I'm not sure this article has been cited here (June 2011).

http://www.gpsworld.com/survey/test-...eceivers-11726

QUOTE

In typical high-precision GPS receivers, IF filters are wide (20 MHz)
but are still designed to block out adjacent bands while allowing GPS
signals to pass through. So why aren't these working near LightSquared
towers? The problem is that their stop-band does not roll off quickly
enough to block a signal next door that is many orders of magnitude more
powerful than GPS. Essentially, they fail due to the overwhelming power
sneaking in the stop-band of the IF. The LightSquared statement "GPS
receivers are looking onto our band" is misleading and tries to deflect
the real issue, which is the brute strength of the LightSquared signal
overpowering the GPS receiver's stop-band.

/QUOTE

QUOTE

One major reason that LightSquared has proceeded as far as it has
seemingly stems from a decision the FCC made in 2003 and one in which
LightSquared keeps referring to. In 2003, a company called Mobile
Satellite Ventures (MSV), a predecessor of LightSquared and Skyterra,
was granted permission by the FCC to erect 1,725 towers broadcasting 77
watts to supplement their satellite-to-earth communications, according
to the Coalition to Save Our GPS website. That wouldn’t necessarily be a
serious threat to GPS. But that has been expanded to 40,000 towers
broadcasting 1,500 watts each. That’s a big stretch, sort of like saying
you’ve already qualified for the Olympics because you won the
seventh-grade 100-meter dash in your physical education class.

/QUOTE

QUOTE

Historically, the L-band frequency that LightSquared is proposing to
transmit from, 1525 MHz to 1559 MHz, has been limited to low-power
satellite-to-Earth communications. What LightSquared is proposing, and
the FCC seems to be open to, is allowing high-powered terrestrial
transmissions a billion times stronger than the GPS signal to be located
next to the GPS spectrum.

/QUOTE




--
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There's been a story going around about how the GPS industry is
negligent for not having followed DoD guidlines for equipment design.
It stems from a letter submitted by Lightsquared's Jeffrey Carlisle to
the FCC.

http://fjallfoss.fcc.gov/ecfs/docume...?id=7021701967

The argument is based upon this paragraph in the Standard Positing
Service Performance (SPS) Document:

Those [SPS performance] assumptions include the use of a GPS C/A code
correlator receiver (and early–minus-late correlator with 1 chip
spacing) that uses “an exact replica of the waveform within an ideal
sharp-cutoff filter bandwidth at 24 MHz with linear phase centered at
the L1 frequency.

From which Mr. Carlisle concludes:

” In other words, the [SPS] specification is premised upon the use of
a receiver with a 24 MHz (1563.42 - 1587.42 MHz) pass band filter. To
comply with these specifications to have a 24 MHz pass band filter,
therefore, a GPS receiver would have to filter out transmissions from
the adjacent band in which LightSquared operates."

Just another twist of Lightsquared's assertion that GPS devices are
poorly engineered and cheaply made.

Well, Mr. Carlisle's argument is premised upon a cleverly worded DoD
disclaimer not an engineering specification. In engineering, an ideal
anything is a design goal that cannot be fully realized. And, of
course a bandpass filter rejects out of band power; by definition.

--- CHAS

On 2011-08-14 18:57 , HIPAR wrote:
> There's been a story going around about how the GPS industry is
> negligent for not having followed DoD guidlines for equipment design.
> It stems from a letter submitted by Lightsquared's Jeffrey Carlisle to
> the FCC.
>
> http://fjallfoss.fcc.gov/ecfs/docume...?id=7021701967
>
> The argument is based upon this paragraph in the Standard Positing
> Service Performance (SPS) Document:
>
> Those [SPS performance] assumptions include the use of a GPS C/A code
> correlator receiver (and early–minus-late correlator with 1 chip
> spacing) that uses “an exact replica of the waveform within an ideal
> sharp-cutoff filter bandwidth at 24 MHz with linear phase centered at
> the L1 frequency.
>
> From which Mr. Carlisle concludes:
>
> ” In other words, the [SPS] specification is premised upon the use of
> a receiver with a 24 MHz (1563.42 - 1587.42 MHz) pass band filter. To
> comply with these specifications to have a 24 MHz pass band filter,
> therefore, a GPS receiver would have to filter out transmissions from
> the adjacent band in which LightSquared operates."
>
> Just another twist of Lightsquared's assertion that GPS devices are
> poorly engineered and cheaply made.
>
> Well, Mr. Carlisle's argument is premised upon a cleverly worded DoD
> disclaimer not an engineering specification. In engineering, an ideal
> anything is a design goal that cannot be fully realized. And, of
> course a bandpass filter rejects out of band power; by definition.

Lightsquared have been cherry picking their statements and quotes as it
suits their purpose. That is no surprise. (Isn't there a power density
statement that goes with that as well?)

No filter is square at the extremes no matter how "sharp" it is made,
and of course any filter, no matter how sharp it is made can be swamped
by a powerful enough adjacent transmitter because the transmitter's
output filters cannot be made sharply enough (either) and will output
some energy into the adjacent spectrum. Given the high power L^2 want
to broadcast, and the high sensitivity of the GPS receivers they will
get whacked.

The third paragraph I quoted also seems to bear heavily. Were L^2
really allowed to re-purpose that spectrum?

Somebody at the FCC screwed up years ago in permitting L^2 to acquire
and re-purpose the spectrum. The FCC has egg all over its face on this
one. The government may get sued by L^2 over this. And once the courts
are into it, the outcome could be ugly (eg: "GPS users shall filter")
but hopefully it could come out as "payoff L^2".

--
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