Our Magellan has been acting a fool for 2 days now. It locks up everytime we sit still more than a couple minutes and have to do a hard reset and let it redo the route and catch back up to is again. It's been a real POS lately. We need to upgrade, but trying to hold off for now. Hope it clears up, going with Turtles solar flare theory!!
GPS can't find it's *&&
- Thread starter mjmsprt40
- Start date
purgoose10
Veteran Expediter
Makes you wonder about I90 and the troubles. Wonder if its the deviding line between satilite feeds. Sounds crazy doesn't it?
The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails). Each of these 3,000- to 4,000-pound solar-powered satellites (with battery backup for solar eclipses) circles the globe at about 12,000 miles altitude, making two complete rotations every day traveling about 7,000 MPH. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.
A GPS receiver's job is to locate four or more of these satellites, figure out the distanc*e to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration (sometimes refered to as triangulation). Trilateration in three-dimensional space can be a little tricky, so it's best to start with an explanation of simple two-dimensional trilateration.
Imagine you are somewhere in the United States and you are totally lost. No clue. You've been kidnapped, blindfolded, drugged and dumped out of the sliding door of a windowless expediter van in the middle of nowhere. What do you do?
You find a friendly local and ask, "Where am I?" He says, "You are precisely 625 miles from Boise, Idaho," and then walks off. Awesome. This is a nice, hard fact, but it is not particularly useful by itself. You could be anywhere on a circle around Boise that has a radius of 625 miles:
You ask somebody else where you are and they say, "You are 690 miles from Minneapolis, Minnesota." Well, OK. Now you're getting somewhere. If you combine this information with the Boise information, you have two circles that intersect. You now know that you must be at one of these two intersection points, if you are 625 miles from Boise and 690 miles from Minneapolis. But where, exactly?
You find a third person and they tell you that you are 615 miles from Tucson, Arizona. Never mind the fact that these three people are the three most obtusely pedantic people on the planet, at least you now have enough information to determine where you are, because the third circle will only intersect with one of these points.
You now know exactly where you are: Denver, Colorado. Why they didn't just tell you that in the first place, I have no idea, but it does help with our little story.
2-D trilateration gives you the exact longitude and latitude on a flat map. This same concept works in three-dimensional space, as well, but you're dealing with spheres instead of circles.
Fundamentally, three-dimensional trilateration isn't much different from two-dimensional trilateration, but it's a little trickier to visualize. Imagine the radii from the previous examples going off in all directions. So instead of a series of two-dimensional circles, you get a series of three-dimensional spheres.
If you know you are 10 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10-mile radius. If you also know you are 15 miles from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle. If you know the distance to a third satellite, you get a third sphere, which intersects with this circle at two points.
The Earth itself can act as a fourth sphere, and only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space. Receivers generally look to four or more satellites, however, to improve accuracy and provide precise altitude information.
In order to make this simple calculation, then, the GPS receiver has to know two things:
Three satellites will give you your longitude and latitude, and the distance to those satellites will give you your altitude. You need all three to determine your exact locations on a lump and bumpy sphere.
The GPS receiver figures both of these things out by analyzing high-frequency, low-power (50 Watts or less, usually) radio signals from the GPS satellites. Better units (higher dollar) have multiple receivers, so they can pick up signals from several satellites simultaneously. A $499 dollar receiver is more than just additional bells and whistles over a $99 receiver.
Radio waves are electromagnetic energy which travels at the speed of light (about 186,000 miles per second in a vacuum). The receiver can figure out how far the signal has traveled by timing how long it took the signal to arrive. This is a fairly elaborate process, but it's the key, crucial part of how GPS works.
At a particular time (pick one, but let's say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code. The receiver begins running the same digital pattern also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern because of the time it takes to travel from the satellite to the receiver. Light travels fast, but it's not instantaneous.
The length of the delay is equal to the signal's travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the distance from receiver to satellite. Easy peasy.
Easy peasy, except, in order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. To make a satellite positioning system using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself. Atomic clocks cost somewhere between $50,000 and $100,000, so there goes that idea.
But, the GPS a clever, effective solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets. The satellite is constantly broadcasting its time, along with other information. So, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy based on the identical clock times of the satellites. As there is only one value for the "current time" that the receiver can use, and it gets that from the satellites, the correct time value will cause all of the signals that the receiver is receiving to align at a single point in space. That time value is the time value held by the atomic clocks in all of the satellites. The receiver sets its clock to that time value, and it then has the same time value that all the atomic clocks in all of the satellites have.
When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point. Three spheres will intersect even if your numbers are way off, but four spheres will not intersect at one point if you've measured incorrectly. Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect.
