Author Topic: Before there was LORAN Charlie, there was...LORAN Alpha  (Read 241 times)

Offline HenryC

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Before there was LORAN Charlie, there was...LORAN Alpha
« on: September 23, 2015, 08:00:42 AM »
 I was a frequent user of LORAN-A during my service as a Quartermaster aboard a missile frigate in 1968. The following remarks should be of historical interest to those curious about obsolete navigational systems, and may prove nostalgic to those old salts out there who used "Not So Sweet Loran" (a pun on a "Country Joe and the Fish" song popular around that time). Keep in mind that these events occurred over 40 years ago; so forgive me if my memory is a bit fuzzy on some of the details.

The navigation officer and the division LPO called a chartroom conference and gave me a special assignment. Our squadron had been operating in the North Atlantic, engaged in NATO exercises with the British, Canadian, German and Dutch navies. It was November, and as could be expected the weather was awful, celestial could not be relied upon, and it was too far from land to use radar. At the limits of its range, LORAN coverage was spotty, at best, and intermittent, but it was all that was available. Accurate navigation was essential to complete our mission; to maximize our chances of getting good positional data, it was decided that LORAN fixes be taken every 15 minutes, 24 hours a day, and be continuously plotted. If LORAN faded out, or if insufficient Lines Of Position were acquired to get a good fix, the best possible Dead Reckoning posit was to be plotted on the bridge chart. The watchstanding Quartermaster was instructed to notify me immediately of every course and speed change. The radar guys in Combat Information Center would keep up an independent DR using their bug table as a reality check. The navigator and our QM1 would handle the daytime fixes, and I would be up all night handling the nightime ones. In return, I was exempted from all other duties except morning muster, man overboard drill, and General Quarters. It seemed like a good deal to me.

All of the QM watchstanders were thoroughly checked out on LORAN by this time, so we knew that in our OPS area the signals were intermittent, faded in and out, and were plagued with ionospheric echoes which delayed their arrival to the receiver. Rarely were all station pairs required for a fix simultaneously operational, so we often had to rely on two or even one LOPs and a running fix from a previous observation. Sometimes we even crossed a LORAN LOP with a DR'd day-old sunline or last night's star shot for a fix. Occasionally, there would be periods of clear reception when it was possible to really nail down an LOP to within 1000 yards (this was in pre-GPS days, remember). It was critical to plot good fixes as carefully as possible so that we could learn to make allowances for the signal as it started to deteriorate due to weather or conditions in the ionosphere. It was possible to watch a signal "drift" from its actual value as the night progressed, and thereby be able to make allowances for it and correct for the error.

Modern electronic navigation systems analyze the radio signals they use and calculate direct lat/long positions with internal processors. These values are then printed on the display, where they can be directly read by the navigator or passed on to other devices for further processing. LORAN-A was completely analog. The signals were visible as spikes on a little Cathode Ray Tube (a little TV-like screen, like the ones you see on analog oscilloscopes). The drill went something like this: you checked the chart and decided which station pair to observe (these were printed as faint colored hyperbolas on charts: you've probably seen them, especially on older charts). The station pairs were given names, like "3HO" which indicated their frequency. This name was punched into the receiver using rotary switches to lock in that master-slave station pair. Looking into the CRT, you could see a horizontal green line of static (radio noise from natural sources in the environment and the equipment itself) with little vertical green lines which represented the master signal and the response from the slave transmitter. By turning a manual hand crank, it was possible to move one signal until it touched the other. At this point, tripping a switch allowed a "zoom" in on the signal pair so you could precisely line up the two spikes. Once the two signals were aligned, the delay time in microseconds could be read from a counter (it looked like an automobile odometer, the old kind, with numbers printed on rolling cylinders). After the microsecond delay time was written down, you would go to the chart and locate two hyperbolas for that station whose numbers bracketed your reading, and use dividers and the "linear interpolator" graphic printed on the chart to interpolate between hyperbolas and draw a line of position. The process was repeated for other station pairs for additional LOPs, until a trustworthy fix was developed. If no pre-printed LORAN chart of the area was available, there were published tables that allowed location of LOPs through a long and involved graphical method on a plotting sheet.

With a little practice, the process took less time to do than it does to explain, and normally, LORAN-A was easy to use and gave excellent results, better than Radar or Celestial. But on this particular cruise, conditions made it a real touch-and-go operation. Weak signals caused by our distance from the stations shrank down into the noise until they were overwhelmed by static. If we turned up the gain to boost the signal spike, it boosted the noise, too. But with a little practice, the eye could locate even a weak signal because it did not move to the left or right, but remained in one spot on the CRT, while the noise jumped around all over (frequency variation). It also required patience and a sharp eye, because often the signals would fade in and out, sometimes the master, sometimes the slave, sometimes both. You had to get lucky and get both of them on the display long enough to identify and zoom in and get a reading. Echoes or bounces were another problem; sometimes a station would show up as a series of spikes, the original signal and delayed echoes off the Heaviside layer up at the edge of space. We were north of the Arctic circle, and auroral activity was constantly distorting the ionosphere and these echoes gave us fits. We had tables to help correct for this effect, but they were practically useless, and we had to rely on trial and error corrections until we started getting consistent fixes.

November nights at those latitudes last a long time, so I got plenty of practice. In spite of all the problems, I soon became a real wizard at operating the equipment and combining all the navigational information I had available to come up with really good posits. I was getting accurate fixes where my fellow QMs couldn't even see the signals! Needless to say, CIC couldn't even come close. When we made a flawless rendezvous with the Royal Navy carrier in Tactical Command of our squadron, we were given an official "well-done" for our "pinpoint navigation under difficult conditions". Praise from the Brits is rare, but always well-deserved. Our skipper passed the compliment on to us with his personal thanks.

There was one other personal triumph for me during this episode. Our ship had a facial hair policy which allowed only "neatly trimmed" beards and mustaches. In practice, what this meant was that if you came aboard with whiskers, you were allowed to keep them, but it was impossible to grow them without getting gigged. With over a month on duty on a blacked-out bridge and chart house it was possible for me to grow a glorious 'stache, right under their noses, so to speak. I still have it to this day.

Offline Craig

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Re: Before there was LORAN Charlie, there was...LORAN Alpha
« Reply #1 on: September 23, 2015, 10:33:07 AM »
Great post Henry! Very interesting and informative. Thanks!
Craig, Horizon Cat "Kailani"  Punta Gorda, FL

Offline deisher6

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Re: Before there was LORAN Charlie, there was...LORAN Alpha
« Reply #2 on: September 23, 2015, 11:02:25 AM »
Thanks too!  great post.
regards charlie

Offline JBC

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Re: Before there was LORAN Charlie, there was...LORAN Alpha
« Reply #3 on: September 23, 2015, 09:31:08 PM »
Very enjoyable read.  Thanks for the post.  Says to me that human skills are an essential part of any navigation system, no matter the technology.  And in your case, those skills seem to be well honed fairly quickly.

Jett