[aprssig] "Smart" vs. "dumb" trackers

Stephen H. Smith wa8lmf2 at aol.com
Mon Jun 23 12:58:00 EDT 2014

On 6/23/2014 11:11 AM, SARTrack Admin wrote:
> On 23/06/2014 17:03, Stephen H. Smith wrote:
>> On 6/23/2014 10:03 AM, SARTrack Admin wrote:
>>> The tinytrak-3 connected to a (cheap) handheld radio will check for an
>>> open
>>> squelch (any noise) before transmitting.
>> Except that this is not a positive carrier detect; i.e COR. All it does
>> is sense audio energy coming from the radio's audio-out.   It gets
>> fooled by dead carriers or under-deviated voice signals on the channel
>> into thinking the channel is clear.
> Correct, but how often do you have 'dead carriers' of voice data on the APRS
> frequency? It basically always is a APRS signal, so it works quite well.

Many operations have used the Mic-E technique of "tail-gating" voice 
transmissions with APRS bursts, allowing a single-frequency radio on a voice 
channel to also inform the net control station where they are after every voice 

Further, as has been pointed out repeatedly on this list, the carrier 
detect/COR/noise detect (or whatever you want to call it) is relatively 
ineffective because the low power handhelds on the ground usually can't hear 
each other.   They mostly hear the digipeater and/or base station due to it's 
presumably superior transmit power and antenna height.    As a result, you can 
(mostly) avoid collisions with the digi's transmit, but not with other trackers.

The only really practical way to avoid collisions between low-power 
low-elevation field units is precise time-slotting.   Meaning all the units 
involved need to be under the control of a single authority ahead of time, so 
that time slots can be assigned to each.

> The original question was about Trackers. Not a full two-way messaging system.

Yes it was....

"Does anyone know of a reasonably priced and reasonable performance "smart" 
tracker? That would be a tracker with a receiver (so it doesn't depend solely 
on timeslotting to avoid packet collisions)? Having some minimal UI on the 
tracker so messages could be sent back to it from the Net Control Station 
position would be a plus."

It appears that the original poster WAS seeking an "el cheapo" full 
send/receive messaging (i.e. display & keyboard) replacement) for a D72 or a VX-8

> Have a look at my original design, using a VX-170 (or Type Approved radio in my
> final design), GPS chip and TinyTrak3 in a small box, and powered from the
> radio. This setup runs for 12 hours, every 30 seconds TX (used on a special VHF
> frequency). You can also use a very cheap Chinese radio on the HAM frequencies.
> http://www.sartrack.co.nz/Trackers.html

Which is still a transmit-only device (albeit nicely packaged) of the type the 
original poster characterized as a "dumb tracker".

Your comments on the problems with digitized voice channels on hand-helds was 
interesting.  I hadn't realized this has become an issue with ordinary 
commodity VHF/UHF hand-helds.

This HAS been an issue for many years,  with trackers and security systems 
kludged into cellphone mic/earphone jacks.   The speech-centric (syllabic-rate) 
digital processing of CDMA and GSM codecs absolutely destroys steady-state 
tones like DTMF and '202 1200-baud modem tones.

[Have you noticed that when you reach an answering machine when calling from a 
cell phone, that the "beeeep" after the outgoing message comes through as a 
wheezing gasping gurgle instead of a pure tone?

Any "DTMF" over-dialing from a cellphone's keypad is sent purely as a digital 
data stream that is converted to actual DTMF only at the point when the traffic 
passes from the cell network to the (analog-emulating) land-line phone network. 
   The DTMF tones you hear in the cellphone earpiece are a "faked" sidetone, 
purely for the benefit of the local user.]



Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
Home Page:          http://wa8lmf.net

  Long-Range APRS on 30 Meters HF

High Performance Sound Systems for Soundcard Apps

"APRS 101"  Explanation of APRS Path Selection & Digipeating

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