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[aprssig] APRS Telemetry Question

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Fri Oct 12 18:42:21 UTC 2012


An APRS message, such as those sent by the APRS-capable radios, is a raw 
packet data type of colon (:). A telemetry packet is a raw packet data 
type of capital T (T). No, an APRS-messaging radio cannot send telemetry 
DATA packets.

That said, you might be able to transmit telemetry DEFINITION packets 
because those are just specially-formatted APRS message (:) packets.  
Transmitting the definitions will allow suitably-capable client 
applications like aprs.fi and/or APRSISCE/32 and others to interpret, 
scale, and label subsequent telemetry data.  However, the client 
application in question must be operating when you knob-twiddle and 
transmit the definitions. This would be true for aprs.fi which is 
(nearly) 24x7, but may not be true of other APRS applications.

Also, here's a quick "anatomy of an APRS packet" that might explain why 
the stuff in front of the colon is the same for two different packets.

MYCALL>TOCALL,PATH1,PATH2:APRSPacketPayload

MYCALL = Source station's callsign (may include -SSID)
 > = Humanly readable separator between source and destination calls.
TOCALL = Packet-defined "To" callsign, but used as an "Application 
Identifer" in APRS.  (Note that this is also replaced with part of the 
coordinate information for a Mic-E-formatted position packet)
,PATH1,PATH2 = Requested (or used if * present) path components
: = Humanly-readable separator between header and payload (not to be 
confused with the : datatype for an APRS message as that's the first 
character of payload)
APRSPacketPayload = The actual APRS packet payload.  The first character 
describes the datatype of the remainder of the payload.

So, an APRS message will have a ::MSGCALL  : as the delimiter and first 
part of the payload.  A telemetry packet would have :T#nnn as the 
delimiter and first part of the payload.

Hopefully this helps.  aprs101.pdf is packed with good stuff, but it 
takes numerous bed-time readings to grok it all.

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

PS.  I haven't re-checked aprs101.pdf, but I do believe the # is 
required.  Some clients use the sequence to detect duplicate and/or 
missing telemetry packets.

On 10/12/2012 2:18 PM, Steve Noskowicz wrote:
>
> --- Heikki Hannikainen wrote, that Steve Noskowicz wrote:
>>> Q:  Is Telemetry a standard APRS message? ...
>> Well, it's a standard APRS message / packet (that's a
>> slightly ambiquous wording), but telemetry data is not
>> transmitted in a TEXT message or bulletin. It's an APRS
>> packet type of it's own, starts with a T# (text messages
>> start with a ':').
> Let me try this from another perspective:
>
>    With these radios we can send what I call a "standard APRS text message" via the knob-twiddle method or an added keyboard/W2FS-adapter.
>    Now...
>    All the data in "telemetry messages" is ASCII, so... My intent of this question is to determine whether the D700/D7 radios can send a telemetry 'message'.
>    These radios *also* allow a "stardard APRS text message" to be sent by sending the text into the serial port using an all ASCII message command "AMSG 00,...".  The text following that command is a text message and the rest of the packet is whatever is appropriate for a "standard APRS message".
>    _If_ the only difference is that the Telem message starts with T and sequence number, then these radios can do it.
>
>
> Q:  One way to re-word this question is:  If, on a D700, I send a message via the knob-twiddle method that starts with T#001, then comma sep groups of 3 ASCII digits, would it be interpreted as Telemetry?  OR, is there some other "sfuff" in a "true" Telemetry packet that distinguishes it as Telemetry and NOT an ordinary message? - OTHER than the fsct that the message is 'to itself".?
>
>
> I also see some inconsistencies...
>    APRS 101 says a Telem msg starts with a "T" and a three character sequence number, but shows "T#xxx" and does not mention the "#" character elsewhere.
> Q: Is the "#" required or optional?
>
> Q:  I also see that KI6TSF-8 has only 2 characters in the sequence numbers, so aparently the telemetry sequence number length is not fixed, and possibly ignored anyway...no?
>
>
>> On the other hand, telemetry channel names and coefficients
>> are sent in APRS text messages
> APRS 101 mentions bulletins....making this confusing.
>
>
>
>> If you wish to see what each packet decodes to...:
>> http://aprs.fi/?c=raw&call=OH7RDA&limit=50&view=decoded
>    As I look at KI6TSF-8 packets, I see that everything up to and including the ":" is the same for the Wx and Telem packets.  This sugests that the only difference between WX, Telem and ordinary messages is the payload (and the destination since a real message goes to another call).  Therefore it appears that Telemetry is simply a regular APRS message with a correctly formatted payload portion...  no?
>
>
> 73, Steve, K9DCI
>
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