5-37. General. GPS baseline solutions are usually generated through an iterative process. From
approximate values of the positions occupied and observation data, theoretical values for the observation
period are developed. Observed values are compared to computed values and an improved set of
positions occupied is obtained using least squares minimization procedures and equations that model
potential error sources. This section discusses general postprocessing issues. Due to the increasing
number and variety of software packages available, consult the manufacturer's guidelines when
appropriate. Processing time is dependent on the accuracy required, the available software, the
computer hardware, the data quality, and the amount of data. In general, high-accuracy solutions, crude
computer software and hardware, low-quality data, and high volumes of data will cause longer
processing times. The user must take special care when attempting a baseline formulation with
observations from different brands of GPS receivers. It is important to ensure that observables being
used for the formulation of the baseline are of common format. The common data-exchange formats
required for a baseline formulation exist only between receivers produced by the same manufacturer,
even though there are some exceptions.
5-38. Processing Techniques. The capability to determine positions using GPS is dependent on the
ability of the user to determine the range or distance of the satellite from the receiver located on the
earth. There are two general techniques used to determine this range--pseudoranging and carrier beat
phase measurement.
a. Pseudoranging. The observable pseudorange is calculated from observations recorded during a
GPS-S. The observable pseudorange is the difference between the time of signal transmission from the
satellite (measured in the satellite time scale) and the time of signal arrival at the receiver antenna
(measured in the receiver time scale). When the differences between the satellite and the receiver clocks
are reconciled and applied to the pseudorange observables, the result is corrected pseudorange values.
The value found by multiplying this time difference by the speed of light is an approximation of the true
range between the satellite and the receiver. The value can be determined if ionosphere and troposphere
delays, ephemeris errors, measurement noise, and unmodeled influences are taken into account during
pseudoranging calculations. The pseudorange can be obtained from either the C/A-code or the more
precise P-code.
b. Carrier Beat Phase Measurement. The observable carrier beat phase is the phase of the signal
remaining after the internal oscillated frequency generated in the receiver is differenced from the
incoming carrier signal of the satellite. The observable carrier beat phase can be calculated from the
incoming signal or from observations recorded during a GPS-S. By differencing the signal over a period
or epoch of time, the number of wavelengths that cycle through the receiver during any given specific
duration of time can be counted. The unknown cycle count passing through the receiver over a specific
duration of time is known as the cycle ambiguity. There is one cycle-ambiguity value per
satellite/receiver pair as long as the receiver maintains continuous phase lock during the observation
period. The value found by measuring the number of cycles going through a receiver during a specific
time, when given the definition of the transmitted signal in terms of cycles per second, can be used to
develop a time measurement for transmission of the signal. Again, the
EN0593
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