Still not correct Mel. All modern PND's are capable of "seeing" every one of the (up to) 32 active GPS satellites in the constellation. Now some of the very newest chipsets can also make use of GLOSNOS or Galileo satellites. Japan has a set of sats specific to them that requires a different chipset altogether. And now the Chinese and Indians are launching their own "GPS" satellites too.
But the US system is what the world currently relies on.
Well then, show us that. As far as I know, it's not entirely true. If it is, then I would like to know what's changed from what I knew to be true.
No. The article you're relying on that claims only 3 satellites is needed is wrong. I was going to post a link, but Solipcism's will do fine.
For full disclosure I've moderated and been active for years at several GPS-centric forums, including TomTomForums, GPSReview.net, GPSMagazine. An occasional editorial contributor at GPSWorld and invited to the Space Center to participate in interviews with the GPS wing of the military for the initial launch of the new generation of GPS satellites a few months ago. I apologize for the lack of links for what I posted, but I didn't rely on any links. Searching out easily understood ones would be extremely time-consuming, but for an excellent overview of GPS, somewhat Garmin-specific, you could try http://home.comcast.net/~ghayman3/ga...s/pageinfo.htm
For more in-depth articles and explanations, visit http://gpsinformation.net/ where I've also been active.
Thank goodness you're here to prove the original poster correct about the conspiracy theory stuff.
Did you even read Google's response about the Street View incident? Here's the link to a nice blog entry about it. Maybe it will help you pull your head out of the dark hole it's apparently stuck in.
I can practice suspension of disbelief when I watch a movie, but not when I read a Google PR piece.
From my Garmin Quick Start Guide. Part No. 190-00161-01 Rev. A January 1999
Page 3:
"If some of the satellites in view are blocked, or "shaded", the receiver can use the remaining satellites (at least three are needed) to maintain its location."
It doesn't say how many are required to initially fix your location but it apparently only needs three to maintain it.
Wikipedia has a fairly well-written article on the subject, explaining why four signals are needed to get an initial lock. Maintaining a nominal lock with three is achieved by some combination of assumptions, usually regarding elevation.
There are a number of articles that say this. Show us a link that says otherwise. I'll be happy to change my views if I'm shown better information, but I need that information first in order to know that what I know is wrong.
From my Garmin Quick Start Guide. Part No. 190-00161-01 Rev. A January 1999
Page 3:
"If some of the satellites in view are blocked, or "shaded", the receiver can use the remaining satellites (at least three are needed) to maintain its location."
It doesn't say how many are required to initially fix your location but it apparently only needs three to maintain it.
Yeah, that?s to maintain because your general position on the planet isn?t likely to have moved much from its surface.
Remember, these satellites are 12,000 miles above the earth. For perspective? Mariana Trench is only 7 miles deep and commercial airlines fly at 7 miles above the Earth, or 1/1,714th the distance.
Correct. Civilian gps chipsets lack the ability to use the additional encrypted signals used by our military.
Civilian GPS does lack the ability to decrypt those signal, which are mainly used for anti-jamming, but not the accuracy. Selective Availability was eliminated during Clinton presidency around the year 2000. Instead of accuracy degrading, the military can now block GPS signals to selective areas as needed for national security.
Sorry, still incorrect. While that might seem to make sense, your gps-enabled device cannot give you an initial location estimate without four satellites locked.
Trust me if I make a statement about GPS
Actually, my old Gamin II and 60GS which I used a lot for off shore sailing locked my position after picking up the signals of three satellites, as long as they were in a useful angle to my position.
Quote:
Originally Posted by solipsism
You?re right, and being knowledgable in a subject is great, but you have to establish that so detailing why and posting links that back up your points are helpful to others and to your point.
This explains it better than I can:
How does GPS "triangulation" work?
For this exercise, you'll need to dust off some simple skills you learned in geometry. Start by imagining a "distance sphere" (in three dimensions) surrounding a GPS satellite. Points on the surface of the sphere are all the same distance from the satellite that's located exactly at the center.
Knowing distance from one satellite places you somewhere on a spherical surface that's centered around the satellite.
Knowing distances from two satellites places you somewhere along a circle that's between the two satellites (defined by the intersection of their "distance spheres?).
