On Sat, 15 Sep 2012 21:22:18 -0500, Jon Elson wrote:> Jim Whitby wrote: > >> I'm asking this here in the hope someone knows something about GPS >> receivers. >> >> I'm a decent tech, but not an engineer... >> >> I have a Tomtom 2535 gps. Nice unit blah blah. >> >> Some ( software? ) engineer decided to change the gps info display. It >> now contains a pic of earth with dots for satellites instead of a bar >> graph. Ok. No biggie. >> >> Except now there is a signal strength percent! Percent of what??? >> >> Full quieting? 20 db quieting? 10 db sinad? or ???? >> >> I ask because at less than about "80%" I get incomplete data ( speed, >> direction, etc ). This is with 7 or more "locked" satellites. >> >> This occurs quite often and I'm trying to figure out if its bad or >> "normal" operation. If its weak signal ( 100% is 10 db sinad ), I can >> understand it. If 80% is 80% of full quieting, then it must be broke. >> >> Does anyone have a clue what 100% is relative to? > Well, the way a GPS receiver works, there is no way to actually measure > the signal. It is below the noise of the receiver! > What they do is autocorrelate the signal with a deciphering key and if > the data passes a checksum, it is good. So, they must be measuring the > ratio of good to bad decoded data blocks.It's worse than just the signal being below the noise: the way a GPS receiver works, the received signal is always relative to the received noise: all but the most expensive GPS receivers pass the signal through a comparator (basically, or a "1 bit ADC", or a "splitter", but it's all the same thing). It works because unless there's a jammer in-band, the satellite signal is buried in noise, so the noise works to linearize the "ADC". So there is no way to measure the signal strength as such, which means there is no way to compare it against any absolute reference. And, that means that the question "100% of what?" is a good one, and (I'll bet) one that you'll not find an answer to easily. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
signal strength on a GPS
Started by ●September 15, 2012
Reply by ●September 16, 20122012-09-16
Reply by ●September 16, 20122012-09-16
On a sunny day (Sun, 16 Sep 2012 00:03:50 +0000 (UTC)) it happened Jim Whitby <james@jameswhitby.org> wrote in <k33516$h5s$1@dont-email.me>:>I'm asking this here in the hope someone knows something about GPS >receivers. > >I'm a decent tech, but not an engineer... > >I have a Tomtom 2535 gps. Nice unit blah blah. > >Some ( software? ) engineer decided to change the gps info display. It >now contains a pic of earth with dots for satellites instead of a bar >graph. Ok. No biggie. > >Except now there is a signal strength percent! Percent of what???Probably signal to noise ratio, in fact bit error rate. In GPS (the US system) all sats transmit at the same frequency, their data xored with some pseudo random code that is different for each sat. So they likely actually use the bit error rate for each satellite. That is how 'clean' the detection of each sat signal is in the total signal after comparing it its code. Google for example: IS-GPS-800B.pdf for a lot of technical details.
Reply by ●September 16, 20122012-09-16
"Jim Whitby" <james@jameswhitby.org> wrote in message news:k33516$h5s$1@dont-email.me...> I'm asking this here in the hope someone knows something about GPS > receivers. > > I'm a decent tech, but not an engineer... > > I have a Tomtom 2535 gps. Nice unit blah blah. > > Except now there is a signal strength percent! Percent of what???Percent of 1-bit correlator output vs ideal full scale output. This is a measure of SNR. Vladimir Vassilevsky DSP and Mixed Signal Consultant www.abvolt.com
Reply by ●September 16, 20122012-09-16
On Sat, 15 Sep 2012 22:17:57 -0700, MrTallyman <MrTallyman@BananaCountersRUs.org> wrote:>On Sat, 15 Sep 2012 20:02:34 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >>On Sun, 16 Sep 2012 00:03:50 +0000 (UTC), Jim Whitby >><james@jameswhitby.org> wrote: >> >>>I'm asking this here in the hope someone knows something about GPS >>>receivers. >>> >>>I'm a decent tech, but not an engineer... >>> >>>I have a Tomtom 2535 gps. Nice unit blah blah. >>> >>>Some ( software? ) engineer decided to change the gps info display. It >>>now contains a pic of earth with dots for satellites instead of a bar >>>graph. Ok. No biggie. >> >>A software engineer probably doesn't know what a dB is. > > You're an idiot.OK, explain to us what a dB is. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●September 16, 20122012-09-16
On Sun, 16 Sep 2012 08:12:38 GMT, Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote:>On a sunny day (Sun, 16 Sep 2012 00:03:50 +0000 (UTC)) it happened Jim Whitby ><james@jameswhitby.org> wrote in <k33516$h5s$1@dont-email.me>: > >>I'm asking this here in the hope someone knows something about GPS >>receivers. >> >>I'm a decent tech, but not an engineer... >> >>I have a Tomtom 2535 gps. Nice unit blah blah. >> >>Some ( software? ) engineer decided to change the gps info display. It >>now contains a pic of earth with dots for satellites instead of a bar >>graph. Ok. No biggie. >> >>Except now there is a signal strength percent! Percent of what??? > >Probably signal to noise ratio, in fact bit error rate. >In GPS (the US system) all sats transmit at the same frequency, >their data xored with some pseudo random code that is different for each sat.If you use a 4-quadrant (Gilbert cell) multiplier (mixer) instead of a XOR gate, the despread signal will definitely have a positive SNR in the 1000/50 Hz bandwidth) and the absolute signal power should be easily measurable.
