[aprssig] APRS freq spec questions

Kenneth Finnegan kennethfinnegan2007 at gmail.com
Mon Mar 16 16:17:22 CDT 2015

Greetings fellow APRSers,

First, a quick note: my master thesis on APRS has been defended and
accepted, so you can now retrieve the final draft of my thesis online
here: http://digitalcommons.calpoly.edu/theses/1341/ There are still
some issues/typos in that document, but it has been set in stone and
I'm moving on with life.

Having taken a hiatus from APRS and writing for a few months in the
search for a job (which still hasn't born fruit yet...), I'm starting
to again poke at my notes from my thesis where topics didn't make the
cut for the limited scope of my thesis. I completely ignored the
contents of APRS packets, so now that I'm FREE of the reigns of an
academic institution dangling a piece of wallpaper over my head, I can
start look at APRS with much smaller brush strokes.

Let us talk about the frequency spec:
http://www.aprs.org/info/freqspec.txt What follows is a few point by
point interpretations, questions, and suggestions based on a casual
reading of the 9 May 12 freq spec.

The way I read it, the freq spec consists of a 10 octet frequency,
followed by a number of optional five or ten octet modifiers. The spec
seems to imply that these modifiers are limited to two, but that seems
arbitrary and limiting to me. I would suggest that an arbitrary number
of modifier fields be explicitly allowed. Clients MUST parse at least
two fields, but may continue to parse modifier fields until they reach
the end of the comment text or a modifier that they do not understand/
appears to have a syntax error.

The freq is expressed via "###.###MHz", or "###.## MHz" if the ones of
kHz doesn't need to be specified. This seems odd to me to complicate
this spec by allowing a space. When would a user only want to specify
a frequency to 10kHz instead of 1kHz resolution?

What is the expected behavior below 100MHz? Should the number be zero
padded ("052.525MHz"), space padded (" 52.525MHz"), or left justified
("52.525MHz")? This last one violates the 10 octet rule for the
primary frequency term.

What is the expected behavior for frequencies >1GHz? Should they
continue to be expressed to one hundredths of MHz? ("1286.200MHz" -
This violates the ten octet rule) Should they be allowed to float the
decimal place? ("1286.20MHz" - This is a bloodly mess for static
parsers) Should they be allowed to specify their frequency in GHz to
the least significant digit that isn't zero? ("1.2862GHz" - Often
violates the ten octet rule and a freaking disaster for stupid
parsers)  I would suggest we allow higher frequencies to break the ten
octet rule and keep everything in MHz.
Edit: I now finally see the MICROWAVES section at the bottom... I
don't know how I feel about this solution vs those considered above...
This plus the delimiter issue (" ", "/", or "" after 7 octet data
extensions) means to me that pretty much the only way to find the
start of a freq spec is to scan the entire comment field for [0-9A-O
]?[0-9][0-9]\.[0-9][0-9][0-9 ][MG]Hz and start there...

Moving on to the modifier terms... Every modifier term consists of a
space, one identifier, and three octets of payload (with the exception
of the rx freq term).

I would suggest that FFF.FFFrx follow the same convention as the
primary frequency in that it is always expressed in MHz, allowed to
violate the ten octet rule for >1GHz frequencies, and follow the same
conventions for <100MHz padding.

There are three squelch access modifiers: T, C, and D. The case of
this identifier specifies the channel bandwidth, so uppercase TCD
indicates 25kHz channels, and lowercase tcd indicates 12.5kHz
channels. I'm ok with limiting this to those two bandwidths.

What is the difference between the T tone and C CTCSS terms? I'd guess
 that it means that a T repeater has tone access but carrier squelch,
where CTCSS allows the users to both encode and decode PL. Is this

What about repeaters with split PLs? (Which I deal with regularly due
to passive intermod issues) I would suggest that we allow two access
modifiers, where the first is the user encode frequency, and the
second is the user decode frequency.
For example, the W6BHZ VHF repeater (which has a 91.5Hz input and
127.3Hz output) would then be expressed as "146.760MHz T091 C127
-060". The UHF side uses 127.3Hz on both input and output, and should
be expressed as "442.300MHz C127 +500"
A repeater using digital input and PL output would be "FFF.FFFMHz D023
C127", PL input and digital output as "FFF.FFFMHz T127 D023", a
repeater using identical digital codes be "FFF.FFFMHz D023" and split
codes as "FFF.FFFMHz D023 D754" (not that I think anyone would want to
do that... but that's a mechanism vs policy issue)

Are all tone bursts >1kHz? 1750Hz is the only one I've heard of. I
dislike the replacing a 1 with an l trick...

I'm ok with limiting the +### and -### to 10kHz-9.99MHz splits.
Anything beyond that includes a FFF.FFFrx modifier instead. Sucks to
be a 23cm repeater on APRS, I guess... (i.e. W6PIY/1.2GHz becomes
"1286.200MHz 1274.200rx C100") I'd be more comfortable with saying
"implicit offsets are implicit" if the freq spec included that list of
standard repeater offsets, but that's another whole can of worms...

I look forward to any comments. I'm sorry I keep showing up just to
stir the pot around here.
Kenneth Finnegan, W6KWF

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