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[aprssig] The APRS AX.25 Frame

Ian Wade G3NRW g3nrw-radio at ntlworld.com
Wed Sep 21 08:13:00 UTC 2011


___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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