Geosynchronous orbit

Can any orbit be geosynchronous?


The period of an orbit varies with distance. Low earth orbit is 90+ minutes. The moon is at a very high orbit (250,000 miles) and orbits in about 29 days. So somewhere in between is the precise distance where the orbital period is exactly 24 hours a day. (I got a headache and would show you the equations otherwise.)

Note that elliptical orbits can be 24 hours in length but only a perfectly circular orbit will maintain a satellite over the same spot on the equator all day.

Now’s where my ibuprofen…

You mean the same spot in the sky ? yes , though i think its called geostationary in England , the Astra and SKY TV networks are geostationary at a high altitude between the moon and the earth and keep a transmission footprint broad cast over western Europe

No. Geosynchronous orbit is a very specific orbit. It has to be at an exact height (35,786 kilometers) and velocity, and it has to be about the equator (zero degrees orbital inclination)

oops didn’t read the question , only equatorial orbits can be geostationary so no any orbit cannot though somebody with a major maths knowhow may proveme wrong

So what is a Polar Geosynchronous orbit?
Are the TV satellites for the Northern and Southern hemispheres in the same orbit about the equator?

Ther is no orbit which will keep a sattelite permanetly over the poles. There are polar orbits, and there are geosynchronous orbits, but they aren’t the same thing.

All geosynchronous orbits are over the equator. So yes, all sattelites for both the north and south hemisphere are in the same orbit; you just have to aim your sattalite dish south if you’re in the norther hemisphere, or north if you’re in the southern.

Thanks, all.

No such thing, but you can actually get close enough to be useful, at least temporarily. A Molniya orbit, named for a series of Soviet communications satellites that used it, can serve almost as well, and satellites using it may be more useful than equatorial ones at high latitudes.

A Molniya orbit is extremely elliptical, with the satellite much further away from Earth when over the Poles than when over the Equator. As seen from the ground at high latitudes, a Molniya-orbit satellite spends a long time in a fairly close angular distance from the Pole. A fixed antenna with a little angular range in its reception capability can therefore track it for enough time to be useful. If you have a series of satellites in the same Molniya orbit, they can be spaced so that there’s always one in range of the fixed ground antenna pointed to the Pole.

Just to add:

If it weren’t over the equator the orbit would kinda be stationary. That is, it’s azimith (compass degree) would stay the same, but it’s declination (angle above the horizon) would wobble up and down by the degree it was inclined to the equator.

Not being particularly useful, I doubt there are any satellites with such non-equatorial, semi-geosychronous orbits.

I guess technically you could have a Polar Geosynchronous orbit … but not for long. :wink:

Unless it was far enough away from the planet, following the Earth’s orbit around the sun, making corrections for seasonal tilt. Probably much more trouble than it’s worth though. Plus it would be a solar orbit not an Earth orbit.

Despite the fact that they are often used as synonyms, my understanding (and I’m supported by this site) is that geosynchronous and geostationary mean two different things.

Geostationary orbits have the same period as the primary and stay above one spot on the ground. They can only be above the equator.

Geosynchronous orbits just have the same period as the primary. A geosynchronous orbit with non-zero inclination has a ground track that’s a figure-8 centered on the equator. It traverses that figure-8 once a day. And yes, you can have a polar geosynchronous orbit (one with an inclination of 90[sup]o[/sup]). It will go from pole to pole.

Perhaps you were thinking of something called a “sun synchronous” orbit:
This type of orbit is used by Earth resource monitoring satellites (e.g., LANDSAT).

What you’re talking about, Hail Ants, is a special case of what dtilque is calling geosynchronous (but not geostationary). The figure-8 ground track in the more general case is a result of the orbit being elliptical rather than circular. On average it will have the same angular velocity as the Earth, but at apogee it will be less than that and so will “fall behind”, then catch up as it is greater at perigee. Coupled with the up-and-down motion in declination you identified, this gives the figure-8. Elliptical orbits are used in place of circular ones because they allow either a lower altitude over the target area or extended time over it.

