Does anyone happen to know at what point in the signal chain a GPS receiver measures SNR, if it reports SNR at all? I'm assuming that it'd be doing so after despreading and bit detection. Thanks. Inquiring minds want to know. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reported SNR of GPS Receivers
Started by ●February 2, 2011
Reply by ●February 2, 20112011-02-02
On Wed, 02 Feb 2011 16:20:47 -0800, Tim Wescott <tim@seemywebsite.com> wrote:>Does anyone happen to know at what point in the signal chain a GPS >receiver measures SNR, if it reports SNR at all? I'm assuming that it'd >be doing so after despreading and bit detection. > >Thanks. Inquiring minds want to know.On the ones I've designed there was an RSSI function (in the 2nd IF). I don't know if the software developed an SNR "report" from that or not. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Remember: Once you go over the hill, you pick up speed
Reply by ●February 2, 20112011-02-02
On Feb 2, 7:20�pm, Tim Wescott <t...@seemywebsite.com> wrote:> Does anyone happen to know at what point in the signal chain a GPS > receiver measures SNR, if it reports SNR at all? �I'm assuming that it'd > be doing so after despreading and bit detection. > > Thanks. �Inquiring minds want to know. > > -- > > Tim Wescott > Wescott Design Serviceshttp://www.wescottdesign.com > > Do you need to implement control loops in software? > "Applied Control Theory for Embedded Systems" was written for you. > See details athttp://www.wescottdesign.com/actfes/actfes.htmlHigher end (including aviation) GPS have SNR reporting. It is important to monitor the signal to noise ratio degradation on airplanes when other avionics are turned on. I believe the SNR calculations are done by looking at the stability of the data at the very end of the receiver chain.
Reply by ●February 2, 20112011-02-02
On 02/02/2011 07:00 PM, brent wrote:> On Feb 2, 7:20 pm, Tim Wescott<t...@seemywebsite.com> wrote: >> Does anyone happen to know at what point in the signal chain a GPS >> receiver measures SNR, if it reports SNR at all? I'm assuming that it'd >> be doing so after despreading and bit detection. >> >> Thanks. Inquiring minds want to know. >> >> -- >> >> Tim Wescott >> Wescott Design Serviceshttp://www.wescottdesign.com >> >> Do you need to implement control loops in software? >> "Applied Control Theory for Embedded Systems" was written for you. >> See details athttp://www.wescottdesign.com/actfes/actfes.html > > Higher end (including aviation) GPS have SNR reporting. It is > important to monitor the signal to noise ratio degradation on > airplanes when other avionics are turned on. > > I believe the SNR calculations are done by looking at the stability of > the data at the very end of the receiver chain.Given the nature of the signal, I kind of figured that would be the only useful SNR estimate. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●February 3, 20112011-02-03
On Feb 3, 4:50�am, Tim Wescott <t...@seemywebsite.com> wrote:> On 02/02/2011 07:00 PM, brent wrote: > > On Feb 2, 7:20 pm, Tim Wescott<t...@seemywebsite.com> �wrote: > >> Does anyone happen to know at what point in the signal chain a GPS > >> receiver measures SNR, if it reports SNR at all?...> > Higher end (including aviation) GPS have SNR reporting. �It is > > important to monitor the signal to noise ratio degradation on > > airplanes when �other avionics are turned on. > > > I believe the SNR calculations are done by looking at the stability of > > the data at the very end of the receiver chain. > > Given the nature of the signal, I kind of figured that would be the only > useful SNR estimate.I would *guess* that the variance of the reported position *formally* depends on the SNR + system-specific factors. If one were able to control the system-specific factors 'sufficiently well' one would probably be able to estimate SNR from variance in position. I have no idea what it would takes to do this - probably some sort of calibration in a controlled environment. Rune
Reply by ●February 3, 20112011-02-03
On Feb 2, 4:20�pm, Tim Wescott <t...@seemywebsite.com> wrote:> Does anyone happen to know at what point in the signal chain a GPS > receiver measures SNR, if it reports SNR at all? �I'm assuming that it'd > be doing so after despreading and bit detection. > > Thanks. �Inquiring minds want to know. > > -- > > Tim Wescott > Wescott Design Serviceshttp://www.wescottdesign.com > > Do you need to implement control loops in software? > "Applied Control Theory for Embedded Systems" was written for you. > See details athttp://www.wescottdesign.com/actfes/actfes.htmlThe SNR appears for each satellite on most GPSs, well at least what I've owned in the last decade. There are algorithms to weight the information of each bird based on SNR before determining position. The timing GPS can do this to, but most use an elevation filtering scheme rather than SNR. They ignore birds on the horizon and up to a cutoff the user selects. There is a usenet forum for GPS.
