SPOT messenger

On May 15, 8:19 am, "Robert M. Gary" wrote:
On May 14, 11:34 pm, Darryl Ramm wrote:

On May 14, 4:37 pm, "Matt Herron Jr." wrote:
The SPOT does not "wait for an opening" to talk to Globalstar, it has
no idea where the Globalstar satelites are, it just sends blind, then
retransmits the same message later. It's pure simplex, there is no
"carrier detect", no handshake, no ACK. Nothing. The Globalstar
satellites are just dumb one-way bent pipe repeaters. Sometime being
simple is a beautiful thing.

I always assumed the 20 minute thing was put into the SPOT system
because sat time is expensive.

I've got to be believe the 20 mins is a wild design guess on their
part. The practical transmit reliability with SPOT across their
intended users has got to be dominated by environmental factors,
device antenna direction, obstructions (trees, mountains, etc. for
hikers, car roofs) and finally satellite position. Modeling the number
and durations between retransmit attempts to deal with these factors
would be very difficult. You'd start with a simplified model that just
factored satellite coverage and message collision probabilities and
guess from there. You want to cap retransmits to reduce battery
consumption and just excess further collisions. For 911 it keeps
retransmitting much longer AFAIK. And the only "cost" for the sat time
is the risk of further message collisions.

Darryl