What is the difference in equipment used to generate an ATIS, AWIS and METAR?
Say you're going into somewhere, a class D aerodrome, say Coffs Harbour. The tower is closed, so there's no ATIS available. My understanding is that you can contact ATC to request the latest METAR information to get a understanding of what to expect, but you can't use the QNH they give you from the METAR.
AIP ENR 1.5-33 5.3.1 states that prior to passing the IAF, pilots are required to set either:
a. the actual aerodrome QNH from an approved source, or
b. the forecast terminal QNH, or
c. the forecast area QNH
It goes on to talk about the 100ft reduction with an actual QNH from an approved source, then clarifies what an approved source is (ATC, ATIS, AWIS and CASA approved met observers) and adds a note, "METAR QNH does not meet this requirement". So if the METAR QNH isn't covered as an approved source, the only choices we seem to have left are:
b. the forecast terminal QNH, or
c. the forecast area QNH
So the QNH generated from the METAR is obviously unreliable compared to the QNH you'd get from an ATIS or AWIS.
My question is then, how is the METAR generated, and if it is from an automatic weather generating device at the aerodrome, why is it not as reliable as the QNH gathered from somewhere with an AWIS, where it IS an approved source? And what equipment do the guys in the tower have access to, to allow them to distribute an ATIS. Obviously better equipment than what is being used to generate the METAR I presume?
In short: what is the difference in equipment used to generated an ATIS, an AWIS and a METAR?