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Re: geostationary packet sattelite



Chris, KC8GOQ suggested:
> Has anyone ever tried or thought of a geostationary packet satellite? All it
> would be is a digi and nothing esle, a basic packet repeater.
> 
> What commercial companies are developing a geostationary bird?  maybe we could
> hitch a ride in one with a small digi (an ht sized transceiver, and a tnc)
> 
> it could help revive packet a bit.....

You get several good answers, but let me expand. F6FAO pointed out that the
commercial GEO spacecraft would rather fly Hydrazine for station keeping. If
you can extract a longer lifetime from the satellite by carrying a bit more
propellant for station keeping, then you can make more money. AMSAT has tried
for more than 25 years to find a piggyback GEO opportunity and has never been
able to find one. 

WB4APR correctly noted that GEO requires a lot more power in both directions
[KM1P was wrong in saying that the signal drops off as the square-root of the 
distance -- it goes as the SQUARE of the distance]. GEO is 40-50 times as far
as LEO, so 1600-2500 times as much power is needed (i.e. about 31-32 dB). And
that kind of power is needed by the user on the uplink and at the satellite on
the downlink. The commercial GEO satellites solve this problem using microwave
frequencies and dishes on both ends of the link. Hardly an "HT-sized" mission!

Let me point out a couple of other problems that would be encountered. If you
are
thinking about a simple FM radio link, remember that FM allows only the
strongest
signal to be used. And yet from GEO, the satellite would see one-third of the 
earth. How many users would be contending for that single uplink? How many
would
step on other users mid-packet? The solution is that the "simple digi" would
have
to act as "boss" telling which users could transmit when and pacing the link
very carefully.

It takes about a tenth of  a second to send a signal to GEO, and then another
tenth 
for it to come back to earth. This time -- plus the length of the data in the
packet constitutes the time interval during which a user must "own" the
channel
if he is not to be clobbered. And that discipline must apply to all the
amateurs
(who can't hear each other!) over a third of the area of the earth.

Problems like this COULD be solved by designing special protocols, by using
modulation techniques like CDMA Spread Spectrum, by enforcing both timing and
power discipline on the users, and by going to microwave frequencies for the
links. But then it is hardly the "HT and a TNC" mission that was suggested.

73 de Tom, W3IWI

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