GPS Written Exam Question

[slick1537]

Hi guys, I have been studying for my written private pilot exam with mywrittenexam.com and also the 2007 asa testprep book. Tonight I was bored and reading some of the 2003 publication of the Pilots Handbook of Aeronautical Knowledge. On page 14-26 it says that 26 satellites make up the GPS network. However on page 10-14 of the 2007 asa testprep book it says that GPS consist of 24 satellites. I was wondering if anyone could clarify what the correct answer would be on the FAA written in case this questions comes up.

[67november]

that depends, do you want the civilian version or the miltary version? both put it in the same area and the rest is classified

Edited December 31, 2007 by 67november

[Goldy]

Hi guys, I have been studying for my written private pilot exam with mywrittenexam.com and also the 2007 asa testprep book. Tonight I was bored and reading some of the 2003 publication of the Pilots Handbook of Aeronautical Knowledge. On page 14-26 it says that 26 satellites make up the GPS network. However on page 10-14 of the 2007 asa testprep book it says that GPS consist of 24 satellites. I was wondering if anyone could clarify what the correct answer would be on the FAA written in case this questions comes up.

 

 

What if there are 26, but two of them dont work !!?

 

The Official test answer will be 24. There are even more than 26 up there, but some are for standby use...and boy they have had to use them ! Did you know each satellite is equipped with nuclear launch detection equipment (not classified). And here you thought the military put them up there to help you with your approaches !

 

They also are not in a geostationary orbit like most other satellites up there( 22,800 miles up). Instead they circle the globe at half that distance...about 12,000 miles.

 

Anything else?

Goldy

[Future206Pilot]

They also are not in a geostationary orbit like most other satellites up there( 22,800 miles up). Instead they circle the globe at half that distance...about 12,000 miles.

 

I'm sorry to get technical, but aren't GPS Satellites geostationary?? That would mean they don't circle, they sit a 12,000 miles...

 

Just a thought

[Goldy]

I'm sorry to get technical, but aren't GPS Satellites geostationary?? That would mean they don't circle, they sit a 12,000 miles...

 

Just a thought

 

 

Nope, get technical ! What part of non geosynchronous dont you understand !! (lol).

 

Nope, they dont sit there like other satellites do. They actually orbit, circling the earth 2X a day...or every 12 hours. I know it sounds a bit strange, but thats the way it works.

 

(added) Oh BTW, nothing can sit at 12,000 and not orbit, its too close to the earths gravity and eventually would just get sucked back in. Thats where 22,800 miles up came from...not too close, not too far.

 

(edited again)- Almost forgot. They just launched another one last week, so I think maybe thats like 32 up there now? Of course, they don't all work !

 

take care, fly safe,

 

Goldy

Edited December 31, 2007 by Goldy

[Witch]

I'm sorry to get technical, but aren't GPS Satellites geostationary?? That would mean they don't circle, they sit a 12,000 miles...

 

Just a thought

Nope, they're moving.

 

During the summer on a moonless night for about an hour after civil twilight, you may actually see one. It's quite neat to see one. Not that one, it's just a flying saucer. It's just a light moving across the sky, and you can see it with your naked eye. I've even had the grandkids looking for them.

 

There's a website that shows the orbit of every satellite. I'll see if I can find it.

 

Later

 

I've found one site that shows some orbits, but it's not the one I'm seeking.

http://satellite.tracks.free.fr/satrace/sa...php?language=en

 

Enjoy

 

Also, for your entertainment pleasure, if you have Dish network, the satellites they use are Echostar 6 and 8. They're more commonly known as 119 and 110 respectively. That's where they're parked over the equator.

 

The Navstar series are the GPS satellites. http://tycho.usno.navy.mil/gpsinfo.html

http://en.wikipedia.org/wiki/Global_Positioning_System

 

The Irridium series-IIRC-are used for satellite phones. The Hotbird series are for TV in the Mideast and Africa. I have no idea about the Cosmos series.

Edited December 31, 2007 by Witch

[Future206Pilot]

Nope, they dont sit there like other satellites do. They actually orbit, circling the earth 2X a day...or every 12 hours. I know it sounds a bit strange, but thats the way it works.

 

I've been had....

 

And another thought, Is there much usefulness to a GPS Satellite in heliosynchronous orbit???

[67november]

OK here we go. GPS satellites are in a geostationary orbit so the signal can bounce to the same satalite.

From the ground, a geostationary object appears motionless in the sky and is therefore the orbit of most interest to operators of artificial satellites (including communication and television satellites). Due to the constant 0° latitude, satellite locations may differ by longitude only.