Distances from three satellites usually intersect at two points, and if you're not flying around, one of these points will be on Earth's surface.
Distances from four or more GPS satellites will intersect at just one point.
This process works by finding the intersection of your distances from three or more satellites. Thus, describing it as "trilateration" is actually better than "triangulation", but neither term seems precisely correct from a technical standpoint.
Thanks solipsism for the refresher. You sent me mentally back to classroom.
Wikipedia has a fairly well-written article on the subject, explaining why four signals are needed to get an initial lock. Maintaining a nominal lock with three is achieved by some combination of assumptions, usually regarding elevation.
The question is more than that. It's also whether receivers use more than three or four satellites these days, or whether none ever use more than that number.
I'm open to more information, if it comes from a trusted source.
Wikipedia has a fairly well-written article on the subject, explaining why four signals are needed to get an initial lock. Maintaining a nominal lock with three is achieved by some combination of assumptions, usually regarding elevation.
Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a less accurate (degraded) position when fewer than four satellites are visible.
Civilian GPS does lack the ability to decrypt those signal, which are mainly used for anti-jamming, but not the accuracy. Selective Availability was eliminated during Clinton presidency around the year 2000. Instead of accuracy degrading, the military can now block GPS signals to selective areas as needed for national security.
That's correct. At one time civilian receivers were relegated to something like 100 yards or something like that. I don't remember the distance. But once the military signals were opened up to public use, it dropped to a one meter radius or so. The military has nothing more accurate than that.
One reason it was done was for civilian development, which opened up new industries at a great benefit for the public. The other was because it was found that other countries could break the encryption anyway.
The question is more than that. It's also whether receivers use more than three or four satellites these days, or whether none ever use more than that number.
I'm open to more information, if it comes from a trusted source.
Again from my quick start guide: you will typically have anywhere from 5 -10 satellites.
The question is more than that. It's also whether receivers use more than three or four satellites these days, or whether none ever use more than that number.
I'm open to more information, if it comes from a trusted source.
A-GPS could easily supply the elevation and general location you’re in. This might be why you access Maps and your initial position then slides magically after a few seconds, especially when you’re up in a building.
I think people just assume it’s three satellites for GPS because of the association with triangulation, even though it’s multiple triangles that all confer using a network of triangles.
Probably not. If Apple's going to limit it to seven days, then I imagine the apps would be required to respect that.
Any app that you permit to access your location can create its own database (remember, receiving location information is one of the permitted background activities). However, the app would not start tracking after a restart of the iPhone until you start the app for the first time (there are no login items in iOS).
The military has sub-meter accuracy. Actually approaching centimeters. Civilian devices are generally accurate to about 5 meters, but all that means is that there's a 50% probability that you are within 5 meters of where your gps device thinks you are. It's an involved explanation.
The question is more than that. It's also whether receivers use more than three or four satellites these days, or whether none ever use more than that number.
I'm open to more information, if it comes from a trusted source.
Yes - I didn't think that the rest of the question really needed addressing. That information is also in Wikipedia (and elsewhere if you distrust WP). Dedicated GPS units are still multichannel for improved accuracy and to maintain lock as individual satellites go in and out of visibility to the unit. Many units include status indicators showing satellites detected and satellites locked.
I don't know what chipset the iPhone uses now, but the early ones were 12 channel, I believe. I'm sure that someone here can elaborate.
A bit late coming in to this discussion (since my last couple of days have been crazy, and I have not been able to get on to AI).
FANTASTIC job, Apple. Straight, to-the-point mea culpa. I say this as someone who was vehemently demanding something like this (despite having taken much abuse from this forum).
Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a less accurate (degraded) position when fewer than four satellites are visible.
Ok. So according to the Wikipedia article, my numbers are correct. While it prefers four, it can use three for a quick and dirty fix, and it can use more for redundant information. And as eight are visible at one time, it can use up to eight.
The only thong about the article that's not quite correct is the current accuracy. It's much more accurate than 65 feet.
The question is more than that. It's also whether receivers use more than three or four satellites these days, or whether none ever use more than that number.
I'm open to more information, if it comes from a trusted source.
