[aprssig] Packet Interpretation - Wind or Motion

[Lynn W. Deffenbaugh (Mr)]

I am suitable cowed, embarrassed, and chagrined.....I thought I 
understood the spec and I thought that an @ packet was ONLY for position 
data.  I now see that on page 66 it further qualifies that a symbol code 
of _ is plainly a weather data indicator.  Silly me, I never noticed 
that specific symbol selection actually drove the packet format!

I guess the real problem is that I THOUGHT instead of RE-READ the spec!  
Thanks for pointing me in the right direction. 

Lynn (D) - KJ4ERJ

PS.  This may read as sarcastic, but it is a truly serious "thank you" 
for the information.


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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>
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