> Date: Fri, 26 Sep 2014 10:07:56 +0100 > From: "Dr. David Kirkby (Kirkby Microwave Ltd)" > <[email protected]> > To: Peter Putnam <[email protected]>, Discussion of precise time and > frequency measurement <[email protected]> > Subject: Re: [time-nuts] Homemade GPS Receiver > Message-ID: > <canx10hbonpsnop5sjtvrzcdw4rhz_cytfcbtdp55ovytess...@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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
