Geosyncrous (sp?)
Geosynchronous.
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.
http://en.wikipedia.org/wiki/Molniya_orbit
Its characteristics:
- 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.)
Steve W3HF
Hi Stephen,
Hey Just came a light bulb above my head, something i never thought about till now,
what does average birds weigh now? and why?
Stephen Melachrinos wrote:
Geosyncrous (sp?)
Geosynchronous.
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.
http://en.wikipedia.org/wiki/Molniya_orbit
Its characteristics:
- 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.)
Steve W3HF
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
----- Original Message ----- From: "Stephen Melachrinos" melachri@verizon.net To: amsat-bb@amsat.org Sent: Friday, July 18, 2008 1:01 AM Subject: [amsat-bb] Re: AMSAT NA
The Molniya orbit **is** a HEO, albeit a special one.
http://en.wikipedia.org/wiki/Molniya_orbit
Its characteristics:
- 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.
Steve W3HF
Hi Steve, W3HF
Here is the problem with the Molniya orbit. Since the inclination is 63.43 degrees then the argument of perigee doesn't shift because the line of the apsides that join the apogee and perigee do not rotate. If after launch the argument of perigee is placed to 270 degrees then the apogee will stay over the Northern emisphere for the full life of the satellite.
More exactly the latitude of perigee will be:
Latitude of perigee = arc sin ( sin 63.43 x sin 270 ) = - 63.43 degrees in the South emisphere
Changing the signe the latitude of the apogee will be + 63.43 degrees in the Northern emisphere
Forever !
In this conditions the satellite will be not useful for South America , South Africa, Australia and so on in the Southern Emispfere.
This is why the Molniya orbit is nice to broadcast TV and make phone communications for the Russian areas but not for us worldwide.
In addition as soon the Molniya satellite is at the apogee the speed is very low and the orbit is unstable because it is strongly perturbated by the forces of the moon and the sun and very soon the perigee will aproach the earth and the the satellite will be lost.
73" de
i8CVS Domenico
participants (3)
-
i8cvs -
Joe -
Stephen Melachrinos