How accurate would you like your GPS?

[lw395]

GPS satellites are not geostationary.
Your receiver knows the paths of the satellites.
Each broadcasts a time code
So the relative delay gives difference in distance between receiver and the different satellites.
That gives you a lot of equations which are easier to solve if you assume the receiver is near the earth's surface and not doing hundreds of mph.

 

[mjcoon]

GPS satellites are not geostationary.
Your receiver knows the paths of the satellites.
Each broadcasts a time code
So the relative delay gives difference in distance between receiver and the different satellites.
That gives you a lot of equations which are easier to solve if you assume the receiver is near the earth's surface and not doing hundreds of mph.

That much I have always understood since I heard of GPS. It's "doppler" that gives me the problem, as I have written here.

Mike.

 

[lw395]

On the face of it, Doppler looks like a hard work way of measuring a speed of something moving obliquely to the satellite at a few knots or even tens of knots, using a relatively low frequency like 1575MHz.

 

[Uricanejack]

A bit of google research.
GPS Satellites are not Geostationary. About 24 or 31 of them depending upon which google article you read are at an altitude of 20000 inclined 55 degrees from the equator. orbiting about twice a day. Supposedly they transmit their position data which they have derived from the ground stations and the receivers. Measure the time it takes for the signal to get from the Satellite.
No mention of doppler shift yet.

While I have found articles with big words like trilateration and Pseudo Random Code. Comparing random codes to get time Difference. To calculate distance from satellite. Which means I have not yet found an article on google yet which can actually describe how GPS works

 

[savageseadog]

GPS is a engineering tour de force, of electronics, Mathematics and digital signal processing. The received signal is below the noise received and receiver noise, it's recovered using correlation between the received signal and internally generated pseudo random sequences. There's countless corrections and tweaks including as I remember a relativistic Einsteinian one. Just when you're absorbing one astounding fact about how it's done another takes your breath away. There was a terrific series of articles about it in the now defunct Wireless World.

 

[Lon nan Gruagach]

A bit of google research.
GPS Satellites are not Geostationary. About 24 or 31 of them depending upon which google article you read are at an altitude of 20000 inclined 55 degrees from the equator. orbiting about twice a day. Supposedly they transmit their position data which they have derived from the ground stations and the receivers. Measure the time it takes for the signal to get from the Satellite.
No mention of doppler shift yet.

While I have found articles with big words like trilateration and Pseudo Random Code. Comparing random codes to get time Difference. To calculate distance from satellite. Which means I have not yet found an article on google yet which can actually describe how GPS works

I thought that satellite position is transmitted in the form of an almanac, not lat/long/height per se. The almanacs for all in the constellation are transmitted cyclically by all in that constellation. That's why it takes some time for an initial fix, you have to wait for them.

Edit add
Also the time from satellite to receiver cannot be directly measured, it is the difference in time delays from different satellites that is used (certainly for the basic coarse reckoning)

 

[mjcoon]

I thought that satellite position is transmitted in the form of an almanac, not lat/long/height per se. The almanacs for all in the constellation are transmitted cyclically by all in that constellation. That's why it takes some time for an initial fix, you have to wait for them.

Edit add
Also the time from satellite to receiver cannot be directly measured, it is the difference in time delays from different satellites that is used (certainly for the basic coarse reckoning)

Yes, the satellite position must be sent in the form of a calculation that the receiver can use to find the position at every instant because it is changing rapidly!

Similarly the receiver does not have an atomic clock with absolute time so that has to be calculated too.

It's all very clever and most users are utterly unaware of the complexity. Just like the rest of the content of their smartphone!

Mike.

 

[RobbieW]

Truthfully I have no clue how GPS works. Magic box, it just does. If I had to explain it for an examiner.
What the response would be?....

PFM innit, thats Pure and Magic with the F being anything you want it to be - at least that was electrics as explained by the ex naval tiffy airframe/engine instructor when I was an apprentice

 

[[70521]]

I still think it is all magic and understand the maths behind it, sad really.

 

[Daydream believer]

When you are looking for the west entrance to Cherbourg Harbour in less than 200m visibility as we were last Saturday, with no radar on board, Mark 1 eyeball alone isn’t enough. Knowing that the chartplotter is showing where you are accurate to a couple of boat lengths is very helpful. Only when we were actually in the marina could we see where we were. It was also helpful that as we knew where we were we could concentrate our efforts on looking out for where other boats might be. Fortunately nobody came within sight of us. The experience has persuaded the skipper that even if he can’t afford radar an AIS receiver is worth investing in. It certainly engenders respect for those that have gone before us without radar, ais or chartplotter.

If it was the same time that i came in i could only see the shore side pier but not the outer side. To make matters worse a cruise ship was coming out and a police launch preceeded it waving at me to get out of the way. In the fog i was not even certain that it was the entrance pier that i could really see
once i reached the inner harbour and avoided the exiting ferry it was as clear as anything

 

[Stemar]

avoided the exiting ferry

The first time I read that I read exciting ferry

 

[Daydream believer]

avoided the exiting ferry

The first time I read that I read exciting ferry

It was actually , if you like that sort of thing

 

[Stemar]

It was actually , if you like that sort of thing

Thanks, but no thanks. I'm a devout coward, especially at sea