Speed of Light, measured tonight…

August 25, 2008 | Amateur Radio | By: Mark VandeWettering

Well, I was bored, and it was getting close to midnight. There was nothing really going on on any of the amateur radio bands, so I got to thinking. Light travels about 186,000 miles per second, or about 186 miles per ms. On 10Mhz, NIST broadcasts a standard time signal from stations in Colorado, and in Hawaii. A little Wikipedia work told me that they are located:

CM87ux -> BL01cx: bearing 254.4°, distance 2479.3 miles (to Boulder)
CM87ux -> DN80xq: bearing  73.8°, distance 1095.7 miles (to Hawaii)

The difference is about 7.44ms. So, I recorded some of the time signal tonight, where I could hear both signals. The start of the minute is indicated by a 1000 hz signal from Boulder and a 1200 Hz signal from Hawaii. Here’s the spectrogram I came up with:

Click on it to see the enlarged picture. Each horizontal pixel is 1ms. Sure enough, you can see that the lower frequency path is about 7 pixels to the left of the upper trace. Physics confirmed!

(Yes, there are better ways to detect the start of each pulse, this is just what I had on hand. Glad to see it worked though.)

Comments

Comment from Eric / WY7USA
Time 8/25/2008 at 6:37 pm

What a coincidence. My friend and I were thinking about a similar problem last night. He lives about 4-5 miles from me and we decided to try out his new 80 meter antenna. We were very surprised to have 9+20 signals running less than 5 watts with a hill between us. So we were considering this to be an NVIS contact. So we tried to see if we could measure a difference in time between an 80 meter signal and a VHF signal. But the difference would be in the sub-millisecond range, which we were not able to measure properly.

Any ideas how to determine if this was NVIS or not?