[aprssig] Group tracking

John Goerzen jgoerzen at complete.org
Mon Apr 30 16:41:15 CDT 2012

Hi folks,

I'd like to get some recommendations on beacon configuration for group 
tracking scenarios.   The main scenarios I have in mind are:

 1. A caravan of 2 or 3 vehicles traveling to a common destination.  The
    trip could be 30 minutes or 15 hours, could involve a mix of highway
    speeds (75 MPH) and city speeds (20 MPH or less).  The point of APRS
    tracking would be to give at least one vehicle the means to find the
    other if needed, due to getting separated in traffic, taking
    different exits, loss of cellphone communication, etc.  The vehicles
    will usually, but conceivably not always, be within simplex range of
    each other (owing to lack of external antennas and high speeds in
    some cases).
 2. Hikers in 2 groups in a large park/wilderness area.  A similar goal
    as before - give at least one group the means to find the other
    assuming nothing but APRS beacons as communication.  Here I would
    assume that the hikers will be within simplex range (say, a mile or
    two) of each other at 5W.

One could also add other similar scenarios, such as coordinating search 
and rescue parties, tracking SKYWARN volunteers, etc.

There are a few special things about these scenarios:

 1. We may not usually care about getting frequent position updates. 
    During the minutes / hours in which one party is not trying to
    locate the other, we don't care too much about getting frequent updates.
 2. When one group is trying to locate the other, default position
    update timings will likely be way too long.  For instance, 5 minutes
    or even 2 minutes at 70MPH is a lot of miles and a lot of potential
    exits that may or may not have been taken.  A lot of SmartBeaconing
    scenarios have 30 minutes between beacons at 20MPH, which is
    unreasonable for tracking someone in city traffic.  (And, according
    to my reading, could actually be even longer at speeds between the
    low and high speed, oddly enough)

One could conceivably use a TinyTrak AIO or similar device for the group 
being tracked, though in my particular case it will likely be a locked 
VX-8GR.  Neither the TinyTrak nor the 8GR responds to APRS position 
query packets (?APRSP), so that is out.  The TH-D72A does, but it is 
also larger and for various reasons is also the stronger tracker device.

For the hiking scenario, we could conceivably use a 440 frequency since 
it is unlikely that a digipeater would be of any help at all.  This puts 
less of an importance on restricting transmit rates.  A lot of testing 
suggests that battery drain on APRS HTs is usually due to engaging the 
RX side, not the TX side, so I would not have battery concerns on a 
simple 1-minute interval.

For the car travel, we would probably want to use WIDE1-1 (are there any 
digis that would answer to WIDE2-1 but not WIDE1-1?) to get a little 
help from nearby digis, due to the likelihood of greater separation and 
more challenges to RF signals getting out.  It seems a bit annoying to 
have a 1-minute beacon going from a restaurant parking lot for an hour 
on 144.390, but perhaps it is not as bad as I am making it out to be.  
It seems that if I set the low speed on SmartBeaconing to, say, 5MPH to 
prevent this, and set the high rate to 2 minutes and the low rate to 5 
minutes, it would lead to some very long intervals at, say 10MPH 
(70/10*2 is 14 minutes).  An odd little gap in the SmartBeaconing 
design, that.

What are suggestions to both meet the objectives and be friendly to 
other RF users?


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