 

a Geosynchronous Orbit is as follows;

A satellite in geosynchronous orbit circles the earth once each day. The time it takes for a satellite to orbit the earth is called its period. For a satellite's orbit period to be one sidereal day, it must be approximately 35,786 kilometers (19,323 nautical miles or 22,241 statute miles) above the earth's surface. That is a lot higher than the Shuttle ever goes (usually about 300 kilometers). We calculate this height using, what are today, common geometric formulas.

 

these satellites are for the most part survey and spy satellites

[Witch]

67!!!

 

GPS is NOT!!! geostationary!!!!!!!

 

THEY MOVE!!!!!!

 

Sorry fer yellin'

[67november]

OK I found It MY BAD

 

GSP Satellite Orbits

 

The satellites orbit the earth with a speed of 3.9 km per second and have a circulation time of 12 h sidereal time, corresponding to 11 h 58 min earth time. This means that the same satellite reaches a certain position about 4 minutes earlier each day. The mean distance from the middle of the earth is 26560 km. With a mean earth radius of 6360 km, the height of the orbits is then about 20200 km. Orbits in this height are referred to as MEO – medium earth orbit. In comparison, geostationary satellites like ASTRA or Meteosat – satellites orbit the earth at 42300 km, which is about twice the distance of GPS satellites.

The satellites are arranged on 6 planes, each of them containing at least 4 slots where satellites can be arranged equidistantly. Today, typically more than 24 satellites orbit the earth, improving the availability of the system.

The inclination angle of the planes towards the equator is 55°, the planes are rotated in the equatorial plane by 60° against each other. This means that the orbits range from 55° north to 55° degrees south. Block I satellites had an inclination of 63° against the equator.

 

 

 

 

Inclination of the orbital planes

By this arrangement of the orbits it is avoided that too many satellites are to often over the north and south pole (like it was the case in the TRANSIT system, where the satellites ran on polar orbits).

However the orbits run far enough to the north and south to guarantee GPS availability in polar regions. Furthermore this arrangement leads to a rather stable constellation, as orbit disturbing factors like solar winds and gravitation fields have about the same influence on all of the satellites.

The number and constellation of satellites guarantees that the signals of at least four satellites can be received at any time all over the world.

The closer you get to the poles, the lower over the horizon the satellites are located. They can still be received very well, but in no case they are directly above. This may lead to a - typically insignificant - loss of the precision of the position determination. This effect, caused by the geometry of the satellite arrangement, happens from time to time on any spot of the earth surface and can be forecasted.

 

 

 

 

Ground-Track (sub satellite path) of the Satellite GPS BIIR-07 (PRN 18) of 18.10.2001 00:00 o'clock to 19.10.2001 00:00 o'clock.

The picture on the right shows the ground track of satellite BIIR-07 (PRN 18) from 2001-10-18, 00:00 ’o clock to 2001-10-19, 00:00 ’o clock. The yellow arrow marks the 00:00 ’o clock time point. It can be seen that the orbit time is slightly shifted (about 4 minutes) in 24 h.

The yellow dot marks the satellite position at 09:30 pm. The satellite is positioned over Ethiopia. The correlating “zone of sight”, within which the satellite signal can be received, is mark in light blue. The graph was compiled with the FreeWare software WinOrbit (and slightly modified).

For a deeper insight into orbits of GPS satellites and other satellites, the following link can be recommended: J-Track 3D. A Java applet on this page can illustrate orbits and information of more than 500 satellites.

Edited December 31, 2007 by 67november

[Witch]

Hey, I found the site that has every satellite currently in orbit.

 

Go to http://science.nasa.gov/Realtime/jtrack/3d/JTrack3D.html

 

AND click on J-track 3D.

 

It'll open up a seperate screen and you can see all satellites. Click on one to see it's name and orbit.

 

Later

[Gomer Pylot]

Most of the GPS satellites orbit over the poles, giving a north-south

movement, but the WAAS satellites are in geostationary orbits, letting them provide constant correction to US locations. Nothing about the GPS system is classified. Information on all satellites and launches is widely and publicly disseminated. I have no idea what the answer for the FAA exam is, but the number of satellites changes frequently, as more are launched and older ones go out of service. That information is completely immaterial to pilots, because it makes absolutely no difference to users. That's the case with many exam questions, though. Just learn the exam, get the rating, and then forget it.

 

 

 

(I deleted your double post for ya, 67N)

Edited December 31, 2007 by 67november

[FUSE]

I want to say that the question will ask what is the minimum number of satellites used for GPS, which the answer would be 24.

