[aprssig] BLENDING APRS: SEEMLESS INTEGRATION IN A MULTI-FREQUENCY ENVIRONMENT

David Dobbins ddobbins at gmail.com
Sat Apr 17 19:37:36 CDT 2010


 Hi all APRS and NWAPRS enthusiasts:

Within the NWAPRS region a couple groups of hams have been developing
alternate APRS networks on 9600bd with great success. The project started
because the saturation of APRS activity on the primary freq, 144.39 at
1200bd, particularly around the Puget Sound region and Seattle. If you'd
tune into the 144.39 freq you'll rarely hear a quiet moment, and we suspect
upwards of 50% of the packet activity never gets through due to collisions
of u/i packets. I suppose this is happening in other larger metro areas
around the country and with our overseas friends.

The desire for further optimization sparked experimentation of 9600bd APRS
on both UHF and VHF frequencies. Success was found on both bands, but one
draw back on UHF was locating a common freq that could be expanded out of
the Puget Sound region. We still haven't discovered that common freq that
will work everywhere as we have on VHF, but we continue experimenting on
440.800Mhz under the direction of Bob (K7OFT) in Seattle. A typical WIDEn-N
digi on 9600bd employs a Kenwood TM-D700 in some locations, or an Icom
IC-207 and Kantronics KPC-9612 in others. Bob continues APRS development
on 220Mhz around the Seattle area as well. Bob (K7OFT) has proven APRS at
9600bd is a viable alternative to the crowded 1200bd activity on 144.39Mhz,
but also notes that while the D700 can DIGIPEAT in APRS Packet mode, it does
so with a limited set of commands. The D700 internal TNC is not fully
compatible with what we want to do within the compatibility
requirements. The D700 is just about the only rig we can use if we stay on
144.350 and 144.390MHz. The reason is because the D700 turns off the
receiver opposite of the band you are transmitting on. So if you transmit on
144.350 then the receiver on 144.390MHz is shut off, and
vice-versa. Otherwise the front end of the listening receiver could suffer
damage because the 40Khz may be too close to not cause problems. In Bob's
experimentation, we believe that the only way we can get around the limited
commands problem is to use Digi_ned at the digi site. Bob (K7OFT) also notes
we should be looking for an easy solution to passing messages and one-liners
to and from the primary and alternate frequencies.
>From early analysis we discovered a new problem of sharing data within the
two environments in what later evolved into the topic of this report,
BLENDED APRS DEVELOPMENT.  At our planning meeting for the NWAPRS Summer
Gathering Scott (N7FSP), brought up "Blended APRS Networks" to allow the
sharing of APRS information across the various networks in the area so that
users were able to better share information between networks without the use
of the Internet. Simply put, we haven't discovered an easy way to implement
the crossband sharing of data specific to a mountaintop WIDEn-N digipeater.
Our work continues.

In steps Scott (N7FSP), and Casey (KJ7XE), who both have interest and most
importantly, mountaintop access, where development of a VHF 9600bd
experiment could continue. Scott settled on 144.35Mhz as it appears widely
under-utilized across the spectrum. The real "jewel" in this development was
the reduction in implementation cost by combining a Kenwood TM-D700 and
external Kantronics KPC-3 to come up with one "easy" digipeater package
resulting in 1200bd and 9600bd APRS on VHF with no need for another (UHF)
antenna, feed line or coax, or other ancillary equipment needed to install a
separate UHF radio at the remote location. The Kenwood D700 is configured to
digipeat APRS on the radio's A band on 144.35Mhz and internal TNC at 9600bd,
while the B band is tuned to 144.39Mhz and uses an external KPC-3 set at
1200bd. The cost savings is better than two separate radio systems at the
mountaintop, and upgrading a typical VHF radio plus KPC-3 digipeater is
easy; just replace the radio with a Kenwood TM-D700 and you're in business
on two freqs, one at 1200bd and the other 9600bd. The XTAL/CRYSTL digipeater
is the FIRST true "Double Sided Digi" (quote to K7MCR) that we know of.

OK, so we have lots of room for growth at the 9600bd environment, which
means more data can be pumped to the users on 9600bd. We're still working
out exactly what we want to expand with, but for the most part there's room
for those 10 second trackers or club announcements and flood closure
warnings or whatever.

Now comes the problem we're attempting to resolve now, with our BLENDED APRS
DEVELOPMENT project. As it exists now, two mobile travelers, one on the
primary freq 144.39Mhz at 1200bd, and the other on the secondary freq
144.35Mhz or 440.800Mhz at 9600bd, both driving down I-5 equipped with
Kenwood TM-D710 radios and AvMap G5 GPSs, neither will know of each other's
presence unless they're both communicating on a common voice frequency. A
nearby home station, typically setup for the primary freq, will only see the
one station unless his APRS application, typically UI-View, is also pulling
data from the APRS-IS. Many don't, relying only on the RF picture plotted on
their screen. Another nearby home station, new and improved, but monitoring
only the secondary freq with a 96kb-only TNC, will see only the one station
unless his APRS application is also pulling data from the APRS-IS. A third
home station, pulling only the APRS-IS feed, may see both stations while the
fourth home station, with deeper pockets, has UI-View running on his
computer connected to a KPC-9612 and two radios, one on 144.35Mhz or
440.800Mhz and the other 144.39Mhz.

>From this comes our goal of blending the various APRS frequencies and baud
rates, preferably at the mountaintop digipeater site, or at a home station,
to where other home and mobile stations are aware of ALL APRS activity in
their area, and the blend makes it transparent for any user to access this
data, and include some filtering or adjusting of path selection and other
variables.

We continue our discussions on the nwaprssig reflector, and have it included
in our list of presentations for the September 10th NWAPRS Summer Gathering,
more info for which can be found on the http://www.nwaprs.info web site.
Many thanks to Bob (K7OFT) and Scott (N7FSP) for their assistance in
preparing this message for discussion.

Regret I won't be able to attend the TAPR DCC in Portland at the end of
September, but wanted to get this out there now in hopes there will be some
discussions on it here in the sigs and at DCC.

David K7GPS
Lead Coordinator NWAPRS
Home QTH Spokane
Sunning by the pool in Tucson
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