[aprssig] APRS vs. SPOT

[jjesson at voyager.net]

>From my ~10 years of designing-in commercial satellite/cellular
tracking and telematics systems:

  SPOT technology
originally was designed by Axonn's then CTO and this technology is based
on simplex protocol talking to Globalstar.... protocol is interesting as
it chips between Globalstar's native CDMA duplex protocol (native to
Globalstar). Their form of simplex is also known as send-and-pray as not
an ARQ protocol with Simplex... but a form of repeat packet transmissions
improve the odds of correctly receiving packets and this is a setting in
their engineering configuration tool.  

Unfortunately, the
fade margin (C/N) is marginal so trees or roof structure attenuate the
signal to the point of not working and the lack of confirmation was a
major issue.  Not exactly the product or wireless satellite protocol
I would use if my life was riding on packet reception but ok for a
lightweight tracking unit.  I used these for a quick light fleet
tracking system.

The new Iridium ASIC provides a true duplex
portable unit and offers a solid ARQ protocol. I have evaluated their ASIC
and it works great and would be my life-saving product of choice. 
Hey, all the ice road truckers use Iridium tracking devices and they do
bet their life on this satellite network!  One word of caution -
Iridium data costs are not for the typical experimenter but if needed
commercially a good deal if you need  very low-latency duplex data
transmission. 

 Incidentally, for the best
cost/packet  and if you can live with a longer latency and larger
antennas - such as for heavy equipment and 53' trailer fleets - you cannot
beat Orbcomm.  While both Globalstar and Iridium use L-band, Orbcomm
uses 136-150 MHz for its operation.  Now you understand the antenna
size issue :-)  Orbcomm is getting ready to relaunch this year and
they will, IMHO, become the leader for global large asset tracking.

73, Joe Jesson, KC2VGL



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