Satellites

[MJG]

I've been pointing out satellites to the pilots I fly with for as long as I can remember. It amazes me how many people "had no idea" that you could actually see a satellite with your eyes.

Best time to catch them is within 3-4 hours before sunrise or after sunset. As the Earth's shadow moves across the sky with the sun, it takes with it the light needed to reflect off of the objects.

Check out Heavens Above. A great website for spotting and tracking Earth orbiting objects.

 

[Big Duke Six]

With all these different orbits, has there ever been a satellite mid-air? That would be ugly but given they are usually maneuverable and that everything up there is tracked, I would be surprised if it's happened. But it IS getting more crowded up there all the time.

I saw Hoot Gibson fly a shuttle high over my house one morning, in very close formation with the Mir as he was preparing to dock with it. VERY COOL!

 

[CL60]

With all these different orbits, has there ever been a satellite mid-air? That would be ugly but given they are usually maneuverable and that everything up there is tracked, I would be surprised if it's happened. But it IS getting more crowded up there all the time.

I saw Hoot Gibson fly a shuttle high over my house one morning, in very close formation with the Mir as he was preparing to dock with it. VERY COOL!

You mean a mid-space collision? There is a "chain reaction" theory that describes the events that would occur if there ever was a collision. One collision would send thousands of randomy flying projectiles out that might also eventually hit other orbiting objects causing more projectiles and other collisions and so on...

Depending on which definition of the word "satellite" you use, the numbers around our planet could be in the millions! That is only if you include 30 years of "space junk' and the resultant debris that goes along with it. Everything from a glove that departed the one of the Gemini missions to nuts and bolts, tools, paint flecks, and other man-made objects to naturally occurring space particulate matter. In order to maintain orbit, an escape velocity of nearly 18,000 miles per hour must be attained. Imagine the closure rates of converging objects at varying inclinations. An actual collision would involve lots of energy.

Most man-made objects are tracked by the US Space Command.

From AF Space Command HQ:

Air Force Space Command operates a worldwide Air Force satellite control network and constantly monitors the operation of satellites in orbit. The command is also responsible for launching and operating military satellites.

The Air Force command tracks and catalogues 8,000 man-made objects orbiting the earth. The objects can range in size from a baseball to large satellites and include everything from space junk to operational satellites. The command assists NASA in ensuring a safe path for space shuttles. It also operates the Global Positioning System and supports the military with navigation and weather information, missile warning, satellite communication, and intelligence.

The Goddard Space Flight Center currently lists 2271 manufactured orbital vehicles in orbit.

 

[EagleRJ]

How can you tell the Space Station from an ordinary satellite?

(Sounds like a silly NASA joke but I'm totally serious)

The Space Station is ordinarily much brighter than other satellites, due to its size. All of its solar arrays and radiators already cover thousands of square feet, so there's a lot of surface area to reflect light.


That 3-D map on the NASA site is a good way to visualize the different kinds of orbits.

The satellites that are arranged in a "ring" around the Earth are in geosynchronous orbit, and remain over a single point on the surface. Those include communications and weather satellites, as well as some deep-space imaging satellites.

The satellites that appear to be just over the surface of the Earth are in LEO. They make one revolution roughly every 90 minutes. The inclination (the angle to the equator) will vary depending on the mission. For spy satellites, radar imaging satellites, crop research, oceanographic study, and other missions that require global coverage, the inclination will be near 90 degrees to the equator (what's called a polar orbit). Polar-orbiting satellites will be able to cross every square mile of Earth's surface over a few days.
The rest of the LEO traffic is at low inclination orbits. That's where you'll find the Space Station, microgravity research, the Hubble space telescope, and lots of lower-cost research and communications satellites.

The satellites that are off on their own in the middle of nowhere are mostly GPS satellites. The GPS constellation has three specific orbits with seven satellites each, ensuring that at least three are visible at any time from any point on Earth.

