| Author |
GPS and Spectrum analyser
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| Is it possible to see the sattelites GPS signal on an ordinary Spectrum
analyser, and if not, why?
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| Sam Wormley 2005-10-16, 5:48 pm |
| RK wrote:
> Is it possible to see the sattelites GPS signal on an ordinary Spectrum
> analyser, and if not, why?
>
>
Signal us buried in the electrostatic noise of the earth.
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>
> Signal us buried in the electrostatic noise of the earth.
But still, the GPS receiver detects it? Is it because it is a burst
transmission and not continous or is it because the signals are so weak that
they are below the analysers noise floor?
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| Sam Wormley 2005-10-16, 5:48 pm |
| RK wrote:
>
>
> But still, the GPS receiver detects it? Is it because it is a burst
> transmission and not continous or is it because the signals are so weak that
> they are below the analysers noise floor?
>
>
GPS receivers are correlation receivers!
See GPS User Equipment Introduction - Sept 1996 (PDF Format)
http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf
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| Brian Morrison 2005-10-16, 5:48 pm |
| RK wrote:
>
>
> But still, the GPS receiver detects it? Is it because it is a burst
> transmission and not continous or is it because the signals are so weak that
> they are below the analysers noise floor?
>
>
The analyser cannot see them because its noise floor is higher than the
signal level of the GPS transmissions in the bandwidth the instrument
uses in its IF.
A GPS receiver can receive them because it uses a correlator to follow
the pseudo-random code of the transmission and hence it effectively
narrows the bandwidth and reduces the noise level and also creates a
processing gain related to the spreading factor.
--
Brian Morrison
please observe reply-to address
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> The analyser cannot see them because its noise floor is higher than the
> signal level of the GPS transmissions in the bandwidth the instrument uses
> in its IF.
>
> A GPS receiver can receive them because it uses a correlator to follow the
> pseudo-random code of the transmission and hence it effectively narrows
> the bandwidth and reduces the noise level and also creates a processing
> gain related to the spreading factor.
Does this tranlate into a more narrow bandwidth in the GPS receiver than
in the analyser (and so a lower noise floor) I mean, before you can
correlate anything the signal has to be recognised by the receiver or what?
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| Sam Wormley 2005-10-16, 5:48 pm |
| RK wrote:
>
>
> Does this tranlate into a more narrow bandwidth in the GPS receiver than
> in the analyser (and so a lower noise floor) I mean, before you can
> correlate anything the signal has to be recognised by the receiver or what?
>
>
See GPS User Equipment Introduction - Sept 1996 (PDF Format)
http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf
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| RK wrote:
> Is it possible to see the sattelites GPS signal on an ordinary Spectrum
> analyser, and if not, why?
>
>
Yes it is. HOWEVER, the power level is so low you WILL need a hi-gain
antenna. We used a 150 foot (as I recall, it has been 30 years) dish,
and probably some amplification.
So in real terms, I guess the answer would be no.
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| Sam Wormley 2005-10-17, 2:48 am |
| Ray wrote:
> RK wrote:
>
> Yes it is. HOWEVER, the power level is so low you WILL need a hi-gain
> antenna.
High gain antenna will amplify the noise just as much as the signal
buried in it. The way to "see" the signal is with a correlation
receiver... not a spectrum analyzer.
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| Sam wrote:
> High gain antenna will amplify the noise just as much as the signal
> buried in it.
No, the noise is coming from all around whereas the GPS signal is
coming from an essentially point source. Using a large steered
parabolic dish aimed at a particular satellite can certainly raise the
signal level above the noise floor. But as Ray indicated it's not the
most practical solution for most of us.
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| Brian Morrison 2005-10-17, 5:48 am |
| RK wrote:
>
>
> Does this tranlate into a more narrow bandwidth in the GPS receiver than
> in the analyser (and so a lower noise floor) I mean, before you can
> correlate anything the signal has to be recognised by the receiver or what?
>
>
Well essentially what the correlator does is to reduce the bandwidth of
the receiver, but the real trick is in knowing the pseudo-random code
applied to the modulation which means that it knows where to find the
next chip in the sequence and so it can maintain the narrow bandwidth
all the time, otherwise it would need to see the whole IF bandwidth and
the signal would then be below noise.
The navigation messages are sent at 50 bits/s but with a 1.023 Mbps
pseudo-random code superimposed on them, hence the bandwidth reduction
is worth about 43dB of noise reduction.
--
Brian Morrison
please observe reply-to address
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| Sam Wormley 2005-10-17, 5:48 pm |
| peter wrote:
> Sam wrote:
>
>
>
> No, the noise is coming from all around whereas the GPS signal is
> coming from an essentially point source. Using a large steered
> parabolic dish aimed at a particular satellite can certainly raise the
> signal level above the noise floor. But as Ray indicated it's not the
> most practical solution for most of us.
