ABSTRACT

Loran-C has become a widely used tool for-radio positioning over the past few years with the development of the range-range mode of operation and the portable Loran ACCUFIX transmitter.

The Austron Model 5000 Loran-C System has been used in over 500 crew months of-operation as a standalone radio positioning system, in conjunction with a satellite receiver, and interfaced directly to integrated navigation systems as a velocity/ heading sensor.

The Loran ACCUFIX transmitter can be used in conjunction with existing Loran-C stations to extend coverage or configured as complete nets to establish coverage where none exists. The portability of the stations makes the system suitable for relatively short term operations.

Loran-C is a long-range radionavigation system operating on a frequency of 100 KHZ utilizing a combination of pulse and phase comparison techniques. Thirty-one stations comprise eight chains which cover a large portion of the northern hemisphere with ranges of 800–1400 miles, depending upon the area of the world. (See figure 1)

Loran-C is normally used in the hyperbolic configuration. This mode uses three stations, each with an independent transmitter controlled by a cesium beam frequency standard. (See figure 3) The end or slave stations are controlled so as to keep their pulse transmissions within a predetermined tolerance (0.2 microseconds) of the master station and to give the appearance that the master signals are received by the slave stations and then retransmitted. The mobile receiver consists of two time interval counters which are started by the receipt of the master station pulse and stopped by the pulses from respective slave stations. The resulting time differences yield hyperbolic lines of position.

In 1969 the U.S. Coast Guard pioneered the first range-range Loran-C system by incorporating a cesium standard in the mobile receiver. (See figure 4) By synchronizing the mobile standard with those controlling the base stations, the mobile standard can be used to start the time interval counters when the base stations transmit. The counters are stopped by the respective base station pulses which give the direct transmission times or ranges to the stations.

The range-range mode of operation has several advantages over the hyperbolic configuration but also has two disadvantages:

  1. Any two stations can be used for range-range operations while a master and two slave stations are required for hyperbolic operation, thereby increasing reliability and the area of coverage.

  2. Range-range geometry is more favorable than hyperbolic geometry.

  3. Range-range readings have less noise than hyperbolic readings since a range reading is determined by comparing a received signal with a clean locally generated reference signal from the cesium standard, while a hyperbolic reading is determined by comparing two received signals, both of which contain noise.

  4. The range-range mode has the disadvantage of drift between the standards controlling the base and mobile stations that must be corrected for.

  5. Calibration may be lost in a range system if the reference signal from the cesium standard is interrupted.

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