Reply by ●February 3, 20112011-02-03
miso@sushi.com wrote:> On Feb 2, 4:20 pm, Tim Wescott<t...@seemywebsite.com> wrote: >> Does anyone happen to know at what point in the signal chain a GPS >> receiver measures SNR, if it reports SNR at all? I'm assuming that it'd >> be doing so after despreading and bit detection. >> >> Thanks. Inquiring minds want to know. >> >> -- >> >> Tim Wescott >> Wescott Design Serviceshttp://www.wescottdesign.com >> >> Do you need to implement control loops in software? >> "Applied Control Theory for Embedded Systems" was written for you. >> See details athttp://www.wescottdesign.com/actfes/actfes.html > > The SNR appears for each satellite on most GPSs, well at least what > I've owned in the last decade. There are algorithms to weight the > information of each bird based on SNR before determining position. The > timing GPS can do this to, but most use an elevation filtering scheme > rather than SNR. They ignore birds on the horizon and up to a cutoff > the user selects. > > There is a usenet forum for GPS.sci.geo.satellite-nav There is very good or very bad info there.
Reply by ●February 3, 20112011-02-03
In article <e900b2b6-72e1-4a28-9aa1-43d2508ce692@a28g2000vbo.googlegroups.com>, Rune Allnor <allnor@tele.ntnu.no> wrote:> On Feb 3, 4:50�am, Tim Wescott <t...@seemywebsite.com> wrote: > > On 02/02/2011 07:00 PM, brent wrote: > > > On Feb 2, 7:20 pm, Tim Wescott<t...@seemywebsite.com> �wrote: > > >> Does anyone happen to know at what point in the signal chain a GPS > > >> receiver measures SNR, if it reports SNR at all? > ... > > > Higher end (including aviation) GPS have SNR reporting. �It is > > > important to monitor the signal to noise ratio degradation on > > > airplanes when �other avionics are turned on. > > > > > I believe the SNR calculations are done by looking at the stability of > > > the data at the very end of the receiver chain. > > I would *guess* that the variance of the reported position > *formally* depends on the SNR + system-specific factors. > If one were able to control the system-specific factors > 'sufficiently well' one would probably be able to estimate > SNR from variance in position. > > RuneIsn't the data in a digital format before it is used to determine position? I don't see how you can determine signal to noise based on errors in a digital stream - rather, you can determine error rate, but that is distinct from SNR. Also, doesn't position depend upon the combination of several inputs? It doesn't make sense to use variance in position to determine SNR when you can't really know which of the many inputs are responsible for the error. Brian Willoughby Sound Consulting
Reply by ●February 3, 20112011-02-03
On Feb 3, 6:05�am, Sound Consulting <Sound_Consulting@_rm_Sounds.wa.com> wrote:> In article > <e900b2b6-72e1-4a28-9aa1-43d2508ce...@a28g2000vbo.googlegroups.com>, > �Rune Allnor <all...@tele.ntnu.no> wrote: > > > > > > > On Feb 3, 4:50�am, Tim Wescott <t...@seemywebsite.com> wrote: > > > On 02/02/2011 07:00 PM, brent wrote: > > > > On Feb 2, 7:20 pm, Tim Wescott<t...@seemywebsite.com> �wrote: > > > >> Does anyone happen to know at what point in the signal chain a GPS > > > >> receiver measures SNR, if it reports SNR at all? > > ... > > > > Higher end (including aviation) GPS have SNR reporting. �It is > > > > important to monitor the signal to noise ratio degradation on > > > > airplanes when �other avionics are turned on. > > > > > I believe the SNR calculations are done by looking at the stability of > > > > the data at the very end of the receiver chain. > > > I would *guess* that the variance of the reported position > > *formally* depends on the SNR + system-specific factors. > > If one were able to control the system-specific factors > > 'sufficiently well' one would probably be able to estimate > > SNR from variance in position. > > > Rune > > Isn't the data in a digital format before it is used to determine > position? �I don't see how you can determine signal to noise based on > errors in a digital stream - rather, you can determine error rate, but > that is distinct from SNR. > > Also, doesn't position depend upon the combination of several inputs? � > It doesn't make sense to use variance in position to determine SNR when > you can't really know which of the many inputs are responsible for the > error.The error rates depend on the SNR of the recieved signal. If the error correction system is seen to be too busy correcting errors, it's at least an indication that the numbers in the associated data stream might be viewed with some suspicion. An error correcting system can only handle so much - there might be errors coming through if the SNR is low enough. Rune
Reply by ●February 3, 20112011-02-03
Sound Consulting <Sound_Consulting@_rm_Sounds.wa.com> wrote:>Isn't the data in a digital format before it is used to determine >position? I don't see how you can determine signal to noise based on >errors in a digital stream - rather, you can determine error rate, but >that is distinct from SNR.Usually you'd take the decoded data pattern, compute the expected constellation points, and compare those to the received signal at the demodulator. I don't recall the GPS spec having an SNR measurement requirement. Some other standards have such a beast. Steve
