Budget Analysis of the Minimum Size of the Correction Message and Baudrate for USCG NDGPS

Byungwoon Park, Jeonghan Kim, Changdon Kee

Abstract:	Before May 1, 2000, the PRC including SA fluctuated so much that the reference station should broadcast the PRC as frequently as possible, and that the baud-rate should be high. Now that the SA has been off, the rate of change with time of the DGPS common errors (atmospheric delay, satellite orbit and clock error) is quite slow. The PRC is far more stationary and robust to the time latency, but the latency error still exists. The time latency error is a kind of trade-off problem. If the DGPS services use a high radio bandwidth, the latency error can be very small, but it’s too expensive for user to use the system. Otherwise a cheap low baud-rate data link can induce a big latency error. Additionally, the position accuracy degradation by separating from the reference station should be considered, because the distance between the reference station and the user enlarge the spatially decorrelated error. Therefore a manager of the reference station should determine the baud-rate considering these temporally and spatially decorrelated errors and the cost. When the users choose the DGPS receivers, they consider their own specific requirements. The costumers, who use DGPS with a rough accuracy, don’t need expensive receiver. Otherwise, the receivers for some high accuracy should meet the users’ required performance. The users should also keep in minds that the accuracy can be lower than the known one, when the reference station broadcast the corrections in lower baud-rate than now. In this paper, we approach theoretically to the accuracy variation by the baud-rate of the reference station, the distance between the user and reference station, and the noise statistics of the user receiver. We identify the minimum and maximum size of correction data, interval time, the coverage range, and the baud-rate which are required to maintain the existing DGPS service to present users. The accuracy degradation value in time-latency and the range can be estimated by considering three effects, which are the receiver noise of reference station, the rate of change with time of both group and phase delays (same sign), and the rate of change with time of group delay and phase advance (opposite sign) as the satellite elevation changes. We use the tropospheric model developed for the WAAS and the ionospheric shell model that incorporates total electron content and its variation within the 11-year solar cycle. Using these models, we consider both of the normal and worst cases to find the function between the baud-rate and the error of PRC. By using this relationship and the noise statistics of the user receiver’s, the user’s actual accuracy after getting the corrections can be estimated. The DGPS users, who apply this result to their system, don’t have to worry about whether their own receivers work well with a present baud-rate. Before purchasing the DGPS receivers, they can estimate the accuracy and judge whether it meets their own requirements. The U.S Coast Guard (USCG) is also interested in this paper, because it should take the user’s required accuracy into consideration to determine the baud-rate of the DGPS correction message. We anticipate the result of our study would be great help to determine the flexible factors both in the reference station and in the DGPS users
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	2761 - 2770
Cite this article:	Park, Byungwoon, Kim, Jeonghan, Kee, Changdon, "Budget Analysis of the Minimum Size of the Correction Message and Baudrate for USCG NDGPS," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 2761-2770.
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