Why Can't I See the GPS Signal Coming Out of My GPS Network With a Spectrum Analyzer?

The GPS signal is different than standard radio signals (FM signal/AM signal, cellular, TV, two way radio, etc.). It is different mainly by the amount of power it has when received by your roof/external antenna.

If you were standing next to the satelllite, you would be able to see a pattern, which is is represented (or known) as the normalized sinc function.

Why can't you see it? The short answer is you are not standing next to the satellite. The long answer is GPS satellites are 10,898 nautical miles in space and they don't transmit with enough power for GPS signals to be as powerful as the terrestrial radio signals that you see with your spectrum analyzer.

Also, standard radio signals have a positive SNRs (signal-to-noise ratio). This means they are stronger than the noise in their band. The GPS signals have a negative SNR, i.e., they are weaker than the noise in their band. If GPS had a positive SNR, you would probably not need an antenna on your roof. It is possible you could receive it indoors without an outside antenna, just like your cell phone or AM/FM radio. But, the GPS signal has a negative SNR. By the time it makes it to the earth's surface, it is about 26dB below the noise floor of the earth. That is why an external/roof antenna is needed. The GPS signal is available, but it is hidden under all the noise.

The GPS signal can be extracted from the noise. A typical GPS receiver has approximately 40dB of processing gain. When it looks at the "noise" it can see/detect something. It does this by performing what is called a correlation of the noise against the same codes that were used to create the GPS (CDMA) signal. A close analogy would be that of a comb with some of the teeth broken out that represents the pattern. If you comb the noise with a comb that is missing the exact same teeth as the comb that created the pattern...at exactly the right instant, you will get a match.

So what is the bump that you are seeing on your spectrum analyzer? What you are seeing is the response/shape of the antenna element (it acts like a filter) and the filter response of the antenna's preamplifier. Their frequency response has a bell (Gaussian) curve shape, so that you see a bell shaped bump on your analyzer. The noise is shaped by the filter, because noise is random. It has equal power across the band, so you wind up with noise I the shape of the filter response.

The GPS signal is there, but it is 26dB below the peak of the bump down in the noise or roughly 1,024 times weaker than the power of the noise. If the GPS signal is 26dB weaker than the signal when it arrives, is the GPS signal 26dB below the noise being measured? No, not necessarily. It is an 'okay" rule of thumb, but keep in mind the exact signal level depends on the bandwidth of your system, the exact gains of all devices, their noise figures, temperature (remember noise figure changes with temperature), etc