Accuracy of shipborne kinematic GPS surveying
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In December 1990 an experiment was conducted at the Naval Postgraduate School in Monterey, California in which four different receivers, mounted on the mast of a ship, collected data simultaneously for several hours at a time. Ashtech LD -XII, Trimble 4000 ST, TI 4100 and Magnavox MX 4200 receivers were used. The reference system consisted of a Krupp Atlas Polarfix laser system set up on the shore at a pre-surveyed site. A two-axis vertical gyro system and a heading gyro gave the ship's 3-dimensional orientation at any instant in time, providing a connection between the laser reflector and the GPS antennas on the ship's mast. This enabled the reduction of the laser reflector's trajectory to the Ashtech and Trimble antennas for subsequent comparison to the kinematic GPS trajectories of these receivers determined by the postprocessing of the data collected. Each data set was processed once with the software provided by the manufacturer and once with an independent software package, OMNI, developed by the National Geodetic Survey. In addition to the software, six factors were examined to determine their effects on kinematic GPS surveys. They included : tropospheric corrections, initialization, satellite geometry, ephemeris type, data interval and multipath. In general the software available is versatile; however each package has some limitations which call for further development. The results indicate that the effects mentioned are small, generally less than the noise in the reference system. An important factor is the initialization of the kinematic process. Bad initialization can cause a reasonable solution for a period of time, but a sudden deterioration when the satellite configuration changes. Accuracy levels of a few decimeters were easily achieved with the systems and procedures used. Both Ashtech and Trimble produced trajectories which were accurate to within the noise level of the laser trajectories. In both cases the solution produced by OMNI differed from the solution produced by the manufacturer's software, only by a few centimeters.
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