[Goldy]

Geez, have we beat this orbit thing to death yet?

[Voluptuary5]

Geez, have we beat this orbit thing to death yet?

 

Nope!

 

I always just assumed the GPS satellites were geosynchronous. Does anyone know the reason/advantage for having them orbit twice a day versus being geosynchronous?

 

Thanks,

 

 

-V5

[Goldy]

Nope!

 

I always just assumed the GPS satellites were geosynchronous. Does anyone know the reason/advantage for having them orbit twice a day versus being geosynchronous?

 

Thanks,

 

 

-V5

 

 

Well, there may be a few. Remember these are DOD satellites first, navigation satellites second. I dont want to even see the math required to calculate not just your position, but the position of each bird at that exact point in time.

 

Here's just one. If one bird fails, you lose a certain shadow of signal. That shadow falls over some portion of the world, and degrades navigation in that area...for days until they can get a spare bird active in its place. If however, they circle every 12 hours, then that shadow is a moving one...not a permanent one....lasting maybe a few hours, instead of days. That shadow of degraded reception is moving all the time. You can anticipate it..track it and plan for it.

 

Just a thought.

[Future206Pilot]

Remember these are DOD satellites first, navigation satellites second.

 

So the second that you lose all signal in what used to be the best place to get it, you should land and start dancing around like a madman?

[Gomer Pylot]

Geosynchronous satellites must, by definition, orbit over the equator. By definition, GPS means global, and it's not possible to provide global coverage from an equatorial orbit. It's necessary to be able to use GPS at the poles, and other places. In a geosynchronous orbit, each satellite can cover only one area, and if it dies, coverage for that area dies. With North-South orbits, every satellite covers the entire earth at some point, and every spot on the earth is covered by some satellites all the time. Plus, the geosynchronous orbit area is almost competely filled already with communications satellites. There is only a finite amount of room up there. That's why the WAAS satellites aren't separate, bandwidth on existing satellites is used for that.

[Goldy]

So the second that you lose all signal in what used to be the best place to get it, you should land and start dancing around like a madman?

 

YES. But make sure the main rotor has stopped turning before you start hopping around.

[RotorRunner]

Another reason the GPS system uses low/med earth orbit satelites, is for better reception using a smaller antenna, making a GPS portable instead of lugging around 3 or 4 dishes for 2D or 3D positioning.

 

This is the same reason the Iridium phones can work so marvelous with a small quadrifilar antenna.

Edited December 31, 2007 by RotorRunner

[Voluptuary5]

Gomer,

 

It may be semantics, I'm not sure, but I think your definition of geosynchronous is actually the definition of geostationary. From what I can tell on a quick read, geosynchronous means the satellites are in orbit at the same speed/altitude to correspond to the speed of one earth rotation. Geostationary appears to refer to a satellite with an orbit over/crossing the equator but is still geosynchronous. So, if I read this right, all geostationary satellites are geosynchronous but NOT all geosynchronous satellites are geostationary; therefore, you could have a geosynchronous satellite orbiting over 45 degrees north latitude for example. I think.

 

Goldy,

 

Yep, can't even begin to image the math even if the GPS satellites weren't moving in space and me moving on the ground. I'm glad there are people much smarter than me figuring all that stuff out.

 

Rotor,

 

Good point on signal strength.

 

 

-V5

[Gomer Pylot]

How are you going to orbit at 45 degrees latitude? The orbit has to be around the center of mass, which is at the center of the earth. True, the definitions of geosynchronous and geostationary are slightly different, but for all practical purposes they're the same. Putting a satellite into an orbit which moves back and forth across the equator, while theoretically possible, isn't practical, and won't happen. The band is simply too crowded.

[helonorth]

Here's the short answer: I highly doubt that question is in ANY of the writtens. But if it

is: pick C.

[Goldy]

How are you going to orbit at 45 degrees latitude? The orbit has to be around the center of mass, which is at the center of the earth. True, the definitions of geosynchronous and geostationary are slightly different, but for all practical purposes they're the same. Putting a satellite into an orbit which moves back and forth across the equator, while theoretically possible, isn't practical, and won't happen. The band is simply too crowded.

 

GPS has many orbits, I dont think you can describe any of them as geo anything. The whole purpose of the system is to cover the earth like a blanket. Here's a photo from US DOD that illustrates the orbits.

 

I'm done. The answer to the original post is 24 !!

 

Goldy

 

 

[FUSE]

Here's the short answer: I highly doubt that question is in ANY of the writtens. But if it

is: pick C.

 

Actually yes it is, it was in my private.

 

And you are correct sir, the answer was C