Hi Bob,
While I do concur that Galileo interference is a "potential but currently unknown" problem, I'm approaching the issue with confidence that their design teams will find ways to handle the potential interference issues they appear to be well aware of.
Just look at their report "Galileo Interference Measurement Campaign" at: http://www.joanneum.at/index.php?id=530&L=1 and you will read about the many sources of potential interference they will need to be concerned about. In all their travels they found only one source of Amateur interference: an Amateur TV repeater. (I guess AO-51 wasn't in the L/S or L/U mode at the time of their trip!)
Not only must the Galileo system be designed to handle terrestrial interference but now that the GPS and Galileo frequencies have overlap they must handle mutual interference. I can reference reports on that issue too.
Then too - the Galileo E6 signal (pilot carrier at 1276 Mhz) has a significant null region around 1260+ Mhz. Now that's the lower end of our Amateur satellite band. P3E has L2 located there (good choice guys and gals!) and I would trust that is where you plan on locating Eagle's L-Band receiver.
By the way, the upper null falls around 1296 MHz so EME activities may also be able to co-exist.
This isn't the first time Amateurs had to deal with interference sources and surely won't be the last (i.e. power line internet), but we can't just admit defeat so early in the game - vacant our use of L-Band - and make the job of regulators who might wish to take the allocation away from us any easier than necessary.
Again, I think we can co-exist - my positive outlook. Naturally we must stay in touch with their progress and I would further suggest we establish a dialog with them early on to indicate to them we are aware of the potential for interference we could cause and try to "work with them" to find areas of mitigation.
Admittedly a tall task but surely its worthy of an attempt. In other words - try to get on their good side early in the game and let's take a pro-active approach to this potential problem!
Regards...Bill - N6GHz
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org]On Behalf Of Bob McGwier Sent: Thursday, September 21, 2006 9:16 AM To: John B. Stephensen Cc: amsat-bb@amsat.org Subject: [amsat-bb] Re: Galileo interference on L band
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@yahoo.com To: amsat-bb@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@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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-- 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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