[aprssig] Packet Interpretation - Wind or Motion

[Robert Bruninga]

Pierre,

It appears you have found a conflict in the spec.  I do remember
(I think) that the intention was (from the WX guys) that we do
speed in MPH.  So if there is a conflict, I think MPH wins.  It
used to be that I woiuld rely on Dale Heatherington to weigh in
on these topics, but I'm not sure he is around anymore.

Since this comes up every few years, I would think that this is
addressed in the APRS11 addendum aprs.org/aprs11.html.  If you
cannot find it there, then we can add it.

Bob, WB4APR


> >   > I suspect that many (most) weather station
> >   > software that are generating APRS data
> >   > are actually sending wind speed in mph.
> >
> >    Yes, that is the spec.  But that is the packet format 
> only.  Every
> >    APRS client software that displays WX data should hve 
> the option for
> >    display in any local units desired.
> 
> Bob,
> 
> Maybe there's something I do not understand in the spec.
> 
> Here's what I read in Chapter 7: APRS Data Extensions:
> 
>     Wind Direction and Wind Speed
> 
>     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.
> 
>     For example: 220/004 represents a wind direction of 220 
> degrees and a
>     speed of 4 knots.
> 
> Further in the spec (Chapter 12), we see that wind speed in 
> "Positionless
> Weather Data" is in mph, but on the following page, we read 
> with regard to
> "Complete Weather Reports with Timestamp and Position":
> 
>     ... Also, the 7-byte Wind Direction and Wind Speed Data
Extension
>     replace the cccc and ssss fields of a Positionless Weather
Report.
> 
> In which case, it is my understanding that the units that 
> should be used
> for transmission have to be knots (not mph) as specified in
Chapter 7.
> 
> (In case attachements are not permitted by the mailing list, 
> I will gladly
> email directly to anyone asking.)
> 
> In the attached UI-View screen capture, I see what I
understand as a
> "Complete Weather Reports with Timestamp and Position", with 
> a wind speed
> 008 and gust 009 as it is beeing transmitted. As you can see
UI-View
> translates the wind speed to 8,0 mph and 3,6 m/sec while if 
> it had taken
> the wind speed in knots, it would have been translated to 9,2 
> mph and 4,1
> m/sec.
> 
> Xastir on his part does the conversions as expected (i.e. the 
> authors of
> Xastir seem to share my understanding of the spec).
> 
> '73 - Pierre
> __
> 
> Pierre Thibaudeau
> VA2RKA/VA2RKB/VE2RIO/VE2RVR/VE2RQF/VE2RTO/VE2LKL/VE2TXD
sysadmin
> 
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