[time-nuts] Homemade GPS Receiver

[Stewart Cobb]

> Date: Fri, 26 Sep 2014 10:07:56 +0100
> From: "Dr. David Kirkby (Kirkby Microwave Ltd)"
>         <drkirkby at kirkbymicrowave.co.uk>
> To: Peter Putnam <peter at ni6e.com>,  Discussion of precise time and
>         frequency measurement <time-nuts at febo.com>
> Subject: Re: [time-nuts] Homemade GPS Receiver
> Message-ID:
>         <CANX10hBOnPSnop5sJtVRZCDW4rHZ_CytFcbtdP55oVyTEssGqA at mail.gmail.com>
> Content-Type: text/plain; charset=UTF-8
>
> 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.
>
> Dave

The units come from the official GPS system specification, which is
available here:

<http://www.navcen.uscg.gov/pdf/IS-GPS-200F.pdf>

Quoting section 3.3.1.6:

"3.3.1.6 User-Received Signal Levels. The SV shall provide L1 and L2
navigation signal strength at end-of-life (EOL), worst-case, in order
to meet the minimum levels specified in Table 3-V. Any combining
operation done by the SV and associated loss is compensated by an
increase in SV transmitted power and thus transparent to the user
segment. The minimum received power is measured at the output of a 3
dBi linearly polarized user receiving antenna (located near ground) at
worst normal orientation, when the SV is above a 5-degree elevation
angle. . . ."

Table 3-V gives the minimum received power level for L1 C/A code as
-158.5 dBW, equivalent to -128.5 dBm.  The old spec (some 20 years
ago) was -160 dBW, but the actual satellites were always a bit hotter
than spec, and they finally decided to just bump the spec up a bit.
Of course, the new satellites are hotter than the new spec, and the
spec actually describes a relatively poor receiving antenna.  Actual
received power levels from a well-sited high-quality antenna (which
all time-nuts should have) can be several dB higher than the spec.

So to answer your question, the original source was using somewhat
sloppy wording.  However, the actual system spec is indeed written in
terms of signal power (dBm) at the antenna feedpoint, not in terms of
field units like V/m or W/m^2.

Cheers!
--Stu