Yes, once position has been established, then it's "the more the merrier" for accuracy, but on a declining scale. Once you've got 6 or 7 sats identified, adding another two or three more doesn't improve the accuracy all that much. Perhaps from an estimated 12' down to 9-10'. But the accuracy of the gps location isn't the big variable. It's the quality of the maps the device uses. All maps used by consumer gps devices are rift with positional errors.
As for how many satellites are generally used for navigation, it's common for 8-11 satellites to be "found" by modern gps navigation devices within a few minutes at most.
Comments
Still not correct Mel. All modern PND's are capable of "seeing" every one of the (up to) 32 active GPS satellites in the constellation. Now some of the very newest chipsets can also make use of GLOSNOS or Galileo satellites. Japan has a set of sats specific to them that requires a different chipset altogether. And now the Chinese and Indians are launching their own "GPS" satellites too.
But the US system is what the world currently relies on.
Well then, show us that. As far as I know, it's not entirely true. If it is, then I would like to know what's changed from what I knew to be true.
For full disclosure I've moderated and been active for years at several GPS-centric forums, including TomTomForums, GPSReview.net, GPSMagazine. An occasional editorial contributor at GPSWorld and invited to the Space Center to participate in interviews with the GPS wing of the military for the initial launch of the new generation of GPS satellites a few months ago. I apologize for the lack of links for what I posted, but I didn't rely on any links. Searching out easily understood ones would be extremely time-consuming, but for an excellent overview of GPS, somewhat Garmin-specific, you could try http://home.comcast.net/~ghayman3/ga...s/pageinfo.htm
For more in-depth articles and explanations, visit http://gpsinformation.net/ where I've also been active.
Thank goodness you're here to prove the original poster correct about the conspiracy theory stuff.
Did you even read Google's response about the Street View incident? Here's the link to a nice blog entry about it. Maybe it will help you pull your head out of the dark hole it's apparently stuck in.
I can practice suspension of disbelief when I watch a movie, but not when I read a Google PR piece.
Someone needs to tell Garmin then.
From my Garmin Quick Start Guide. Part No. 190-00161-01 Rev. A January 1999
Page 3:
"If some of the satellites in view are blocked, or "shaded", the receiver can use the remaining satellites (at least three are needed) to maintain its location."
It doesn't say how many are required to initially fix your location but it apparently only needs three to maintain it.
Wikipedia has a fairly well-written article on the subject, explaining why four signals are needed to get an initial lock. Maintaining a nominal lock with three is achieved by some combination of assumptions, usually regarding elevation.
Correct. Civilian gps chipsets lack the ability to use the additional encrypted signals used by our military.
Ah, they're encrypted, that explains it. So anyone can receive them but they will be gibberish.
No. The article you're relying on is.
There are a number of articles that say this. Show us a link that says otherwise. I'll be happy to change my views if I'm shown better information, but I need that information first in order to know that what I know is wrong.
Someone needs to tell Garmin then.
From my Garmin Quick Start Guide. Part No. 190-00161-01 Rev. A January 1999
Page 3:
"If some of the satellites in view are blocked, or "shaded", the receiver can use the remaining satellites (at least three are needed) to maintain its location."
It doesn't say how many are required to initially fix your location but it apparently only needs three to maintain it.
Yeah, that?s to maintain because your general position on the planet isn?t likely to have moved much from its surface.
Remember, these satellites are 12,000 miles above the earth. For perspective? Mariana Trench is only 7 miles deep and commercial airlines fly at 7 miles above the Earth, or 1/1,714th the distance.
Correct. Civilian gps chipsets lack the ability to use the additional encrypted signals used by our military.
Civilian GPS does lack the ability to decrypt those signal, which are mainly used for anti-jamming, but not the accuracy. Selective Availability was eliminated during Clinton presidency around the year 2000. Instead of accuracy degrading, the military can now block GPS signals to selective areas as needed for national security.
Sorry, still incorrect. While that might seem to make sense, your gps-enabled device cannot give you an initial location estimate without four satellites locked.
Trust me if I make a statement about GPS
Actually, my old Gamin II and 60GS which I used a lot for off shore sailing locked my position after picking up the signals of three satellites, as long as they were in a useful angle to my position.
You?re right, and being knowledgable in a subject is great, but you have to establish that so detailing why and posting links that back up your points are helpful to others and to your point.
This explains it better than I can:
Thanks solipsism for the refresher. You sent me mentally back to classroom.
