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Government in and in , Qualcomm successfully completed tests of live assisted GPS on a mobile phone. In order to calculate location, a GPS device must be able to read the signal from at least four satellites. Each satellite in the network circles the earth twice a day, and each satellite sends a unique signal, orbital parameters, and time. At any given time, a GPS device can read the signals from six or more satellites, but there needs to be at least four. A single satellite broadcasts a microwave signal, which a GPS device picks up and uses to calculate the distance from GPS device to the satellite.
To better understand this, we could use a simple two-dimensional example. Instead of dealing with spheres, we could look at circles. When a satellite sends a signal, it creates a circle with the radius being the distance of the GPS device from the satellite. When we add a second satellite, it creates a second circle and the location is narrowed down to two points — the two points where the circle intersects.
A third satellite is used to determine the location of the device. The device is at the intersection of all three circles produced by the distance of the device from a given satellite.
In reality we live in a three-dimensional world, which means that each satellite produces a sphere not a circle. The intersection of three spheres produces two points of intersection — the point nearest Earth is chosen.
A fourth satellite is also required to move us into the third-dimension and calculate the altitude of a device. As a device moves, the radius distance to the satellite changes. As the radius changes, new spheres are being produced, giving us a new position. We can use that data, combined with the time from the satellite, to determine velocity, calculate the distance to our destination, and the time it will take.
The term GNSS is used to refer to all satellite systems with global coverage.
Satellite Navigation - Global Positioning System (GPS)
Other countries are working to build their own satellite systems. The EU has been working on Galileo, which is expected to reach full operation capacity by A GNSS receiver has 60 satellites available for viewing. While a device only needs three satellites to determine its location, accuracy is improved with a larger number of satellites. The chart below shows an example of number of satellites available shown in green , along with its signal strength height of the column , to a GPS receiver.
NAVSTAR Global Positioning System (GPS)
In this case, 12 satellites are available. A GNSS device can see more satellites. In the chart below, there are 17 available satellites. A larger number of satellites providing information to a receiver allows the device to calculate location with greater precision. For example, when driving through central Seattle where high-buildings, greater signal reflections, and frequency noise exists, GNSS is more accurate than a GPS device. More satellites give a device a better chance of getting a positional fix when the receiver has calculated the location of the user.
Lastly, power consumption would be slightly higher than with GPS receivers as it connects to more satellites and runs the calculations to determine location. The accuracy of a GPS device is dependent on many variables; the number of satellites available, the ionosphere, and the urban environment.
Public Interface Control Working Group and Forum for NAVSTAR GPS Set for Sept. 12
For example, accuracy tends to be higher in open areas with no adjacent tall buildings, known as urban canyons. This could lead to miscalculations of the satellite distance. In general, high-quality receivers provide better than 2.