[aprssig] Packet Interpretation - Wind or Motion
Pierre Thibaudeau
ve2prt at sympatico.ca
Thu Aug 13 09:16:31 EDT 2009
P.S.
I suspect that many (most) weather station software that are generating
APRS data are actually sending wind speed in mph. Another compelling
argument to re-write the spec with regard to DIR/SPD.
'73 - Pierre
__
Pierre Thibaudeau
VA2RKA/VA2RKB/VE2RIO/VE2RVR/VE2RQF/VE2RTO/VE2LKL/VE2TXD sysadmin
Today, 07:55 -0400, Pierre Thibaudeau wrote:
> Today, 07:20 -0400, Lynn W. Deffenbaugh (Mr) wrote:
>
> > To start a new topic in the group,
> >
> > As most of you know, I'm working on a new APRS-IS client. I'm working on
> > improving my parser to parse, buffer, and display more datatypes coming in
> > over the APRS-IS stream and I've just started looking at weather packets.
> > I've stumbled across a station that appears to be moving (using @ position
> > updates) with a course and speed (CSE/SPD), but when looking at the comment
> > text visually, it more appears to be a weather packet wherein the CSE/SPD
> > should be the wind direction and speed.
> >
> > Any guidance on how to interpret these packets from K3YJP-4?
> > http://aprs.fi/?c=raw&call=K3YJP-4
>
> It is indeed a weather report; see APRS 101.
>
> Chapter 7 (p.27) specifies the DIR/SPD and later in Chapter 12 the section
> "Complete Weather Reports with Timestamp and Position" describes all the
> fields in such a message (explanation for the other fields after the
> DIR/SPD are given on the previous page, with "Positionless Weather Data".
>
> It must me noted that (to open yet another debate) the APRS specifications
> specifies that (in this format) wind speed is in knots and gust speed is
> in mph. However, APRS clients like findu.com, aprs.fi, UI-View are all
> interpreting the wind speed in mph (altough they all interpret the speed
> of moving objects in knots). Xastir on his part is "correctly"
> interpreting wind speed in knots. It drives me crazy as I "have" to create
> weather reports for APRS clients that do not follow the spec, even if it
> means that my APRS client of choice displays incorrect data.
>
> Note that for positionless weather reports, wind speed is expressed in
> mph. But discrepencies like this are to be expected. As mentioned in APRS
> 101:
>
> "For historical reasons there is some lack of consistency between units
> of data in APRS packets — some speeds are in knots, others in miles
> per hour; some altitudes are in feet, others in meters, and so on. It
> is emphasized that this specification describes the units of data as
> they are transmitted on-air. It is the responsibility of APRS
> applications to convert the on-air units to more suitable units if
> required."
>
> With UI-View beeing in a frozen state, the spec should probably be revised
> (and Xastir and others be "fixed").
>
> '73 - Pierre
> __
>
> Pierre Thibaudeau
> VA2RKA/VA2RKB/VE2RIO/VE2RVR/VE2RQF/VE2RTO/VE2LKL/VE2TXD sysadmin
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