OCEAN ELECTRONIC NAVIGATIONAL AIDS 17 



BANDWIDTH. — Early designs incorporated a total bandwidth of the order 

 of 80 kilocycles at 6 db. down. Current trends are toward considerable reduc- 

 tion and total bandwidths of 45 kilocycles or less at the same decibel limitation 

 are contemplated. 



DIFFERENTIAL GAIN AMPLIFIER. — The receiver incorporates a differential ampli- 

 fier which operates in synchronism with the incoming signal recurrence 

 frequency to permit amplification of each of the two signals received at differ- 

 ent ratios. This feature permits presentation to the cathode ray indicator 

 of signals of equal amplitude. Sufficient range of operation is provided to 

 permit accommodation of incoming signals having a ratio of strengths as 

 high as 1,000 to 1. Earlier equipments had an operating limit of roughly 

 100 to 1. 



INDICATOR 



FUNCTIONAL PURPOSE. — The indicator unit contains the necessary circuits to 

 perform all of the timing functions of the equipment with the required pre- 

 cision. It contains the sweep generators and the cathode ray tube for presen- 

 tation of the signals received. 



MASTER OSCILLATOR. — The basic timing medium of the equipment is a pre- 

 cision, crystal-controlled, master oscillator. The oscillator possesses a high 

 order of short time stability, in the order of a few parts in 10 million. Manual 

 means of adjustment is provided to vary the frequency over a range of more 

 than 200 parts in a million. This adjustment permits cycling the oscillator 

 until the timing of the receiver-indicator is in exact step with the recurring 

 pulses received from the transmitting stations. 



TIMING MARKERS. — Through the medium of its timing circuits, the indicator 

 provides a sequence of precise timing markers spaced at convenient intervals 

 to facilitate measurement of time sequences with a basic accuracy in the order 

 of plus or minus 1 microsecond. 



TIME BASE. — The sweep generators provide a slow sweep as outlined in the 

 system specification which covers the entire recurrence interval by means 

 of a divided trace. A fast sweep lasting in the order of 200 microseconds or 

 less is provided and by means of delay controls may be positioned to examine 

 the particular section of the time base at which a signal appears. For con- 

 venience in identification and for purposes of triggering and delay measure- 

 ment a pedestal or raised rectangular pulse appears on the slow sweep pres- 

 entation in the portion covered by the fast sweep. Timing arrangements 

 are made to permit the fast sweep generator to fire at a predetermined point 

 on both the upper and lower traces. 



MATCHING PULSES. — To measure Loran time differences, the time base is 

 cycled with respect to the transmissions until the master and slave signals 

 appear at convenient points on the upper and lower traces, respectively, with 

 adjustment being made such that the fast sweep generator fires precisely 

 in the region of both signals. The signals are examined on the scope operat- 

 ing on fast sweep and a fine adjustment is made until the pulses are super- 

 imposed or matched with respect to time. Time-difference measurements 

 are made by means of timing markers, or, in the newer equipments, are read 

 directly from a mechanical counter. 



EQUIPMENT POWER REQUIREMENTS. — Loran receiver-indicators for shipboard 

 installation are designed to operate on 115-volt (nominal), 60-cycle, single 

 phase, alternating current, and require from 200 to 300 watts. 



Aircraft equipments are made to operate on voltages of from 80 to 115, 

 single phase, alternating current, and on frequencies from 360 to 2,460 cycles. 

 Equipments require less than 300 watts. 



RECEIVING ANTENNA. — Receiv'ng antenna installation is simple since only a 

 vertical wire is required. A length of 50 to 60 feet is considered desirable, 

 although satisfactory operation is experienced when physical conditions 

 require considerable decrease in the effective antenna length. 



METHOD OF OBTAINING LORAN READINGS. — The majority of Loran receiver- 

 indicator equipment now in use was developed and manufactured during 

 the war. This equipment requires that time difference determination's be 

 made by matching master and slave pulses and then, by switching to addi- 

 tional scope selection positions and by reference to a set of markers, counting 

 the divisions and thereby arrivine at the reading. Receiver-indicators are now 

 being manufactured and installed which incorporate direct-reading counters ; 

 thus once the pulses are matched, all that is necessary to determine the time 

 difference reading is to refer to a counter device which shows directly the 

 proper numerical reading. 



