Reliable cycle selection in LORAN-C system

In a LORAN-C receiver wherein received pulses are envelope-derived to provide a derived signal in which the kth and subsequent carrier cycles are phase-reversed relative to carrier cycles in each received pulse, and including means for generating a tracking pulse and means for rendering said tracking pulse time coincident with any zero crossing having a predetermined slope polarity in the received pulse carrier, cycle selection apparatus for rendering said tracking pulse time coincident with the zero crossing between the k - 1 and kth carrier cycles of the received pulse, said cycle selection apparatus comprising:

• means for sampling the polarity of said derived signal in a set of at least four samples at times corresponding to one-quarter and three-quarters of a carrier cycle before and after each of n tracking pulses, where n is a predetermined number;
• decoding means responsive to each set of polarity samples taken during each tracking pulse repetition interval for providing early and late indications if the tracking gate is positioned earlier or later, respectively, than the zero crossover between the k - 1 and kth carrier cycles of the received pulse;
• means for registering the difference between the number of early and late indications provided after n sets of samples;
• and
• means responsive to said difference for shifting subsequent tracking pulses one carrier cycle later if the early indications exceed the late indications by more than a pre-established difference and for shifting subsequent tracking pulses one carrier cycle earlier if the late indications exceed the early indication by a pre-established difference.

In a LORAN-C receiver wherein received pulses are envelope-derived to provide a derived signal in which the k + h and subsequent carrier cycles are phase-reversed relative to carrier cycles in each received pulse, and including means for generating a tracking pulse and means for rendering said tracking pulse time coincident with any zero crossing of predetermined slope polarity in the received pulse carrier, a method for rendering said tracking pulse time coincident with the zero crossing between the k - 1 and kth carrier cycle of the received pulse, said method comprising the steps of:

• sampling the polarity of said derived signal at least at four times corresponding to one-quarter and three-quarters of a carrier cycle before and after each of n tracking pulses, where n is a predetermined number;
• in response to each set of polarity samples taken for each tracking pulse, providing early and late indications if the tracking gate is positioned earlier or later, respectively, than the zero crossover between the (k - 1)th and kth carrier cycles of the received pulse;
• registering the difference between the number of early and late indications provided after n sets of samples;
• and
• in response to said difference, shifting subsequent tracking pulses one carrier cycle later if the early indications exceed the late indications by more than a pre-established difference and for shifting subsequent tracking pulses one carrier cycle earlier if the late indications exceed the early indications by a pre-established difference.

In a LORAN-C receiver, a method for locating and tracking with a tracking gate a particular zero crossover point in the carrier of a received LORAN-C pulse, said method being characterized by the steps of:

• generating an envelope-derived signal from said received pulses wherein a carrier phase reversal is provided at said particular zero-crossover point;
• repetitively sampling the same plurality of half cycles of said envelope-derived signal for multiple received LORAN-C pulses;
• comparing said samples to pre-stored data to determine whether the tracking gate is earlier or later than said carrier phase reversal;
• registering the difference between early and late determinations made during said multiple received pulses;
• and
• re-positioning the tracking gate earlier or later in accordance with the size and sense of said difference.