[time-nuts] Sun Outage
jimlux at earthlink.net
Fri Oct 10 13:25:47 UTC 2014
On 10/10/14, 4:21 AM, Bob Camp wrote:
> Bringing this back to GPS…
> The TV stuff is at about 8X the frequency of GPS. That makes a difference in terms of things like rain. That said, yes, there probably is some giant rainstorm that would impact GPS accuracy. Much more likely in a “I can only see a small patch of sky” situation.
It's a huge difference..
The rain fading is typically represented by an equation like
gamma = a * Rate^b (dB/km) (Rate is the rainfall rate in mm/hr)
at 1 GHz, a = 0.0000387 and b=0.912
at 10 GHz, a=0.0101 and b = 1.276
at 30 GHz, a = 0.187 and b=1.021
The "a" term mostly has to do with the size of the raindrops vs the
wavelength. Very few raindrops are close to 20cm in diameter (1500
MHz). the "b" term is mostly "how many drops are in a given volume"
For lower frequencies (wavelength longer than drop size), it's mostly a
Rayleigh scattering problem.
For TV, the other factor that is important is that the rain depolarizes
the signal, so you could lose half your signal power because of cross
pol, and have an interfering signal become equal to the desired signal
(because they use polarization diversity to "double up" channels)
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