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[aprssig] The APRS AX.25 Frame

Joseph M. Durnal joseph.durnal at gmail.com
Wed Sep 21 13:16:28 UTC 2011


Thanks Ian, I have http://www.aprs.org/doc/APRS101.PDF printed, that
was my starting point.  Some of it is still over my head :)

On Wed, Sep 21, 2011 at 4:13 AM, Ian Wade G3NRW
<g3nrw-radio at ntlworld.com> wrote:
> ___Original Message_________________________________________
> From: Joseph M. Durnal <joseph.durnal at gmail.com>
> Date: Tue, 20 Sep 2011   Time: 15:09:29
>
>>I'm trying to explain an APRS packet to many non-APRS users and I want
>>to get it right, but I've confused myself
>>
>
>
> Hi Joseph
>
> To get the full information you need, take a look at the APRS
> Specification:
>
>    http://www.aprs.org/doc/APRS101.PDF
>
> This includes all the details of packet structure, APRS commands etc.
>
> When writing the specification, I wrote an APRS decoder (APRSDEC) that
> breaks down each received packet into readable English:
>
>    http://homepage.ntlworld.com/wadei/aprs.htm
>
> Here is a snippet of the APRSDEC output:
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Record #3
> G3NRW*>APRS:!5157.97N/00029.38W-                              ~ Basic
> posit
>  APRS Data Type= Posit w/o time. No APRS
>  Lat= 51 deg 57.97 min N  Long= 0 deg 29.38 min W
>  Icon= House QTH VHF   Overlay= (none)
>
> --------------------------------------------------------------
> Record #4
> WB4APR*>APRS:!5157.97N/00029.38W-                             ~ Exactly
> the same posit, but Bob is a long way from home!
>  APRS Data Type= Posit w/o time. No APRS
>  Lat= 51 deg 57.97 min N  Long= 0 deg 29.38 min W
> **QUESTION: Is this lat/long position reasonable?
>            It seems a long way from home for this callsign.
>  Icon= House QTH VHF   Overlay= (none)
>
> --------------------------------------------------------------
> Record #5
> G3NRW*>APRS:TheNet X-1J4!5157.97N/00029.38W-                  ~ "TheNet"
> string preceding posit
>  APRS Data Type= Posit w/o time. No APRS
>  Lat= 51 deg 57.97 min N  Long= 0 deg 29.38 min W
>  Icon= House QTH VHF   Overlay= (none)
>
> --------------------------------------------------------------
> Record #6
> G3NRW*>APRS:@012345z5157.97N/00029.38W-                       ~ Basic
> posit with timestamp
>  APRS Data Type= Posit w/ time. With APRS
>  Day= 01  Time= 23 hours 45 mins UTC
>  Lat= 51 deg 57.97 min N  Long= 0 deg 29.38 min W
>  Icon= House QTH VHF   Overlay= (none)
>
> --------------------------------------------------------------
> Record #7
> G3NRW*>APRS:@5157.97N/00029.38W-                              ~ Same
> again, but timestamp missing (a common mistake)
>  APRS Data Type= Posit w/ time. With APRS
> **WARNING: Bad/missing timestamp. Rest of data may not decode correctly.
>  Lat= 61 deg 58.47 min N  Long= 111 deg 41.36 min W
>  GPS Fix= Current   NMEA Source= RMC   Compression Origin= Digipeater
> conversion
>  Course= 216 deg   Speed= 2 knots (2.3 mph  3.7 kph  1.0 m/s)
>  Icon= **INVALID ICON   Overlay= **BAD OVERLAY CHAR 'N' not allowed with
> this symbol
>
> --------------------------------------------------------------
> Record #8
> G3NRW*>APRS:!5157.97N/00029.38W-/A=000056                     ~ /A=
> altitude
>  APRS Data Type= Posit w/o time. No APRS
>  Lat= 51 deg 57.97 min N  Long= 0 deg 29.38 min W
>  Icon= House QTH VHF   Overlay= (none)
>  Altitude= 56 feet (17 meters)
>
> --------------------------------------------------------------
> Record #9
> G3NRW*>APRS:=5157.97N/00029.38W-023/050                       ~ CSE/SPD
>  APRS Data Type= Posit w/o time. With APRS
>  Lat= 51 deg 57.97 min N  Long= 0 deg 29.38 min W
>  Icon= House QTH VHF   Overlay= (none)
>  Course= 023 deg   Speed= 050 knots (57.5 mph  92.6 kph  25.7 m/s)
>
> --------------------------------------------------------------
> Record #10
> G3NRW*>APRS:=5157.97N/00029.38W-.../...                       ~
> Indeterminate CSE/SPD
>  APRS Data Type= Posit w/o time. With APRS
>  Lat= 51 deg 57.97 min N  Long= 0 deg 29.38 min W
>  Icon= House QTH VHF   Overlay= (none)
>  Course= (Indeterminate) deg   Speed= (Indeterminate)
>
> --------------------------------------------------------------
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
> Go to http://aprs.fi for a good source of raw data to play through
> APRSDEC.
>
> Also, someone else in this thread mentioned MIC-E. The details of MIC-E
> packets are explained in the specification, and my MICENC encoder
> program (also at http://homepage.ntlworld.com/wadei/aprs.htm) lets you
> generate a MIC-E packet from a standard uncompressed packet.
>
> Hope this helps.
>
> --
> 73
> Ian, G3NRW
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