[aprssig] Mobile Digipeating

[Tom Russo]

On Fri, Sep 16, 2011 at 10:34:30PM -0600, we recorded a bogon-computron collision of the <dnchls at gmail.com> flavor, containing:
> I am in search and rescue and use APRS so others can keep track of me.

You're in good company.

> I
> live in New Mexico where we have many digipeaters on mountain tops, but we
> still have many dead zones. 

I've been watching your tracks.  I'm not sure the dead zones are as dead as
all that.  Still, there are some notable places where there are searches and
that have spots where you can't be heard.

> I have a YAESU VX-8DR ht that I carry with me
> both when practicing and on actual searches. I am considering buying a
> Kenwood TM-D710A. I would like to use the 710 in my mobile as a digipeater
> to either get back to search base on a search or to a mountain digi when I'm
> practicing so my family can know where I'm at on APRS.fi. I would typically
> be within a couple of miles from my truck when I transmitted an APRS signal
> from my ht. How much power does the 710 transmit when in digipeater mode?

It transmits with the same power that you have set for its own APRS 
transmissions.

> Would this be a situation where using a mobile would be useful? Presently,
> it just doesn't seem I have the punch I need when using the ht.

It would help if your truck is generally within range of your handheld, the
truck is in a good place where it is getting digipeated, and you don't have 
terrain in between you and the truck that prevents the truck from hearing you.

In this case you could use your truck's callsign as the first hop in your
digi path, and WIDE2-2 next in the path.  That'd let your truck beacon you
with more ooomph.

The downside of this is that if you wind up in a spot where your truck's
radio can't hear you, you won't get anywhere even if a WIDE can hear you,
because they won't premptively digipeat before your truck.

It's unfortunate that Yaesu chose not to implement "Proportional Pathing" in the
VX-8*R series radios.  The VX-8R lets you set 8 paths, but rather than
cycling through them, it simply concatenates them for every transmission.  I'm
unfamiliar with the VX-8DR, maybe it does have proportional pathing.  With
proportional pathing, you can set multiple digi paths, but rather than using
them all at once, it cycles through them for each transmission.  Then you
could have "TRUCK,WIDE2-2" for every third transmission, 
"WIDE1-1,WIDE2-2" for every third transmission, and "WIDE2-2" for every third.
That would take care of making sure that at least SOME of your packets get
to a wide digi --- either directly, or through your truck, or through a fill-in.

> Unfortunately, the people I know both in SAR and amateur radio have never
> played with mobile digipeaters and are pretty clueless on the matter.

Clueless?  I don't think that's a fair statement.  

To date, most SAR activities in the state have managed without pressing need 
for mobile digis.  NM SAR Support's comms trailer does have a TNC programmed 
to digi WIDE1-1, so if your path is WIDE1-1, WIDE2-2 (which is OK in NM for 
low-powered stations and ONLY low-powered stations... there was a big 
discussion on that subject recently, and you should check the archives 
about it, I won't open that can of worms again), then the comms trailer in 
base will serve as a fill-in digi if needed.  We also have a portable digi we 
can place in the search area when necessary.  It is rarely necessary, but it
does get deployed now and then.

You should also set your D710 to do TEMPn-N digipeating, but I'm sure 
Bob B. will pipe in here quickly about that one.  Right now, very few people
in the SAR community have their radios set up for it, so counting on the
availability of TEMPn-N on searches is not a safe bet just yet.  But if you
set your own truck to do TEMPn-N, then when you know you aren't reaching
a wide with your handheld on a search, and you know you can hear your truck,
you can tinker with your path to use TEMPn-N for a while to be sure of getting
out.  You'd use "TEMP1-1,WIDE2-2" in that case, and as long as your truck 
can hear you you're good.

One thing you should NOT do is set your truck to digipeat WIDEn-N as if it
were permanent infrastructure.  Some folks in NMSAR did do that for a while
and it was a terrible mistake --- it meant that ALL traffic heard by the 
incident comms trailer was getting digipeated if it had any wide hops left.
That meant the APRS radio in the trailer was transmitting all the time, and
desensing lots of nearby voice radios every few seconds.  

-- 
Tom Russo    KM5VY   SAR502   DM64ux          http://www.swcp.com/~russo/
Tijeras, NM  QRPL#1592 K2#398  SOC#236        http://kevan.org/brain.cgi?DDTNM
 "One man alone can be pretty dumb sometimes, but for real bona fide
 stupidity, there ain't nothin' can beat teamwork." - Edward Abbey