[BC] Lightsquared asks for declaratory ruling

[Jonathan E. Hardis]

On Dec 21, 2011, at 12:58 PM, Tom Spencer wrote:

> What they're saying is that their system meets emission limits, so  
> they
> shouldn't be held responsible for someone else's poorly-designed  
> receivers.

Yeah, yeah, yeah.   Here's a Fourier transform problem for you:

GPS receivers work by measuring the arrival time of signals.  Roughly,  
if you want to known your location to a foot, you need to measure  
signal-arrival time to a nanosecond.  It gets complicated because  
you're listening to several satellites (information redundancy), the  
satellites are moving (Doppler shifts), the signals are continuous,  
and so on, but to an order of magnitude you're playing a nanosecond- 
level game.  At 10 meters resolution, which what vanilla GPS is meant  
to provide, we're talking 30 ns, or thereabouts.

Now, how wideband must your receiver be in order to have an  
uncertainty in time determination of 30 ns or less?  That is, for how  
wide must the receiver have flat frequency response?  Now, suppose you  
wanted to put in a brick-wall filter that attenuated a gazillion dB.   
How can you do that without introducing variations in group delay and  
phase delay across your required band?

No -- casting this as a problem of GPS receivers deliberately "looking  
into" LightSquared's spectrum is a bit of an over-simplification.

It's even more bizarre because many high-precision GPS receivers use  
additional signals sent via geostationary satellites in LightSquared's  
spectrum.  LightSquared and Intelsat sell that service!  The contracts  
allows them to move the frequency around within the spectrum block, so  
the receivers had to be built to capture the signal wherever it  
happens to be.

      - Jonathan