Reported SNR of GPS Receivers
Tim Wescott
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
Jim Thompson
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
brent
> 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
>
> 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.
Tim Wescott
Given the nature of the signal, I kind of figured that would be the only
useful SNR estimate.
Rune Allnor
> 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
mi...@sushi.com
> 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
>
> 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.
Richard Owlett
sci.geo.satellite-nav
There is very good or very bad info there.
Sound Consulting
<e900b2b6-72e1-4a28...@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.
Brian Willoughby
Sound Consulting
Rune Allnor
<Sound_Consulting@_rm_Sounds.wa.com> wrote:
> In article
> <e900b2b6-72e1-4a28-9aa1-43d2508ce...@a28g2000vbo.googlegroups.com>,
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
Steve Pope
>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
Eric Jacobsen
wrote:
>On 02/02/2011 07:00 PM, brent wrote:
I don't know enough about the specifics of the GPS signals to say for
certain, but usually there are multiple ways to estimate SNR depending
on the signal and what you want to do.
The received constellation can be sliced so that the error vector is
measured from the sliced constellation point. This works well from
mid- to high-snr but when the noise starts saturating in the dynamic
range of the slicer the quality of the estimate degrades pretty
quickly.
There are a number of techniques where the incoming raw error rate can
be estimated and then compared against the matched filter bound for
that modulation type. That's often reasonably accurate across a very
useful range of SNR.
For some signalling, and I think GPS falls into this category, there
are chunks of the signal that are known apriori, either Unique Words
for framing, or chunks of known sequence used for synchronization or
channel estimation, etc. Since the sequences or UWs are known and
appear in known locations in the signal, they can be used pretty
effectively for SNR estimation if the noise statistics are stationary
enough (which they usually are, and I'd expect so for GPS).
Eric Jacobsen
http://www.ericjacobsen.org
http://www.dsprelated.com/blogs-1//Eric_Jacobsen.php
Tim Wescott
If you're doing it at the receiver you should be able to do things like
compute the variance of the amplitude of the output of the
integrate-and-dump right before dump, or like Eric said, take advantage
of known data sequences.
TheGlimmerMan
wrote:
That's all of Usenet in a nutshell.
brent
> 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
>
> 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
Here is a little more info:
In GPS world the "SNR" is discussed in terms of C/N or carrier to
noise level. Look up carrier to noise on google and you will get a
little help.
The book : Undersatanding GPS: Principles and Applications by Elliot
Kaplan (he is the credited author, but it is compilation of many
authors works) Is the definitive book on GPS when diving into the
engineering aspects of how to design GPS receivers.
I worked on some of this stuff 5 years ago or so, but my memory fails
me on the details.
Eric Jacobsen
Or the whole internet! ;)
upsid...@downunder.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.
When the GPS system was first revealed to the general public, there
was claims of systems working down to -20 dB SNR :-).
Of course, this was calculated comparing the signal power to the
thermal noise in 1.023 MHz bandwidth. However, since the actual
despread net data rate is only 50 bit/s, that -20 dB SNR would
translate to something like +23 SNR :-).
Jim Thompson
I designed a PLL data extractor that could function on a TACAN signal
at around -6dB SNR (all analog, of course ;-). That's the one that
Gardner (of PLL book fame) opined couldn't work, until I demonstrated
it to him.
I was 28 at the time, and a wee bit scared of confronting Gardner.
But I had already cleaned Larry Huelsman's clock a few years before,
proving his paper on desensitizing NIC's was a bunch of crap, so I was
experienced :-)
TheGlimmerMan
wrote:
>
>In GPS world the "SNR" is discussed in terms of C/N or carrier to
>noise level. Look up carrier to noise on google and you will get a
>little help.
That because it is from a satellite, and in the satellite world,
everything is noise, even the signal.
A valid term here is "noise floor". Then, there becomes a point at
which it invades your "good noise" so much. Then, "bit error rate"
becomes a valid term. That threshold is more important than s/n numbers.
The carrier is analog, but all of the signals are digital, heavy FEC
laden packets.
TheGlimmerMan
wrote:
>I worked on some of this stuff 5 years ago or so, but my memory fails
>me on the details.
I am currently working on our battlefield tracking and comms solutions.
http://en.wikipedia.org/wiki/Blue_Force_Tracking
Our GPS antenna is a very precisely positioned octal array. Of course
the whole system goes far deeper than the antenna.
TheGlimmerMan
wrote:
But ESPECIALLY Usenet, and usually toward the latter type.
That being the case, how much can you count on my opinion... either?
The way is that *some* of us can see the glimmers through the sewage
waterfall.
Greegor
Archie > I am currently working on our
Archie > battlefield tracking and comms solutions.
What kind of security and handlers does
your employer provide to keep their pet
sociopath from becoming a public embarassment?
Sound Consulting
<2e11c5b1-4eeb-426a...@d12g2000vbz.googlegroups.com>,
Rune Allnor <all...@tele.ntnu.no> wrote:
> 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.
> >
> > 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.
