[amsat-bb] Re: Galileo interference on L band
John B. Stephensen
kd6ozh at comcast.net
Fri Sep 22 08:10:31 PDT 2006
Part of the concern about using L as the primary digital uplink is the fact
that the ground stations will be high duty cycle emitters. BPSK has a very
low crest factor and one of the uses for a 256 kbps link is streaming video,
so it will be very much like an ATV repeater. Given the equatorial orbit,
Eagle will also be closer to the horizon than previous amateur HEOs.
Even a restriction similar to the one in place for U uplinks in areas of the
U.S. (1 kW EIRP) would make high-speed uplinks unavailable.
----- Original Message -----
From: "Bruce Rahn" <brahn at woh.rr.com>
To: <K3IO at verizon.net>
Cc: "AMSAT BB" <amsat-bb at amsat.org>
Sent: Friday, September 22, 2006 05:42 UTC
Subject: [amsat-bb] Re: Galileo interference on L band
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.]
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
> * 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)
> * Amateur and Radar QRM reported at a 1999 technical meeting:
> * A tutorial that shows how interfering signals can affect a spread
> spectrum GPS rcvr (caution -- contains more
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
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
Wisdom has two parts:
1. having a lot to say; and
2. not saying it!
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