
GPS industry rages: LightSquared 4G network would "defy" laws of physics - joelhaus
http://arstechnica.com/tech-policy/news/2011/06/how-gps-interference-could-derail-a-new-national-4g-networkgps-industry-rages-lightsquared-4g-network-would-defy-laws-of-physics.ars
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coin
One thing not mentioned is LightSquared misuse of the spectrum. The spectrum
in question is the L1 band, which was set aside for satellite communications.
These satellite transmitted signals appear weak from the ground.

A loophole in the FCC rules allows the use of ground based stations to fill-in
weak areas of satellite coverage. This is called Auxiliary Terrestrial
Component (ATC). LightSquared's plan is to use the ATC loophole to build out a
terrestrial ground-based network with very little satellite usage. They are
essentially getting valuable ground-base spectrum at the low price of
satellite spectrum.

LightSquared blaming of GPS receiver manufactures is a bit ridicules. The GPS
unit manufactures weren't expecting out-of-spec usage of the neighboring L1
band. The L1 band was set aside for satellite signals, not terrestrial.
Somehow none of the press is pointing this out.

Here's a good write-up [http://www.directionsmag.com/articles/whats-the-deal-
with-th...](http://www.directionsmag.com/articles/whats-the-deal-with-the-
lightsquared-gps-clash/179780)

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jevinskie
You can find the report that LightSquared submitted in the right-hand column
of the page at [0]. I'm reading it now to see if their suggestion to use
filters (which was very vague in the linked article) really has merit. The
"Save our GPS" consortium surely doesn't think it does.

[0] <http://www.pnt.gov/interference/lightsquared/>

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thaumaturgy
I'm reading the Recommendations portion of the report. It is surprisingly
light on technical details. LightSquared's claim seems to be that there is
only a conflict on the "edge" of one of their licensed signal bands, primarily
because a small subset of GPS devices (mostly survey, mining, etc. use
devices) are improperly accepting signal in adjacent bands, either
accidentally or intentionally in order to carry a little additional custom
data. (?)

So far, after reading most of the Recommendtions report and without being
knowledgeable in this specific area, I'm inclined to side with LightSquared.
Groups advocating protection of wireless spectrum are frequently hysterical
when it comes to anything that _might_ affect their signal. (e.g., the hams'
strong and vocal objection to various proposals to carry telecom signals over
power lines.)

~~~
patrickyeon
I've glanced at the same doc, got a slightly different take from it. The
higher precision devices are actually receiving data in LightSquared's
licensed band (by having more receive bandwidth, they get better accuracy), on
purpose. This is not the GPS devices' band to use, so IMO tough luck on them.
(If you like analogies, it's like carving out a block of unassigned IPs for
yourself, and then complaining when someone is actually assigned those IPs and
wants to use them).

The one band they tested is the one that is further away, and therefore easier
for devices to reject. They claim it's not a problem for "over 99% of devices"
when they are at full power there. They concede that operation the the higher
frequency band (that LightSquared has licensed) would cause trouble for "a
significant portion of legacy devices."

The root cause of most of the problem (not the part where ultra-precision
receivers use bands that aren't theirs) is that the GPS receivers expected
things in neighbouring bands to remain quiet, when there was no guarantee that
such would happen. This assumption made for easier and cheaper designs for
them, and now all this hardware is out there that is susceptible to the
interference.

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thaumaturgy
Right. I might've done a bad job describing my take on the report, because I
agree with all that.

It's also worth noting that LightSquared is offering to provide circuitry --
which they claim to have already developed -- to any device manufacturers that
want to filter LightSquared's signal.

Regardless, I'm not yet seeing a single thing anywhere in this part of the
report that would lead me to conclude that they're proposing something that
"defies physics".

~~~
patrickyeon
The "defies physics" part would be a filter that is physically impossible to
implement. As you need sharper roll-off of your filter (more rejection of out-
of-bands, and sooner), you start making sacrifices and attenuating the signals
in your passband as well. GPS already needs very sensitive receivers, and
attenuating the signals more than they already are could make them unusable.
("Oh!" you say, "just put more amplifiers behind the filter!" Well, no, at
that point the signal is inderminable from thermal noise, which you will
amplify just the same. And you can't put one in front of the filter, because
then the LightSquared signals don't get filtered out and they interfere...)

Without seeing the specs they are being asked to hit, I can't speak to the
challenges of realizing these filters.

And just to be clear, there is no way for LightSquared to use their upper band
and not interfere with the ultra-precision stuff that's effectively squatting
there. There's no happy compromise in that situation, but as I implied
earlier, I have little sympathy for the guys who weren't playing by the rules.

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46Bit
Does anyone have any real worthwhile info of this entire issue? All I've seen
is the few incomprehensible technical arguments and then plenty of over-
popularised articles like this one. I think it's obvious that LS can't go
ahead if these problems are real, but the whole debate seems to lack much real
detail.

~~~
sbierwagen
Lightsquared has a license to broadcast in the 1525-1559 MHz band. Since their
transmitters are on the ground, the signals are powerful.

GPS is 1559—1610 MHz. These signals are very weak, since the transmitters are
in orbit.

The pissing match is that even though on _paper_ the two ranges don't overlap,
in _practice_ the Lightsquared transmissions tend to stomp all over the lower
end of the GPS range, which make various interested parties unhappy.

There's additional humor in that while it's certainly _possible_ to get around
interference like this, in practice that's dual-use technology which can be
used to evade military GPS jamming.

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blhack
If anybody was going to be outraged by this, wouldn't it be the FAA and the Us
Military?

~~~
mikeknoop
The presumption is that more expensive equipment based on GPS (aircraft
navigation, etc) have the appropriate "filters" in place to prevent
interference.

If LightSquared is to be believed, the problem is that commodity GPS makers
did not take adequate precautions against interference.

~~~
gamble
There's probably a grain of truth to both positions. Dealing with RF
interference is something of a black art, so given that LightSquared hasn't
actually deployed their network yet, it's not surprising that existing GPS
devices don't filter out those signals perfectly. At the same time,
LightSquared seems to have assumed that they could avoid interfering with the
GPS band to a greater degree than they were ultimately able to deliver.

Of course, it's also a question of money - from the GPS manufacturers'
perspective, they will have to assume all the costs while LightSquared derives
the benefits. From LightSquared's perspective, they paid to acquire their
spectrum while the GPS manufacturers get access to government infrastructure
for free and are effectively blocking access to a significant chunk of
spectrum just to reduce their per-unit costs.

