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Old 03-01-2020, 08:49 PM   #1
borgward
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Off Subject GPS


Are some GPS receivers configured to a specific set of satellites and other GPS modules configured to another set of satellites, or does any GPS unit run off of any satellite?

Wifes Fords Navigation map works around here but goes out on the way to work 30 miles away. A "no GPS" symbol appears on the screen even when the map displays correctly with in 5 miles of home.

Ford sent a notice mapping needs to be updated ($149) That's why I asked about about whether or not GPS navigation software is configured to a specific set of satellites or not.
 
Old 03-01-2020, 09:04 PM   #2
michaelk
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Yes, depends on make and model of receiver. I would say the typical commercial GPS navigation units you can by in the US at Best Buy etc. only use the USA satellites.

https://en.wikipedia.org/wiki/Satellite_navigation

There are certain areas around the country were GPS does not work well and could be due to man made interference or other physical surroundings.

I would guess those are just map updates and not anything to do with receiver firmware or satellite systems. The basic information that GPS satellite systems provide to the car are receiver position i.e latitude, longitude, height and time. All the mapping and other fancy stuff is in database that gets updated.

Last edited by michaelk; 03-01-2020 at 09:22 PM.
 
Old 03-02-2020, 12:53 AM   #3
rnturn
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Quote:
Originally Posted by michaelk View Post
There are certain areas around the country were GPS does not work well and could be due to man made interference or other physical surroundings.
Some of the interference is due to Mother Nature. GPS signals are L-band which is highly directional, easily blocked by structures, and reflects off some objects very nicely giving the receiver a hard time. Downtown areas, for example, with the reflections off the glass building faces make it particularly difficult for GPS receivers to provide good positions. Back in the early days of GPS, the testing of the military receivers was done outside Yuma, AZ. The thinking was, I suspect, that testing at a military base in the desert would minimize signal blockages due to terrain for lower elevation SVs (the ones you want to track if you're interested in an accurate lat/long position). Unfortunately, desert sand turned out to be an excellent reflector at L-bands. The team I was working with had to scramble to understand the problem. The strong reflections were throwing the receiver's tracking loops wa-a-ay off, causing the ranges to the low elevation SVs to be off, and the position solutions were awful. In some cases, I think the receiver would even lose track of an SV and have to reacquire it. (Remember "ghosts" back in the analog TV days? Similar idea.)
 
Old 03-03-2020, 12:32 PM   #4
michaelk
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That was a bit before my time... An ex-coworker of mine worked on the initial program and helped develop all the math.

To further explain GPS. There are around 33 US GPS satellites currently in orbit and since they are not geosynchronous move in and out of view of your vehicle constantly. The GPS receiver is multichannel and can typically track 12 satellites at a time. GPS phone apps and handheld units typically can display what satellites the built in receiver is currently tracking and their signal status. I'm sure the data is available for your car but just can not be displayed. It takes a minimum of 3 satellites to calculate a position and 4 to calculate height. As posted the signal can be easily blocked.

Last edited by michaelk; 03-03-2020 at 12:38 PM.
 
Old 03-03-2020, 01:25 PM   #5
rtmistler
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We moved towards using both US GPS satellites as well as GLONASS satellites. Sure you need 4 sats for a high confidence solution, but we've found that the more satellites in view, the better. There are multi-constellation modules out there. SIRFStar V and ublox both have them, as do others. There's also at least one other constellation, but there's not many sats up as yet, from China, I believe.

Or, you can just use the raw streams and design your own Kalman filters, but that's a moderately large project. I have to assume there's some free code out there that gets all that started and helps you to derive solutions as it stands, and
 
Old 03-03-2020, 01:41 PM   #6
michaelk
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For one of my projects a few years ago we also purchased receivers with dual constellation capabilities. I am vaguely familiar with SIRFStar but unfortunately I have not kept up with the current hardware.
 
Old 03-05-2020, 04:07 AM   #7
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I was quite impressed to learn that my 2017 'phone uses at least four different constellations (GPS, GLONASS, Galileo, and BeiDou), and reports 20+ satellites in use in a flat area. Even so the position can be out by 50 metres or more near a mountain or wood - my guess is that's partly reflection, and partly all the visible satellites being clustered in a relatively small patch of sky.
 
  


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