[amsat-bb] Re: AMSAT NA
melachri at verizon.net
Thu Jul 17 16:01:53 PDT 2008
But actually, you probably mean geostationary. Geosynchronous just means that the orbital period is integrally related to one sidereal day, so the ground track repeats on a daily basis. There are lots of possibilities for geosynchronous orbits, but only a few are practical. Geostationary is a special case of geosynchronous, where the period is exactly equal to one sidereal day, the eccentricity is zero, and the inclination is also zero. The result is that the satellite does not appear to move (is "stationary") when viewed from an observer on the earth. That's the orbit typically referred to as a "GEO," and the one that's most in use for commercial and military communications satellites, direct broadcast satellites, and civil weather satellites.
>Ok, how difficult is that weird orbit that a few of
>the Russian Birds had, called something like Molylna,
>or something like that? They semed awesome and the
>best of both worlds,
The Molniya orbit **is** a HEO, albeit a special one.
- Its apogee is over the North, so it spends most of its time (better than 80%) above the equator.
- Its period is equal to one-half of one sidereal day, so its ground track repeats exactly each day.
- Its inclination (63.4 degrees) is such that its argument of perigee doesn't shift (from 270 degrees) so the apogee stays over the North Pole.
Because the apogee is so high (about 40,000 km), it's a relatively high-energy orbit, meaning it's expensive (fuel-wise) to launch. It's not quite as bad as GEO, but much worse than LEO. My rules of thumb put a factor of two between each of these. In other words, if you could launch 100 kg to LEO, you could have either:
- a 100 kg spacecraft at LEO, or
- spend 50 kg of fuel to put 50 kg into a Molniya-like HEO, or
- spend 75 kg of fuel to put 25 kg into GEO.
(Note that these are approximations. There are variations based upon the altitude of the LEO, the inclination of launch, the Isp of the propulsion system, and the accuracy of your orbital knowledge and execution of maneuvers.)
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