Cumulative General Science/Technology Quiz

[nonconformist]

Is it that when a GPS powers up, it sends a signal to a single geostationary satellite which sends a signal right back, which should take a fair precise time. If the atmosphere is denser because of interference and such, the signal is delayed and the density of the atmosphere calculated using the Beer-Lambert relationship?

 

[History_Buff]

Is it that when a GPS powers up, it sends a signal to a single geostationary satellite which sends a signal right back, which should take a fair precise time. If the atmosphere is denser because of interference and such, the signal is delayed and the density of the atmosphere calculated using the Beer-Lambert relationship?

GPS receivers don't broadcast (or more properly, GPS satellites don't listen for recievers).

Do the satellites send out a "signal" signal (...something like a single-frequency, standard amplitude signal, at standard intervals in time, outside of the frequency range of the "data" signal) that is known to the GPS receiver? Any deviation from this known signal (noise, assumed to be as a result of atmospheric interference) can be used to estimate the effect of atmospheric interference.

This is on the right track, you've got the technique in here.

Specifically though, how do they isolate atmospheric interference without solving for things like clock-error in the reciever?

 

[ainwood]

They do it by sending out signals at a set frequency (time between pulses). Atmospheric interference will not affect all pulses equally, so by looking at the time difference between a number of pulses, the atmospheric effects can be determined.

 

[History_Buff]

They do it by sending out signals at a set frequency (time between pulses). Atmospheric interference will not affect all pulses equally, so by looking at the time difference between a number of pulses, the atmospheric effects can be determined.

Also the right idea, but you work out the difference between something slightly different.

EDIT: Just to be extra specific about it, I mean the Ionospheric component of atmospheric interference, when doing single point positioning. And that doesn't eliminate any previous answer.

 

[History_Buff]

Alright, that's 72 hours.

You do it by using two different frequencies. GPS satellites broadcast on two different channels, called L1 and L2. Because the Ionosphere affects the two frequencies differently, you can use that and a whole lot of math to get a pretty good idea of how active the Ionosphere is, and how much its delaying (or advancing, which does happen) your signal.



Easy one this time:

How many GPS satellites do you need to see in order to get a valid position fix, and why?

 

[uppi]

How many GPS satellites do you need to see in order to get a valid position fix, and why?

Four: One for every dimension. Your postion has three degrees of freedom, so you need three signals to fix it. The forth is needed to fix the time (the forth degree of freedom in space-time)

 

[History_Buff]

Four: One for every dimension. Your postion has three degrees of freedom, so you need three signals to fix it. The forth is needed to fix the time (the forth degree of freedom in space-time)

Bingo. Your turn.

 

[uppi]

Under what conditions can an object levitate in a static magnetic field?

 

[Mise]

We had that question before, btw ^^ (History buff's second question that is).

@uppi: if it's spinning?

 

[uppi]

We had that question before, btw ^^ (History buff's second question that is).

@uppi: if it's spinning?

No, it doesn't have to spin. Spinning might stabilize it, but I don't think it helps in the levitation part.

Hint: To levitate the object needs a certain property.

 

[History_Buff]

We had that question before, btw ^^ (History buff's second question that is).

@uppi: if it's spinning?

I figured you might have. I just love it because most people will say 3 without thinking. And this was slightly better than leaving an open floor.

 

[EdwardTking]

If the object is itself magnetic or has a magnetic field from an electric current going through it.

 

[uppi]

If the object is itself magnetic or has a magnetic field from an electric current going through it.

No. There is no stable equilibrium for two magnets (or a magnet in a magnetic field), only unstable ones. There are some tricks to stabilize it, but that's not my question.

Another hint: Water can levitate.

 

[Mise]

EDIT: nevermind...

 

[nonconformist]

It needs to be frozen I believe, or at least at very low temperatures.

 

[Perfection]

It's hoverin' over a superconductor.

 

[uppi]

It needs to be frozen I believe, or at least at very low temperatures.

No, depending on the material it can also work at room temperature. (It is way easier to get a sufficient magnetic field at very low temperatures, though)

It's hoverin' over a superconductor.

Superconductors can levitate very easily, as they are a perfect example of the needed property. But they're only a special case, it works with other materials, too.

 

[dutchfire]

Induction!

 

[nonconformist]

So I assume you need a field and a current?
A flow of electrons?

 

[uppi]

Induction!

Induction only works if something is moving. In the case of levitation there isn't anything moving (at least not down, and you would need that for an upward force)

So I assume you need a field and a current?
A flow of electrons?

You need a field, but there is no need for a current (apart from the current needed to generate the field). The right object has to be at the right place, but nothing else is needed.