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 Jim
signal strength on a GPS
Started by ●September 15, 2012
Reply by ●September 15, 20122012-09-15
On Sun, 16 Sep 2012 00:03:50 +0000 (UTC), Jim Whitby <james@jameswhitby.org> wrote:>Does anyone have a clue what 100% is relative to?Can I guess? 100% is full scale on some manner of RSSI (relative signal strength indication) buried inside whatever chipset is used in the TomTom 2535. I run into the same thing in cell phones and wi-fi chipsets. The relationship between receive signal level in -dBm and whatever the chipset produces is not linear. The output is usually in the form of an 8 bit number from 0 to 255. In order to convert that into signal level in -dBm, a lookup table would be needed. For cell phones, they just coarsely quantize it into 4 or 5 "bars" for the user, but produce the real signal level in -dBm in the "service mode" page. Looks like TomTom took the easy way out and just produced a percentage of full scale. I suspect that if you dive deep into the well hidden GPS diagnostic pages in your TomTom, you might find the signal levels in -dBm. Wi-Fi is more of the same. Some GPS and Wi-Fi chipsets now have an internal lookup table for the conversion. Either way, the number produced is still bogus if you have an external GPS antenna amplifier. The indicated signal level will be what appears at the receiver input, not what is at the antenna. Disclaimer: I couldn't find the chipset used and therefore could not RTFM to see if my guess is correct. You might get a more specific answer in sci.geo.satellite-nav on TomTom support site: <http://us.support.tomtom.com/app/home/locale/en_us/> or on multiple TomTom user forums. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Reply by ●September 15, 20122012-09-15
Reply by ●September 15, 20122012-09-15
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. Jon
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.A software engineer probably doesn't know 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 Sat, 15 Sep 2012 20:02:34 -0700, the renowned 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.dataBase? Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by ●September 16, 20122012-09-16
On Sat, 15 Sep 2012 18:04:12 -0700, Jeff Liebermann wrote: <snip>> > You might get a more specific answer in sci.geo.satellite-navThanks will chk there.>on TomTom > support site: <http://us.support.tomtom.com/app/home/locale/en_us/> > or on multiple TomTom user forums.Not much help there, from what I've seen. Can't get to anyone at Tomtom that might know what the answer is ( I suspect (sh)he is locked up in a Faraday cage). Agin thanks for lead to sci.geo.satellite-nav Jim
Reply by ●September 16, 20122012-09-16
On Sat, 15 Sep 2012 20:02:34 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:>A software engineer probably doesn't know what a dB is.dBm as in "don't bother me". Using Occam's Razor, I suspect your simple answer may also be the correct answer to the question. It may also have been something equally simple, such as running out of program space or running out time to code the output in dBm. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Reply by ●September 16, 20122012-09-16
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.
Reply by ●September 16, 20122012-09-16
On Sat, 15 Sep 2012 21:22:18 -0500, Jon Elson <elson@pico-systems.com> 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. > >JonThey measure timing flag packet arrival times from three satellites.
