Study On ASF Corrections In Integrated Navigation Systems Of Loran-C

It is necessary to utilize an enhanced-Loran-C as a backup and enhancing technique for satellite navigation and to form an integrated navigation system with improved performances. In this case, it is the major source of positioning error that the Additional Secondary Factor (ASF) of Loran-C signal propagation over land path in this integrated navigation system.The study of this paper has been commenced with the 3262 construction of the Longwave Timing System, carried out by the National Time Service Center, Chinese Academy of Sciences, awarded the first degree of National Advances of Science & Technique in 1988, and with the projects of"Ground Conductivities Tested by GPS Technique"and"Study of ASF Correction on the Integrated Navigation System", supplied by the National Natural Science Foundation of China.It has been introduced and discussed in this paper that the factor parameter and several models as following:ground conductivity, atmospheric refraction over the ground surface, calculation and determination for the distance of radio propagation path, propagation model over an uniform smooth path, propagation model over a non-uniform smooth path ( Millington method ), propagation model over an inhomogeneous path ( numerical integral method ).The calculation software of ASF for Millington method and a numerical integral method has been programmed with MATLAB technique. ASF got in this Millington method agrees with the result of"Computational Methods of LW Ground-wave Transmission Channels". The result of (SF+ASF)with the numerical integral method agrees with the result showed by Dr. Paul Williams in his paper. There is larger difference of (SF+ASF) calculated by use of Millington method and numerical integral method. But, it has been discovered in further study that there is only a little difference of ASF calculated with Millington method and numerical integral method over land path.The software of numerical integral method for calculating delay & field strength of groundwave signal has been employed for ASF correction with a GIS software. In October 2004 and July 2005, actual ASF measurement over an area around the Huangshan mountain was carried out. The ASF correction by the numerical integral method is better and more suitable than the traditionary Millington method at a mountain area, compared with the actual measured ASF.The partial measurement around the Huangshan mountain indicates that the error of ASF correction is possible reduced to less than 0.5 microsecond if using the numerical integral method combined with Millington method and the GIS software, and checked by the actual measured ASF. on some typical path.