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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.<br>
<br>
First, from aprs101.pdf, I read that the Positionless Weather data s
and g terms are in mph:<br>
<br>
<blockquote type="cite">s = sustained one-minute wind speed (in
mph).<br>
g = gust (peak wind speed in mph in the last 5 minutes).</blockquote>
<br>
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:<br>
<br>
<blockquote type="cite">The 7-byte DIR/SPD Data Extension can be
used to represent the wind<br>
direction and sustained one-minute wind speed in a Weather Report.<br>
The wind direction is expressed in degrees (001-360), clockwise
from due<br>
north. The speed is expressed in knots. A slash / character
separates the two.</blockquote>
<br>
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"
(<a class="moz-txt-link-freetext" href="http://wiki.sandaysoft.com/a/Wxnow.txt">http://wiki.sandaysoft.com/a/Wxnow.txt</a>) says:<br>
<br>
<blockquote type="cite">
<pre>Feb 01 2009 12:34
272/010g006t069r010p030P020h61b10150</pre>
</blockquote>
<blockquote type="cite">
<p>The second line is the wx report, in the format used in APRS
"complete weather reports". The format is:
</p>
<ul>
<li>272 - wind direction - 272 degrees
</li>
<li>010 - wind speed - 10 mph
</li>
<li>g006 - wind gust - 6 mph
</li>
<li>t069 - temperature - 69 degrees F
</li>
</ul>
</blockquote>
<br>
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).<br>
<br>
I checked the 1.1 and 1.2 addenda mentions of weather, but found
nothing at either:<br>
<br>
<a class="moz-txt-link-freetext" href="http://www.aprs.org/aprs11/spec-wx.txt">http://www.aprs.org/aprs11/spec-wx.txt</a><br>
<a class="moz-txt-link-freetext" href="http://www.aprs.org/aprs12/weather-new.txt">http://www.aprs.org/aprs12/weather-new.txt</a><br>
<br>
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.<br>
<br>
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.<br>
<br>
Lynn (D) - KJ4ERJ - Author of APRISSCE for Windows Mobile and Win32<br>
<br>
PS. The <a class="moz-txt-link-freetext" href="http://www.aprs.org/aprs12/weather-new.txt">http://www.aprs.org/aprs12/weather-new.txt</a> 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:<br>
<br>
<blockquote type="cite">
<pre>Thus, the usual APRS
weather report string similar to:
DIR/SPD/Gggg/Tttt/Rrrr/Pppp etc
is extended by adding these additional parameters for </pre>
</blockquote>
<br>
but this is more like what I'd expect from aprs101.pdf:<br>
<br>
<blockquote type="cite">
<pre>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</pre>
</blockquote>
<br>
but then it goes on to say:<br>
<br>
<blockquote type="cite">
<pre>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...</pre>
</blockquote>
<br>
I remain confused by the conflicting specifications...<br>
<br>
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