4 ms·

There's a few other restrictions as well as the 1000mph speed limit, as far as I'm aware. GPS is still slightly inaccurate in its publicly available form: ther

There's a few other restrictions as well as the 1000mph speed limit, as far as I'm aware. GPS is still slightly inaccurate in its publicly available form: there's a second frequency broadcasting an encrypted signal, and with both frequencies you can better account for variations in the ionosphere. Galileo I believe will shortly be the only system with worldwide coverage and publicly available multiple-frequency broadcast.

michaelt 6y ago
GPS is (slowly) adding new signals [1]; L2C a non-encrypted second frequency, and L5, a non-encrypted third frequency Should be pretty exciting when it becomes available. To the extent one can be excited about something that takes 20 years to be delivered. [1] https://www.gps.gov/systems/gps/modernization/civilsignals/ https://www.gps.gov/systems/gps/modernization/civilsignals/
- gorkish 6y ago
  You don't have to wait for the full fleet to cycle before you can use L2C or L5. L2C is already 15 years into its rollout and is widely available despite still being "pre-operational" due to it not having 100% coverage at all times. You also missed discussing L1C, the 4th civilian signal being introduced with GPS III. This will work primarily with L5 to enhance indoor accuracy.
gorkish 6y ago
ITAR regulations restrict GPS for civilian use to a maximum altitude and maximum velocity, but this is solely a software limitation that is increasingly becoming less relevant. Although selective availability has been disabled for a very long time, even before SA was turned off it was easily possible to get a precision measurement by integrating over time as the SA error was pseudo-random. It is also possible to use things like the phase information from the non-civilian 'encrypted' signals to increase accuracy even though the data cannot be decoded. Some survey-grade receivers were doing this even before SA was disabled, and it's pretty standard now in the precision GPS world. Your understanding of GPS is quite out of date. GPS began adding a second L2C frequency for consumer use 15 years ago, and started ramping it in 2014. They are in fact in the process of adding a third civilian signal called L5. You can get accuracy from the consumer signals <10cm while in motion today with the right receivers and antennas. L5 will make it more robust indoors and give higher accuracy with cheaper antennas. Galileo is great, but I wouldn't be holding it up as a crown jewel of GNSS. They have had some major missteps and operational issues in the recent past. For consumer applications, multi-GNSS receivers are really where it's at. Combining GPS + GLONASS + Galileo + BaiDou is not simply an excercise of comparing the resultant positions given by each networ, but actually being able to combine the information to produce a single faster or more reliable measurement. For instance, getting a 3d position from 2 GPS satellites + 2 Galileo satellites when ordinarily you would not even be able to get a 2d position from either network in that situation.
- phicoh 6y ago
  I wonder how combining 2 GPS satellites with 2 Galileo satellites would work in a realtime setting. As far as I know, GPS and Galileo have a different timebase. If you don't know the time difference, you can be way off.

michaelt 6y ago

GPS is (slowly) adding new signals [1]; L2C a non-encrypted second frequency, and L5, a non-encrypted third frequency Should be pretty exciting when it becomes available. To the extent one can be excited about something that takes 20 years to be delivered. [1] https://www.gps.gov/systems/gps/modernization/civilsignals/ https://www.gps.gov/systems/gps/modernization/civilsignals/

gorkish 6y ago
You don't have to wait for the full fleet to cycle before you can use L2C or L5. L2C is already 15 years into its rollout and is widely available despite still being "pre-operational" due to it not having 100% coverage at all times. You also missed discussing L1C, the 4th civilian signal being introduced with GPS III. This will work primarily with L5 to enhance indoor accuracy.

gorkish 6y ago

ITAR regulations restrict GPS for civilian use to a maximum altitude and maximum velocity, but this is solely a software limitation that is increasingly becoming less relevant. Although selective availability has been disabled for a very long time, even before SA was turned off it was easily possible to get a precision measurement by integrating over time as the SA error was pseudo-random. It is also possible to use things like the phase information from the non-civilian 'encrypted' signals to increase accuracy even though the data cannot be decoded. Some survey-grade receivers were doing this even before SA was disabled, and it's pretty standard now in the precision GPS world. Your understanding of GPS is quite out of date. GPS began adding a second L2C frequency for consumer use 15 years ago, and started ramping it in 2014. They are in fact in the process of adding a third civilian signal called L5. You can get accuracy from the consumer signals <10cm while in motion today with the right receivers and antennas. L5 will make it more robust indoors and give higher accuracy with cheaper antennas. Galileo is great, but I wouldn't be holding it up as a crown jewel of GNSS. They have had some major missteps and operational issues in the recent past. For consumer applications, multi-GNSS receivers are really where it's at. Combining GPS + GLONASS + Galileo + BaiDou is not simply an excercise of comparing the resultant positions given by each networ, but actually being able to combine the information to produce a single faster or more reliable measurement. For instance, getting a 3d position from 2 GPS satellites + 2 Galileo satellites when ordinarily you would not even be able to get a 2d position from either network in that situation.

phicoh 6y ago
I wonder how combining 2 GPS satellites with 2 Galileo satellites would work in a realtime setting. As far as I know, GPS and Galileo have a different timebase. If you don't know the time difference, you can be way off.