[time-nuts] FTS4060 Continuing Saga, GPS rain

[Brooke Clarke]

Hi Tom:

I think the problem is related to the FTS4060.  During the brown out my 
computer and the SR620 counter reset, but the two green LEDs on the 4060 
stayed on, at the time my thought was "cool".  But the yellow "Align" 
light behind the door has turned on and so far I have not been able to 
get it to turn off.  But after trying a bunch of stuff the jitter value 
is now back in the 9 ns area.  I have not been able to find anything in 
the manual about operation with the "Align" light on and with the "Lock" 
and "AC Pwr" LEDs on.  Does anyone have any ideas about it?

On the rain thing.  I think the key parameter of water that matters is 
the dielectric constant which is about 80.  The propagation constant is 
proportional to 1 / SQRT( e ) or about 11% of what it is in air.  If 
it's 1 ns per foot in air then it's 8.9 ns per foot in water

I haven't figured out how to calculate the total volume of water in the 
column between the satellites and the receiving antenna.  I did find a 
web page that shows the terminal velocity of rain and it varies from 
0.02 MPH for small drops to 20 MPH for big drops.  But this would need 
to be coupled with the rate of rain fall (inches/hour) to determine how 
much water was in the column.  Assuming big drops (20 MPH = 352 IPS) and 
0.1 IPS rain rate says that the rain takes up 1 part in 3520.  If the 
height of the rain is 20,000 feet then there's 5.6 feet of rain for a 
delay of 50.5 ns???

Have Fun,

Brooke

Tom Van Baak wrote:

>>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
>
>
>
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