>
The principle is sound, but we are dealing with a very broadband, very
weak DSSS signal here and I would be very much surprised to see it in
any spectrum analyzer real time!
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|
| >> No, the noise is coming from all around whereas the GPS signal is
>
>
> The principle is sound, but we are dealing with a very broadband, very
> weak DSSS signal here and I would be very much surprised to see it in
> any spectrum analyzer real time!
>
>
Thanks everyone, I'm not familiar with the nature of the GPS signal. I'm
still a little puzzled though, how can a relatively wideband amp be used to
repeat the signal for indoor use (as I have seen some do) wouldn't an extra
amp raise the noise floor even more? And if so, what would be the best way
to repeat the signal for indoor use?
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| robertharvey@my-deja.com 2005-10-17, 5:48 pm |
| RK wrote:
> Thanks everyone, I'm not familiar with the nature of the GPS signal. I'm
> still a little puzzled though, how can a relatively wideband amp be used to
> repeat the signal for indoor use (as I have seen some do) wouldn't an extra
> amp raise the noise floor even more? And if so, what would be the best way
> to repeat the signal for indoor use?
The repeaters I have used are essentially a normal GPS antenna with a
small linear amplifier and a re-radiating antenna. The normal antenna
gives a few dB of gain because it is not an omni, more of a
cardiod-like vertical lobe. The linear gives a few db of gain,
probably no more than 6dB. But the coupling to the unit indoors is
very good, typically a new 10s of mm away.
The loss budget from the transmitter to the reciever was based on early
electronics and recent models are probably 3db better off, so there is
a small amount in hand. Then the clever nature of the demodulation
allows it to work down to very infeasible SNR (around unity), and again
modern receivers are very good at signal recovery giving a bit more
headroom.
So a technique that 20 years ago might have seemed unlikely will
succeed by using up some of the slack in the system.
Example: In strong tropical rain a fixed installation will loose
perhaps a quarter of the satellites. A re-radiator system to an indoor
unit under test will probably loose lock altogether.
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| peter 2005-10-17, 11:48 pm |
| RK wrote:
> Thanks everyone, I'm not familiar with the nature of the GPS signal.
You may want to look for some of the many descriptions of CDMA (Code
Division Multiple Access). Most will describe the applications for
cellular/PCS telephones, but the basics of spreading the signal with
codes and recovery through correlators using those codes are the same
for the GPS signals.
> I'm still a little puzzled though, how can a relatively wideband amp be used to
> repeat the signal for indoor use (as I have seen some do) wouldn't an extra
> amp raise the noise floor even more? And if so, what would be the best way
> to repeat the signal for indoor use?
The re-radiating external antennas do amplify both the signal and the
noise at that frequency. They're used because the GPS receiver indoors
may be almost completely blocked from receiving the signals by any
metal (or other absorbing) layer in the surrounding construction. So
all that the receiver would be able to see is the noise with no signal.
Placing a GPS antenna outside lets it pick up the proper signal in
addition to noise and deliver that combination inside through a coax
cable and then retransmit it from a second antenna placed close to the
GPS receiver.
So the key to their operation isn't so much the wideband amp as the
antenna placed outside the building where it can pick up the GPS
signals.
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| budgie 2005-10-18, 2:48 am |
| On 17 Oct 2005 19:12:22 -0700, "peter" <prathman@comcast.net> wrote:
>RK wrote:
>
>You may want to look for some of the many descriptions of CDMA (Code
>Division Multiple Access). Most will describe the applications for
>cellular/PCS telephones, but the basics of spreading the signal with
>codes and recovery through correlators using those codes are the same
>for the GPS signals.
>
>
>The re-radiating external antennas do amplify both the signal and the
>noise at that frequency. They're used because the GPS receiver indoors
>may be almost completely blocked from receiving the signals by any
>metal (or other absorbing) layer in the surrounding construction. So
>all that the receiver would be able to see is the noise with no signal.
> Placing a GPS antenna outside lets it pick up the proper signal in
>addition to noise and deliver that combination inside through a coax
>cable and then retransmit it from a second antenna placed close to the
>GPS receiver.
>
>So the key to their operation isn't so much the wideband amp as the
>antenna placed outside the building where it can pick up the GPS
>signals.
Equally important is the amplifier noise contribution.
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| As others have noted the GPS signal is way below the background noise (due
to temperature of the antenna, etc) but also each satellite is transmitting
on the same frequency but with its own pseudo random code which I believe is
1023 bits long.. The GPS receiver time shifts the signal until it matches a
specific code for a given satellite. Newer receivers do this in parallel so
they can detect all visible satellites at once. Truly amazing technology.
"RK" <R@J> wrote in message
news:43528058$0$3864
0$edfadb0f@dread12.news.tele.dk...
> Is it possible to see the sattelites GPS signal on an ordinary Spectrum
> analyser, and if not, why?
>
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