I believe its’ the opposite - the satellite is highest (furthest) when it’s above North Americal.
Orbital speed is fastest at the lowest point, and slowest at the highest point. (Basically Kepler’s 2nd Law). So the satellite spends more time at the “high” part of the orbit than at the “low” part. Now if you tilt this highly elliptical orbit so that the highest point of the orbit is above the Northern hemisphere, the satellite spends most of its time above the Northern hemisphere. Which is perfect if you’re mostly broadcasting to North American customers.
Sure, but since Sirius is only licensed (AFAIK) for the US & Canada, they’re going to tell me not to try it, or that it’s impossible, or some other platitude that satisfies their legal department.
There is internet service. It seems likely that is where they are getting it. Sirius or XM do not broadcast to Europe or Africa.
I know the volume may be down on this keyboard but I will try again. I was on a Caribbean island. Sirius worked just fine.
The coverage map from Sirius shows anything west from and including Puerto Rico will be covered. If you go east of Puerto Rico you may run into signal problems.
Before we had the radio sent we asked customer service and they said they knew it worked in Puerto Rico but were not given information about anywhere else. If you look at their map you can figure out the rest.
That is pretty much what I said too, but then from the link provided it looks more like the Sirius satellites are “hovering” instead of “orbiting”. I think that would be a curious use of the term just because the orbit is eccentric due to propulsion to optimize usefulness.
Are you saying that a circular orbit must be over latitude 0 (the equator), and that an elliptical orbit can be inclined to the equator, but that any object in space that is not in either of those types of orbits is not in orbit at all?
If so, then what exactly is it in?
And nobody would confuse a satellite with a helicopter, so what is it? Hubble for instance is in a low earth orbit, I foget the details, but it is certainly not geostationary nor would I think it is either strictly circular or elliptical. Is it not in orbit at all?
I worked in operations on another satellite, don;t know if it is still out there or not, IUW (launched circa 1977 I think). Its orbit was “near geostationary”, in that instead of a 24 hour trip around the earth, it was 23 hours and 58 minutes or so, and once a month it would have lost an hour with regard to the 8 hour operations shifts. Which led to the occasional but predictable result that the night shift sometimes worked during the day as they stayed with the shift, not the clock.
It was a hypothetical. I agree there are no good ways known to use propellant efficiently to move around and not run out rapidly.
No. Earlier you claimed that a “stationary” orbit can be achieved above any point on the earth. I was trying to explain why this is incorrect: If a satellite was “fixed” above anywhere other than the equator (e.g. if it was directly above Boston all the time), then the center of this orbit would not be at the center of the earth. So such an orbit would be impossible.
Of course orbits can be elliptical, or inclined, or both. But the earth must occupy one of the foci of the ellipse.
The flight path of a powered aircraft/spacecraft is, well, a flight path. Or trajectory. As far as I know, the word “orbit” refers to the trajectory of an inert object.
Been decades since I worked on this stuff, and even then it was more attitude then orbit.
I no longer have the definitive (or any for that matter) texts I once had.
So can you explain why this is? I will plead ignorance for now.
OK, but now we seen to be quibbling about if an object is powered or not . If so, it is not in orbit, but rather in flight. If not, then it is in orbit.
Maybe in a technical sense, but that is not the common vernacular.
Are the current Sirius satellites in orbit or on a fllight path in outer space to you?
Still, it is not clear to me that, other then degradation due to gravity, friction with the remnants of the atmosphere, and matters of that sort, that you couldn’t have a non-powered object hover over Boston for instance.
Here is a scenario. Feel free to knock it down, but let’s use it as a starting point for a discussion.
Shuttle launches with non-powered (what I think you mean by inert) device.
Shuttle travels to point over Boston, traveling parallel to the equator (iow perpendicular to the axis defined by the poles).
Shuttle travels and a ground velocity/angular velocity such that it appears to hover from the ground. It takes Boston 24 hours to travel the circle of its latitude, whose circumference is pid where d is the diameter of the Earth. Shuttle takes 24 hours to travel the circumference pi (d+a) where a is the altitude.
I am too lazy right now to look up the constants, but I am sure everyone here knows what I mean, and agrees we could calculate the speed the shuttle needs to be traveling given the altitude.
So shuttle achieves this, opens bay, with robotic arm extends object. Object has the same velocity as shuttle. Shuttle gently releases grasp on object and backs away.
What’s to say the object is not in unpowered geostationary orbit at that point? It would be carrying the same inertia as when it was part of the shuttle, and be on the same flight path. If we ignore minor friction, and less minor gravity concerns, that is all it takes, right?
And let’s go back to friction and gravity - they do affect the orbit of all objects, so afik all satellites meant to last DO have power systems to adjust the orbit, if not continuously, then at least periodically.
Are your earlier definitions to say that other then space junk, missions meant to degrade permanently, and naturally occurring objects such as the moon, that nothing is in orbit at all, but rather the rest of the man made objects are merely on some flight path?
