[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: Another view



>From: Phil Karn <karn@ka9q.net>  {edited}
>The original comment to which I was responding made some blanket
>claims about the fundamental limitations of LEO satellites. I was
>attempting to elicit the assumptions behind those claims.
>
>A Globalstar satellite in LEO has the same geometric limitations as
>any other satellite in the same orbit, including amateur. Even the
>path losses are approximately the same as a hypothetical amateur
>L/S-band LEO satellite.
>
>That leaves the likely higher EIRP of Globalstar, the likely greater
>capacity of a Globalstar satellite (due to multiple beams) and
>satellite availability with the likely smaller size of an amateur LEO
>constellation. Let's address each in turn.
>
>The lower EIRP of the amateur L/S LEO satellite will admittedly
>require ground antennas larger than those on a Globalstar phone, but
>still well within limits for a transportable station. Certainly much
>smaller than an AO-40 class station, so I don't think that was an
>issue.
>
>The smaller capacity of the ham satellite is not likely to matter
>unless we are *very* successful. I would like to have this become a
>problem, just as Globalstar would like to have the limited capacity of
>their satellites become a problem. :-)
>
>As for the number of satellites in the constellation, we need to know
>not only the fraction of time that each LEO satellite can be seen from
>a given point on the earth as compared to a spacecraft in a P3D-like
>orbit, but also how many LEO spacecraft could could be built and
>launched for the cost of each large P3D-class spacecraft.
>
>The fact that two separate commercial LEO constellations have been
>built implies that at least two sets of systems people felt that doing
>it that way had some advantages over using fewer spacecraft in higher
>orbits.  

{BTW there seem to be two or three threads runnning here on about the same
topic} {below is just re-phrasing what Phil has said for the most part}

Obviously there are two philosophies working here:  
1.  build low-earth-orbiting satellites with decent power to provide easy
access with minimal ground equipment, but requiring a constellation of
satellites to provide constant coverage and/or linking for global coverage, or
2.  Build a geo-sync or high orbit satellite(s) that will provide 1/3+
earth coverage, but require more extensive ground station capability.

Individual, unlinked, Leos suffer the limitation of small footprint and
short term accessibility.  But they are easy to work with modest equipment.
 They also do not afford much channel capacity, to date {popularity for
AO-27 is not a problem....using it???}.  So if this philosophy is to be
pursued for ham-sats, I think inter-satellite {or I-gate} linking,
muti-channel, and digital mod schemes are ultimately the way to go.  Now if
a 100 KHz passband FM bird were practical, it would compare with the
through-put of the ssb/cw passband sats.  I think satellite power-budgets
are the problem, though.

Molinya orbiting satelites do require more ground station capability, but
solve the "footprint" issue; cw/ssb solves the through-put problem.

Regarding ground station needs, in general:  The small tripod/dish compact
station is achievable by going to microwave frequencies {mode L/S is most
obvious}.  This is easier to track with "philosophy #2" satellites then the
Leos.  For the Leos, a hemi-sperical omni antenna system for the ground
station is desirable, which requires certain minimum EIRP levels from the
Leo {exactly where the commercial sats have gone}.

So bottom-line is what can amateur satellite community afford to do?  (this
is left as a homework problem for the rest of you ;-)

Now be Happy!  Enjoy the Holidays.  Pray for each other!  {and maybe for
little AO-40, too}

Ed
----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home