[time-nuts] Iridium source?
jimlux at earthlink.net
Wed Oct 7 08:37:04 EDT 2015
On 10/7/15 1:16 AM, Tom Van Baak wrote:
> Thanks for the heads up. If you can leak anything from the upcoming paper please let us know.
> Since you're an Iridium expert, would you be able to answer the question? The OP (John Todd) was asking about reception deep indoors, where GPS signals fail. In case you deleted it, the posting is here:
I would worry about the varying propagation time through the building.
Over the past couple years, I've been propagating 3 GHz signals through
a variety of buildings, piles of rubble, and so forth.
There's two effects I can think of with respect to timing:
a) the propagation path is highly scattered, and there is a lot of
fading (on a scale comparable to wavelengths.. so if the satellite is
moving at 7 km/s, and the wavelength is 20 cm, you're going to get fades
at 35 kHz).. More to the point, the phase shift is wildly fluctuating in
the short term, but pretty stable in the long term average sense.
b) The "average" path length is going to change as the satellite to
ground terminal angle changes, because you're going through more or less
of the building (unless the building happens to be a big symmetrical
radome that you are in the center of<grin>)
Dense rubble has a propagation velocity of about 50-70% of free space.
Buildings are pretty close to free space: the effect is one of bouncing
around in an indirect path, rather than going through walls, although
there's some of both effects.
In any case, at 1 foot/nanosecond in free space, going through, say, 3-4
feet of construction material at 0.70 Velocity is going to add a
nanosecond to your time delay.
The attached is a modeled gaussian pulse propagating through 5-7 meters
of mixed rubble (I don't recall the exact geometry.. the rubble pile is
5x5x3 meters in this model, and I think the transmitter was at one
corner and the receiver is buried near the opposite corner)
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