GPS Accuracy Levels

Major Accuracy Divisions

There are several different levels of accuracy that can be achieved. The difference is equipment and techniques. In the commercial world, there have been roughly 4 generations of equipment, with some from the last three still in service. There are also different techniques, many made possible by LSI Chips and faster/cheaper microcomputers.

The major difference in the accuracy level achieved today in a receiver is the processing techniques. This is driven by the error level on the range measurements. (See Accuracy Factors.) For standalone users, the extent use of phase is a major factor.

The noise on the phase is typically 1 mm, that on the range 10 cm to 1 m. Therefore the use of phase data in any one of several ways can improve the solution. The multipath is also drastically reduced for the phase measurement.

The second technique to remove errors is differential GPS. Here errors in the signal outside the antenna are observed at a reference station. Those errors are sent to the user by one of several methods. For realtime users this can be the US Coast Guards "beacon" transmitters, or a commercial system using satellite communication links or FM subcarriers etc. These are usually L1 only systems.

The post processed differential user typically uses L1 and L2 data from high end receivers. Special techniques (called Kinematics) make use of the phase to achieve very high accuracies . However they normally can only be used out to ranges of 25 to 50 km. Many manufacturers of high end receivers have vendor specific real time version of this. You almost always have to have the same type of receiver at each end and a dedicated communication link.

The PPS accuracy level is specified to be 16 m, spherical error probable (SEP). GPS position errors are always about twice as bad in height as along either horizontal axis. This means a horizontal error of about 10 m. The graph below lists the official current level. The actual level is much less. Horizontal errors for PPS (and SPS) in the 3-5 m range are common.

Since Selective Availability was turned off, the main operational difference between the civilian and military systems (SPS vs PPS) is the availability of a L2 for real time removal of the ionosphere. Inexpensive receivers with only L1 still have to deal with the ionosphere that mainly affects the height. Even this difference is removed for the more expensive receivers that use complex techniques to track L2.

The civilian (SPS) level is set by DoD policy. When the intentional degradation of accuracy was in place (called Selective Availability or SA) SPS accuracy was 100 m horizontal 5 % of the time ( 2 standard deviations). On May 1 2000, SA was set to a level of 0 -- effectively being turned off.

Levels of Accuracy

The graph shows the accuracy levels of all currently available systems. The vertical axis is the expected accuracy or error level, shown both in centimeters and meters. The horizontal axis is the distance along the earth's surface between the reference station and the remote user. If there is no reference station, the line is drawn all the way to 10,000 km, all over the earth.

Accuracy vs Baseline Different Receivers/Techniques

Static Vs. Dynamic

Almost all levels quoted here are for dynamic users, including aircraft. The exception is the survey levels at the bottom of the figure and along the sloping line.

Standalone Accuracy

The accuracy of GPS with an individual receiver, only getting the signals from the satellites and nothing else, is called standalone operation. There are two standalone levels, that available to the civilian world, called the Standard Positioning Service (SPS) and that available to the military, the Precise Positioning Service (PPS). Prior to May 1, 2000, there was a large difference between the accuracy between SPS and PPS. After that date the horizontal accuracy became essentially the same. The vertical accuracy in the SPS is still slightly less for inexpensive receivers.

Differential GPS

Most of the errors in two receivers relatively close to one another are identical. Therefore setting a reference receiver on a known point allows one to measure the errors in real time on each satellite visible. These can then be broadcast over some communications channel to other users who use this information to correct their measurements in real time. This is differential GPS ( DGPS ).

The accuracy obtained in this way depends mainly on the:

These are listed in decreasing order of importance. The last factor, multipath, involves signals reaching the antenna both from a direct path to the satellite and from a path that bounces off some reflector. Multipath is normally tightly controlled at reference sites by careful antenna site selection. This is not included in the chart.

Levels of Differential GPS Accuracy

The "Standard DGPS" belongs to the first generation of equipment used in large volume. It has an update rate of once per 30 sec per satellite. This was available in the mid 1980's and achieved 3 to 4 m accuracy. It is the original US Coast Guard design, but today the USCG is better than this.

The next level, labeled second generation, began coming on line about 1990. This generation achieved "sub-meter" accuracy, about 70 to 100 cm. It uses about a 5 - 10 sec update cycle. Multipath is now an important consideration.

In 1990, a small firm specializing in high bandwidth digital communications, came up with a new approach to measuring ranges in GPS. This was called the "narrow correlator" technology. It has a narrow correlator in the time domain, making use of the sharp transition of bit edges. The receiver measurement noise was reduced by a factor of 5 initially.

Using narrow correlator techniques, and the civilian SPS signals, 20 cm DGPS has been demonstrated using only ranging signals. This has been done even in aircraft. The communications rate requirement goes up to 1 Hz for aircraft applications. There are now several manufacturers achieving 10 cm accuracy levels.

Between these last two generations of commercial systems is the best current military (PPS) based DGPS. At the Naval Postgraduate School, 35 cm DGPS has been achieved using PPS signals. An important side benefit of PPS is a low communication rate for this accuracy, once per few minutes. This benefit became available to the SPS when SA was turned off.

There is another measurement available in GPS. This is usually called the Phase Variable. It has less than 1 mm of noise and is affected less by multipath. There are some technical drawbacks, mainly having to do with initializing a solution and high communications rates. It has been used from the mid 1980's in a post processing mode and since 1990 has been used real time. It is usually call "Kinematic" DGPS. Using either the civilian or military system, 5 cm accuracies can be achieved in aircraft after initialization.

3 Dimensional Accuracy, 1 Standard Deviation

The following is an example of DGPS errors taken with a 1996 vintage receiver. Although the Ashtech Z12 used is a dual frequency receiver, the real time DGPS output only uses L1 C/A code.

DGPS Error vs Time - USCG as Reference

Notice that late at night US Coast Guard stations from far away, over 1000 km in this case, can be picked up and used. This causes a large increase in the error. There were other single frequency 12 channel receivers using the same DGPS data stream. They had the same pattern.

Static Surveying

Kinematic DGPS grew out of the GPS surveying communities. Here data is taken over a few hours on static sites and brought together for post processing. The errors here are a function of the baseline length. Early 1980's science produced 1 cm per 10 km of baseline (one part per million). This are now a turnkey commercial products from many vendors. There is also a higher accuracy system, that corrects the orbits. This has also moved from science to commercial application. Current science is 5 to 10 times better than that, having an error of 1 - 2 cm on baselines from Los Angeles to Boston.

James R. Clynch
Naval Postgraduate School
February 2001

A Government System with The Standard Disclaimer