How are altimeter settings estimated? So, I spotted another weird METAR from the same place that spawned the first one I asked about (it's a USAF base if you're wondering what kind of 'drome it is):


  K... 060846Z 00000KT 10SM OVC002 18/18 A2995 RMK AO2A SLPNO ALSTG ESTMD $


While I can reckon that the barometric pressure sensor there's on the fritz (again) from the SLPNO and the $, they also have the "ALSTG ESTMD" remark on there.  While the estimate looks to be not too far out, or at least more reasonable than the completely bogus figure from last time, it raises a question: how are they estimating an altimeter setting in the absence of the AWOS's barometer?  Or does it just mean that their backup barometer isn't as good as the one on the AWOS, so the altimeter setting might be a few ULPs out?
The instrument they are using is not certified, hence the ESTMD.  When they get a certified one, the ESTMD will drop off the METAR.

Technically, the primary certified instruments are inop or unavailable, so backup equipment, believed to be reliable, but not certified, is being used.