Wikipedia has a fairly well-written article on the subject, explaining why four signals are needed to get an initial lock. Maintaining a nominal lock with three is achieved by some combination of assumptions, usually regarding elevation.
The question is more than that. It's also whether receivers use more than three or four satellites these days, or whether none ever use more than that number.
I'm open to more information, if it comes from a trusted source.
Wikipedia has a fairly well-written article on the subject, explaining why four signals are needed to get an initial lock. Maintaining a nominal lock with three is achieved by some combination of assumptions, usually regarding elevation.
Ok good to know.
http://en.wikipedia.org/wiki/Global_Positioning_System
Basic concept of GPS
last paragraph, last sentence:
Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a less accurate (degraded) position when fewer than four satellites are visible.
Civilian GPS does lack the ability to decrypt those signal, which are mainly used for anti-jamming, but not the accuracy. Selective Availability was eliminated during Clinton presidency around the year 2000. Instead of accuracy degrading, the military can now block GPS signals to selective areas as needed for national security.
That's correct. At one time civilian receivers were relegated to something like 100 yards or something like that. I don't remember the distance. But once the military signals were opened up to public use, it dropped to a one meter radius or so. The military has nothing more accurate than that.
One reason it was done was for civilian development, which opened up new industries at a great benefit for the public. The other was because it was found that other countries could break the encryption anyway.
The question is more than that. It's also whether receivers use more than three or four satellites these days, or whether none ever use more than that number.
I'm open to more information, if it comes from a trusted source.
Again from my quick start guide: you will typically have anywhere from 5 -10 satellites.
The question is more than that. It's also whether receivers use more than three or four satellites these days, or whether none ever use more than that number.
I'm open to more information, if it comes from a trusted source.
A-GPS could easily supply the elevation and general location you’re in. This might be why you access Maps and your initial position then slides magically after a few seconds, especially when you’re up in a building.
I think people just assume it’s three satellites for GPS because of the association with triangulation, even though it’s multiple triangles that all confer using a network of triangles.
Probably not. If Apple's going to limit it to seven days, then I imagine the apps would be required to respect that.
Any app that you permit to access your location can create its own database (remember, receiving location information is one of the permitted background activities). However, the app would not start tracking after a restart of the iPhone until you start the app for the first time (there are no login items in iOS).
The question is more than that. It's also whether receivers use more than three or four satellites these days, or whether none ever use more than that number.
I'm open to more information, if it comes from a trusted source.
Yes - I didn't think that the rest of the question really needed addressing. That information is also in Wikipedia (and elsewhere if you distrust WP). Dedicated GPS units are still multichannel for improved accuracy and to maintain lock as individual satellites go in and out of visibility to the unit. Many units include status indicators showing satellites detected and satellites locked.
I don't know what chipset the iPhone uses now, but the early ones were 12 channel, I believe. I'm sure that someone here can elaborate.
FANTASTIC job, Apple. Straight, to-the-point mea culpa. I say this as someone who was vehemently demanding something like this (despite having taken much abuse from this forum).
All is forgiven!
Ok good to know.
http://en.wikipedia.org/wiki/Global_Positioning_System
Basic concept of GPS
last paragraph, last sentence:
Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a less accurate (degraded) position when fewer than four satellites are visible.
Ok. So according to the Wikipedia article, my numbers are correct. While it prefers four, it can use three for a quick and dirty fix, and it can use more for redundant information. And as eight are visible at one time, it can use up to eight.
The only thong about the article that's not quite correct is the current accuracy. It's much more accurate than 65 feet.
The question is more than that. It's also whether receivers use more than three or four satellites these days, or whether none ever use more than that number.
I'm open to more information, if it comes from a trusted source.
Yes, once position has been established, then it's "the more the merrier" for accuracy, but on a declining scale. Once you've got 6 or 7 sats identified, adding another two or three more doesn't improve the accuracy all that much. Perhaps from an estimated 12' down to 9-10'. But the accuracy of the gps location isn't the big variable. It's the quality of the maps the device uses. All maps used by consumer gps devices are rift with positional errors.
As for how many satellites are generally used for navigation, it's common for 8-11 satellites to be "found" by modern gps navigation devices within a few minutes at most.