Not so. Being well north of the equator, Boston’s 24-hour circle has a diameter substantially smaller than the Earth’s (it’s actually D[sub]earth[/sub] * cos (Boston’s latitude)). For a more dramatic example, think of someone a few feet from the North Pole - what’s the size of his circle?
The critical point is that the plane of Boston’s circle does not include the center of the earth, something the plane of any orbiting body must do (as scr4 has noted). Why? Because the force (gravitation) that holds the object in orbit always acts on a line between the center of mass of the two bodies.
(Note that this ignores tiny effects such as the way the moon and sun perturb orbits).
You are absolutely right, and I alluded to this in my earlier post. Told you I was feeling lazy today
Agreed.
Hmmm.
That seems right to my lazy self right now. Seems that would cause a force to pull in a southerly direction (towards lower latitudes)Mayb.
Even so, propulsion could counteract that force.
Still waiting for clarification from scr4 on his distinction between orbit and flight.
Maybe I should have said geostationary flight in space instead of geostationary orbit to satisfy him.
Because we have these Sirius satellites on some kind of path, and depending onthe size of the spot, you can say they are hovering over it.
Reduce the spot to increase precision, and modify the path and the conclusion remains. Take that to the limit and you are left with a path that is essentially a hover and it could be over any arbitrary location.
But it would involve propulsion and it that makes it flight instead of orbit, then I would be interested in a cite to that. If you pressed me, I’d say flight is travel through the atmosphere, with or without propulsion, and orbit is travel in space, with or without propulsion either in an elliptical path around he earth or at a consistent altitude above it (depending on operational requirements), again with or without propulsion.
By the latter def, I mean to rule out trips that head away from earth to never return, although if they do return for a swing-by because of some complex path meant to cause acceleration for an eventual trip away for good, we can argue if it is in orbit or simply leaving at some arbitrary point in the trip, but that is the edge case and not really the issue here.
I think I can speak for scr4 by saying that orbit means freefall - object creates no force of its own, is influenced only by gravity. Flight involves some force in addition to gravity, generated by the object itself.
Not for an orbiting body it couldn’t, for the reasons given above. “Hovering” at any altitude over, say, Boston requires following a path whose plane does not contain the center of the earth. Since the only force at work is one directed at the center of the earth, the object will immediately be pulled out of its hovering position.
Have you agreed that Boston’s 24-hour circle is smaller than the diameter of the Earth? And thus that any object “hovering” above Boston does not have the center of the Earth in the center of its 24-hour circle? Then here’s a mental exercise: think of a weight on a string - you hold the string in your hand and swing the weight in a vertical circle. Can you cause the weight to swing in a circle that does not have your hand at its center?
Motion of an object around another due solely to gravity is rather important, and it’s proved quite useful to have a term for this. Since you are giving “orbit” a different meaning, can you suggest a term that could now take on its former meaning?
The Sirius constellation of satellites (three of them) occupy what are called “Tundra” orbits. This orbit has a 24-hour period, but it is not geostationary. Instead, it’s highly elliptical, and also highly inclined to the equator. Because its orbit is highly elliptical, a Sirius satellite spends very different amounts of time in different parts of its orbit. It moves very slowly when it’s far away, and very fast when it’s close.
The perigee (closest approach to earth) happens somewhere off the coast of South America. So when the Sirius satellite is in the Southern Hemisphere, it’s near perigee, and it whips around that side of the earth very quickly. It only spends about 8 hours of its 24-hour orbital period below the equator. The other 16 hours are spent over the Northern Hemisphere, where it slows down on the way to apogee (farthest point), and then slowly speeds up on its way “back down”. The apogee occurs over Canada, somewhere just north of the border between Manitoba and Nunavut.
Because of the highly elliptical orbit, the Sirius satellite is moving quite slowly in that part of its orbit over the Northern Hemisphere. It nearly hovers over the north-central part of the US. A look at the ground track shows a figure-8 pattern. That’s because when the satellite is in the slow-moving “apogee dwell” portion of its orbit, it’s actually being outpaced by the Earth rotating under it. It appears to move backwards in the sky for a time, before it gains traction and catches back up. This jockeying for position between the satellite and the part of North America underneath helps to maximize the time the satellite is visible to the people with the receivers.
So the bottom line is that Sirius satellites are placed in orbits that maximize time spend over the northern US, and minimize the “wasted time” spent over areas they don’t serve. The system is set up so that there are always 2 of the three satellites over the Northern Hemisphere at any given time. The three satellites are 8 hours apart in the same orbit. So when one is at apogee, the second is crossing the equator northbound and the third is leaving the Northern Hemisphere.
Nor to Alaska. When I called them a few years ago, I got a bunch of gibberish about “overseas licensing”, but I suspect it has more to do with profits.
