[time-nuts] Homemade GPS Receiver

[Dr. David Kirkby (Kirkby Microwave Ltd)]

On 26 Sep 2014 13:01, "Andrea Baldoni" <erm1eaae7 at ermione.com> wrote:
>
> On Fri, Sep 26, 2014 at 10:07:56AM +0100, Dr. David Kirkby (Kirkby
Microwave Ltd) wrote:
>
> > I don't understand the units of signal strength
> >
> > "The L1 carrier is spread over a 2 MHz bandwidth and its strength at the
> > Earth's surface is -130 dBm. Thermal noise power in the same bandwidth
is
> > -111 dBm"
> >
> > Then goes on to talk about the signal being 20 dB below the noise.
>
> Hello David.
>
> It could be because there is a "process gain" associated in demodulating a
> spread spectrum signal.
>
> Best regards,
> Andrea Baldoni

But fundamentally to say

"its strength at the Earth's surface is -130 dBm"

makes no sense, UNLESS the author is trying to say that the earth receives
a signal of -130 dBm if you add up all the powers over every square metre
of the earth,  which I doubt is the meaning.

Recovering signals below the noise is certainly possible,  but that is not
my real concern.  It does however seems as though the author is comparing a
thermal noise in Watts to something that is not well defined.

I am guessing that the -130 dBm is the power collected by a dipole or
isotropic radiator, but whatever it is, the units in the text make no sense
to me.

Dave