And while I don’t know for certain offhand, I suspect that, far from being non-existant, such geosychronous orbits are more useful and common than simple geostationary ones, since they provide better coverage of high latitude areas like North America and Europe. A common configuration is to have three satellites in related orbits sweeping out the same ground track over the course of the day to give continuity.

What sort of orbits do GPS satellites use?

GPS satellites are in low earth orbit, and therefore move across the sky pretty rapidly. That’s why we need to have a constellation of GPS satellites in orbit instead of just three. The last time I looked, there were 21 GPS satellites in orbit, that being the number required to make sure that there are always three within line-of-sight anywhere on earth.

You need four GPS satellites in order to resolve a position in three dimensions. If you can only lock on to three of them, you lose altitude tracking. If you can only lock on to two of them, you can determine your position but not very accurately. With just one satellite, you can determine your position within a circle determined by all the possible solutions for the fixed distance measured to the satellite. Imagine a cone from the satellite to the ground - with one satellite, your position can be determined to be anywhere inside the cone. With two satellites, the intersection of two cones determines your position, which will be a much smaller ellipse. With three, you get the intersection of three cones. With four, you can add altitude information, and you also get maximum resolution because the intersection area is where all four cones overlap.

The reason GPS satellites aren’t in geostationary orbit is because accuracy would suffer greatly. Low-Earth Orbit is 150-250 miles up - Geostationary orbit is 24,000 miles up. The ‘cone’ from the GPS satellite is now going to be HUGE. So even with three or four satellites, the intersection area would still be miles in size.

I think you’d run into all kinds of other technical problems as well. Maybe pertuberations in the orbit due to the shape of the Earth or pull of the Moon or something would put a hard limit on accuracy. Anyway, it’s not an option.

How does a GPS sat “know” where it is? Does it triangulate with ground stations?

Not necessarily. Suppose we have a circular, geosynchronous (but not geostationary) orbit, with some nonzero inclination. The speed of the satellite will be constant, but the azimuthal component of the speed won’t be, due to the inclination. Specifically, when it’s crossing the equatorial plane, it’s moving at an angle, and the azimuthal component of its velocity would be v[sub]0[/sub]cos([sym]q[/sym]), and would be moving slower than the ground beneath it. At the high-latitude points of its orbit, by contrast, the satellite’s azimuthal motion would be v[sub]0[/sub], and its angular speed about the Earth’s axis would be greater than average, since it’d be closer to the Earth’s axis. The eccentricity of the orbit would have the effect of distorting the shape of the figure 8, such as perhaps making one lobe larger than the other.

carnivorousplant, the GPS satellites know where they are because they know what orbit they were put into, and the math for working out their future positions based on that has been known since Newton. For more precise information, you also want data from ground stations, but it’s not really necessary.

So they have a clock? Worthy of my physics prof touching the bowling ball pendulum to his nose and releasing it.

He did blink.

The receiver knows where the satellite is. Each GPS receiver contains an ‘almanac’, which tells it the position of each GPS satellite at any arbitrary time. When a receiver picks up a GPS signal, the identity of the satellite is contained in the data stream. The receiver does a lookup in the almanac, and retrieves the position of the satellite.

This works because orbits are very predictable. If you know the position of a satellite and its orbital parameters, you can predict where it will be a month or a year from now with high accuracy. You can download programs from the web that will predict the precise locations of orbiting objects like the International Space Station or Iridium Satellites with pretty good accuracy, without having to communicate with anything.

Over time, the orbits do change slightly because the mass of the earth is not uniform, and because the atmosphere changes randomly and puts random amounts of drag on satellites in LEO. So the GPS satellites have a mechanism for transmitting updated almanac information to the receiver at various intervals. I don’t know the exact protocol used in GPS transmission, but I’d guess that the initial handshake consists of the satellite transmitting its ident, and the receiver returning the date of its almanac section for that satellite. If the almanac is out of date, the satellite will transmit a new one to the receiver, just for its own positions. That’s my guess, anyway.

How does the satellite update its own almanac? My further guess is that the actual orbit is periodically measured from military ground stations and a new almanac is uploaded to the satellite when the orbit deviates substantially from what’s predicted in the last almanac. But this is outside my realm of knowledge, so maybe someone else can fill in that detail.