> Rain should have an effect on the timing of the signal, since the > propagation speed of radio waves through water is different from that > through air. It will also attenuate the signal, causing worse S/N > ratio which would cause the lower-elevation satellites to not be seen.
David, Maybe we can figure this out. First, the refractive index of water is about 1.3. So I think this means the propagation speed of radio waves in water is down to about 0.75 c, right? Then, how much water are the GPS signals traveling through? Let's assume the typical amount of rain in a heavy storm is a couple of inches. All that water is either puddles already on the ground, drops on their way down, or moisture still in the clouds waiting to come down. The total amount of water in a cross section column of the atmosphere that the GPS signals travel though is thus a couple of inches total, max. Let's assume a worst case -- 6 inches. So, those GPS signals go through 20,000 km of empty space and atmosphere containing a total of 6 inches of water; in which it slows down by 30%. At a ns/foot, this comes to 25 ps per inch of water content in the air; a total of 150 ps in my worst-case example above. My conclusion is that rain or snow, light or heavy, has no effect, even at the ns level. Can someone who really knows double check this back of the envelope calculation? Thanks, /tvb _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