My point is that packets with errors should never be used to calculate
position, and thus any variance in position cannot be used to determine
the error rate. At least I see no reason to assume that digital errors
would cause the position to vary.
Also, I should point out that error detection is much easier than error
correction, and I assume that GPS has bullet-proof error detection. You
are correct that error correction has its limits, but error detection
should not be difficult at all.
Brian Willoughby
Sound Consulting
Rune Allnor
<Sound_Consulting@_rm_Sounds.wa.com> wrote:
> In article
> <2e11c5b1-4eeb-426a-b9c2-067a57605...@d12g2000vbz.googlegroups.com>,
I'm just speculating. I don't know error correction and I don't know
the specifics of GPS, so I might be totally lost. I am only projecting
the principles I know from other fields, where internal variables
can be used to estimate the quality of the desired computed result.
Run
Rick Lyons
<To-Email-Use-Th...@On-My-Web-Site.com> wrote:
>On Thu, 03 Feb 2011 19:50:39 +0200, upsid...@downunder.com wrote:
>
>>On Wed, 02 Feb 2011 16:20:47 -0800, 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.
>>
>>When the GPS system was first revealed to the general public, there
>>was claims of systems working down to -20 dB SNR :-).
>>
>>Of course, this was calculated comparing the signal power to the
>>thermal noise in 1.023 MHz bandwidth. However, since the actual
>>despread net data rate is only 50 bit/s, that -20 dB SNR would
>>translate to something like +23 SNR :-).
>>
>
>I designed a PLL data extractor that could function on a TACAN signal
>at around -6dB SNR (all analog, of course ;-). That's the one that
>Gardner (of PLL book fame) opined couldn't work, until I demonstrated
>it to him.
>
>I was 28 at the time, and a wee bit scared of confronting Gardner.
>
>But I had already cleaned Larry Huelsman's clock a few years before,
>proving his paper on desensitizing NIC's was a bunch of crap, so I was
>experienced :-)
>
> ...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 | |
Hi Jim,
You're in Phoenix, huh?
Not wanting to be accused of not adding something useful
to this thread, I'll add the following.
One of the scariest movies ever made was Stanley Kubrick's
'The Shining', starring Jack Nicholson as the main
character Jack Torrence. My favorite scene in the movie
was the super-weird conversation between Jack and the ghost
of Delbert Grady in the Overlook Hotel's men's room.
The interior of men's room was painted with an unbelievably
garish all-red color. A truly strange, other-worldly, room.
That men's room was modeled after a Frank Lloyd Wright-designed
bathroom in Phoenix�s famed Arizona Biltmore Hotel.
The next time I'm in Phoenix I'm gonna visit the Biltmore
Hotel to check out that men's room. (I hope Mr. Grady
doesn't show up though.)
[-Rick-]
TheGlimmerMan
wrote:
The idiot greegor can fuck off and die.
Paul Hovnanian P.E.
[snip]
>
> 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.
But in many digital systems the relationship between SNR and error rate is
nonlinear. There's a knee in the curve, beyond which the error rate rises
dramatically. Watching the SNR as it approaches this knee might be useful.
--
Paul Hovnanian pa...@hovnanian.com
----------------------------------------------------------------------
Have gnu, will travel.
Greegor
Archie > I am currently working on our
Archie > battlefield tracking and comms solutions.
G > What kind of security and handlers does
G > your employer provide to keep their pet
G > sociopath from becoming a public embarassment?
Archie > The idiot greegor can fuck off and die.
Is it a big locked gorilla cage or a pedded cell?
Greegor
Archie > battlefield tracking and comms solutions.
G > What kind of security and handlers does
G > your employer provide to keep their pet
G > sociopath from becoming a public embarassment?
Archie > The idiot greegor can fuck off and die.
G > Is it a big locked gorilla cage or a pedded cell?
Does DRS Technologies or other Defense Contractors
hire brain disorder or mental case people for defense work?
http://www.drs-ts.com/article-armycontract.htm
You do BRAT?
( What does BRAT stand for?
Beyond Line-Of-Sight Reporting and Tracking (US Army) )
It's more like you ARE a BRAT, Archie!
Then again, if you Google battlefield tracking, lots
of the hits are for computer games by Electronic Arts
and others. Is Archie living a rich fantasy life?
josephkk
wrote:
>On Wed, 02 Feb 2011 19:50:46 -0800, Tim Wescott <t...@seemywebsite.com>
GPS uses direct sequence spread spectrum (DSSS) which admits even less
than QAM and trellis coded (with a demodulation constellation) systems of
normal SNR estimation techniques. Also, like Paul Hovanian said the error
rate is very non-linear with SNR and has a distinct fall off the cliff
effect (like terrestrial and satellite digital TV). The different
chipping patterns are used to separate and distinguish the different
satellites (all visible or above the horizon satellites can all be
received at the same time on a single antenna). Also, packets ala
Ethernet do not exist. Unique Words ala LAPB are at a higher decoding
level than the base DSSS demodulation. SNR does not really make sense for
GPS signals, even error rate does not apply cleanly.