[aprssig] Packet Interpretation - Wind or Motion

[Bob Bruninga]

> 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.

> Another compelling argument to re-write 
> the spec with regard to DIR/SPD.

Not compelling at at all.  THe spec could be in furlong's per fortnight just as well, how you DISPLAY that data is up to you and SHOULD offer both metric, knots, and kph and MPH as the users choice.  But the on-air format MUST be in only one defined unit or the recepient has no clue what it means.
Or how to display it or convert it.

Bob, WB4APR


>'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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