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[aprssig] Wind/Gust Units Clarification

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Thu Aug 30 23:11:04 UTC 2012


With ISAAC rampaging through the United States, I've had occasion to 
review my parsing of various weather packets and have encountered a 
discrepancy between several wind speed specs.  The basic question is 
what are the units of the three different representations of wind and 
gust data in APRS weather packets.  There's the s and g terms of 
Positionless Weather Data and the Wind Direction and Wind Speed Data 
Extension as it replaces the s term in a Complete Weather Report.

First, from aprs101.pdf, I read that the Positionless Weather data s and 
g terms are in mph:

> s = sustained one-minute wind speed (in mph).
> g = gust (peak wind speed in mph in the last 5 minutes).

The Complete Weather Report doesn't re-define the g term, so I assume it 
is also in mph.  However, the Wind Direction and Wind Speed Data 
Extension (DIR/SPD) appears to be in knots:

> The 7-byte DIR/SPD Data Extension can be used to represent the wind
> direction and sustained one-minute wind speed in a Weather Report.
> The wind direction is expressed in degrees (001-360), clockwise from due
> north. The speed is expressed in knots. A slash / character separates 
> the two.

Ok, so that's what I was about to cement into my code, but then I 
noticed that the wxnow.txt documentation which claims to be the format 
used in APRS's "complete weather reports" 
(http://wiki.sandaysoft.com/a/Wxnow.txt) says:

> Feb 01 2009 12:34
> 272/010g006t069r010p030P020h61b10150
>
> The second line is the wx report, in the format used in APRS "complete 
> weather reports". The format is:
>
>   * 272 - wind direction - 272 degrees
>   * 010 - wind speed - 10 mph
>   * g006 - wind gust - 6 mph
>   * t069 - temperature - 69 degrees F
>

So, is the speed mph (per wxnow.txt) or knots (per aprs101.pdf)?  I 
suspect that many APRS weather transmitting programs simply trust the 
wxnow.txt contents and wrap it in a packet to transmit it. aprs.fi 
certainly seems to interpret it this way (both as mph).

I checked the 1.1 and 1.2 addenda mentions of weather, but found nothing 
at either:

http://www.aprs.org/aprs11/spec-wx.txt
http://www.aprs.org/aprs12/weather-new.txt

I don't personally own a weather station, so I'm not able to compare the 
actual readings to what values are transmitting in a packet, but I'd 
really like to make sure that APRSISCE/32 is interpreting based on 
reality and not blindly implementing a spec that others have not adhered 
to before me.

If there is anyone with a good memory as to which is correct, I'd 
appreciate that knowledge.  If there's anyone with a real weather 
station, it would be interesting to know what station through what 
software is putting what values/units into their raw APRS packets. At 
this point, I don't think we can trust what any particular client 
displays, but I'd like a comparison of actual measured values vs raw 
packet contents.

Lynn (D) - KJ4ERJ - Author of APRISSCE for Windows Mobile and Win32

PS.  The http://www.aprs.org/aprs12/weather-new.txt seems to describe a 
packet format that I've also not seen anywhere else with slashes between 
every term.  Is this a case of not referring to the current spec when 
authoring this page or is it truly specifying a new format?  Or am I not 
remembering something mentioned elsewhere? Specifically:

> Thus, the usual APRS
> weather report string similar to:
>
>    DIR/SPD/Gggg/Tttt/Rrrr/Pppp etc
>
> is extended by adding these additional parameters for

but this is more like what I'd expect from aprs101.pdf:

> SInce the wind direction and speed and temperature are required fields,
> this means the minimum weather report with flood info is
>     
> WX symbol -plus-   .../...t...VxxxFxxxx

but then it goes on to say:

> Examples are:
>
> Weather Station reports with flood gage readings:
>
>   DDMM.hhN/DDDMM.hhW_ddd/sss/Gggg/Tttt/Fxxxx...
>   DDMM.hhN/DDDMM.hhW_ddd/sss/Gggg/Tttt/Fxxxx/Vvvv...
>   DDMM.hhN/DDDMM.hhW_ddd/sss/Gggg/Tttt/Rrrr/PppppFxxxx...

I remain confused by the conflicting specifications...










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