[time-nuts] FTS4060 Continuing Saga, GPS rain

[Tom Van Baak]

> 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