[aprssig] Packet Interpretation - Wind or Motion

[Pierre Thibaudeau]

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