[aprssig] Packet Interpretation - Wind or Motion

[Pierre Thibaudeau]

N.B.

Unfortunately, initial posting of the message below was rejected because 
of the attachement. I will gladly forward it personally to anyone 
interested.

'73 - Pierre
__

Pierre Thibaudeau
VA2RKA/VA2RKB/VE2RIO/VE2RVR/VE2RQF/VE2RTO/VE2LKL/VE2TXD sysadmin

---------- Forwarded message ----------
Date: Thu, 13 Aug 2009 12:12:44 -0400 (EDT)
From: Pierre Thibaudeau <ve2prt at sympatico.ca>
To: Bob Bruninga <bruninga at usna.edu>
Cc: TAPR APRS Mailing List <aprssig at tapr.org>
Subject: Re: [aprssig] Packet Interpretation - Wind or Motion

Today, 11:14 -0400, Bob Bruninga  wrote:

>   > 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