Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: CliffordK on 23/05/2012 00:11:48
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One of the problems with most of our satellites is that they use chemical propellants to maintain orbit. As the chemical propellants run out, the orbits eventually destabilize, and the satellites are lost.
Could one design a polar orbiting satellite to use an electromagnet to maintain orbit using the Earth's magnetic field? One might choose a polar orbit because of the generally N/S orientation of the magnetic field, so one would orient the thrusting magnet with the Earth's magnetic field. Energy could be derived with solar panels.
I realize the field isn't that strong, but in a moderately high orbit, one wouldn't need a lot, and a large solar array would be expensive to drive the magnet. Perhaps one could use a large rare earth permanent magnet and a counterbalance to orient the field as desired.
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... Perhaps one could use a large rare earth permanent magnet and a counterbalance to orient the field as desired.
I think the spacecraft would rotate to align its magnet with Earths magnetic field, like a compass needle.
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I think the problem is that there is always a certain amount of friction. It's not much, but over sufficient time, some of the sat's kinetic energy is lost.
A sat can easily capture solar energy, but it can only convert that energy into kinetic energy if it has some mass that it can accelerate. I think that's the idea behind ion engines.
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... Perhaps one could use a large rare earth permanent magnet and a counterbalance to orient the field as desired.
I think the spacecraft would rotate to align its magnet with Earths magnetic field, like a compass needle.
One could always orient the magnet using a gyroscope and counterbalance. However, it may not be a problem if it orients itself with the magnetic field, as that is probably what is desired.
A permanent magnet may in fact be problematic, especially near the poles. One should be able to simulate with a strong bar magnet and a small magnet orbiting it.
However, with an electro-magnet, one could always just use the attractive force.
I.E. Just use it on the northward journey from the equator to near the N Pole, then again on the southbound journey from the equator the the S Pole again.
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I presume the energy gained from the electromagnetic coupling would be subtracted from the satellite's momentum, causing its orbit to decay.
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I think the problem is that there is always a certain amount of friction. It's not much, but over sufficient time, some of the sat's kinetic energy is lost.
A sat can easily capture solar energy, but it can only convert that energy into kinetic energy if it has some mass that it can accelerate. I think that's the idea behind ion engines.
The friction is with the atmosphere. I don't see why some of the atoms/molecules of gas that cause the friction could not be captured, ionised and accelerated to a high velocity using power from the solar panels. There would have to be a high percentage captured and then accelerated to a higher speed than that of their impact speed but, at least in principle, it should be possible. It also has the advantage that the ability to counteract the decay would be proportional to the frictional cause of the decay.
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The polar orbiting weather satellites orbit at 800km which is sufficiently high for drag not to be of any concequence, 20 year old satellites are still in orbit although not transmitting anything useful
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The polar orbiting weather satellites orbit at 800km which is sufficiently high for drag not to be of any concequence, 20 year old satellites are still in orbit although not transmitting anything useful
Hmmm,
It may be that the location of an orbiting weather satellite may not be as critical as the location of a geosynchronous communications satellite. However, many of the weather satellites have experienced diurnal and potentially altitude drift which is a major issue for long-term climate monitoring. Whether a magnetic field thrust could compensate for diurnal drift would be another issue.
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Geosynchronous orbits can be quite eccentric (in both meanings of the word) but when you refer to telcomms satellites being geosynchronous it usually means the special case of them being geostationary. In this case the satellites are 22,236 miles above the equator and not subject to significant decay from interaction with the atmosphere. Low Earth Orbiting (LEO) satellites are the most problematic for decaying, GPS satellites being an example.
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Graham - don't get your last. GPS satellites are not in LEO - they are in MEO at around 22000km altitude. I don't think they are subject to significant orbital decay.
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Imatfaal, you are quite right. As someone who worked on the design of one of the first (military) GPS receivers I should know that!! They will not be affected by atmosheric drag either.
There are quite a few LEO satellites up there (because it's cheaper I guess) for specific purposes. There are some being used to track positions of cargo containers for example. As you are in a related business you may know of this.
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I am unsure what the individual container-trackers use - all of our ships have multiple GPS stations, both integral to ship, to all tenders and handheld. I have always assumed that some of the container tracking that is advertised is a little bit of a fudge - they know which ship the container is upon and they know where the ship is; rather than actually knowing where the container is. On board a large "postpanamax" container vessel - a unit could be 8-10 containers deep into the stack in every direction; I cannot believe a GPS unit would still get a signal with that much steel around.
The inmarsat communications network that we rely upon for comms and GMDSS (http://en.wikipedia.org/wiki/GMDSS) are geostationary (ie 22000 miles) orbit.
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I don't think they are supposed to be in contact when stacked but that their last point at which contact disappears and then subsequently reappears is sufficient. I guess this is sufficient and they can certainly be tracked if on the road. I am unsure about the software control but know a bit about the hardware.
