Global Positioning System

Sources of GPS measurement errors

Ideally, GPS receivers would easily be able to convert the C/A and P(Y)-code measurements into accurate positions. However, a system with such complexity leaves many openings for errors to affect the measurements. The following are several causes of error in GPS measurements.

Clocks

Both GPS satellites and receivers are prone to timing errors. Satellites often possess cesium atomic clocks. Ground stations throughout the world monitor the satellites to ensure that the atomic clocks are accurate. Receiver clock error is unknown and often depends on the oscillator provided within the unit. However, it can be calculated and then eliminated once the receiver is tracking at least four satellites.

Ionosphere

The Ionosphere is one of the leading causes of GPS error. The speed of light varies due to atmospheric conditions. As a result, errors greater than 10 meters may arise. To compensate for these errors, the second frequency band L2 was provided. By comparing the phase difference between the L1 and L2 signals, the error caused by the ionosphere can be calculated and eliminated.

Multipath

The antenna receives not only direct GPS signals, but also multipath signals: reflections of the radio signals off the ground and/or surrounding structures (buildings, canyon walls, etc). For long multipath signals, the receiver itself can filter the signals out. For shorter multipath signals that result from reflections from the ground, special antenna features may be used such as a ground plane, or a choke ring antenna. Shorter multipath signals from ground reflections can often be very close to the direct signals, and can greatly reduce precision.

Selective availability

In the past, the civilian signal was degraded, and a more accurate Precise Positioning Service was available only to the United States military, its allies and other, mostly government users. However, on May 1, 2000, then US President Bill Clinton announced that this Selective Availability would be turned off, and so now all users enjoy nearly the same level of access, allowing a precision of position determination of less than 20 meters.

Continue to Techniques to improve GPS accuracy.

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