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GPS and sprectrum spreading

senmeis

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Hi,

as far as I know the GPS signal is spectrum spreaded which means it is buried in noise. In the attachment a GPS spectrum is shown together with its signal path. There the spectrum can be seen clearly. Why? Is it the signal after the despreading?
 

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  • GPS.pdf
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kaz1

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I am not sure what your question is. GPS modulation results in spread spectrum. This does not mean it is buried in noise. It has to have enough power to be detected.
 

KlausST

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Hi,
it´s not clear what you mean..
Maybe "CDMA spread-spectrum technique"?

Did you read the GPS documents availiable in the internet?
or at least the wikipeda information?

Klaus
 

kaz1

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The GPS signal power on the ground is -157 dBW. The thermal noise power is ca. -104 dBm for 10 MHz bandwidth according to https://www.electronics-notes.com/a...al-noise-calculations-calculator-formulas.php. Can we say the GPS signal is burried in noise based on this relationship?
Your thermal noise is ok for 10MHz as ithe rule is-174dBm/Hz so for 10MHz add 10log(10^7/1) = +70 giving -104dBm.
For GPS I have seen -160dBm/Hz for centre lobe, this is higher than thermal noise for 1Hz comparison.
 

FvM

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Can we say the GPS signal is burried in noise based on this relationship?
I can agree to this view. The GPS navigation signal has a data rate of 50 bit/s, it's spread to 10 MHz by modulating a pseudo random sequence. Without knowledge of this sequence, you see a noise bandwidth of 10 MHz, a receiver synchronized to the sequence reduces the noise bandwidth.
 

senmeis

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kaz1

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Please refer to P.17 of this presentation: https://insidegnss.com/download/ins...wpdmdl=179382&refresh=5fe15a7d771bd1608604285

It indicates: received GNSS signals are 1000 times below the noise floor. OK, I must agree it has nothing to do with spreading sprectrum, but can we see the above mentioned GPS spectrum which is so small without additional techniques?
SNR can be low or high for a variety of reasons. If GPS signal is below noise I believe yes it can be recovered in a good receiver as correlation is applied to pick up the flooded signal.
 

hagster

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The original pdf is presented without any context so it's difficult to understand what points the author is making with those figures.

For GPS L1 c/a (2.046MHz BW) code the GPS signal is below the noise floor(in any resolution bandwidth you care to select)...once it reaches earth...with a low gain antenna.

Neither of the plots on the original pdf look like the expected sinc shape of the GPS waveform.
 

senmeis

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According to the block diagram in the first slide (Novatel 7000 receiver) the spectrum is taken at the post decimation stage after ADC. I’m not sure if these statements are correct:

1. The GPS spectrum will not be visible without processing (1000 times smaller than noise).

2. Only after the correlation the spectrum can be observed as above (left chart in the second slide).

3. This receiver has analog correlation (before ADC), but this is somehow strange for me.
 

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