How GPS time sync works ??

[AskerOfGeeks]

i think gps works lik this

a gps receiver which knows the positions of all gps satellites in space, receives the signal sent by gps satellites and calculates the distance between the satellite and the receiver and does this with more than 5 satellites to get an accurate output.

the reviver knows the position of the sat that sent the signal
knows the speed of radio wave
will know the time wen the signal was sent frm the sat
then will calculate the time taken for the signal to travel frm sat to receiver
and thus will calculate the distance between the sat and receiver

the part that i didnt understood is how do the receiver knows the current (present) time wen it received the signal frm sat. i mean the receiver's clock cant be reliable as we all are having inaccurate clocks ( i think ) utc or gmt wat ever time system the sat uses sud be synched with the receiver but the receiver is just a receiver and dosent send back signals right ??

so tell me how its still working ?? wat am i missing here ??

 

[Brevor]

The satellite has a very accurate cesium clock onboard. This time is relayed to the receiver.

 

[AskerOfGeeks]

The satellite has a very accurate cesium clock onboard. This time is relayed to the receiver.

its going to cause error bro (i think)
For Example:
the sat is sending the time coded signal at time ' T '
it takes 1 sec to reach that signal at the receiver and receiver syncs its clock ' T-1 ' as the current time and thus a 1sec error (here a lag) will happen in this example.

pls guys help me with a proper explanation

 

[ChrisP58]

The satellites are all in sync with each other such that they all send out the same time stamped burst at precisely the same instant.

A GPS receiver doesn't need to know what time it is. It only needs to be able to accurately discern the time difference between the signals it receives from 3 or more satellites to calculate where it is.

 

[Electroenthusiast]

time is not required at all. It only requires the calculate the time taken by the wave to make to propogate. read about 'traingulation'.

 

[AskerOfGeeks]

The satellites are all in sync with each other such that they all send out the same time stamped burst at precisely the same instant.

A GPS receiver doesn't need to know what time it is. It only needs to be able to accurately discern the time difference between the signals it receives from 3 or more satellites to calculate where it is.

thats wat i call a proper explanation thankuuuu ChrisP58

 

[Electroenthusiast]

Does that mean that the software should an algorithm to calculate the latitude and longitude, Since the signal received has nothing more than time stamp. And the satellite should also be in database.

 

[ChrisP58]

You're welcome AOG.

The time stamp is the same, but each individual satellite also sends individual information, including where it is at that moment.

With info from at least three satellites, the receiver knows where it is with respect to each of those satellites, and can calculate it's own location.

 

[Electroenthusiast]

You're welcome AOG.

The time stamp is the same, but each individual satellite also sends individual information, including where it is at that moment.

With info from at least three satellites, the receiver knows where it is with respect to each of those satellites, and can calculate it's own location.

I'm still not sure because the signals will keep reaching the receiver in real time. And the signal from GPS satellite is continuous, so the signal received now, will have a different time stamp in the very next moment.

 

[Mike odom]

In a 3D world, you need 4 sats for an accurate fix. 3 will only give an estimated posititon. Same as on a 2D chart, you need 3 sightings to give a fix, 2 will only net in an estimated position.

It's like this... one sat yields an answer somewhere on the face of a sphere, 2 sats intersect in a circle, 3 yields 2 points, and while that may be good enough if you know your altitude (which point am I at?)... it takes 4 for the final answer. 3 won't necessarily give you the correct lat/long, the line intersecting the two points may not be perpendicular to local horizontal, so each point may have a different lat/long, as well as altitude.

 

[Electroenthusiast]

2 satellites are enough, from what i learnt in geometry. Can you elaborate a bit?

 

[ronsimpson]

2 satellites are enough, from what i learnt in geometry. Can you elaborate a bit?

I think two satellites will only work in a 2D world. There you and both satellites are on a plain.
In a 3D world where you and the satellites are not on a plain you will need......3 to 4. The more the better.

 

[jpanhalt]

Even in a 2D world, a time difference will define a line, not a point.

John

 

[Mike odom]

I think two satellites will only work in a 2D world. There you and both satellites are on a plain.
In a 3D world where you and the satellites are not on a plain you will need......3 to 4. The more the better.

In a perfect 2D world, 2 would satisfy... the two circles would intersect in a single point, no problem. But in the real world, you have real time errors, and those errors will cause the two distance circles to either intersect at two points (bigger circles), or not intersect at all. Or could, in fact, intersect in a single point in error, one circle being larger than it should, and one being smaller. And you still wouldn't EXACTLY know where you were, so it would be an ESTIMATED POSITION, not a FIX. You don't know which sat is in error, or both, and in which direction. You need an additional one to bring another arc into play. These will not intersect at an exact point, but will from a little triangle. YOU are somewhere inside this triangle. NOW you have a fix. So it takes 3 points to fix your position on a 2D chart (not map). This comes right out of Dutton's Navigation Manual, not to mention my time on the pond tracking our own and other ship's positions.

If you extrapolate into a 3D world, now you need FOUR points to fix your position in a space.

 

[AskerOfGeeks]

LOL wats going on in here
I am the owner of this thread and for ur information my doubt was cleared but how com ur's wasnt ?? !!
Anyway, i dont mind u guys discussing inside my thread. Just dont forget to turn off the lights wen u leave ok peace.... v

 

[ChrisP58]

LOL wats going on in here
I am the owner of this thread and for ur information my doubt was cleared but how com ur's wasnt ?? !!
Anyway, i dont mind u guys discussing inside my thread. Just dont forget to turn off the lights wen u leave ok peace.... v

You are the original poster, but not the 'owner' of this thread. No one 'owns' a thread on ETO. Any thread is an open forum for discussion. Your questions may have been answered, but it obviously has sparked some interest of others on the subject. So ongoing discussion is to be expected, and should be welcomed.

At least (so far) the further discussion is still on topic.

 

[AskerOfGeeks]

You are the original poster, but not the 'owner' of this thread. No one 'owns' a thread on ETO. Any thread is an open forum for discussion. Your questions may have been answered, but it obviously has sparked some interest of others on the subject. So ongoing discussion is to be expected, and should be welcomed.

At least (so far) the further discussion is still on topic.

i wasnt complaining bro. i was just dropping by to see wats going on in here and wanted to comment somthing stupid tats all u guys carry on

and i dont mind u guys discussing "wat ever" in here

 

[Electroenthusiast]

Even in a 2D world, a time difference will define a line, not a point.

John

uncertain about this. Change in location, changes the time difference from both the satellites, so at no another point you'll find same time differences. (in 2D world)