Global Positioning System: Encyclopedia II - Global Positioning System - Sources of GPS measurement errors

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.

Global Positioning System - Clocks

Both GPS satellites and receivers are prone to timing errors. Ground stations throughout the world monitor the satellites to ensure that their atomic clocks are kept synchronized. Receiver clock errors depend upon the oscillator provided within the unit. However, they can be calculated and then eliminated once the receiver is tracking at least four satellites.

Global Positioning System - 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.

Global Positioning System - 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 delay multipath signals, the receiver itself can filter the signals out. A variety of receiver techniques, most notably Narrow Correlator spacing, have been developed to mitigate multipath error contributions to pseudorange measurements. For shorter delay 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.

Global Positioning System - GPS Jamming

A large part of modern munitions, the so-called "smart bombs" or precision-guided munitions, use GPS. GPS jammers are available, from Russia, and are about the size of a cigarette box. The U.S. government believes that such jammers were used occasionally during the U.S. invasion of Afghanistan. Some officials believe that jammers could be used to attract the precision-guided munitions towards noncombatant infrastructure, other officials believe that the jammers are completely ineffective. In either case, the jammers are attractive targets for anti-radiation missiles.

The U.S. Air Force conducted GPS jamming exercises in 2003. A detailed description of how to build a GPS jammer was posted on a hackers' site by an anonymous author. And there has been at least one well-documented case of unintentional jamming; if similar, but stronger, signals were generated on purpose, they could interfere with aviation GPS receivers at a range of 50 km. According to the reference below, "IFR pilots should have a fallback plan in case of a GPS malfunction".

There were also incidents of unintentional jamming, traced back to malfunctioning TV antenna preamplifiers.

• GPS jamming
• The hunt for an unintentional GPS jammer
• GPS Anti-Jamming Protection

Global Positioning System - Selective Availability

When it was first deployed, GPS included a "feature" called Selective Availability (or SA) that introduced intentional errors of up to a hundred meters into the publicly available navigation signals, making it difficult to use for guiding long range missiles to precise targets. Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, its allies and a few others, mostly government users.

SA typically added signal errors of up to about 10 m horizontally and 30 m vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern US area might read 30 m off, but 30 m off everywhere and in the same direction. In order to improve the usefulness of GPS for civilian navigation, Differential GPS was used by many civilian GPS receivers to greatly improve accuracy.

In the 1990s the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance of their own, less accurate, radio navigation systems. The military resisted for most of the 1990s, but SA was eventually turned off in 2000 following an announcement by then US President Bill Clinton. allowing all users to enjoy nearly the same level of access.

Although global SA has been turned off [1], for military purposes the US maintains the capability to use "Selective Deniability" to, in effect, jam civilian GPS units in a war zone or global alert while still allowing military units to have full functionality. In reality, the shortage of military GPS units and the wide availability of civilian ones among personnel resulted in disabling the Selective Availability in the time of the Gulf War. However, European concern about the level of control over the GPS network and commercial issues has resulted in the planned Galileo positioning system. Russia already operates an independent system called GLONASS (global navigation system), although with only twelve active satellites as of 2004, the system is of limited usefulness.

Military (and selected civilian) users still enjoy some technical advantages which can give quicker satellite lock and increased accuracy. The increased accuracy comes mostly from being able to use both the L1 and L2 frequencies and thus better compensate for the varying signal delay in the ionosphere (see above). Commercial GPS receivers are also required to have limits on the velocities and altitudes at which they will report fix coordinates; this is to prevent them from being used to create improvised cruise or ballistic missiles.

• noaa.gov Selective Availability Factsheet (pdf) or [2]

Adapted from the Wikipedia article "Sources of GPS measurement errors", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki