GPS orbits

My answer:

It depends on how high the satellite is above the Earth.

Generally, there are 3 basic kinds.

1.) Low earth orbit: About every hour or 90 minutes

2.) Some Sats are called Geo-stationary, or Geosynchronous . This means that they rotate around the earth, at the same speed as the rotation of the earth, or once every 24 hrs.

3.) The moon, is a satellite. Once every 28 days or so.

But the DOD sats that we usually are using are usually the low earth orbit ones.

Nate

My answer:

> It depends on how high the satellite is above the Earth.
>
> Generally, there are 3 basic kinds.
>
> 1.) Low earth orbit: About every hour or 90 minutes
>
> 2.) Some Sats are called Geo-stationary, or Geosynchronous . This means that they rotate around the earth, at the same speed as the rotation of the earth, or once every 24 hrs.
>
> 3.) The moon, is a satellite. Once every 28 days or so.
>
> But the DOD sats that we usually are using are usually the low earth orbit ones.
>
> Nate

geo stationary would be a tv satellite, so that the tv 'dish' antenna has signal all the time. their relative position is stationary

geo synchronous is an orbit in 24 hours. gnss sat's are semi synchronous, one orbit in 12 hours, very nearly. i don't think they are low earth(will check) they orbit at something like 20200 km altitude, so the curvilinear velocity is 3.87kilometers per hour!!!

dave, have you ever used a wild di4? i need some help on one

My answer:

Dave: the orbits are independent of the earth's rotation. The earth rotates while the orbits stay where they are. Geosynchronous orbits are those that have the same period as the earth's rotation. The orbit still stays the same, but the satellite is moving at the same sidereal rate as the earth, 23h56m. The GPS satellites make two revolutions per sidereal day. Because they are 4 minutes less than 24 hours, the orbits "advance" 4 minutes per day, 2 hours per month, 24 hours per year (so it repeats yearly).

This makes little difference now, but in the past when there were only a limited number of satellites up, the window we could use would change by two hours per month. What this meant was that the window where we had four or more satellites was in the daytime in the summer and in the middle of the night in the winter (at least in the eastern US).

Anyone who used GPS back in the late 80's/early 90's probably has (not so) fond memories of surveying at 3 AM in the middle of winter.

My answer:

I'm not talking about the orbital period, but the orientation of the orbital plane.

My answer:

Haven't used a DI4, but I am just getting a DI1000 working so I may be of some help. Give me a call.

Dave

My answer:

I think this answers my question. Thanks!

just checked

ok

LEO, or low earth orbit. approx 400 km altitude. used for CHAMP, GRACE, GOCE, landsat and TOPEX/POSEIDON (gravity and remote sensing)

MEO, or medium earth orbit. approx 20000 km altitude. used for GPS,GLONASS, GALILEO, etc.

GEO, or geosynchronous. approx 36000 km altitude. used for communications.

just checked

Don't quote me on this, but I think the orbits are diagonal as they cross the sky. In other words, are you asking to the GPS SV's go east to west? Is so, I don't think they do.

just checked

> Don't quote me on this, but I think the orbits are diagonal as they cross the sky. In other words, are you asking to the GPS SV's go east to west? Is so, I don't think they do.

yes, their 'apparent' orbits are diagonal as a result of earth rotation

My answer:

Back in the late 1980s in Argentina the locals referred to us as "Vampire Surveyors."

GPS Satellite Orbits

GPS satellites orbit the earth at of 3.9 km per second and an orbit time of 12 sidereal hours or about 11 h 58 min earth time. This means that the same satellite reaches a certain position about 4 minutes earlier each day. 4 minutes x 365 days / 60 minutes per hour = 24 hour. That extra day accounts for the fact that in 1 year the earth rotates 365 times relative to the Sun at noon being overhead. But the earth rotates 1 extra day relative to space. I believe by having the satellite orbits based on space rotation the mathematics required to keep the solar panels pointing toward the Sun is simplified.

The average distance from the center of the earth to the orbit 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. It is easy to see this if you open a GPS RINEX file in a text processor like WordPad and look at the observables labeled C1, P1 and P2. For an example from a recent GPS observation on 4/06/2013 I include some C1 observations for Satellite PRN 22:

at time 16:27:40 the C1 value is 20,521,589.245
C1 is a ranging distance in meters from the PRN 22 antenna to my GPS receiver antenna in meters.

at time 16:27:50 the C1 value is 20,521,584.802
because it is less than the previous distance it tells me that PRN 22 is getting closer to my position.

at time 16:27:50 the C1 value is 20,521,592.600
because it is a greater distance it tells me that PRN is now getting farther away from my position.

Somewhere in that 20 second interval PRN 22 was almost exactly overhead. I can confirm that by opening my planning software and checking in a skyview the position of PRN 22 at that time.

The satellites are situated in 6 orbital planes, with 5 or 6 satellites in each. The planes are 60° apart. At any one time it is possible to have at least 1 satellite from 5 different planes in view in the sky above us. The planes are inclined at 55° to the equator. When a satellite crosses the equator it is moving in the direction of N 35° E. Because of the satellite distance from the earth's center, here in CONUS, it is possible to have a satellite that is 150° opposite our position peeking over the Arctic Circle and sending us useful signals. The intent was to have 24 satellites in orbit, but it is possible to use all 32 PRN numbers at one time. Because certain receivers get confused with 32, the Air Force limits GPS to 31 active.

The following is from memory so may not be as accurate. Russia being farther North chose to put GLONASS satellites in orbits inclined 62° to the equator, that is more useful to them. GLONASS are in 4 orbital planes and a full constellation is kept at 24.

Europe elected to have Galileo satellites in orbital planes inclined 56° from the equator. The extra degree gives minor improvements in northerly observations. A bigger improvement was made by having the satellites in significantly higher orbits giving better opportunity to have far side satellites peeking over the NW and NE horizons.

Paul in PA