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

Bruce Rahn brahn at woh.rr.com
Thu Sep 21 22:42:36 PDT 2006

Tom Clark, K3IO wrote:
> Let me add a few comments about why I'm so concerned. The reason that
> the Galileo E6 (functionally the same as GPS L2, and overlaying the
> amateur 1260-70 MHz uplink allocation) is important for some uses is
> that it, when used in combination with the primary 1.57± GHz "L1"
> frequency (which is what all your cheap hand-held GPS receivers use),
> can be used to correct the ionospheric errors; the ionosphere adds
> upwards of 10 meters to the pseudo-range for each GPS satellite. Because
> of geometric factors (expressed quantitatively in VDOP), this can in
> turn yield errors in height of up to about 30 meters. [The WAAS and
> EGNOS signals provide some correction for these biases to the few meter
> level, but cannot be relied on during severe ionospheric storms.]
Dr. Clark,

There are several issues at play here. 

First off, the Galileo E6 is currently specified to serve "Commercial 
Services" and "Public Regulated Service".  Several messages in this 
thread have discussed "Safety of Life" concerns.  E6, as currently 
publicly stated, will not be used for aircraft navigation.  Thus, 
"Safety of Life" does not come into play when discussing the impacts 
amateur transmissions may have on E6 users.  Amateur transmissions MAY 
impact commercial and public regulated services, however, the Galileo 
consortium emphasizes that provisions are being made to mitigate 
intentional and unintentional interference to these users.

For Galileo users, ionospheric error corrections can be made using E5 
and E2/E1 (basically L1) signals.  If memory serves me, the greater 
frequency spread between E5 and E2/E1 should provide slightly more 
accurate ionospheric error corrections than would be achieved using 
either E2/E1 and E6 or E5 and E6.
> There is a lot of factual evidence that when dual-frequency geodetic GPS
> receivers (costing ~$25,000 -- hardly cheap!) have been used in
> proximity to terrestrial amateur L-band stations, the GPS performance is
> seriously degraded. I direct your attention to several reports on the topic:
>     * Must reading -- GPS/GLONASS vs L-band digipeaters (Also see GPS
>       World, Oct.2002) (warning contains numbers and equations, as well
>       as uncomplimentary comments about digipeaters)
>       http://elib.uni-stuttgart.de/opus/volltexte/1999/278/pdf/278.pdf#search=%22gps%20interference%20%20digipeater%22
>     * Amateur and Radar QRM reported at a 1999 technical meeting:
>       http://www.navcen.uscg.gov/cgsic/meetings/summaryrpts/33rdmeeting/Presentations/Weber.ppt
>     * A tutorial that shows how interfering signals can affect a spread
>       spectrum GPS rcvr (caution --  contains more
>       numbers): http://www.rin.org.uk/SITE/UPLOAD/DOCUMENT/Vuln-Owen.pdf#search=%22gps%20amateur%20interference%22
I know very well, first hand, the susceptibility of GPS to unintentional 
and intentional interference.  No arguments there.  There is a lot of 
on-going work in both the military and commercial sectors to mitigate 
this vulnerability.  An example of this is the much more robust waveform 
L5 will employ as well as higher downlink power levels (as both future 
GPS and Galileo satellites will employ).  There are other technologies 
at play too.  However, the unintentional sources of interference are not 
just limited to 'problems' caused by the amateur radio community.  There 
was a documented real-world case of a faulty Radio Shack TV antenna 
amplifier on some pleasure craft in the L.A. area (if memory serves me 
again) taking out L1 over a wide area in the mid 90's.  We won't even 
talk about the intentional, real-world, threats that are out there.  GPS 
interference mitigation is big business and technology for both the 
military and commercial sectors will be greatly improved as time marches on.

The amateur radio cases cited in the above references appear to all be 
high duty cycle emitters with radiation patterns generally aimed at the 
horizon.   I doubt our earth to space uplinks would have the same duty 
cycles and most of the intentional radiation will be directed above the 
horizon.  This reduces the probability of unintentional interference to 
GPS, Galileo, and other future systems.
> Even though US amateurs may feel that Galileo is a "European only"
> problem, read carefully Rick's (W2GPS) comments -- in his real life for
> many years he was a VP with ARINC (the people who worry about standards
> in the airline industry) and was on many FAA and ICAO committees that
> decide on airline safety.
Not all US amateurs feel this way.  All one has to do is look at the 
years of effort spent in negotiations between the US and Europeans over 
Galileo-GPS compatibility issues and spectrum reutilization.  That's why 
E5 and E1/E2 can overlap the GPS L2 and L1 allocations. 

Again, from an airline perspective, I will argue there are far more 
critical threats to GPS/Galileo signal integrity  than amateur radio 
which have to and are being addressed.  These threats cannot be 
regulated or controlled as we amateurs are.
> Also realize that the Europeans are absolutely determined to develop
> their own GNSS (Global Navigation Satellite System) call Galileo -- in
> part because they don't really trust a system that depends on the US
> Military, and in part because they see a giga-Euro business opportunity
> for the EC. Who knows how long it will survive, but the Russians have
> their competing GLONASS system. And both the Chinese and Japanese see
> that they need to enter into the GNSS race is they are to be world-class
> technical competitors.
And it will be interesting to see how this all plays out.
> We, the Eagle technical team, have never said that L-band won't work NOW
> or 5 years from now. But our vision for Eagle is that when the first one
> flies 4-5 years from now, we want it to be a useful resource for at 
> least a 10 year lifetime. We are very concerned about making a several
> million dollar (after you count the volunteer builder's blood, sweat &
> tears) investment only to have it blown away right after launch by the
> GNSS cartels just because we picked L-band to be anything like a
> "primary" uplink.
The same concerns could be expressed about ANY of the uplink or 
downlinks that are planned.  ANY of them could be taken away at any 
time.  Spectrum auctions are a wonderful cash cow to Uncle Sam and who 
knows what special interest groups our elected officials will cater to 
in the future.  However, one thing is historically proven...that if we 
do not use our amateur allocations we stand a high probability of 
loosing them.  Do we want to make it easy for the government to take 
them away?

I believe the noise level over the San Diego meeting is because many 
users feel economically threatened.  Collectively, these users have 
spent millions of dollars equipping themselves to use L-Band...to use 
S-Band...because that is what they were encouraged to do.  These are the 
same users who contributed to the construction of our satellites.  It is 
not a resistance to growth or new technology as some have tried to build 
a case for.  Rather a resistance to overnight obsolescence after being 
urged to adapt these bands, the perceived lack of strong engineering 
reasons to make such drastic changes, and a lack of input into the 
decision making process.

Be well -- Bruce

Bruce Rahn

Wisdom has two parts:
1.  having a lot to say; and
2.  not saying it!

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