LEO or MEO for satellite mobile communication

LEO every time.

LEO satellites have less ground coverage, so they would have more precise and frequent pointing needs. They also have highly eccentric orbits most of the time, the average period of a LEO satellite is 1,5 hours. This means you would "see" a given location for minutes only at a pass. The power required for transmission would change with altitude (which is not constant); but it requires less power to operate overall communications. LEO satellites are mostly used for remote sensing and military purpose/spy satellites. They could be relatively small and (relatively) cheaper to build and launch.

MEO satellites can have a more stable orbital profile, even in circles in equatorial plane. They have a much higher period, going as far as 12 hours, so they could see a given area for much longer. So an array of MEOs can provide better coverage, and more reliability. But they require more power. I think that could be fixed with an adequate power subsystem. They would also be bigger and cost more. As far as I know, they are already being used for mobile/wireless communication.

So, MEO would be more suitable for general mobile communication, while LEO could be used for "private" communication and information needs.

<humour> Also, LEO is pretty crowded as of now, and since the recent satellite crash, it might not be very safe either </humour>

GA, what's your take on GEO with respect to the original post

Cheers,

ethobil

GEO is a very distant specific orbit (36000 km from the surface of Earth) and has an exactly 24 hour period. It is also called Clarke Orbit, because of its founder, Arthur C. Clarke, the famous science fiction writer. (Of course, the orbit was always there, but Clarke has calculated that at that specific distance the period would be 24 hours ) Its 24h period lets a satellite stay suspended above a fixed location on Earth for all times (that's why you can always turn your antenna dish to a particular direction and get a signal, the satellite is always there with respect to you).

GEO satellites have to be fairly large (and thus expensive) because they are designed for longer lifetimes, and thus have more propellant, more redundant systems etc. They are launched to a very far away and specific location; so it is costly to launch one, too. They are also more sensitive to perturbations, being so far away from Earth and closer to the moon than other satellites (it counts!) and having to stay on a very specific orbit and such. This increases the need for propellant and better ADCS (attitude determination and control system). Ergo, weight and cost! And considering that you'd need an array of satellites for coverage of entire Earth (unless you're interested only in your own area or plan to join a network of satellites for cooperation), it could be more feasible just to stick with more but smaller MEO satellites.

Besides, GEO orbit is very valuable for communications and too many satellites in there may cause interference between them, so it is regulated by International Telecommunication Union. There are "slots" in the GEO, and almost all of them are allocated to countries. So, even though you decide to put something there, it just might not be possible politically, if not technically.

So, I'd stick to the MEO unless I was the government of a country who feels strong enough to scrape off a slot off GEO if I don't have it already

Thank you archmagus and with respect to the original post, you have rightly established that LEO, MEO and GEO can be used for mobile communication.In terms of feasibility and considering the indicated disadvantages in the three, MEO is preferred.

I concur.

Cheers,

ethobil

Thanks a lot for the comment! It feels good to know that one's information is correct and acknowledged.

I think the main reason is the delay. It has a single hop delay of 250ms and with processing it will come close to 300ms.If it has more than one hop then it becomes relatively useless for mobile communication.

And yes thank you very much for the comparison you made on MEO and LEO.

By the way is it costly to implement a MEO system using intersatellite links?

Unfortunately, I have very limited information regarding real-time transmissions so I don't have anything to say about that. But what you say makes perfect sense; real-time communication would be useless after a certain amount of delay.

About implementing the system; yes, it would be costly. The satellite(s) and launch costs will be more than enough to call it costly. Current communication satellite "constellations" can number as much as 24 satellites, so they don't cover the entire Earth with just 3 or 4 and get on with it. That is extremely redundant; which is good because an Iridium satellite crashed with an old Russian satellite lately; and if there were only 3 in that constellation, you'd lose your entire communication. Not to mention the bandwidth required for mobile communication; there's simply no way a few satellites can cut it.

If you mean contractual arrangements by intersatellite links, I guess it would be costly too (if they put the satellite there, they would want to squeeze its cost out of people) but of course not as much as making and launching satellites yourself. As far as I know, there is a lot of deals going around like that in the world of communications. But then again; I'm no authority on the subject. :)

hmm.. well I was referring to antennas, tracking etc when it comes to intersatellite communication..Anyways thank you for your answer

Well, what I can say about that is visibility matters. The antennas you use shouldn't make a huge difference in cost, but more satellites means more visibility; these things are always on the move and you want to make sure any given satellite sees at least a few others at the same time. You'll also want to transmit the signal towards a particular direction I assume. So it better see a satellite in that direction. You can achieve that by monitoring the poisition of each satellite and calculating the necessary route of transmission.

You can design the antenna placement so that they have a visual all around, so that you can transmit the signal in any direction wihtout turning the satellite. This would save a lot of propellant, and in turn, lenghten the service life. The money saved is huge when compared to the cost of a few more antennas per satellite...

By the way , what is the best form of modulation scheme and the access method that can be used?

The best form of modulation scheme and the access method that can be used depends on what the communication satellite is intended to achieve considering the pros and cons of the available modulation schemes and access method

Especially for the access method, considering the subdivisions of the access methods viz;

Frequency Divsion Multiple Access (FDMA), Time Divsion Multiple Access (TDMA) and

Code Divsion Multiple Access (CDMA)

If the objective is for, high spectral efficiency (Bandwidth availability) and excellent channel partitioning, TDMA and FDMA are ideal in the order of preference.

Conversely, for non interference between adjacent channels and multipath effects, CDMA is ideal as this is immune to interference between channels.

I will add that the best modulation scheme should depend on the access method used (I will check and though more informed comments are welcome here)

Cheers,

ethobil

well I was referring to a mobile satellite system.. the type of modulation and access method that can be and is used for that application

May be for power efficiency, QPSK is very popular and likely the best of the modulation schemes for Mobile satellite communication.

Cheers,

ethobil