How many GPS satelites currently in sky?

Question

what is current constelation of GPS satelites and how many are needed for a location fix?

Answer 1

There are currently 24 satellites working for GPS. You need 3 for a fix and 4 for an altitude. The more you have the more accurate the position.They are located such that you can get up to 12 at one time.There are 2 new geostationary satellites, one over the East Coast and one over the West Coast.

Answer 2

I believe you have 24 satellites, plus 3 for backup (satellites are regularly retired / replaced).

By definition just 3 satellites would be needed to have a position, but most GPS systems will prefer to lock on 4 or even 5 before they can give you good precision (say, down to 30 feet or less).

The EU has a project for satellite navigation, called GALILEO. First satellite was launched in Dec '05, a total of 30 are planned, and the system is due to be operational by 2010. Apart from the political implications (want to be independent from GPS, which the US could chose to scramble any time in case of a conflict or major crisis), Galileo should be more precise than GPS (open service will be about 4 metres, and commercial service will be about 1 metre).

For those who are interested, clocks on the satellites are affected by two relativistic effects, the first due to their speed in orbit (small relative to the speed of light, but still), and the second due to the gravitational field of the earth which is a bit weaker where they are, than on the surface, where we use the signals. As a result the clocks had to be adjusted for this.

Answer 3

There are currently 30, with one more to be launched in November:
http://www.navcen.uscg.gov/gps/gpsnotices/50SW_GPSW_letter.pdf

Current Status:
http://www.navcen.uscg.gov/ftp/GPS/status.txt

Answer 4

64 and 3