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USC&GS and the speed of light


BuckBrooke

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Greetings, all. I thought you might find this interesting. Let's see if Dave D. runs across this post.

 

This is an excerpt from a book I'm casually reading by Louis Brown, "A Radar History of World War II: Technical and Military Imperatives" 1999.

 

Chapter 10: The Measure of Radar

10.1 Navigation Transformed

10.1.5 Radio Navigation and the velocity of light

 

Shoran was a hyperbolic-coordinate navigation system that used the same principles as Gee-H but with an automatic pilot's direction indicator. It had an on-board computer allowing a continuous presentation of the data that permitted blind bombing. It proved to have the accuracy of Oboe or visual bombing but with each bomber capable of using it independently. It was introduced into combat in April 1945 and consequently had little effect on the course of the war[17].

 

The US Coast and Geodetic Survey learned of the accuracies suspected of the various radio-navigation systems and wished to evaluate this new method of measuring the Earth. With the Army Air Forces they set up as a test, Shoran ground stations at locations where there was surveyed control. The results showed precision of two parts in a million but systematic discrepancies from the control data of 50 parts per million, which could be effectively removed if the velocity of light was altered from the accepted value[18].

 

The tabulated value in 1941 was 299,776 +/- 4 km/s. This had resulted primarily from extensive measurements initiated by A A Michelson and carried to completion after his death by his assistants. The experiments had used long, evacuated paths and incorporated what was thought to be the ultimate in experimental accuracy[19]. It was significantly lower than Michelson's previous value, and the experimental uncertainties ascribed each measurement made them incompatible one with the other. Any warning provided by this was swept aside by a confirmation experiment at Harvard that used the Kerr-cell light switch rather than Michelson's rotating mirror and that allowed electronic timing. The result was 299,764 +/- 15 km/s[20]. The Coast and Geodetic Survey found that a value of 299,792 +/- 2.4 km/s made their Shoran data compatible with their ground-surveyd data, very near Michelson's older value.

 

William Hansen had proposed to use a resonating cavity to measure the velocity of light. By knowing the frequency at which the cavity resonated, its dimensions and the rate of its power dissipation, one could calculate the velocity of the waves. This experiment had been carried out at the National Physical Laboratories in England before the Shoran experiment, and gave 299,793 +/- 9 km/s [21], a value also significantly in disagreement with the then accepted value. Other confirmations of the new value were to follow with microwaves and light. The laser was eventually to take accuracy to the currently accepted value of 299,792.458 +/- 0.0012 km/s, but for a time the blue ribbon belonged to microwaves.

 

....

 

[17] Henry E Guerlac Radar in World War II New Yoprk: Tomash-American Institute of Physics Publishers, 1987. pp 525-529

[18] Carl I Aslakson, Velocity of Electromagnetic Waves Nature Vol 164, pp 711-712, 1949

[19] A A Michelson, F G Pease and F Pearson, Measurement of the Velocity of Light in a Partial Vacuum Ap. J. VOl 82, pp 26-61, 1935

[20] Wilmer C Anderson Rev. Sci. Inst. Vol 8, pp 239-247, 1937

[21] L Essen, Velocity of Electromagnetic Waves Nature Vol 159, pp 611-612, 1947

 

I tried to get the USC&GS Nature article from 1949, but they've only scanned their articles back to 1950 (darn it). There's a 1951 article as well, but I can't get to it from home.

 

Here's an NGS article with excerpts from Aslakson's memoir. He's a pretty cool character.

Edited by BuckBrooke
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Here's another little piece of the history, from triangulation station ANTONIO (EV3805):

 

"DESCRIBED BY COAST AND GEODETIC SURVEY 1923 (CLG) ... AND IN TOP OF WESTERNMOST PIER WHICH WAS SET BY DR. A.A. MICHELSON FOR SUPPORT OF HIS INSTRUMENTS. MARKED BY STANDARD STATION DISK. REFERENCE MARKS ....."

 

I've been wanting to get up there, and have been to Mt San Antonio (Mt Baldy) a couple times, but not enough "juice" left for the trek down to the station (an extra 2 miles down & back up), or via other possible routes....

