Modes of Measurement and Post-processing of Data
There are two, general operating modes through which GPS-derived positions can be obtained:
To determine the receiver’s point position at any time, the satellite coordinates as well as a minimum of four ranges to four satellites are required. The receiver gets the satellite coordinates through the navigation message, while the ranges are obtained from either the C/A-code or the P(Y)-code, depending on the receiver type (civilian or military). However, the measured pseudoranges are contaminated by both the satellite and receiver clock synchronization errors with respect to the stable GPS time. Correcting the satellite clock errors may be done by applying the satellite clock correction in the navigation message; the receiver clock error is treated as an additional unknown parameter in the estimation process. This brings the total number of unknown parameters to four: three for the receiver coordinates and one for the receiver clock error. This is the reason that at least four satellites are needed.
GPS relative positioning, on the other hand, employs two GPS receivers simultaneously tracking the same satellites. If both receivers track at least four common satellites, a positioning accuracy level on the order of a few meters to millimeters can be obtained . Carrier-phase and pseudorange measurements can be used in GPS relative positioning, depending on the accuracy requirements. The former provides the highest possible accuracy. GPS relative positioning can be made in either real-time or post-mission modes. GPS relative positioning is used for high-accuracy applications such as surveying and mapping, GIS, and precise navigation.
Since the satellite coordinates are given in the WGS 84 system, the obtained receiver coordinates are also in the WGS 84 system. However, most GPS receivers provide the transformation parameters between WGS 84 and many local datums used around the world.
Also known as absolute positioning, standalone positioning or autonomous positioning, involves only one GPS receiver. That is, one GPS receiver simulta-neously tracks four or more GPS satellites to determine its own coordinates with respect to the center of the Earth. This is the most common military and civil application of NAVSTAR GPS for real-time navigation. When operating in this passive, real-time navigation mode, ranges to NAVSTAR GPS satellites are observed by a single receiver positioned on a point for which a position is desired. This receiver may be positioned to be stationary over a point (static) or in motion (kinematic [such as on a vehicle, aircraft, missile, or backpack]). Two levels of absolute-positioning accuracy may be obtained – the Standard Positioning Service (SPS) and the Precision Positioning Service (PPS). With specialized GPS receiving equipment, data-processing refinements, and long-term static observations, absolute-positional coordinates can be determined to accuracy levels of less than 1 meter. These applications are usually limited to worldwide geodetic-reference surveys.
The Standard Positioning Service: The SPS user is able to achieve real-time, 3D (point-positional) absolute positioning. The SPS is the GPS signal that DoD authorizes to civil users. This level of accuracy is due to the deliberate degradation of the GPS signal by DoD for national security reasons. DoD degradation of the GPS signal is referred to as selective availability (S/A). DoD has also implemented antispoofing (AS), which denies the SPS user the more accurate precision code (P-code).
The Precision Positioning Service: Using the PPS requires DOD authorization for a decryption device that is capable of deciphering the encrypted GPS signals. Army topographic surveyors are authorized users; however, actual use of the equipment has security implications. Real-time, 3D absolute-positional accuracies of 16 to 20 meters are attainable through the PPS.
Differential positioning, also called relative positioning, employs two (or more) GPS receivers simultaneously tracking the same satellites to determine their relative coordinates. Of the two receivers, one is selected as a reference, or base, which remains stationary at a site with precisely known coordinates (pre-surveyed). The coordinates of the other receiver, known as the rover or remote receiver, are unknown. They are determined relative to the reference using measurements recorded simultaneously at the two receiv-ers. The rover receiver may or may not be stationary, depending on the type of the GPS operation.
A minimum of four common satellites are required for relative positioning. However, tracking more than four common satellites simultaneously would improve the precision of the GPS position solution. Carrier-phase and/or pseudorange measurements can be used in relative positioning. A variety of positioning techniques are used to provide positioning information in real time or at a later time (i.e. postprocessing). Generally, GPS relative positioning provides a higher accuracy than that of autonomous or point positioning. Depending on whether the pseudorange or carrier-phase measurements are used in relative positioning, an accuracy levelof a few meters to millimeters, respectively, can be obtained. This is mainly because the measurements of two (or more) receivers simultaneously tracking a particular satellite contain more or less the same errors and biases. The shorter the distance between the two receivers is, the more similar the errors. Therefore, if we take the difference between the measurements of the two receivers (hence the name differential positioning), common errors will be removed and those that are spatially correlated will be reduced depending on the distance between the reference receiver and the rover.
REAL-TIME VERSUS POST-PROCESSING:
The term real time means that the results are obtained almost instantaneously, while the term postprocessing means that the measurements are collected in the field and processed at a later time to obtain the corrected results. Each of these modes has some advantages and some disadvantages.
The first advantage of the real-time mode is that the results as well as the accuracy measures are obtained while in the field. This is especially important for real time kinematic (RTK) surveying, as the user would not store the displayed coordinates unless the ambiguity parameters are shown to be fixed at integer values and centimeter-level accuracy is achieved. This leads to a higher productivity compared with the postprocessing mode, as only enough GPS data to obtain a fixed solution is collected. In addition, processing the GPS data is done automatically in the field by the built-in software. This also means that no postprocessing software training is required. The user also saves the time spent in data processing.
There are, however, some advantages in the post-processing mode as well. The first of these is that more accurate results are generally obtained with the post-processing mode. One reason for this is more flexibility in editing and cleaning of the collected GPS data. There is also no accuracy degradation due to data latency. Another important advantage is that communication link-related problems, such as signal obstruction or limitation of coverage, are avoided. In some cases, the input parameters, such as the base station coordinates or the antenna height, may contain some errors, which lead to errors in the computed rover coordinates. These errors can be corrected in the post-processing mode, while they cannot be completely corrected in the real-time mode.
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