[aprssig] How to detect silent/stealth digis/Igates?

Tom Hayward esarfl at gmail.com
Fri Feb 21 12:26:26 EST 2014

On Fri, Feb 21, 2014 at 5:20 AM, Andrew P. <andrewemt at hotmail.com> wrote:
> Although sensitivity testing is good, what I'm really looking for is a simple up/down test, as in "Does it still work, or has the weather emergency taken this one out?" Considering the "wonderful" ice storm we had recently in my area (southeastern Pennsylvania) that had broken some of our antennas and had power out for several days in some parts of the county, being able to proactively test "is this digi still working?" during an incident (preferably without having to leave the EOC or shelter) would be very useful (as in, "do we need to deploy alternate communications relays?").

My recommendation to do some sort of distant test was not unfounded. I
know of at least one digipeater where the antenna was busted by ice.
The isolator sends all of the reflected power into a dummy load, and
the radio and TNC happily continue to digipeat at flea power in the
vicinity of the site. If you park your truck by the tower, the digi
appears to work fine, but it has disappeared off the map for distant
stations (and not because beaconing was disabled ;-). We fired up a
hot-spare digi at the site and now both show on the map--the spare can
hear the digi with the busted antenna, because they live on the same

There are [at least] two benefits to testing digipeaters from afar:
1. Not having to leave your warm shack
2. Detecting antenna problems by watching for changes in coverage

By the way, if your digi beacons something, you can detect changes
without testing. For example, aprx beacons telemetry containing the
number of packets received and transmitted. If this goes outside of
the normal range, you've got a problem!

I'm halfway through a project that will automatically send me a
message when telemetry values go outside of an expected range. The
piece I need to complete is an always-on listener to pull telemetry
values off APRS RF or APRS-IS.


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