With all these different orbits, has there ever been a satellite mid-air? That would be ugly but given they are usually maneuverable and that everything up there is tracked, I would be surprised if it's happened. But it IS getting more crowded up there all the time.

Operating satellites are assigned spots in orbit, and traffic-control works a lot like ATC. Normally, conflicts don't happen.

The real problem is debris, and old satellites that are no longer functional or controllable. The orbits of every object over about 4" in diameter is tracked by NORAD and other facilities around the world, and occasional maneuvering burns are needed to avoid another object.
The biggest danger comes from the small stuff that NORAD can't see. The Shuttle has come back with deep pits in its windows from hitting paint chips. Due to the high velocities involved, even a small object can pack a punch in kinetic energy. If the Shuttle was hit by an object the size of a penny that was in polar orbit (courses converging at nearly 90 degrees), the resulting energy from the impact would be about the same as a hand grenade!

Most of the small debris up there dates from the Cold War. The Soviets were so paranoid about their spy satellites falling into US hands that they equipped the satellites with scuttling charges. Once they were done with the satellite, they would send a radio signal and blow it up, adding thousands of little pieces of debris to LEO.
Thankfully, LEO is slowly self-cleaning. The Earth's extreme upper atmosphere expands during periods of increased sunspot activity, and the additional drag causes small pieces to lose velocity and fall into the upper atmosphere, where they burn up.

 

[A Squared]

Those include communications and weather satellites, as well as some deep-space imaging satellites.

Many weather satellites ... but not all. You probably know that but your words might leave a different impression. There are number of Wx sats in inclined orbits, the geostationary ones don't give very good imagery in the more northern and southern lattitudes.

The satellites that are off on their own in the middle of nowhere are mostly GPS satellites. The GPS constellation has three specific orbits with seven satellites each, ensuring that at least three are visible at any time from any point on Earth.

Actually the GPS sats are in 6 orbital planes, inclined 55 degrees from the equator and rotated 60 degrees from each other. ( 360/6=60 ) Each orbital plane was originally to have contained 3 sats for a total of 18 in the "full" constellation, plus additional "in orbit spares" Initially the system was intended to have 3 in orbit sapares for a total of 21. this was suppoesed to give 4 visible sats (the minimum for a 3 d position) worldwide. The number of in orbit spares has been steadily increased so that now there are 28 operational sats up there with another launched in September 05 which is in the testing phase and should be operational any day now.

 

[gkrangers]

There are a lot of weather satellites that are polar orbitters (north to south, sweeping a different slice of earth each pass)...which is unfortunate because while its nice to get to sample the whole surface of the earth, if you are focusing on one particular feature, like a hurricane, you only get 1 pass every so many hours...particular useful is microwave data.

 

[EagleRJ]

Actually the GPS sats are in 6 orbital planes, inclined 55 degrees from the equator and rotated 60 degrees from each other. ( 360/6=60 ) Each orbital plane was originally to have contained 3 sats for a total of 18 in the "full" constellation, plus additional "in orbit spares" Initially the system was intended to have 3 in orbit sapares for a total of 21. this was suppoesed to give 4 visible sats (the minimum for a 3 d position) worldwide. The number of in orbit spares has been steadily increased so that now there are 28 operational sats up there with another launched in September 05 which is in the testing phase and should be operational any day now.

Roger. I was going from memory- probably should have looked it up for greater accuracy [rimshot].

GPS only needs three satellites for a position fix, the system's original purpose. A fourth satellite is needed to establish altitude.
Due to the widespread use of GPS in the civilian sector, there is a plan to launch a European system (Galileo) that would work with, and compliment, GPS. Foreign countries are understandibly uneasy about relying on a system that was developed for, and is maintained by, the US military.

At one time, there was another geopositioning system called GLONASS that was operated by Russia. I haven't heard anything about it in years- the Russians probably couldn't afford it and let it die.

