Private Pilot Flight Training and Instruction Flight Training Article Library | Back To 4VFR.COM The LORAN System (I instructed LORAN at 58th Bomb Wing Training School on Tinian Island of the Marianas during the last year of WWII) I also instructed in the use of the APQ-23 which was the first radar set to have offset bombing and DME. Today offset bombing is called RNAV. Loran is a hyperbolic system of position fixing with long range capability. Loran combines the words long and range. Loran-A began as a naval shipboard system during WWII. The equipment was reduced in size to about two units 14'x20"x24" which became the airborne AN/APN-4. By the end of the war the APN-9 required only one such unit. By 1990 the Loran APN (design) number was well into the APN-30's and smaller than a cigar box. The aircraft position is determined by timing the difference in milliseconds it takes the signal to reach the aircraft from the slave and master stations. In WWII these had to be counted on a cathode ray tube. Now it is automatically and continuously computed on a microprocessor. Loran Stations transmit radio pulse signals with a wait time determined by the range of the system. This prevents subsequent pulses from causing interference. Such pulses can be of very high power. Two loran transmitters, several hundred miles apart, made a master/slave pair on the same frequency. The slave would not transmit until it was triggered by the master pulse. It should be noted in passing that there was no apparent effort by the Japanese to jam Loran frequencies during WWII. The Loran set would be tuned to a Loran master/slave pair and could receive the pulses. The pulses would be shown along a cathode ray tube (CRT) line with a space between to be measured as a difference in reception time. By carefully adjusting the frequency of the electronic sweep to the pulse frequency the two pulses could be made to appear with the same space between them on the time base line. If the frequencies were different the pulses would creep forward or backward. The initial line could be greatly magnified and the signals could be electronically superimposed by fine tuning the delay knob. Once the two signals were detected and superimposed new switches brought up a CRT electronic clock. Post WWII stations sometimes had two slave stations. The CRT electronic clock divided the sweep of the phosphorus ray across the tube into a series of spaced divisions much like a ruler. The larger spaces were repeatedly subdivided and could be remagnified and subdivided with additional settings. With training, the divisions on the scope could be counted down from tens of thousands to one millisecond. A skilled operator could do the entire operation is less than one minute. Aircraft position It was now necessary for the operator to make reference to a Loran chart. This consisted of a Mercator chart over printed with hyperbolic Loran lines. they were drawn across the entire chart with numerals to mark the calibrated milliseconds of different pulse times between the master and slave stations of a given pair. At least two pairs of stations were calibrated for each chart. The lines for each station were of different colors. In 1977 there were still 65 Loran A chains in operation. As of 1991 no A Chains are in operation in the U. S. One of the difficulties with Loran-A, initially, was that from a centerline between the stations there were always two possible lines with the same microsecond difference. The operator had to know somewhat his general reference to the station pair to prevent using the incorrect hyperbola. Post-war Loran-A used a coded delay as well as odometer to solve this problem and give instantaneous readings. Loran transmitters would produce both ground wave pulses and one or more sky wave pulses. It was necessary for the operator to distinguish the difference by referring to pulse amplitude until beyond 1000 nautical miles. Beyond this distance all waves would be sky waves. Weak ground waves were to be preferred to sky waves since the charts were based on ground wave differences. When sky waves were used a correction table had to be incorporated into the chart use. Written by Gene Whitt Flight Training Article Library | Back To 4VFR.COM