Technology to Evaluate eLoran Performance

B. Peterson, K. Dykstra, K. Carroll, M. Narins

Abstract:	The U. S Coast Guard and Federal Aviation Administration are cooperating in a major effort to define and analyze the performance of an Enhanced Loran (eLoran) system. Among other things Enhanced Loran will include improved timing at the transmitters for better synchronization to UTC and improved receiver technology including all in view solutions to take advantage of better transmitter timing control. The purposes of the specific portion of the effort described in this paper are to model and bound the diurnal, seasonal and spatial variations in propagation and their effects on navigation and timing performance. Other goals are to look closely at short term changes in TOAs due to noise and interference, to study issues such as early skywave and cross rate interference, and to determine to what extent the performance of the transmitter affects that of the receiver. At present, the USCG operates the North American chains via System Area Monitoring (SAM) sites located throughout the coverage area. The timing of secondary signals is adjusted to maintain the observed Time Differences (TDs) at these SAM sites at specified values. The eLoran system assumes time-of-emission (TOE) control where each station transmission is based on UTC timing. Therefore the exact UTC transmission time is required for each station in order to analyze eLoran and all-in-view receiver performance. This data is available from US master stations but not at secondary stations. Therefore, a time of transmission monitoring (TOTM) unit was designed and placed at selected secondary stations. We explain the TOTM units with respect to functionality, synchronization to UTC via GPS and calibration of each device in the unit and overall installation. We explore the US definition of synchronizing Loran transmissions to UTC and one method to calibrate the system based on that definition as it relates to TOE control. A comparison between how the US and Northwest Europe Loran-C System (NELS) define Loran synchronization is discussed. The receiver part of the effort is the development of technology to remotely measure the Times of Arrival (TOAs) relative to UTC and the analysis of data collected using this equipment. The technology includes a system which utilizes LocusTM LRSIIID monitor receivers installed at existing System Area Monitor sites. The receiver is put in the TOA mode which means it provides TOA information relative to the same arbitrary reference for all signals. The RF gate output on the Locus receiver is phase locked to the strongest signal, and is time tagged using a GPS timing receiver. The time tag data from the GPS receiver plus the standard Loran information is read into a computer which calculates and logs the TOA offset of the strongest signal from the time tag information and the offsets of all other signals from this offset combined with their TOAs relative to that of the strongest signal. In order to determine to what extent the performance of the transmitter affects that of the receiver a signal analysis system was developed to collect timing jitter with respect UTC, envelope-to-cycle difference and amplitude of each pulse. A description of designed functionality and calibration is given. The initial results of data collection efforts with respect to integrity and accuracy are shown for legacy, (1970's and 80's vintage solid state transmitters (SSXs) and timing equipment) and the new prototype equipment (timing suite and SSX).
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	1327 - 1344
Cite this article:	Peterson, B., Dykstra, K., Carroll, K., Narins, M., "Technology to Evaluate eLoran Performance," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 1327-1344.
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