 

[A Squared]

GPS only needs three satellites for a position fix, the system's original purpose. A fourth satellite is needed to establish altitude.

Not true, or at least not true as you stated it. You seem to be saying that if you don't care about altitude, you can determine your horzontal position with only 3 sats. That is not true. what *is* true is that if you *know* your altitude to a sufficient degree of accuracy, and you put that information into tyour GPS receiver, it can determine your horizontal position using only 3 satellites.

The same could be said if you knew your latitude, that could be put into your receiver and the receiver could solve for your altitude and longitide with only 3 satellites...it's just htat GPS receivers aren't programmed that way, and it's tough to know your latitide independently.

The reason is there are 4 variables the GPS receiver is solving for, your position in x,y,z, and a time offest from it's internal clock to GPS time. If you recall from algebra, if you have 4 unknowns (x,y,z, and t), you need to have 4 independent equations which you solve simultaneously to figure the 4 unknowns. Each observation from a different satellite forms an equation to go into the solution. Along those same lines if you have 4 variables (x,y,z, and t) and somone tells you what the value of one (z) is, you only need 3 equations.

Foreign countries are understandibly uneasy about relying on a system that was developed for, and is maintained by, the US military.

Can't say as I blame them.

At one time, there was another geopositioning system called GLONASS that was operated by Russia. I haven't heard anything about it in years- the Russians probably couldn't afford it and let it die.

Yeah, there was great hope for receivers that could use both GPS and Glonass sats .... but it went away for pretty much the reasons you describe.

 

[EagleRJ]

Not true, or at least not true as you stated it. You seem to be saying that if you don't care about altitude, you can determine your horzontal position with only 3 sats. That is not true. what *is* true is that if you *know* your altitude to a sufficient degree of accuracy, and you put that information into tyour GPS receiver, it can determine your horizontal position using only 3 satellites.

All right, you made me go and look it up to make sure I wasn't crazy. Here is how Garmin describes it:

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude).

You're right that using only three satellites can introduce errors that four satellites would be able to factor out. That's all more of a concern for ground users who have a portion of the sky blocked by terrain or structures. In most aircraft installations, many more than four satellites are visible most times.

 

[A Squared]

All right, you made me go and look it up to make sure I wasn't crazy. Here is how Garmin describes it:


A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude).

Yeah, and that's true as far as it goes. the page you referenced is a cursory overview not a detailed explanation. What they don't mention on that page is that in order to compute a 2d fix from 3 satellites, the receiver has to have an altitude. If you had dug deeper into the Garmin website you would have found a little more detailed guid which states: "you'll need to enter your approximate altitude into the GPS receiver. THis will allow the receiver to calculate a two dimensional position (latitude ad longitude) However by adding a fourth satellite the reciever can determine your three dimensional position (latitude, longitude, altitude)"

The altitude either has to be input by the user, or some receivers are programmed to use the last altitude from the previous use. THe second option obviously has limits in a plane in flight, or if say you used it last on the Dead Sea and you were now on Pike's Peak.


If the number of tracked satellites drops to three then a 3D solution is no longer possible and the receiver will use the last available altitude and compute a 2D fix for horizontal position. Here's a website which explains in a little more detail:

http://www.gpsinformation.org/dale/theory.htm

On it you will find this statement:

If the number of tracked satellites drops to three then a 3D solution is no longer possible and the receiver will use the last available altitude and compute a 2D fix for horizontal position.

You're right that using only three satellites can introduce errors that four satellites would be able to factor out.

No, it's not a matter of "introducing errors", without 4 satellites or 3 satellites and an elevation the receiver can't compute a position. If your receiver is operating in 2D mode, it *is* using an elevation from somewhere, it might be the elevation computed in the last 3D fix, it might be input by you, it might be input electronically by an encoding altimeter, but it's there. With only 3 satelites and no infomation on its elevation, a GPS receiver is lost.