wind direction clarification

[nuegli]

Hallo guys...!!!
as new-by on this site I can not do everything better that brig to you a problem to solve.
I'm not a pilot but I'm responsible for an ATIS system that my company provides to some different airports all over the world.
In last days I went to one caribbean country to install and test a new ATIS system that, as required, does not receive data from AFTN line (and so by METAR), but directly from some metereological sensors installed directly on the field. This feature allows the system to continously update metereological data at each ATIS VHF trasmission.

The problem I had was that they required us to change the wind direction from the magnetic value incoming from the anemometer to a true value as reported in METAR.

Before to make any modification I would like to check what ICAO say according tha wind direction on ATIS.

In the ICAO Annex 10 nothing is specified according to the correct value to be used. However, ATIS system commonly take wind data from METAR or from METREPORT and, on those messages, according to ICAO Annex 3 wind as to be reported as true value....

As you have well understood I don't know how to retrieve information on that issue.
Does anyone know wich is the correct value to be reported into the ATIS message ???
Can anyone indicate any specification or document can I use?

Thank you...

[tow-bar]

true value of wind is commonly used

[Spitoon]

In a METAR the wind is T, when passed to the pilot on the radio it is M. I think the references you're looking for are ICAO Annex 3 for the content of a METAR and Annex 11 and or PANS-ATM for what goes into an ATIS broadcast.

I think there is an opportunity for confusion between METARs, which technically are coded messages beyond the aerodrome, typically over the AFTN (or whatever it's called these days) and local reports which are used at the aerodrome of origination but I'm afraid I can't recall the detail and don't have the books to hand.

[ZOOKER]

I remember in the early 1970s, the EGLL ATIS had the word 'magnetic' following the wind direction, and any variability thereof.

[Piltdown Man]

The standard would be to report the wind using the datum as the runway, which is normally magnetic. A simple way of remembering what should be true and what should be magnetic is 'If it's written, it's true." So if it's voice, like ATIS, it has to be magnetic. The UK Air Pilot contains this information (http://www.ead.eurocontrol.int/eadbasic/pamslight-CDCA3A374CE978208E8FE5A77E52729C/7FE5QZZF3FXUS/EN/AIP/GEN/EG_GEN_3_5_en_2011-01-13.pdf). Look under Observing Systems and Operating Procedures.

PM

[Denti]

According to my LIDO manual ATIS and D-ATIS should be in magnetic, METAR in true. This fits nicely into what i was tought all those years ago during my ATPL training.

I would think the relevant standards are set out by ICAO and WMO, however i do not have a link at the moment to any documentation of either or both organizations about it.