How does GPS work?

[alpha]

Loring's "rant" reminded me of a recent video I found interesting:

http://tinyurl.com/cl3ulu6


Bob

[Tom Slick]

I recently installed an AutoFarm RTK GPS system on a tractor at work. It keeps the tractor on-line with sub-inch accuracy even on hill sides. You can come back 6 months later and drive on the exact same lines with sub-inch accuracy. Amazing technology.

[durango dude]

waidaminute ---- you mean to tell me that a GPS isn't run by little green men?

[LCHIEN]

GPS is great, you get to use a $10 billion dollar satellite system with your $100 to 200 receiver and get the benefit of near perfect positioning.

Thank you US Government.

[LCHIEN]

I have some issues with the lady's explanation in the link given by Alpha.

Here's mine:
There's a constellation of some 24 satellites in main use and a number of spares orbiting in semisynchronous orbits around the earth. They are spread evenly in three different orbital paths and so from any given location on the surface of the earth some dozen satellites might be visible in an unobstructed sky. But it only takes four to get a complete reading of position and time - your receiver probably tracks as mnay as 11 and uses the strongest signals and possibly more than 4 satellites to ensure accuracy even when in city or forest locations where parts of the sky are obstructed.

The keystone of the GPS satellites is an incredibly accurate time keeping system in each satellite - each as good as the clock at the National time standard labs in Boulder Colorado. Each clock in each satellite is precisely synchronized with every other satellite. Each satellite transmits a sequenced signal giving its position and time. The difference in times that the receiver sees is the delay to the satellite. There's only one X-Y-Z position in space and time that satisfies the three delays. As a result the receiver solving a 4-variable, 4 unknown math problem resolves X, Y, and Z and the true time (to about 1 microsecond precision or better) The X-Y-Z coordinates should match a point on the surface of the earth or in the air if you are flying. The coordinates can then be translated to Latitude and Longtitude and elevation - e.g. cartesian coordinates to azimuth, elevation and radius relative to the center of the earth, that's just a simple 3D geometry transformation.

In the end the satellites positions are given such that an accuracy of about 10 feet or less is possible with commercial receivers. If you care to store the data and ask the government for the more precise satellite orbit positions, a few weeks later you can resolve you location to inches. ANother interesting fact is that differential measurements, e.g. nearly simultaneous at two spots are only accurate to 10 feet maybe but the distance between them that you calculate may be accurate to inches. GPS velocity, because it uses very precise measurements of time, and the difference between two consecutive measurement points, is also very, very accurate.

[cabinetman]

In light of the GPS discussion and technology, the following is just a joke and not intended to be political, or cast aspersions on any brand of manufacture, or any persons' employment, or how some animals may appear to some people. I may have left out some details in this mini disclaimer, which indicates that I couldn't think of everything.

.

Bud the Cowboy

A cowboy named Bud was overseeing his herd in a remote mountainous pasture in Montana when suddenly a brand-new BMW advanced toward him out of a cloud of dust.

The driver, a young man in a Brioni® suit, Gucci® shoes, RayBan® sunglasses and YSL® tie, leaned out the window and asked the cowboy, "If I tell you exactly how many cows and calves you have in your herd, will you give me a calf?"

Bud looks at the man, who obviously is a yuppie, then looks at his peacefully grazing herd and calmly answers, "Sure, why not?"

The yuppie parks his car, whips out his Dell® notebook computer, connects it to his Cingular RAZR V3® cell phone, and surfs to a NASA page on the Internet, where he calls up a GPS satellite to get an exact fix on his location which he then feeds to another NASA satellite that scans the area in an ultra-high-resolution photo.

The young man then opens the digital photo in Adobe Photoshop® and exports it to an image processing facility in Hamburg, Germany ...

Within seconds, he receives an email on his Palm Pilot® that the image has been processed and the data stored. He then accesses an MS-SQL® database through an ODBC connected Excel® spreadsheet with email on his Blackberry® and, after a few minutes, receives a response.

Finally, he prints out a full-color, 150-page report on his hi-tech, miniaturized HP LaserJet® printer, turns to the cowboy and says, "You have exactly 1,586 cows and calves."

"That's right. Well, I guess you can take one of my calves," says Bud.

He watches the young man select one of the animals and looks on with amusement as the young man stuffs it into the trunk of his car.

Then Bud says to the young man, "Hey, if I can tell you exactly what your business is, will you give me back my calf?"

The young man thinks about it for a second and then says, "Okay, why not?"

"You're a Congressman for the U.S. Government", says Bud.

"Wow! That's correct," says the yuppie, "but how did you guess that?"

"No guessing required." answered the cowboy. "You showed up here even though nobody called you; you want to get paid for an answer I already knew, to a question I never asked. You used millions of dollars worth of equipment trying to show me how much smarter than me you are; and you don't know a thing about how working people make a living - or about cows, for that matter. This is a herd of sheep.”

“Now give me back my dog.”


.

[LCHIEN]

My critique here is that you don't "call up a GPS satellite" to find your location. You listen to 4 or 5 of them and them figure it out yourself, or at least your receiver will do this for you.

[cabinetman]

On the rare occasions I use my GPS, I have no idea which satellites are being used, and I don't think my receiver specifies. After living in this area over 50 years, I know the area very well. Actually the unit was for my wife before she got a car with a built in nav system. She either used the unit or called me on the phone. I couldn't give her a map, but a very good turn by turn.

.

[phrog]

There's a great explanation of GPS at the following link but it is very long if you read all of them:

http://www.trimble.com/gps_tutorial/howgps.aspx

[woodturner]

I have some issues with the lady's explanation in the link given by Alpha.

The other thing missing from the explanation is that the satellites provide accuracy only to about 3 feet, best case. To get accuracy to within inches requires a reference station and carrier phase enhancement or similar technique to develop the greater accuracy.

[Tom Slick]

The other thing missing from the explanation is that the satellites provide accuracy only to about 3 feet, best case. To get accuracy to within inches requires a reference state and carrier phase enhancement or similar technique to develop the greater accuracy.

The RTK system used in agriculture uses a base station. With an implement steering system both the tractor and implement are controlled by GPS. The tractor stays on line but has some error due to the steering system and length of the tractor and implement. Adding a gps guided side-shifting system to the implement allow to implement to make fine adjustments keeping it very close to its line as the tractor moves to keep itself on line.

It's very strange to push a button, let go of the steering wheel, and watch the tires turn left and right as it acquires a line and drive on a pre-programmed path.

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

https://www.youtube.com/watch?v=zI59yyN7Tyw

[phrog]

The RTK system used in agriculture uses a base station. With an implement steering system both the tractor and implement are controlled by GPS. The tractor stays on line but has some error due to the steering system and length of the tractor and implement. Adding a gps guided side-shifting system to the implement allow to implement to make fine adjustments keeping it very close to its line as the tractor moves to keep itself on line.

It's very strange to push a button, let go of the steering wheel, and watch the tires turn left and right as it acquires a line and drive on a pre-programmed path.

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

https://www.youtube.com/watch?v=zI59yyN7Tyw

That sure ain't my grandpa's tractor.

[michael.scheller]

There's only one X-Y-Z position in space and time that satisfies the three delays.

Very good understanding and explanation. However, there are two positions which would match the time delay of the 4 satellite time signals but the receivers are designed to eliminate the one which is nearly impossible without space travel.

The other element which exists for some enabled receivers is WAAS or Wide Area Augmentation System which is a ground based transmitter. Since the only true error involved in GPS system is the error in imprecise amount of correction for how long it takes the time signal to arrive through the atmosphere, the WAAS system helps to correct for that. The WAAS transmitter is placed at a precisely surveyed position and receives the satellite signals, errors and all and compares the computed position to the actual position and then computes and transmits a correction signal to WAAS enabled receivers. This system is used for aviation for precision near airports (approaches more precise than the much older ILS system), for surveying and the like. The accuracy then becomes less than 1 meter.

Reading through this: http://en.wikipedia.org/wiki/Wide_Ar...ntation_System really helped my understanding. Now I just wish my aircraft had a WAAS enabled receiver.

[durango dude]

Back in my college days (80s), I worked commercial fishing.

We used an older form of electronic navigation (Loran-C) - which utilized land-based radio signals.

I also remember back in the late 90s, we had to pay lots of extra money for a unit that was "differential ready." We fished near some shallow reefs - so location really did matter.

Humorously, the DoD used to provide slightly degraded signals to civilian systems, to ensure it could not be used precisely by the enemy
(as if being 20 feet off would matter for a tactical missile).

[LCHIEN]

Very good understanding and explanation. However, there are two positions which would match the time delay of the 4 satellite time signals but the receivers are designed to eliminate the one which is nearly impossible without space travel.

The other element which exists for some enabled receivers is WAAS or Wide Area Augmentation System which is a ground based transmitter. Since the only true error involved in GPS system is the error in imprecise amount of correction for how long it takes the time signal to arrive through the atmosphere, the WAAS system helps to correct for that. The WAAS transmitter is placed at a precisely surveyed position and receives the satellite signals, errors and all and compares the computed position to the actual position and then computes and transmits a correction signal to WAAS enabled receivers. This system is used for aviation for precision near airports (approaches more precise than the much older ILS system), for surveying and the like. The accuracy then becomes less than 1 meter.

Reading through this: http://en.wikipedia.org/wiki/Wide_Ar...ntation_System really helped my understanding. Now I just wish my aircraft had a WAAS enabled receiver.

yes, you are correct that two positions can match. But a small matter of logic eliminates one of them. I left that out to keep it simple, my explanations run to the longish anyways, or so my wife says.