-- Klemmer

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And believe it or not, Loran is still in operation (well, Loran-C). I used Loran-B in the 70's in USAF aircraft (a real pain), and Loran-C in various watercraft thru the 80's (until GPS became useful / affordable). Seems like Loran-C might be on the way out soon. See the "Future of..." link on the USCG site linked above. But I think Shoran was a bit before my time....

Edited by Klemmer & TeddyBearMama
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I do not know that much about either one.

 

I can't even remember where that article I have is about when they changed the speed of light to it's new definition to more correctly fit some model designed by. uhhh. :laughing:

 

edit:

Oh yeah it was the metre thingy.....

Since the fundamental SI unit of length, the metre, has been defined since October 21, 1983 in terms of the speed of light; one metre is the distance light travels in a vacuum in 1/299,792,458 of a second. Converted to imperial units, the speed of light is approximately 186,282.397 miles per second, or 670,616,629.384 miles per hour, or almost one foot per nanosecond.

 

I can not even comprehend that.

my eyes just aint that good anymore.

Edited by GEO*Trailblazer 1
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I do not know that much about either one.

 

I can't even remember where that article I have is about when they changed the speed of light to it's new definition to more correctly fit some model designed by. uhhh. :laughing:

 

edit:

Oh yeah it was the metre thingy.....

Since the fundamental SI unit of length, the metre, has been defined since October 21, 1983 in terms of the speed of light; one metre is the distance light travels in a vacuum in 1/299,792,458 of a second. Converted to imperial units, the speed of light is approximately 186,282.397 miles per second, or 670,616,629.384 miles per hour, or almost one foot per nanosecond.

 

I can not even comprehend that.

my eyes just aint that good anymore.

 

I would really like to meet the guy who had his thumb on the stop watch button. :huh:

 

Seriously how did they measure the speed of light? It’s something I’ve pondered for a long time. It’s not like they could set up with a light on one mountain top and an observer on another one with radios to communicate.

Edited by 68-eldo
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Measurement of the speed of light is also connected with the development of EDM (Electronic Distance Measurement) technology, which uses infrared light signals to determine distance. This can be accomplished accurately because the speed of light is known accurately. Conversely, the same technology, when employed over a distance that is already known accurately, can be used to determine the speed of light!

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Ah Wikipedia is great. It says the first experiments were done in 1667 using lanterns about a mile apart. It does not go into detail as to how the experiments were done, but I would imagine the reaction time of the people operating the lights would render the results useless. But then this experiment may have been more about finding out if the speed of light is finite or infinite then finding the actual speed.

 

The first quantitative measurement was done in 1676 by Ole Rømer observing the appearance or disappearance of the moon Io as it orbited Jupiter. The appearance/disappearance would happen later than predicted as the Earth moved away from Jupiter and sooner as the Earth moved closer.

 

With modern radar and lasers such measurements are much easier.

 

Sorry about the topic migration.

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Ole did pretty well, getting within 1/3 of 1%.

 

My understanding of the USC&GS experiment is that have a triangle of three accurately surveyed positions, thus you know the distance between them. If you then send pulsed radio signals from two points, and receive them at the third, and calculate the path lengths, you should be able to compare that to your known distance to determine how fast the pulses took. Shoran uses hyperbolic paths, etc. so it's a little more complicated, but if your control network is long enough, and you do enough measurements, you will see any discrepancy in the speed of light show up. The error in the published speed of light that he detected was something like one foot in four miles.

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In fact, at it's most simple level, that is how the GPS system works, measuring differences in received time from the various GPS sats. I believe that one of the given constants that the system uses in the calculations is the speed of light (radio waves), including the speeds through the various media (vacuum, air of various densities, etc).

 

So really, these "speed of light" experiments formed some of the basis on which the GPS system is based (among many other things).

 

Full circle, huh?

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In fact, at it's most simple level, that is how the GPS system works, measuring differences in received time from the various GPS sats. I believe that one of the given constants that the system uses in the calculations is the speed of light (radio waves), including the speeds through the various media (vacuum, air of various densities, etc).

 

So really, these "speed of light" experiments formed some of the basis on which the GPS system is based (among many other things).

 

Full circle, huh?

 

Radar is also very dependent on the speed of light. The time it takes for the transmitted pulse to get to the target and the echo to return divided in half is then multiplied by the speed of light to determine the distance to the target.

 

Conversely if the distance to the target is known precisely, then the speed of light can be determined from the time it takes for the echo to return to the antenna.

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