How does GPS work?

2oldman wrote:
My stupid Garmin can take upwards of 15 minutes to get a fix, and then I often hear "lost satellite signal." If cell service is available google maps is much better.

15 minutes is a little extreme even for the older dumb units but if the antenna is positioned such that it can't see a significant portion of the sky, it may have difficulty finding enough satellites.

Keep in mind if you have a cluster of 6 satellites in a small section of the sky, it will still struggle to get a good fix. You want them well distributed across the sky for best results.

Phone based GPS has an advantage that it can use cell tower locations to jump start the process.

garmin inreach for gps AND duplex sms capability with iridium sat system and emergency extraction if needed. All I need in one unit.
I just turn the unit on at least 10 minutes before we leave. If i'm near the location I turned it off at, get GPS in a minute or less. Going from PNW and then turning it on in south america... 10 minutes for a fix.

2oldman wrote:
I read somewhere it usually needs 4 at a minimum.

My stupid Garmin can take upwards of 15 minutes to get a fix, and then I often hear "lost satellite signal." If cell service is available google maps is much better.

Well Oldman,
I believe you have a unit that has either a bad antenna or has never been updated. My oldest sitting next to me has acquired three satellites since I started typing. That is a 20yo 48.

I have been lots of places where there was a single cell tower and the handset did not have a real GPS (most do now) and Google maps was lost and useless and I had not even over driven the loaded map yet.

Matt

2oldman wrote:

My stupid Garmin can take upwards of 15 minutes to get a fix, and then I often hear "lost satellite signal." If cell service is available google maps is much better.

It's sad to see companies, who were pioneer in their field, that refuse to keep up with advance of technology - then get clobbered in modern time. The day that people, especially young ones, waiting forever for a Position Fix is over.

2oldman wrote:

ktmrfs wrote:
the more sat's it can access the faster and more accurate you'll get a good "fix".

And yes, my units are usually off during traveling when not needed and it can often take 5 minutes or more to get a good location fix, especially on smaller handheld units.

I read somewhere it usually needs 4 at a minimum.

My stupid Garmin can take upwards of 15 minutes to get a fix, and then I often hear "lost satellite signal." If cell service is available google maps is much better.

needs 4 or more to get accurate location accounting for elevation and determining elevation, My understanding is In a pinch 3 will get reasonable location, but with limited accuracy. with 3 sat's it assumes you are on a flat location at mean sea level.

So for 3 dimensional location (latitude, longtitude and elevation) 4 sat's are needed at a minimum.

if you are moving much above a walking speed, I think it also needs another sat to determine velocity and take into account time and frequency shifts

ktmrfs wrote:
the more sat's it can access the faster and more accurate you'll get a good "fix".

And yes, my units are usually off during traveling when not needed and it can often take 5 minutes or more to get a good location fix, especially on smaller handheld units.

I read somewhere it usually needs 4 at a minimum.

My stupid Garmin can take upwards of 15 minutes to get a fix, and then I often hear "lost satellite signal." If cell service is available google maps is much better.

valhalla360 wrote:

ktmrfs wrote:

valhalla360 wrote:
Excellent description if you are looking for the minutia and how they addressed many of the implementation issues.

If you are trying to explain it to grandpa...the system measures the distance from your GPS receiver to multiple satellites of known position. If you are polling enough satellites and you know how far away they are, there is only one position you can be in.

system needs at least 3 sat's for position, (latitude and longtitude) 4 sat's for position and elevation.

Technically yes but it becomes problematic using 3 satellites as the earth is not flat. If you are out in the open plains generally it will give you an OK result. If you are in steep mountain terrain, it needs to either directly calculate elevation or the device needs a map that includes elevation to get an accurate location.

There are some things they can do to "cheat". These are especially useful on startup to get you a position quickly.

A simple one is to check the location of the cell tower the phone is operating on (presuming a phone based GPS). The system can then rule out locations that are not within a few miles of the tower.

It can also assume, the device is close to it's last known position. This is often the reason after getting off a plane that it takes a while to figure out where you are because it can mistakenly assume you were at the departure airport but the calculated position isn't matching up correctly. Eventually it gets readings from enough satellites and it figures out the position.

Back when the govt had selective availability turned on (purposely degrading the time signal so it only provided accuracy within 30m), industry came up with Differential GPS. A series of towers were installed that could in real time correct the time signal error and transmit it. Then a device with the capability, could decode the time error. Eventually, they just turned off selective availability. It is still used for some high precision uses like survey as variations in atmospheric density can distort the time signals.

correct. that's why I said "at least". the more sat's it can access the faster and more accurate you'll get a good "fix".

And yes, my units are usually off during traveling when not needed and it can often take 5 minutes or more to get a good location fix, especially on smaller handheld units.

The location fix time is one reason why my inreach unit with search and rescue and sat communication is turned on long before we start relying on it so it has a good location fix at the start. I don't want to run into a 5-10 minute delay in getting location when in an emergency situation.

ktmrfs wrote:

valhalla360 wrote:
Excellent description if you are looking for the minutia and how they addressed many of the implementation issues.

If you are trying to explain it to grandpa...the system measures the distance from your GPS receiver to multiple satellites of known position. If you are polling enough satellites and you know how far away they are, there is only one position you can be in.

system needs at least 3 sat's for position, (latitude and longtitude) 4 sat's for position and elevation.

Technically yes but it becomes problematic using 3 satellites as the earth is not flat. If you are out in the open plains generally it will give you an OK result. If you are in steep mountain terrain, it needs to either directly calculate elevation or the device needs a map that includes elevation to get an accurate location.

There are some things they can do to "cheat". These are especially useful on startup to get you a position quickly.

A simple one is to check the location of the cell tower the phone is operating on (presuming a phone based GPS). The system can then rule out locations that are not within a few miles of the tower.

It can also assume, the device is close to it's last known position. This is often the reason after getting off a plane that it takes a while to figure out where you are because it can mistakenly assume you were at the departure airport but the calculated position isn't matching up correctly. Eventually it gets readings from enough satellites and it figures out the position.

Back when the govt had selective availability turned on (purposely degrading the time signal so it only provided accuracy within 30m), industry came up with Differential GPS. A series of towers were installed that could in real time correct the time signal error and transmit it. Then a device with the capability, could decode the time error. Eventually, they just turned off selective availability. It is still used for some high precision uses like survey as variations in atmospheric density can distort the time signals.

2oldman wrote:

valhalla360 wrote:
..the system measures the distance from your GPS receiver to multiple satellites of known position.

Technically, the time it takes for the signal. That's some incredible precision.

Other way around. Time is used as a means of measuring distance from receiver to satellite.

valhalla360 wrote:
Excellent description if you are looking for the minutia and how they addressed many of the implementation issues.

If you are trying to explain it to grandpa...the system measures the distance from your GPS receiver to multiple satellites of known position. If you are polling enough satellites and you know how far away they are, there is only one position you can be in.

system needs at least 3 sat's for position, (latitude and longtitude) 4 sat's for position and elevation.