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[aprssig] Project-25 for amateur radio

Robert Bruninga bruninga at usna.edu
Sun Aug 17 04:53:50 UTC 2008


> 48 bits of data can get you a position...
> 36 bits can represent a 6-character, 6-bit callsign. 
> Seems like even accounting for some overhead,
> a 1.5 or 2 second transmission would be plenty 
> to get a basic position out.

Scott, looks like you can have a new project.  Make a dongle for
these P25 radios that takes GPS input and sends out the right
bits for APRS.  Lets define the format... Howabout something
like:

CCCCCC-SS>PX:AAAAGGGG,CC,SS,AAA/$,freetext up to 17 bytes (or
more)

PX is some code for your dongle that does this process
Where AAAAGGGG are the 8 bytes of lat/long
CC and SS are course and speed
AAA is the altitude
/$ is the symbol and
Freetext can go on the end.
But free text can also contain FREQ if it is available

But in general, the FREQ will be added when this is received at
the Repeater site and conerted to APRS.  The repeater that
receievd this and does the conversion KNOWS what freq it is on.

THen you also have to make the dongle at the repeater to convert
this to APRS.  I suggest this standard conversion.

CCCCCC-SS>AP25xxx,WIDE2-2:!DDMM.mmN/DDDMM.mmW$CSE/SPD/FFF.FFFMHz
17-bytes-of-free-text!DAO!

Notice that FFF.FFFMHz is inserted by the digi.
!DAO! Is on the end so that it does not mess up text displayed
on all existing radios.  Notice that if there is no GPS data in
the user uplink data, then the repeater dongle inserts its own
position, and a P25 generic SYMBOL with a randomised LATITUDE to
make all users on this repeater appear as a list on all APRS
maps.  Seee details on this process on the
www.aprs.org/aprstt.html web page.

By doing it this way, evevn if the P25 radio user does not use
GPS, he still appears in APRS with a frequency and location...
If he has  your dongle.

I have no idea how many P25 users there are...  But we need to
get all radios on APRS eventually...

Bob, WB4APR

> John Habbinga wrote:
> > Bob, there is a low-speed data mode transmitted along with
digitized
> > voice on Project 25.  However, it passes data at 88.9 bit/s.
As far
> > as I know, there are no applications that has made use of
this
> > feature.  I doubt that it would be very practical to use
"APRS over
> > P25," but I could be wrong.
> > 
> > On Fri, Aug 15, 2008 at 3:38 PM, Robert Bruninga 
> <bruninga at usna.edu> wrote:
> >> I noticed the QST article on Project 25 using digital voice
on
> >> repeaters.
> >>
> >> I noticed a sentence about simple text transfer. And
intended to
> >> bring the article to work with me for noon time reading,
but
> >> forgot it.
> >>
> >> If the radios are capable of digitally exchanging
callsigns,
> >> then there is no reason why they cannot appear on APRS just
like
> >> DTMF users.  SO if anyone can point me to a point of
contact
> >> within that project group, I'd like to invite them to
consider
> >> joining the global APRS system too.
> >>
> >> See http://www.ew.usna.edu/~bruninga/aprstt.html
> >>
> >> Bob, WB4APR
> > 
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> > 
> > 
> 
> 
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