Post by Stephen H. SmithThe 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
transmission.
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
http://wa8lmf.net/aprs/HF_APRS_Notes.htm
High Performance Sound Systems for Soundcard Apps
http://wa8lmf.net/ham/imic.htm
http://wa8lmf.net/ham/uca202.htm
"APRS 101" Explanation of APRS Path Selection & Digipeating
http://wa8lmf.net/DigiPaths