GB1516243A - Coastal navigation aid - Google Patents

Abstract

1516243 Radio navigation STANDARD TELEPHONES & CABLES Ltd 5 Feb 1976 04553/76 Heading H4D A coastal marine navigation system comprises a group of n (e.g. 6) beacons each transmitted bearing information signals in the form of an audio frequency signal modulated onto a VHF carrier common to the group, the beacons transmitting one at a time in a time sharing sequence Fig. 2, in which, for each beacon, during some sequences said bearing information signal is of a first kind, having a long (e.g. 62 sec) duration, and directly utilizable aurally by a mariner equipped only with a VHF radio receiver and during other sequences said signal is of a second kind, having a short (e.g. 1 sec) duration, and only utilizable by an automatic auxiliary unit coupled to said VHF radio receiver, the transmissions during successive sequences being such that every n+1<SP>th</SP> (e.g. 7th) from the group is the first type and all the others are the second type. Each beacon is generally of the kind described in Specification 1,429,743 with the beacon bearing being indicated by the timing of a minimum in the amplitude modulation. The keying of the amplitude modulation during the long signal is as shown in Fig. 3, the start of the cycle of relative phase variation between the signals tranmitted by the two aerials being indicated by a 4 sec continuous start pulse, the modulation then being interrupted for 1À05 sec to 1À55 sec to indicate the beacon sequence number, and the modulation then being keyed for 64 audio beats (as heard on the VHF radio receiver) each lasting À6 sec, spaced by À15 sec. An interruption of modulation then follows 1À1 sec to 1À7 sec, indicating how many degrees of azimuth are represented by each of the 64 beats (i.e. the granularity), and the long signal ending with a morse code identification of the beacon. A receiver, with auxiliary unit Fig. 6, analyses the durations of the various parts of the long signal in timing circuit 60 and feeds the beacon sequence and the granularity to a store 63 for use in a computer 64, and feeds the beacon cycle start time direct to the computer. This enables the computer from the number of the beat exhibiting minimum amplitude to control a display 65 showing the bearings to beacons within range of the receiver, against their number. The short pulses Fig. 5 (as received) have a 0À2 sec initial portion of reduced (half) amplitude, with a level detector 62.in the receiver determining the increase in amplitude and starting a cycle count in the computer 64 until the bearing indicative amplitude minimum. The system thus gives a position fix for both mariners equipped with a VHF receiver only and those equipped with the receiver and auxiliary unit, the latter obtaining much faster fixes. If the mariner knows the sequence numbers of the beacons in range order, he may feed these into the computer and shorten the time he has to wait for all the fix giving bearings (Fig. 7, not shown).