Unexploited Potentials of Loran-C

Abstract

Current Loran-C radio navigation equipment has demonstrated a repeatable precision index of less than 0.01 μs/km. Thus, two co-located receivers measuring the same time difference will deviate less than this amount as to the standard deviation of their measured time difference. It is therefore concluded that operation of Loran-C in a differential mode is a feasible technique for such practical matters as collision avoidance, instrument landing systems, air traffic control, precise location of surface vessels used out of sight of land in underwater exploration, and ground based vehicle location in the non-urban environment. By differential operation mode, we mean that two vehicles can determine their distance apart, their rate of closing or separation, and indeed their direction of relative motion, if the measured time difference information on each vehicle is relayed between vehicles. Loran-C can be compared quantitatively with other systems such as Omega operating in a differential mode. A comparison of Loran-C data with the Beukers-Nard Omega data reveals a possible 20 to 100 accuracy improvement factor if we use Loran-C instead of Omega in the 0 to 700 km differential separation range. We conclude that the fundamental reason for part of the improvement is the reciprocal phase measurement improvement with frequency. The remainder of the improvement is a consequence of the greater stability of the pulsed ground wave propagation mechanism of Loran-C as compared with the single frequency ionospheric waves of Omega. Frequencies greater than 100 kHz would give greater improvement with a loss of range or coverage area. We have found that Loran-C is an excellent propagation measuring device for the detection and identification of effects of irregular, in-homogeneous terrain in the spatial domain, and the study of meteorological variations in the time domain. Furthermore, the Loran-C skywave signal can be used as a D-region diagnostic to study particle precipitation events and D-region variations with latitude, season and solar zenith angle. We make the observation from our Loran-C studies that the first prerequisite to a capability for prediction of propagation phenomena is the ability to separate, identify, and understand the physical nature of the observed radio measurements. This prediction capability is essential to the successful operation of radio navigation systems in both the normal and the differential mode of operation.