Re: What is this?
There is an old saying, "Often wrong but never in doubt."
GPS satellites aren't in Geosync orbit, they are in an inclined orbit at about 13,000 mile high And for decent accuracy you need three satellites, four if the unit is doing an altitude measurement. That's one more than for simple geometry reasons, but your receiver doesn't have a precision clock. The extra link eliminates the clock error. Following from Wikipedia.
Spilker.
A visual example of the GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45?N, changes with time.
The space segment (SS) comprises the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes,[22] but this was modified to six planes with four satellites each.[23] The orbital planes are centered on the Earth, not rotating with respect to the distant stars.[24] The six planes have approximately 55? inclination (tilt relative to Earth's equator) and are separated by 60? right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).[25] The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.[26]
Orbiting at an altitude of approximately 20,200 kilometers about 10 satellites are visible within line of sight (12,900 miles or 10,900 nautical miles; orbital radius of 26,600 km (14,500 mi or 15,400 NM)), each SV makes two complete orbits each sidereal day.[27] The ground track of each satellite therefore repeats each (sidereal) day. This was very helpful during development, since even with just four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.
As of March 2008[update],[28] there are 31 actively broadcasting satellites in the GPS constellation, and two older, retired from active service satellites kept in the constellation as orbital spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements.
There is an old saying, "Often wrong but never in doubt."
GPS satellites aren't in Geosync orbit, they are in an inclined orbit at about 13,000 mile high And for decent accuracy you need three satellites, four if the unit is doing an altitude measurement. That's one more than for simple geometry reasons, but your receiver doesn't have a precision clock. The extra link eliminates the clock error. Following from Wikipedia.
Spilker.
A visual example of the GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45?N, changes with time.
The space segment (SS) comprises the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes,[22] but this was modified to six planes with four satellites each.[23] The orbital planes are centered on the Earth, not rotating with respect to the distant stars.[24] The six planes have approximately 55? inclination (tilt relative to Earth's equator) and are separated by 60? right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection).[25] The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface.[26]
Orbiting at an altitude of approximately 20,200 kilometers about 10 satellites are visible within line of sight (12,900 miles or 10,900 nautical miles; orbital radius of 26,600 km (14,500 mi or 15,400 NM)), each SV makes two complete orbits each sidereal day.[27] The ground track of each satellite therefore repeats each (sidereal) day. This was very helpful during development, since even with just four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.
As of March 2008[update],[28] there are 31 actively broadcasting satellites in the GPS constellation, and two older, retired from active service satellites kept in the constellation as orbital spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements.
