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--- blog:articles:general:orbital_shenanigans [2019/03/14 15:33] – Phil Ide
+++ blog:articles:general:orbital_shenanigans [2019/08/03 11:25] (current) – Phil Ide
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 Sometimes when you do some research – actually, quite often – you find out some really interesting stuff and end up changing your mind. In my story, I had some people on the ground on Mars, and wanted a spacecraft in a geostationary orbit above them to give them communications between them at all times. Just for info, when talking about geostationary orbits, the accepted term for Mars is aerostationary. I’ll use geostationary and geosynchronous because it’s my blog and although the aero prefix is accepted, it isn’t mandatory.
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-Since I’ve written a program that can calculate orbits around Mars (see the download section), figuring out the orbital parameters is easy enough for me, but I decided to check my results against published scientific papers. I like to be thorough in my research. This resulted in a delightful piece of serendipity, which I’ll talk about in a moment, and turned up some fascinating facts about these peculiar types of orbit that I’d never heard of.
+Since I’ve written a [[blog:articles:software:orbital_calculator|program to calculate orbits]] (including around Mars), figuring out the orbital parameters is easy enough for me, but I decided to check my results against published scientific papers. I like to be thorough in my research. This resulted in a delightful piece of serendipity, which I’ll talk about in a moment, and turned up some fascinating facts about these peculiar types of orbit that I’d never heard of.
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 To begin, I should explain the difference between geostationary and geosynchronous. I’ll use Earth as an example to make the explanations easier. The International Space Station lies at an elevation of 250 miles. It completes an orbit once every 92 minutes. The further away from the planet the orbit, the less the gravitational influence, and so the orbital speed is reduced. So the satellite is moving slower, but the circumference of the orbit is getting bigger, hence the orbital period – the time to complete a single orbit – gets longer. Keep moving out, and eventually you reach a point where the period of the orbit equals one day. This equality is a geosynchronous orbit, because the period of the orbit and the rotation of the planet are synchronised.
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 So, back to the drawing board. Let’s tackle this another way. Another orbit, what you might call a ‘regular’ orbit, doesn’t have this problem. At least, not so much. One suggestion was to use a lower orbit at an elevation of 5,000km. Consider that at the geostationary orbit (17,025km), the satellite can see 75 degrees of the planet either side of the point it is above. That’s a 150 degree spread. At 5,000km, this reduces to 107 degrees.
-A 5,000km orbit has a period of approx 0.26 days (that’s Earth days, not Martian ones, which are about half an hour longer). Using my orbital calculator, I finessed the orbit down to 4,696km. That gives it an orbital period of 6hrs 9mins and 13secs. If you do the sums in your head, you’ll see that four orbits come to approximately 24hrs 39 mins 53 secs – the same as a Martian day. So now we have exactly four orbits per day.
+A 5,000km orbit has a period of approx 0.26 days (that’s Earth days, not Martian ones, which are about forty minutes longer). Using my orbital calculator, I finessed the orbit down to 4,697km. That gives it an orbital period of 6hrs 9mins and 13secs. If you do the sums in your head, you’ll see that four orbits come to approximately 24hrs 39 mins 53 secs – the same as a Martian day. So now we have exactly four orbits per day.
-At any part of the planet the satellite can see means, from an observer's point of view from the ground, the satellite is above the horizon. At 4,696km the satellite can see 29% of the Martian surface, with a spread of 104.45 degrees. However, that is its //visual// footprint. It probably has a radio transceiver aboard to talk to objects/people on the ground, and that is very likely to have a much smaller //radio// footprint. On Earth, this is typically 60 degrees, and if our satellite had such a radio footprint, it would be high in the sky when radio contact could be established, reducing the ill-effects of mountains etc. casting radio shadows.
+At any part of the planet the satellite can see means, from an observer's point of view from the ground, the satellite is above the horizon. At 4,697km the satellite can see 29% of the Martian surface, with a spread of 104.45 degrees. However, that is its //visual// footprint. It probably has a radio transceiver aboard to talk to objects/people on the ground, and that is very likely to have a much smaller //radio// footprint. On Earth, this is typically 60 degrees, and if our satellite had such a radio footprint, it would be high in the sky when radio contact could be established, reducing the ill-effects of mountains etc. casting radio shadows.
 The satellite will be above the same point on the ground every 6hrs 9mins and 13secs. The radio footprint is 60 degrees. 60 degrees is exactly one sixth of a circle. This means a ground station will be in the radio footprint for approximately one hour. It will also be in in this footprint four times a day at exactly the same times each day.