PART B - ABSOLUTE PRECISE POSITIONING
5-10. General. Absolute positioning involves the use of a single passive receiver at one station location
to collect data from multiple satellites to determine the station's location. It is not sufficiently accurate
for precise surveying and positioning uses. It is, however, the most widely used GPS positioning
method for real-time navigation and location.
5-11. Accuracy. The accuracy of absolute positioning is dependent on the user's authorization. The
SPS user can obtain real-time point-positional accuracy of 100 meters. The lower level accuracy using
SPS is due to intentional degradation of the GPS signal by DOD selective availability (S/A). The PPS
user (usually a DOD-approved user) can use a decryption device to achieve a 3D accuracy in the range
of 10 to 16 meters with a single-frequency receiver. Accuracy to less than one meter can be obtained
from absolute GPS measurements when special equipment and postprocessing techniques are employed.
5-12. Ephemerides. By using broadcast ephemerides, the user is able to use pseudorange values in real
time to determine absolute point positions with an accuracy of between 3 meters in the best of
conditions to 80 meters in the worst of conditions. Using postprocessed ephemerides (precise), the user
can expect absolute point positions with the accuracy of near 1 meter in the best of conditions and 40
meters in the worst of conditions.
5-13. Pseudorange. When a GPS user performs a GPS navigation solution, only an approximate range
or pseudorange to selected satellites is measured. In order to determine the user's precise GPS location,
the known range to the satellite and the position of those satellites must be known. Using
pseudoranging, the GPS user can measure an approximate distance between the antenna and the satellite
through correlation of a satellite-transmitted code and a reference code created by the receiver. It is not
necessary to factor in corrections for errors in synchronization between the clock of the transmitter and
the receiver. The distance the signal travels is equal to the velocity of the transmission of the satellite
multiplied by the elapsed time of transmission. Satellite signal velocity changes due to tropospheric and
ionospheric conditions are also considered.
5-14. Accuracy. The accuracy of the positioned point is a function of the range-measurement accuracy
and the geometry of the satellites, reduced to spherical intersections with the earth's surface. The
dilution of precision (DOP) is a description of the geometrical magnification of uncertainty in a GPS-
determined point position. Repeated and redundant range observations will generally improve range
accuracy; however, the DOP remains the same. In a static mode (where the GPS antenna remains
stationary), range measurements to each satellite can be continuously remeasured over varying orbital
locations of the satellites. The varying satellite orbits cause varying positional intersection geometry.
Additionally, simultaneous range observations to numerous satellites can be adjusted using weighting
techniques based on the reliability of the elevation and pseudorange measurements.