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Celestial navigation, a fairly simple concept

By Col. Stan Gudmundson

Fri, Dec 20th, 2013
Posted in All Commentary

In some ways, I am a dinosaur, the last of a specialty that has been bypassed by technology. I was trained in the art of celestial navigation using nothing but stars, the moon, the sun, and some of the planets to navigate airplanes.

Celestial navigation is based on a fairly simple concept. Determine the direction a heavenly body is in degrees from true north. Then measure the height of that body, also in degrees, above the horizon. Using only this simple procedure, one can determine important information about position on earth with only one star and one only. That is the North Star (Polaris). Measure its position to the horizon in degrees and that is the exact latitude you are on.

Determining position based on other celestial bodies is a little more complex. That’s because the earth is rotating. We are also in an orbit around the sun so celestial bodies change with the seasons. The corriolis effect that has to be accounted for. Think of it as water running counter-clockwise down a drain applied to a large air mass. Finally there is the little matter of being in an airplane traveling more than 500 miles per hour.

Fortunately, astronomers have created reference sources (Air Almanac and HO 249 Star Volumes) containing the information we need. Other basics needed are a blank chart containing just lines of latitude and longitude to plot, among other things, course and position, celestial precomputation forms, a very accurate time, a sextant, clear skies, and a rough idea of where we are.

We would start with what we called a DR, that is a position we anticipate being at after a period of time. For example, if we are over Hawaii heading west, we would have to be somewhere within an approximate area given direction and time. DR stands for dead reckoning and has been the basis for navigation for centuries. Still is really.

Based on that position we would use our reference materials to accomplish celestial precomputations (precomps) on a form for just that. These calculations allowed us to locate the “heavenly body” we were looking for and then “shoot” it. The sextants we used had a bubble level that we positioned over an internal crosshair. We weren’t able to use the horizon as a required level reference, therefore the bubble.

Using a knurled knob to position the star in the bubble in the middle of the crosshairs, we would then begin the shot exactly at a predetermined time (based on the precomps). We started an internal timer on the sextant and, by turning the knurled knob, kept the star centered in the middle of the bubble aligned in the crosshairs. At the end of two minutes, the timer would terminate the shot. Then, lining up a couple of indices, the sextants fancy internal mechanisms provided an average height of a star above the horizon. Using that number, we could then go back to the precomp and determine a “line of position.”

That line of position, was drawn perpendicular to the star’s direction on the chart. We did that two more times with two other stars as close to 120 degrees apart as possible. The three lines of position form a triangle and the center is our position. That is what is called a fix, i.e. a fixed position where we know where we are.

Obviously, “shooting” three stars can’t be done at the same time. Typically we shot one 12 minutes prior to fix time, one at nine before the fix, and another six prior. Consequently, based on the speed we were flying, lines of position had to be adjusted forward to coincide for the time of the fix. This also gave us the ability to adjust the course of the aircraft at fix time if we were off track.

Celestial navigation during daylight hours was less accurate since we had only the sun to use and sometimes the moon. During the day we would “shoot” the sun three times and then average those LOPs. We would compare the resulting averaged LOP with our dead reckoning position and modify the DR as much as we had confidence in our sun shots. These we called “Most Probable Positions” (MPPs).

Accuracy? Well, that all depends. Turbulence was often a problem. The most important factor however, was the skill of the navigator. Some were simply brilliant and others less so. Unfortunately, as in every human endeavor, some succeeded and others didn’t. I was fortunate, I survived.

Today, our navigation systems provide accuracies to within very few feet. Inches almost. We tried to be perfect but to be within about five miles at night and ten miles of a daytime celestial navigation termination point was pretty fair. That’s on just your run of the mill average sortie.

The winners of the Strategic Air Command’s annual navigation competitions usually took it to another level though. The winner’s average was frequently within a couple of miles. Sometimes much closer. The British were always tough competitors.

On my longest B-52 sortie I was the radar navigator (bombardier) on one of two aircraft flown from Mather AFB near Sacramento. We flew out west of the International Date Line to do a little reconnaissance on Soviet naval vessels. We had nothing but handheld 35mm cameras but the US wanted to show the flag as it were.

As the lead aircraft, we were responsible for getting both aircraft to the recon area and also for navigating back home. We were the first B-52s to ever do that.

After nine or 10 hours of flying over water without any navigation aids, wandering about in the designated search area and the mission accomplished, we began our flight back home figuring that the DR position we began with was somewhere in the vicinity of a 50 or so mile circle. During daylight we used the sun and resulting MPPs and took two night celestial fixes prior to our termination point. We arrived at the end celestial navigation position within two miles and within 30 seconds of our estimated time of arrival. Not bad, not bad at all.

I’d like to brag about my role on this mission but the primary credit goes to the navigator I was crewed with, the staff navigator who accompanied us, and our Electronic Warfare Officer (EWO) who operated the sextant. Okay, the pilots deserve credit too. Well, a little anyway.


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3:21:07, May 4th 2014

rwmacevoy says:
I'm a former tanker navigator who flew the KC-135A model from 1976 until conversion to the E model about 1980 than going to C-130's in 1983. I loved the A model because it had basically nothing in terms of modern navigation systems. Some of our birds had real antiquated doppler systems that gave a position readout in left or right cross-track and plus or minus track. This was almost better than the coordinate read out system since both were analog and you had to run a DR to perform an update to the one with geographic coordinates since it took time to slew in the new information. With the cross-track you could just "bump in" the updates.

On top of that it often didn't matter that much since the doppler information was always suspicious. I learned in the tanker to trust my own ability to judge how the winds were changing.