Yes, Inmarsat satellites are geostationary. The problem that results is that the phones have to use a fair bit of transmit power as a result so need biggish batteries (or have a short talk-time without recharging). You can get away with smaller batteries with a directional aerial of course (the bigger the better). It does not matter on a boat but is a bind if you want something in your pocket whilst walking in the desert (say). Still it is a good system for guaranteed contact wherever you are in the world.
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The containers are secured up and across to their fellows at the corners by a doogit called a twistlock (jeez I would have loved to have patented that one!) The gap is maybe an inch between the corners of adjacent containers and a couple to half a dozen inches between the slightly recessed sides - Could you really get a signal through those sorts of gaps? (Signals and electronics has always been a dark art to me)
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And when you mentioned hand-held phones it dawned on me which ones definitely use LEO - Iridium Satellite Phones (http://en.wikipedia.org/wiki/Iridium_satellite_constellation)
and a really nice diagram showing orbits
http://upload.wikimedia.org/wikipedia/commons/b/b4/Comparison_satellite_navigation_orbits.svg
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So, you're saying a really big cell-phone would keep a satellite in orbit?
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One like this might just work
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Now that's what I call a phone! Only really important people can afford one that size. Where can I get one?
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The containers are secured up and across to their fellows at the corners by a doogit called a twistlock (jeez I would have loved to have patented that one!) The gap is maybe an inch between the corners of adjacent containers and a couple to half a dozen inches between the slightly recessed sides - Could you really get a signal through those sorts of gaps? (Signals and electronics has always been a dark art to me)
Probably not. Certainly the lower ones would struggle. I think the application software (for containers) may be able to deal with the transfer onto a boat, then just track the boat until the individual container is visible again.
As a matter of interest he RF receiver has to allow for the Doppler shift from the LEO satellites as they pass over.
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The containers are secured up and across to their fellows at the corners by a doogit called a twistlock (jeez I would have loved to have patented that one!) The gap is maybe an inch between the corners of adjacent containers and a couple to half a dozen inches between the slightly recessed sides - Could you really get a signal through those sorts of gaps? (Signals and electronics has always been a dark art to me)
Probably not. Certainly the lower ones would struggle. I think the application software (for containers) may be able to deal with the transfer onto a boat, then just track the boat until the individual container is visible again.
As a matter of interest he RF receiver has to allow for the Doppler shift from the LEO satellites as they pass over.
Yeah I can imagine the need for an allowance - what speed are they doing relative to ground 5-10 km/s ?
Now that's what I call a phone! Only really important people can afford one that size. Where can I get one?
I thought that might appeal to the geezer's love of the mobile bephoned businessman. After posting it yesterday it was truly at the front of my mind when I had to take a business call on the train this morning - "Yes - we can counter at 3.1million USDollars..." suddenly everyone in the carriage wants to know what I am buying/selling and I realise I am Mr Annoying-MobilePhone-Man
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It sounds like the individual container tracking is much easier to do when the containers are loaded onto a train than when in the bottom of a hull deep within a cargo ship.
If one can't really track the individual containers, then the system is not much better than using bar codes.
Would a local ship-board repeater help?
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A repeater would not help as it would still need to communicate with each container. This would be no easier than the satellite doing it. Ships themselves can be tracked though as well as any containers at the top of the stacks, so if the software is clever enough, it would know that these containers were on the same boat. I guess that the system only has to know the last position before it goes out of communication. If that position is on a boat or in a stack somewhere, that is good enough. It is probably more important to note if a container is somewhere it should not be i.e. misplaced or being stolen.
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Some container vessels will provide power for chilling to containers (really not sure if the big ones can do this) - if they did that they could link up individuals to main GPS beacon; but that sounds highly unlikely.
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GPS satellites are transmit only and there is no reason for the containers to know where they are if they can't tell the LEO satellite network where they are. In theory the containers could be connected to some master system but I don't think they are; they all just operate independently. All that is needed is for the system to know they are on the boat and then for the system to know where the boat is. The containers don't have to communicate at all once aboard (even if they try to). I think this is how it works however I don't know much about the system software from my limited perspective.
http://www.satmagazine.com/cgi-bin/display_article.cgi?number=27078890
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Periodically containers do wash overboard. Although, I presume those are most likely those on top of the stack, revealing the next lower conex boxes that would suddenly show up on the net.
Assuming there is adequate power for the system, it would be easy enough to make a local ad-hoc network with each container communicating with other local containers. Then you could, in fact, transmit reefer unit info to the bridge, as well as the company.
I suppose I would be leery of a system that would just assume there would be 100% tracking of containers as they are loaded onto a ship. So, if a crane is moving a stack of a half dozen conex boxes, the bottom one would get not get missed. And, there is never a momentary system hiccup in which data isn't collected.
As mentioned, if it is merely knowing when a conex box is loaded and unloaded, then one could simply track the boxes with bar codes and bar code readers that the transfer equipment would periodically pass.
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The idea is to track the containers on road, rail, sea, stored or stolen. Obviously the company who own the satellites are trying to sell this service which may or may not be flawed in some ways. There has been a fair investment in the concept so far though.