[amsat-bb] Re: Galileo interference on L band

Thu Sep 21 10:21:44 PDT 2006

High-accuracy receivers use multiple downlink frequencies to compensate for 
errors, such as those induced by the ionosphere. There are probably 3 
downlink frequencies in case 1 fails.

73,

John
KD6OZH

----- Original Message ----- 
From: <sco at sco-inc.com>
To: "amsat bb" <amsat-bb at amsat.org>
Sent: Thursday, September 21, 2006 16:54 UTC
Subject: [amsat-bb] Re: Galileo interference on L band

>
> I understand this argument. BUT what i do not understand is the idea
> that an airliner would only be receiving one of the three bands that
> Galileo is saying that it intends to use for GPS. The L band is in
> just one of those three bands that the airplane would be receiving.
> Would the system not require the receiver to take data from the best
> signal? Could we not ask the FAA to require any airborne receiver
> that it certify must receive all three bands and switch bands if it
> receives any interference?
>
> As a practical matter G GPS is not now in orbit, it would take years
> before it could be funded, launched and be operational. Then it would
> take the FAA probably another 10 years before they would certify it
> for use.  I see 10-15-20 years before Galileo might be a problem to hams.
>
> Why not fly Eagle with a backup L/S linear and use C/X for the
> digital transponder?
>
> Les
>
>
> At 12:15 PM 9/21/2006, Bob McGwier wrote:
>>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!
>>
>>Bob
>>N4HY
>>
>>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.
>> >
>> >73,
>> >
>> >John
>> >KD6OZH
>> >
>> >----- 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
>> >
>> >
>> >
>> >
>> >>John,
>> >>
>> >>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:
>> >>
>> >>http://www.southgatearc.org/articles/galileo.htm
>> >>
>> >>
>> >>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
>> >>>towards terrestrial
>> >>>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.
>> >>>
>> >>>73,
>> >>>
>> >>>John
>> >>>KD6OZH
>> >>>
>> >>>
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>> >>
>> >
>> >_______________________________________________
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>> >
>> >
>>
>>
>>--
>>Robert W. McGwier, Ph.D.
>>Center for Communications Research
>>805 Bunn Drive
>>Princeton, NJ 08540
>>(609)-924-4600
>>(sig required by employer)
>>
>>
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>
>
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