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Why Isn’t GPS Perfectly Accurate?
can cause inaccuracies in distances when it comes to finding a position. Fortunately, there’s an easy solution to this mess, and that is, your receiver picks up
information from a fourth satellite. By using a fourth measurement, the clock
errors can be corrected for and your position can be accurately determined.
The position you’ll receive is measured using the WGS84 datum.
Why Isn’t GPS Perfectly Accurate?
If GPS can be used to find a location on Earth’s surface, the next question to
ask is “just how accurate is GPS?” Through the use of pseudorange measurements from four satellites, it would seem the system should only give the user
one accurately measured location. However, in average everyday use with an
ordinary off-the-shelf civilian receiver, accuracy would be about 15 meters (or
less) in the horizontal direction. This doesn’t mean that the location is always
off by 15 meters—it could be off by 5, 10, or 15 meters. This is due to a number
of factors that can interfere with or delay the transmission or reception of the
signal—and these delays in time can cause errors in position.
One of the biggest sources of error was actually a deliberate one worked
into the system to intentionally make GPS less accurate. The United States introduced Selective Availability (SA) into GPS with the goal of intentionally
making the C/A signal less accurate, presumably so that enemy forces couldn’t
use GPS as a tool against the United States military. Selective Availability introduced two errors into the signals being transmitted by the satellites—a delta error (which contained incorrect clock timing information) and an epsilon error
(which contained incorrect satellite ephemeris information). The net effect of
this was to make the accuracy of the C/A about 100 meters. As you can imagine,
this level of accuracy certainly limited GPS in the civilian sector—after all, who
wants to try to land an airplane with GPS when the accuracy of the runway
location could be hundreds of feet off? In the year 2000, the U.S. Government
turned Selective Availability off and has no announced plans to turn it back on
(in fact, the U.S. Department of Defense has announced that new generations
of GPS satellites will not have the capability for Selective Availability).
While Selective Availability is an intentional error originating from the
satellites, other errors can naturally occur at the satellite level. Ephemeris
errors can occur when the satellite broadcasts incorrect orbit position information. These types of errors indicate that the position of the satellite is not
where it is supposed to be and a correction of ephemeris data hasn’t been
updated from a control station. Typically, these errors can introduce about
2 meters (or so) of error.
Even the arrangement of satellites in space (that is, how they are distributed)
can have an effect on the position determination. The further the satellites are
spread out from one another and the wider the angle between them, the more
accurate a measurement will be obtained by the receiver. The error introduced by
a poor geometric arrangement of the satellites is referred to as the Position Dilution of Precision or PDOP. Some receivers will calculate a value for PDOP for you
Selective Availability
the intentional
alteration of the
timing and position
information transmitted
by a GPS satellite.
PDOP the Position
Dilution of Precision—it
describes the amount
of error due to the
geometric position of
the GPS satellites.
Why Isn’t GPS Perfectly Accurate?
can cause inaccuracies in distances when it comes to finding a position. Fortunately, there’s an easy solution to this mess, and that is, your receiver picks up
information from a fourth satellite. By using a fourth measurement, the clock
errors can be corrected for and your position can be accurately determined.
The position you’ll receive is measured using the WGS84 datum.
Why Isn’t GPS Perfectly Accurate?
If GPS can be used to find a location on Earth’s surface, the next question to
ask is “just how accurate is GPS?” Through the use of pseudorange measurements from four satellites, it would seem the system should only give the user
one accurately measured location. However, in average everyday use with an
ordinary off-the-shelf civilian receiver, accuracy would be about 15 meters (or
less) in the horizontal direction. This doesn’t mean that the location is always
off by 15 meters—it could be off by 5, 10, or 15 meters. This is due to a number
of factors that can interfere with or delay the transmission or reception of the
signal—and these delays in time can cause errors in position.
One of the biggest sources of error was actually a deliberate one worked
into the system to intentionally make GPS less accurate. The United States introduced Selective Availability (SA) into GPS with the goal of intentionally
making the C/A signal less accurate, presumably so that enemy forces couldn’t
use GPS as a tool against the United States military. Selective Availability introduced two errors into the signals being transmitted by the satellites—a delta error (which contained incorrect clock timing information) and an epsilon error
(which contained incorrect satellite ephemeris information). The net effect of
this was to make the accuracy of the C/A about 100 meters. As you can imagine,
this level of accuracy certainly limited GPS in the civilian sector—after all, who
wants to try to land an airplane with GPS when the accuracy of the runway
location could be hundreds of feet off? In the year 2000, the U.S. Government
turned Selective Availability off and has no announced plans to turn it back on
(in fact, the U.S. Department of Defense has announced that new generations
of GPS satellites will not have the capability for Selective Availability).
While Selective Availability is an intentional error originating from the
satellites, other errors can naturally occur at the satellite level. Ephemeris
errors can occur when the satellite broadcasts incorrect orbit position information. These types of errors indicate that the position of the satellite is not
where it is supposed to be and a correction of ephemeris data hasn’t been
updated from a control station. Typically, these errors can introduce about
2 meters (or so) of error.
Even the arrangement of satellites in space (that is, how they are distributed)
can have an effect on the position determination. The further the satellites are
spread out from one another and the wider the angle between them, the more
accurate a measurement will be obtained by the receiver. The error introduced by
a poor geometric arrangement of the satellites is referred to as the Position Dilution of Precision or PDOP. Some receivers will calculate a value for PDOP for you
Selective Availability
the intentional
alteration of the
timing and position
information transmitted
by a GPS satellite.
PDOP the Position
Dilution of Precision—it
describes the amount
of error due to the
geometric position of
the GPS satellites.