The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock. The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.
In order for the distance information to be of any use, the receiver also has to know where the satellites actually are. This is pretty easy to do, since the satellites travel in very high and predictable orbits. The GPS receiver simply stores an "almanac" that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly, but the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' normal signals.
OK, so now you've got your four satellites and you're ready to go. Except that the speed of light is a constant only in a vacuum, and the Earth's atmosphere slows the electromagnetic energy down somewhat, particularly as it goes through the ionosphere and troposphere. Clouds and rain can also affect it. The delay varies depending on where you are on Earth, which means it's difficult to accurately factor this into the distance calculations. Problems can also occur when radio signals bounce off large objects, such as skyscrapers, mountains and canyon walls, giving a receiver the impression that a satellite is farther away than it actually is. On top of all that, satellites sometimes just send out bad almanac data, misreporting their own position. There are methods of error correction used to overcome these problems. DGPS (Differential GPS) is one way. WAIS and SirF chips also play into it.
So, the essential function of a GPS receiver is to pick up the transmissions of at least four satellites and combine the information in those transmissions with information in an electronic almanac, all in order to figure out the receiver's position on Earth. That's really all it does.
Once the receiver makes this calculation, it can tell you the latitude, longitude and altitude (or some similar measurement) of its current position. To make the navigation more user-friendly, most receivers plug this raw data into map files stored in memory. They also can plot navigational routes, which is what we use them for. A standard GPS receiver will not only place you on a map at any particular location, but will also trace your path across a map as you move. If you leave your receiver on, it can stay in constant communication with GPS satellites to see how your location is changing. With this information and its built-in clock, the receiver can give you several pieces of information:
That last one is where a lot of problems crop up with GPS receivers, where the software used to calculate a laundry list of things may have bugs. These bugs can cause the receiver to lock up, reboot, or report inaccurate mapping or location. GPS is a complicated process even when things work perfectly. When you add in the software for mapping and routing, it gets even more complicated, and more error prone.
When a GPS unit goes a little screwy, the chances really and truly are that it's the software or the quality of the receiver's electronics, or some random, errant FR or EM interference, sunspots and solar flares, or for whatever reason the receiver can only see 2 or 3 satellites instead of 4 or more, that are the causes of the problem. It could be intentional jamming by those ever popular and quite evil gubmint folks, but that's actually a far more complicated scenario than the already complicated complexity of GPS itself.
A GPS receiver's job is to locate four or more of these satellites, figure out the distanc*e to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration (sometimes refered to as triangulation). Trilateration in three-dimensional space can be a little tricky, so it's best to start with an explanation of simple two-dimensional trilateration.
Imagine you are somewhere in the United States and you are totally lost. No clue. You've been kidnapped, blindfolded, drugged and dumped out of the sliding door of a windowless expediter van in the middle of nowhere. What do you do?
You find a friendly local and ask, "Where am I?" He says, "You are precisely 625 miles from Boise, Idaho," and then walks off. Awesome. This is a nice, hard fact, but it is not particularly useful by itself. You could be anywhere on a circle around Boise that has a radius of 625 miles:
You ask somebody else where you are and they say, "You are 690 miles from Minneapolis, Minnesota." Well, OK. Now you're getting somewhere. If you combine this information with the Boise information, you have two circles that intersect. You now know that you must be at one of these two intersection points, if you are 625 miles from Boise and 690 miles from Minneapolis. But where, exactly?
You find a third person and they tell you that you are 615 miles from Tucson, Arizona. Never mind the fact that these three people are the three most obtusely pedantic people on the planet, at least you now have enough information to determine where you are, because the third circle will only intersect with one of these points.
You now know exactly where you are: Denver, Colorado. Why they didn't just tell you that in the first place, I have no idea, but it does help with our little story.
2-D trilateration gives you the exact longitude and latitude on a flat map. This same concept works in three-dimensional space, as well, but you're dealing with spheres instead of circles.
Fundamentally, three-dimensional trilateration isn't much different from two-dimensional trilateration, but it's a little trickier to visualize. Imagine the radii from the previous examples going off in all directions. So instead of a series of two-dimensional circles, you get a series of three-dimensional spheres.
If you know you are 10 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10-mile radius. If you also know you are 15 miles from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle. If you know the distance to a third satellite, you get a third sphere, which intersects with this circle at two points.
The Earth itself can act as a fourth sphere, and only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space. Receivers generally look to four or more satellites, however, to improve accuracy and provide precise altitude information.
