| Abstract: | Accuracy requirements for positioning a vessel in the near shore area have increased in recent years. To meet these requirements several electronic positioning systems have been developed. One such system is the Cubic Autotape Model DM-40. The Autotape is a circular positioning system which utilizes the phase comparison between transmitted and received microwave signals to measure the respective distance to the ends of a known base line. Since line of sight transmission is used, the accuracy is higher than many other systems, but the useable range is limited to approximately 30 miles. To evaluate the instrument under dynamic conditions, a second order trilateration control network was established and adjusted using least squares techniques. Autotape measurements were then recorded together with a measure of the velocity of the interrogator antenna as the instrument moved past a series of seven test stations. The relationship between the velocity and the observation errors was then investigated. The instrument was used to measure known base lines and the positions of the test stations under static conditions. The static measurements were tested to determine the effects of classification by day and mode using a three factor analysis of variance program. Test station coordinates were computed from the static measurements and compared to control values. Scale factors were also computed from these measurements. The dynamic observations were scaled and the relationship between the scaled observation errors and the velocity was also investigated. Although contrary to expectations, the dynamic measurement errors tend to decrease as the velocity increases. A possible explanation involves the filtering of the received signal. Coordinates were also computed from the dynamic measurements and were found to exhibit the same general trend as the range measurements. Final results indicate that for the conditions of this project the relative error of position for the repeatability of the instrument is one part in 3330 and the relative error of the survey is one part in 2610. |
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