[time-nuts] Homemade GPS Receiver

Brian Lloyd brian at lloyd.aero
Fri Sep 26 10:15:57 EDT 2014

On Fri, Sep 26, 2014 at 4:07 AM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby at kirkbymicrowave.co.uk> wrote:

> On 17 Sep 2014 23:38, "Peter Putnam" <ni6e at sbcglobal.net> wrote:
> >
> > Greetings,
> >
> > The link below describes a homemade GPS receiver.
> >
> > It is presented in a detailed and elegant manner that is certain to
> appeal to this reflector's subscribers.
> >
> > Peter
> >
> >
> > http://www.aholme.co.uk/GPS/Main.htm
> 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.
> Unless the -130 dBm is over the whole surface area of the earth,  which I
> doubt, the units make no sense to me. The units of signal strength should
> be V/m, A/m or W/square metre.
> The noise power should be in Watts or dBm. So taking the difference (19 dB,
> which is approximately 20 dB) between these figures seems odd to me.

I think you may be confusing field strength with signal strength. What
comes out of the feed-line and reaches the antenna connector at the
receiver, i.e. signal strength, is indeed measured in watts and can
therefore be compared directly with thermal noise power to produce an
effective S/N.

The antenna itself subtends an "effective" area (aperture) on the surface
of a theoretical sphere where the emitter is at the center. So the units of
area found in the field strength, i.e. the m^2 in W/m^2, are canceled by
the units of the effective area of the antenna, leaving only units of power
to appear at the feedline connection.

Every unit tells a story.

Brian Lloyd
Lloyd Aviation
706 Flightline Drive
Spring Branch, TX 78070
brian at lloyd.aero
+1.210.802-8FLY (1.210.802-8359)

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