Not so fast, LightSquared!

[Robert E. Seastrom]

"Bob Bruninga " <bruninga at usna.edu> writes:

> Lightsquared can filter all they want.  Even with zero emissions in
> the GPS band, HOWEVER, that is not the problem!
>
> THe problem is front end overload of the GPS receivers in the
> presence of STRONG nearby, Light Squared transmitters who should
> never have been allowed in a weaksignal Satellite Downlink Band!!!

Precisely so (pardon the pun).  I've been doing the math for people at
work to illustrate it; this should be old hat for people here, and
oversimplifies things like transistor saturation, but the illustration
works fine just on loud/soft:

Let's say you have a LightSquared transmitter a couple of miles away,
that's running, oh, let's say 100 watts ERP.  That's +50 dBm at the
source.  You're a couple of miles away, so 100 dB free space path
loss.  Fair enough, now we're at -50 dBm at the receive antenna.  And
further, let's say you've got a pretty darned nice filter for a pocket
device that has pushing 50 dB of out of band isolation (a pipe dream?
most that I've seen do 40).  Now we're at -100 dBm signal strength
inside your receiver for the unwanted signal.

Guess what...  GPS received signal strength is around the -150 to -165
dBm level.  60 dB difference.  This is like trying to whisper at a
rock concert.

What you need is a filter with pushing 110 dB of out of band rejection
and minimal loss in-band.  Is it even possible at those frequency
differences with the technology we have today?  What are the
trade-offs?  I'm thinking an array of cavity filters and a "hand held"
GPS receiver the size of a lunch box.  Is that what we really want?

-r

PS: Someone send me first aid for a big 'ol nasty bite on my tongue
this afternoon; this morning I am going to be in a presentation that
includes a certain former head of the FCC and I expect that I will
need it.