[amsat-bb] Re: Galileo interference on L band
n4hy at idaccr.org
Thu Sep 21 09:15:34 PDT 2006
Allow me to add (AGAIN) for emphasis that the issue is the near/far
problem. Maybe I should explain this in more detail as I think John and
I are assuming that while you might not necessarily be able to calculate
the exact path loss, you had an intutive understanding for the
problem. The differences in distances are ENORMOUS. The Galileo
satellites will typically be something like the circumference of the
earth away from you and the best case is 1/2 the circumference of the earth.
The path loss from Galileo to your location is in the very best case
given the planned orbit is approximately 180 dB.
The path loss from you to your neighbor a km away is about 93 dB.
100km improves this by approximately 55 dB. That is, ANY
interference from your station given equal powers is inherently 87 dB
stronger at the ground station than Galileo at 1 km and at 100 km, it
is inherently ~30 dB stronger. Even if you factor in different powers
on the spacecraft and ground, and losses from circularity,
polarization, sidelobes, blah blah, 10^9 is a BILLION times
advantage for your signal to clobber the Galileo signal before you take
these into account. This calculation does not include any shaping of
the beam on your part so a few dB gain in the direction of the ground
station and the problem is worse. As the airplane approaches your
location from 100 km, you will overload the front end without drastic
measures taken by the manufacturers.
Please understand the engineers designing the Galileo system understand
these issues well. They will argue very strongly that the interference
sources be removed since they do not wish to notch you by 90 dB!
John B. Stephensen wrote:
>The article predicts that there may be limitations on the amateur service.
>The biggest problem is sidelobes from the antenna that can be of either
>polarization sense. A 16 kW EIRP uplink can easily generate 500 W EIRP
>sidelobes (15 dB down) within the Galileo receiver passband and, replicating
>the calculations outlined in the article, they can cause interference from
>42 km away.
>----- Original Message -----
>From: "Marc Franco" <lu6dw at yahoo.com>
>To: <amsat-bb at amsat.org>
>Sent: Wednesday, September 20, 2006 19:03 UTC
>Subject: [amsat-bb] Galileo interference on L band
>>Galileo is circularly polarized, so using the opposite
>>polarization sense will help.
>>An excellent paper on Galileo interference was written
>>by Peter Blair, G3LTF, a well known moonbounce
>>authority and outstanding engineer. The paper can be
>>found following this link:
>>73, Marc N2UO
>>--- "John B. Stephensen" <kd6ozh at comcast.net> wrote:
>>>Unfortunately, the Gaileo downlink covers 1258-1299
>>>MHz, the first satellite
>>>has been lanched and the satellites in the
>>>constellation will be on over the
>>>entire world. Our uplink antennas have sidelobes
>>>that are 10-20 dB down, so
>>>a 1 kW EIRP SSB uplink results in 10-100 W radiated
>>>receivers. A 256 kbps uplink would require 16 kW
>>>EIRP and be 0.5-1 MHz wide.
>>>P3E has a second L receiver tuned to a null in the
>>>Galileo signal (there is
>>>only one null in the 1260-1270 MHz band) but no one
>>>knows if this will help.
>>>SSB users can move to the U uplink if L is a
>>>problem. However, this only
>>>works for narrowband signals. A wideband uplink
>>>won't fit in the null and
>>>can't move down in frequency.
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>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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Robert W. McGwier, Ph.D.
Center for Communications Research
805 Bunn Drive
Princeton, NJ 08540
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