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The development of the colac timer and the Loran Replacement Equipment package has provided two sources of dramatic improvement of the performance of Loran–C transmitting stations: solid state reliability and the ability to have control locally or remotely exerted. The improvement due to solid state reliability has been well documented over the past several years while the exploitation of the remote control capability has begun with the recent test deployment of a calculator based remote chain control unit. Results to date have indicated that chain control tolerances on the order of 40 nanoseconds can be attained. At the heart of any such tightly controlled system is the requirement for a reliable communications network such as is readily and economically available from commercial sources on the east coast of the U. S. Plans for worldwide implementation, however, hit a snag in the form of the poor reliability of the communications links available at many of the remote or isolated stations such as those of the Central Pacific or Alaskan chains. For these stations the Coast Guard Two Pulse Loran–C Communications system has been developed. The system, consisting of pulse position modulation (PPM) of two of the pulses of a Group Repetition Interval, has only two performance requirements and these are unique among the various Loran–C communications schemes. One requirement is the transmission of short, infrequent control messages which can be received at baseline distances and, in the face of all the classical interference sources of the Loran-C channel, with a small probability of error. A second requirement is that the PPM impact insignificantly, if at all, on the navigation channel, since improved navigational performance is the primary objective of the system. As will be developed, these two requirements are not at all inconsistent. A complete description of the features of the transmission scheme and their effects is presented. |
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