signal result in error to the GPS range value. Ionospheric range effects are frequency dependent.

d. Tropospheric Delays. GPS signals in the L-band level are not dispersed by the troposphere, but

are refracted. The tropospheric conditions causing refraction of the GPS signal can be modeled by

measuring the dry and wet components.

e. Multipath Effects. Multipath describes an error in positioning which occurs when a receiver

receives a signal from more than one path. This is caused by the reflection of the GPS signal by a

nearby object, which produces a false signal at the GPS antenna. Multipath normally occurs near large

reflective surfaces, such as buildings with reflective surfaces, chain-link fences, and antenna arrays.

GPS signals received as a result of multipath give inaccurate GPS positions when processed. Newer

receiver and antenna designs and thorough mission planning can aid in minimizing these errors.

Averaging GPS signals over a period of time can also reduce multipath effects.

f. Receiver Noise. Receiver noise includes a variety of errors associated with the ability of the

GPS receiver to measure a finite time difference. These errors include signal processing, clock and

signal synchronization and correlation methods, receiver resolution, and signal noise.

g. Selective Availability and Antispoofing. S/A purposely degrades a satellite signal to create

position errors by dithering the satellite clock and offsetting the satellite orbits. The effects of S/A can

be eliminated by using differential techniques. AS is implemented by interchanging the P-code with a

classified, encrypted P-code called a Y-code. This code denies access to users who do not possess an

authorized decryption device. Manufacturers of civil GPS equipment have developed methods such as

squaring or cross correlation to make use of the P-code when it is encrypted. The absolute value of

range accuracies obtainable from GPS is largely dependent on which code is used to determine

positions. The range accuracy (for example, user-equivalent range error [UERE]), when coupled with

the geometrical relationships of the satellite during the position determination (for example, DOP),

result in a 3D ellipsoid that depicts uncertainties in all three coordinates. Given the changing satellite

geometry and other factors, GPS accuracy is time and location dependent. Error propagation techniques

are used to define nominal accuracy statistics for a GPS user.

h. Root-Mean-Square (RMS) Error Measures. Two-dimensional GPS positional accuracies are

normally estimated using an RMS radial error statistic. A 1-sigma RMS error equates to the radius of a

circle in which the position has a 63 percent probability of falling. A circle of twice this radius (2

sigmas) represents about a 97 percent probability that the position is within the circle. This 97 percent

probability circle, or 2D RMS, is the most common positional-accuracy statistic used in GPS surveying.

In some instances, a 3D RMS or 99 percent or more probability is used. This RMS error statistic is also

related to the positional-covariance matrix. An RMS error statistic represents the radius of a circle and

therefore is not preceded by a ... sign.