Challenges to Precision



What Impacts the Accuracy of GPS


The positioning accuracy offered by GPS varies depending upon the type of service and equipment available. For security reasons, two GPS services exist: the Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). The SPS uses a code modulated onto the signal for measurements and is referred to as the Coarse Acquisition code (C/A code). The US Department of Defense (DoD) reserves the PPS for use by its personnel and authorized partners. The PPS uses a different code than the SPS, referred to as the Precise Code (P-code) and contains more resolution than the C/A code. The DoD provides the SPS free of charge, worldwide, to all civilian users. Currently, autonomous GPS is able to provide accuracy on the order of 10-15 meters, depending on the sophistication of the GPS engine. For many positioning and navigation applications, this level of accuracy is not sufficient, and differential techniques must be employed.


     Atmospheric Errors


Atmospheric errors are the most significant source of errors of GPS. With the satellites orbiting at about 20,000km above the earth, the GPS signals have to travel through the ionosphere and the troposphere layers before reaching the receiver antenna. Ionosphere is the collective term for the various layers of ionized particles and electrons found at altitudes of 80-250 km in the atmosphere. Ionization is caused primarily by short-wavelength (Xray and ultraviolet) solar radiation during the daytime. Ionospheric activities have the biggest impact on GPS accuracy.





Dilution Of Precision


There is no fixed indication of what is considered a good or bad DOP. Knowing that the ideal DOP value is 1, different applications will require different accuracies and allow higher DOP fixes. Usually, 1 to 2 is excellent, 3 to 4 is good, 5 to 7 is fair and 8 and above is poor. Companies requiring only about 5m accuracy and increased productivity will work with PDOP's as high as 8-12. Differential correction will not compensate for DOP errors. Instead, since the DOP is computed by the receiver, most GPS software will offer filters to prevent operation or recording when the DOP reaches a preset value.


Multipath                 


Multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies, mountains, trees and buildings. Differential correction will not compensate for multipath errors. Certain precautions will minimize GPS antenna sensitivity to these reflected signals, like operating away from large reflective structures such as buildings. A high-end receiver/antenna combination will be more robust in rejecting multipath, whereas a consumer-level receiver will tolerate a higher amount of multipath.


Orbit And Timing Errors


GPS satellites carry very accurate atomic clocks and follow very precise orbits. But drifts in both clock and orbit are inevitable and very small amount can cause significant errors in a receiver on the ground. Even though their clocks and orbits may not be adjusted, their offsets are computed by the GPS Ground Segment then sent back to the satellites. The satellites then broadcast the clock and ephemeris message to the end-user. There is some latency between the actual occurrence of the offsets and the time they are computed and broadcasted. Depending on the type of differential correction used (local or global), the effects of satellite orbit and timing errors can greatly be compensated.