm systems.
>
> Basically, this is a long-winded way of saying that you need to be
> constantly resetting altimeters.  When they are set properly they are
> very accurate.  However they need to be reset frequently to remain
> accurate.
>
> One tip: the barometric pressure doesn't usually change significantly
> over short distances.  Most of the local airports will have the same
> setting, so you could theoretically just dial the weather at, say,
> Kennesaw (770-425-3406), to get a local altimeter setting that should be
> "close enough" for you.  Worst case you'll be maybe +/-50 feet off, but
> is that really too much error for your use-case?
>
> -derek
>
> -- 
>        Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
>        Member, MIT Student Information Processing Board  (SIPB)
>        URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
>        warlord at MIT.EDU                        PGP key available
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-- 

(PS - If you email me and don't get a quick response, you might want to
call on the phone.  I get about 300 emails per day from alternate energy
mailing lists and such.  I don't always see new email messages very quickly.)

Ron Frazier
770-205-9422 (O)   Leave a message.
linuxdude AT techstarship.com

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