Re: [time-nuts] Homemade GPS Receiver
> Date: Fri, 26 Sep 2014 10:07:56 +0100 > From: "Dr. David Kirkby (Kirkby Microwave Ltd)" > > To: Peter Putnam , Discussion of precise time and > frequency measurement > Subject: Re: [time-nuts] Homemade GPS Receiver > Message-ID: > > 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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
On 9/28/14, 10:09 PM, Peter Monta wrote: How hard is the beam steering relative to everything else? It's a weighted sum of the antenna signals (as with any phased array), so the cost is the extra arithmetic to do this (on a per-satellite basis). The weights can be computed open-loop from the positions of the satellites, provided you're interested only in gain and not antijam performance. The multiple antenna elements, downconverters, and ADCs add to the expense somewhat, of course. Since many GPS receivers use a single bit quantizer with direct conversion, the front end can be pretty simple. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
> How hard is the beam steering relative to everything else? > It's a weighted sum of the antenna signals (as with any phased array), so the cost is the extra arithmetic to do this (on a per-satellite basis). The weights can be computed open-loop from the positions of the satellites, provided you're interested only in gain and not antijam performance. The multiple antenna elements, downconverters, and ADCs add to the expense somewhat, of course. Cheers, Peter ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
it is relative complicated, since you need to look multiple satellites almost in the same time 73 Alex On 9/26/2014 5:12 PM, Hal Murray wrote: he...@pericynthion.org said: Since the GPS signals come from all parts of the sky this is pretty much required, unless you're using fancy beam steering techniques. How hard is the beam steering relative to everything else? ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
he...@pericynthion.org said: > Since the GPS signals come from all parts of the sky this is pretty much > required, unless you're using fancy beam steering techniques. How hard is the beam steering relative to everything else? -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
On Fri, Sep 26, 2014 at 8:25 AM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote: > As I say, without stating the properties of the receiving antenna, > absolute power levels are not a sensible unit. Indeed, there is an implicit assumption of a ~omnidirectional (hemisphere-pattern) antenna, with a typical effective gain around 1 dBi. Since the GPS signals come from all parts of the sky this is pretty much required, unless you're using fancy beam steering techniques. Perhaps slightly sloppy terminology but the meaning is clear, to me anyway :) Henry ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
On Fri, Sep 26, 2014 at 4:07 AM, Dr. David Kirkby (Kirkby Microwave Ltd) < drkir...@kirkbymicrowave.co.uk> wrote: > On 17 Sep 2014 23:38, "Peter Putnam" 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 br...@lloyd.aero +1.210.802-8FLY (1.210.802-8359) ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
On 26 September 2014 13:52, Azelio Boriani wrote: > From gpsinformation.net: > > In the frequency allocation filing the L1 C/A power is listed as 25.6 > Watts. The Antenna gain is listed at 13 dBi. Thus, based on the > frequency allocation filing, the power would be about 500 Watts (27 > dBW). > > Now, the free space path loss from 21000 km is about 182 dB. Take the > 500 Watts (27 dBW) and subtract the free space path loss (27 - 182) > and you get -155 dBW. The end of life spec is -160 dBW, which leaves > a 5 dB margin. I have not checked your figures, but assuming they are right, the -155 dBW would be based on a receiver antenna with 0 dBi gain, since that is what the free space path loss assumes. Any lossless antenna will have a peak gain of > 0 dBi. > And if you really get into it, you'll discover ALL of the following > represent the same approximate signal strength for GPS on the face of > the earth (m stands for milliwatts and m2 stands for meters squared): > > -160 dBW, -130 dBm, -135 dBW/m2, -105 dBm/m2, -223 dBW/Hz, -163 > dBW/MHz, -193 dBm/Hz, -198 dBW/m2/Hz, -138 dBW/m2/MHz As I say, without stating the properties of the receiving antenna, absolute power levels are not a sensible unit. Dave ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
It is because of the processing gain and I believe these to be normal figures. Not an expert here but when the system de-spreads the signal the information pops up above the noise since the noise is random and the spread carrier only appears random. Regards Paul WB8TSL On Fri, Sep 26, 2014 at 7:22 AM, Andrea Baldoni 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 > ___ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
On 26 Sep 2014 13:01, "Andrea Baldoni" 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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
>From gpsinformation.net: In the frequency allocation filing the L1 C/A power is listed as 25.6 Watts. The Antenna gain is listed at 13 dBi. Thus, based on the frequency allocation filing, the power would be about 500 Watts (27 dBW). Now, the free space path loss from 21000 km is about 182 dB. Take the 500 Watts (27 dBW) and subtract the free space path loss (27 - 182) and you get -155 dBW. The end of life spec is -160 dBW, which leaves a 5 dB margin. And if you really get into it, you'll discover ALL of the following represent the same approximate signal strength for GPS on the face of the earth (m stands for milliwatts and m2 stands for meters squared): -160 dBW, -130 dBm, -135 dBW/m2, -105 dBm/m2, -223 dBW/Hz, -163 dBW/MHz, -193 dBm/Hz, -198 dBW/m2/Hz, -138 dBW/m2/MHz On Fri, Sep 26, 2014 at 1:22 PM, Andrea Baldoni 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 > ___ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
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 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
On 17 Sep 2014 23:38, "Peter Putnam" 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. Dave ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Homemade GPS Receiver
Quite a project and extremely well presented and executed. Thanks for the link to a fascinating read! Didier KO4BB On September 17, 2014 5:34:38 PM CDT, Peter Putnam 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 >___ >time-nuts mailing list -- time-nuts@febo.com >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. -- Sent from my Motorola Droid Razr HD 4G LTE wireless tracker while I do other things. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