Reply by ●September 16, 20122012-09-16
On a sunny day (Sun, 16 Sep 2012 19:48:12 +0300) it happened upsidedown@downunder.com wrote in <c90c58l6qps7sh5adeegdcfadjhp39jj3t@4ax.com>:>On Sun, 16 Sep 2012 08:12:38 GMT, Jan Panteltje ><pNaonStpealmtje@yahoo.com> wrote: > >>On a sunny day (Sun, 16 Sep 2012 00:03:50 +0000 (UTC)) it happened Jim Whitby >><james@jameswhitby.org> wrote in <k33516$h5s$1@dont-email.me>: >> >>>I'm asking this here in the hope someone knows something about GPS >>>receivers. >>> >>>I'm a decent tech, but not an engineer... >>> >>>I have a Tomtom 2535 gps. Nice unit blah blah. >>> >>>Some ( software? ) engineer decided to change the gps info display. It >>>now contains a pic of earth with dots for satellites instead of a bar >>>graph. Ok. No biggie. >>> >>>Except now there is a signal strength percent! Percent of what??? >> >>Probably signal to noise ratio, in fact bit error rate. >>In GPS (the US system) all sats transmit at the same frequency, >>their data xored with some pseudo random code that is different for each sat. > >If you use a 4-quadrant (Gilbert cell) multiplier (mixer) instead of a >XOR gate, the despread signal will definitely have a positive SNR in >the 1000/50 Hz bandwidth) and the absolute signal power should be >easily measurable.No, you misunderstand the GPS system. The satellites all transmit at the same frequency, their digital signals are xored before modulation with a pseudo random code, a so called 'Gold code'. This code is different for each satellite. Basically this is done with a shift register. In the receiver AFTER demodulation the digital signal is run through a similar shift register, and one by one tested for the correct Gold code (each sat has its own code). In case of noise, you do not get an exact match against the know codes, and the number of wrong bits is then a measure for the signal to noise ratio. 100% good signal, no bit errors. Weak signal lots of bit errors. http://en.wikipedia.org/wiki/GPS_signals
Reply by ●September 16, 20122012-09-16
On Sun, 16 Sep 2012 00:03:50 +0000 (UTC), Jim Whitby <james@jameswhitby.org> wrote:>I'm asking this here in the hope someone knows something about GPS >receivers. > >I'm a decent tech, but not an engineer... > >I have a Tomtom 2535 gps. Nice unit blah blah. > >Some ( software? ) engineer decided to change the gps info display. It >now contains a pic of earth with dots for satellites instead of a bar >graph. Ok. No biggie. > >Except now there is a signal strength percent! Percent of what??? > >Full quieting? 20 db quieting? 10 db sinad? or ???? > >I ask because at less than about "80%" I get incomplete data ( speed, >direction, etc ). This is with 7 or more "locked" satellites. > >This occurs quite often and I'm trying to figure out if its bad or >"normal" operation. If its weak signal ( 100% is 10 db sinad ), I can >understand it. If 80% is 80% of full quieting, then it must be broke. > >Does anyone have a clue what 100% is relative to? > >Thanks > >JimIf it's Trimble, the measurement is relative to AMU (Arbitrary Mystery Units). http://gauss.gge.unb.ca/papers.pdf/SNR.memo.pdf
Reply by ●September 17, 20122012-09-17
On Sun, 16 Sep 2012 18:45:35 GMT, Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote:>>>Probably signal to noise ratio, in fact bit error rate. >>>In GPS (the US system) all sats transmit at the same frequency, >>>their data xored with some pseudo random code that is different for each sat. >> >>If you use a 4-quadrant (Gilbert cell) multiplier (mixer) instead of a >>XOR gate, the despread signal will definitely have a positive SNR in >>the 1000/50 Hz bandwidth) and the absolute signal power should be >>easily measurable. > >No, you misunderstand the GPS system. >The satellites all transmit at the same frequency, their digital signals are >xored before modulation with a pseudo random code, a so called 'Gold code'. >This code is different for each satellite. >Basically this is done with a shift register. >In the receiver AFTER demodulation the digital signal is run through a similar >shift register, and one by one tested for the correct Gold code (each sat has its own code). >In case of noise, you do not get an exact match against the know codes, and the number of wrong >bits is then a measure for the signal to noise ratio. >100% good signal, no bit errors. >Weak signal lots of bit errors. > http://en.wikipedia.org/wiki/GPS_signalsThe GPS signal is an ordinary direct sequence spread spectrum (DSSS) signal. It can be received with a mixer/multiplier/xor gate by multiplying the received signal with the same chip clock (in this case 1.023 MHz satellite specific PRN sequence) _synchronized_ with the transmitter modulator chip clock. After the demodulator the about 1 MHz wide spread spectrum signal is despread to something about 1 kHz. At this point you could also make power measurements. After this, the 50 bit/s data signal is extracted. The shift register and bit error detection you are talking about has to do with signal acquisition _before_ the receiver chip clock generator has been synchronized with the transmitter chip clock.
Reply by ●September 17, 20122012-09-17
Jan Panteltje wrote:>Google for example: > IS-GPS-800B.pdf >for a lot of technical details.I didn't understand a damn thing in that document!
Reply by ●September 17, 20122012-09-17
Jan Panteltje wrote:>No, you misunderstand the GPS system. >The satellites all transmit at the same frequency, their digital signals are >xored before modulation with a pseudo random code, a so called 'Gold code'. >This code is different for each satellite. >Basically this is done with a shift register. >In the receiver AFTER demodulation the digital signal is run through a similar >shift register, and one by one tested for the correct Gold code (each sat has its own code). >In case of noise, you do not get an exact match against the know codes, and the number of wrong >bits is then a measure for the signal to noise ratio. >100% good signal, no bit errors. >Weak signal lots of bit errors. > http://en.wikipedia.org/wiki/GPS_signalsDo they x-mit a different freq. for military use?
