Distance Measuring Equipment

D Vautier
11/2006

When you run an Army survey the idea is to know your location on the earth, or more accurately, where you are in relation to the rest of your army, especially the big guns.  By knowing this you can tell the artillery pieces and all other available attack elements which way to shoot, how much to shoot and how far away the enemy is.  This is a good way to make the those enemy guys very uncomfortable.

The most easiest method to extend geographic co-ordinates, or “control” is by traverse.  A survey traverse consist of a series of “legs” of known direction (asmuth) and distance.  By applying trigonometry, it’s easy to calculate the coordinates of the next point, and then the next, and so on.  It is also a good idea to close the traverse on another known point or on the same point.  That way you can figure your overall accuracy.  Closing on the same point verifies distance but not asmuth (direction).  Closing on another point does both.

When we were in military training at Fort Sill, we had little access to good distance measuring equipment.  There was no electronic equipment (DMEs) at all so we got to “chain” distances between stations using a 100 meter steel tape.  The lead chainman ran forward dragging the tape, and when properly lined up on the forward station, the rear chainman hollered "stick" and the forward guy would drop his plumb bob, trying not to hit his foot.  If he got a good stick, he would holler "stuck", mark the spot with a witness pin and drag the tape another 100 meters.  If you needed to go over a hill you had to "break tape", a process to horrible even to describe.  After a few hours of this "stick-stuck" business you tend to go completely insane.

When I first got to Germany, I discovered to my undying happiness that nobody did the stick-stuck thing at all (nevertheless people still tended to go completely insane anyway).  Instead they used these wonderful DME devices, which were accurate to 4 parts per million, plus or minus 1.5 centimeters, and that sounded pretty damn fine to most of us (the 1.5 centimeters came from the width of the cathode because the last crystal couldn’t tell which side of the wave it was on—thought you'd never ask).

The DME devices were big boxes crammed with electronics with parabolic domes on the front of them.  They measured distances by means of wave comparison.  There were 6 crystals.  Stations were designated “master” and “slave” and switched around after a measurement was completed to reduce error.  The DME operator ran through the six crystals; 1000 meter, 100 meter, 10 meter, 1 meter, 1 centimeter, and finally 1 millimeter.  He would turn a knob until the needle came to zero.  He was then reading the point where the wave was being returned in the same phase that it was sent out at.  Each reading was recorded and by simple addition, the exact distance was calculated.

Pretty slick stuff—lots better than “stick stuck” technology for sure.  DMEs could measure up to maybe 20 Ks (kilometers) but the recommended maximum distance was 15 Ks.

I was on my fifth survey or thereabouts.  It was a long one.  We were sitting on a gentle hill overlooking a little German town in the valley below us. It was about 7 PM.  Far off in the distance over the fog was the spire of a castle.  My party chief drove up:

Chief:  We’re going to close this survey on that castle.
Me:  Sounds good chief.  So where is our forward station?
Chief:  I just told you.  It’s that castle.
Me:  Er, ahh, excuse me chief, what?
Chief:  It’s that castle.
Me:  Er, Ahhh, I hate to bring this up but we have a few minor problems here, er, chief.  That castle looks like its wwwwaaaayyyyy far away, like, 20 Ks or so.  Our DMEs will never work over that distance.  And besides how are you going to get line-of-sight to that castle with all the fog?

Party chief:  We’ll do a night shot.  We’ll wait for the fog to clear.  I’m going over there now.  Sit tight.  Stay alert.  Talk to you on the horn.

And that’s all there was to it.  Here we were stuck up here on this miserable hill, it was getting close to nightfall, and we were expected to close this damn survey before tomorrow morning, and the chief wanted to try a 20 K shot.  He's dreaming.

In the distance was this castle shrouded in fog, well over what our DMEs could do, and besides that, we were supposed to shoot an angle to it, and record a distance?  Fat chance.  Is this fantasyland?

It was also getting cold, real cold.  We huddled around our ¾ ton truck and took turns in the cab where there was some heat.  I stretched out on the warm hood to stay warm.  It was about 1 AM when I fell asleep on the hood.  At about 3 AM I heard the charming voice of Tucker, a member of our forward team loud and clear on the radio.

“Good morning all you boys and girls.  I am now speaking from the castle.  Can you see our glorious light?”

I glanced over to the castle and the fog was gone, and I saw a light.  I grabbed my recorder book and a flashlight.  I said to Bonner. “Shoot it, quick shoot the angle”.  He began calling out numbers and I started repeating them back and recording them.  Just after he was finishing the last reading, the light disappeared in the fog.  I carefully checked the angle.  It was within tolerance.  We breathed a huge sigh of relief.  We got our angle.

So now our attention was completely focused on the DME operator.  He started calling out numbers.  The cold air and night must have helped, because all he got earlier was static.  He was able to get what looked like a good distance.  It was just a little over 24 Ks.  He had to do it three times.  We transmitted the information to the guys who did the math.  About 2 hours later we got the message.  “Good job.  The survey closed real tight.  Come on home”  We were all exhausted.

The survey closed better than fine.  Thanks to that 24 K leg, it’s accuracy was 1 over 30 thousand.  Our requirement was 1 over 5 thousand for forth order work.  Nobody believed that we actually could take a 24 K DME shot, but we did it.