In order to make this simple calculation, then, the GPS receiver has to know two things:
- The location of at least three satellites above you
- The distance between you and each of those satellites
Three satellites will give you your longitude and latitude, and the distance to those satellites will give you your altitude. You need all three to determine your exact locations on a lump and bumpy sphere.
The GPS receiver figures both of these things out by analyzing high-frequency, low-power (50 Watts or less, usually) radio signals from the GPS satellites. Better units (higher dollar) have multiple receivers, so they can pick up signals from several satellites simultaneously. A $499 dollar receiver is more than just additional bells and whistles over a $99 receiver.
Radio waves are electromagnetic energy which travels at the speed of light (about 186,000 miles per second in a vacuum). The receiver can figure out how far the signal has traveled by timing how long it took the signal to arrive. This is a fairly elaborate process, but it's the key, crucial part of how GPS works.
At a particular time (pick one, but let's say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code. The receiver begins running the same digital pattern also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern because of the time it takes to travel from the satellite to the receiver. Light travels fast, but it's not instantaneous.
The length of the delay is equal to the signal's travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the distance from receiver to satellite. Easy peasy.
Easy peasy, except, in order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. To make a satellite positioning system using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself. Atomic clocks cost somewhere between $50,000 and $100,000, so there goes that idea.
But, the GPS a clever, effective solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets. The satellite is constantly broadcasting its time, along with other information. So, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy based on the identical clock times of the satellites. As there is only one value for the "current time" that the receiver can use, and it gets that from the satellites, the correct time value will cause all of the signals that the receiver is receiving to align at a single point in space. That time value is the time value held by the atomic clocks in all of the satellites. The receiver sets its clock to that time value, and it then has the same time value that all the atomic clocks in all of the satellites have.
When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point. Three spheres will intersect even if your numbers are way off, but four spheres will not intersect at one point if you've measured incorrectly. Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect.
The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock. The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.
In order for the distance information to be of any use, the receiver also has to know where the satellites actually are. This is pretty easy to do, since the satellites travel in very high and predictable orbits. The GPS receiver simply stores an "almanac" that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly, but the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' normal signals.
OK, so now you've got your four satellites and you're ready to go. Except that the speed of light is a constant only in a vacuum, and the Earth's atmosphere slows the electromagnetic energy down somewhat, particularly as it goes through the ionosphere and troposphere. Clouds and rain can also affect it. The delay varies depending on where you are on Earth, which means it's difficult to accurately factor this into the distance calculations. Problems can also occur when radio signals bounce off large objects, such as skyscrapers, mountains and canyon walls, giving a receiver the impression that a satellite is farther away than it actually is. On top of all that, satellites sometimes just send out bad almanac data, misreporting their own position. There are methods of error correction used to overcome these problems. DGPS (Differential GPS) is one way. WAIS and SirF chips also play into it.
So, the essential function of a GPS receiver is to pick up the transmissions of at least four satellites and combine the information in those transmissions with information in an electronic almanac, all in order to figure out the receiver's position on Earth. That's really all it does.
Once the receiver makes this calculation, it can tell you the latitude, longitude and altitude (or some similar measurement) of its current position. To make the navigation more user-friendly, most receivers plug this raw data into map files stored in memory. They also can plot navigational routes, which is what we use them for. A standard GPS receiver will not only place you on a map at any particular location, but will also trace your path across a map as you move. If you leave your receiver on, it can stay in constant communication with GPS satellites to see how your location is changing. With this information and its built-in clock, the receiver can give you several pieces of information:
- How far you've traveled (odometer)
- How long you've been traveling
- Your current speed (speedometer)
- Your average speed
- A "bread crumb" trail showing you exactly where you have traveled on the map
- The estimated time of arrival at your destination if you maintain your current speed or on software calculated figures accounting for road types and speed limits.
That last one is where a lot of problems crop up with GPS receivers, where the software used to calculate a laundry list of things may have bugs. These bugs can cause the receiver to lock up, reboot, or report inaccurate mapping or location. GPS is a complicated process even when things work perfectly. When you add in the software for mapping and routing, it gets even more complicated, and more error prone.
When a GPS unit goes a little screwy, the chances really and truly are that it's the software or the quality of the receiver's electronics, or some random, errant FR or EM interference, sunspots and solar flares, or for whatever reason the receiver can only see 2 or 3 satellites instead of 4 or more, that are the causes of the problem. It could be intentional jamming by those ever popular and quite evil gubmint folks, but that's actually a far more complicated scenario than the already complicated complexity of GPS itself.
purgoose10
Veteran Expediter
Turtle my friend, you have entirely to much time on your hands. Freight slow?
lol!
lol!