[ralferoo]

> One question I would have about the comms theory is whether the amount of power being used would be reasonable for that use-case. Jamming tends to be much higher power than just communicating, but also GNSS signals are very low bandwidth as comms channels go

GPS is suprisingly low power. I believe the satellites themselves transmit between 20W and 50W, and in general the signal is quieter than the background noise threshold. It's only by correlating with the PRNG stream [1] that the data signal can be detected at all [2].

[1] The PRNG stream is 1023 bits at 1.023Mbps, so repeats every 1ms, and only autocorrelates with the correct stream when they are aligned. When the streams are not aligned, the data looks like random noise, and each transmitter has a different LFSR configuration to provide a different sequence such that each stream has a low level of correlation with another.

[2] The PRNG stream bits at 1.023Mbps are exclusive-or'd with the data stream at 50bps, so when the decoder is using the correct PRNG and sequence offset, exclusive-or'ing with that produces detectable long pulses at the expected 50bps.

[trumpdong]

FWIW this is how almost every communication system works. They're all weaker than background noise (e.g. sunlight) but you extract them by correlating with some kind of carrier signal (often but not always a sine wave)

[labcomputer]

Err what?

No, conventional radio broadcasts can be received with a low noise amplifier and a tuned filter.

The received GPS signal, at ground level, is lower than the thermal noise floor. And the 1.023MHz code is modulated on the RF carrier anyway.

[trumpdong]

> and a tuned filter

So correlating it with a sine wavelet?

[mrguyorama]

No? Old AM radio required only rectification. You can receive it with accidental diodes.

[rcxdude]

You do generally need a tuned filter before the rectification, unless you have an extremely large signal dominating the local airwaves. Which is precisely the parent poster's point: with RF you are almost always doing something to demodulate the signal. Whether you are doing it with a sine wave or something more complicated is not that fundamentally different. (and if you're looking at a spectrum analysis, that is looking at the radio signal from the point of view of that sinusoidal modulation scheme, so you will see such signals 'above' the noise floor more readily than something using a different modulation).

[ralferoo]

I'd argue that "correlation" is an accurate description of what you're doing with Gold codes - you're testing the known sequence of the output of a PRNG against the received signal, and only accepting it when the data correlates, otherwise you're adjusting the offset and trying again until you find a high correlation (strong +ve and -ve spikes) or you give up and assume there's no transmission. There's nothing in the received signal that tells you there is a real signal there at all, without correlating against every possible offset.

If you compare that to the majority of radio transmissions modulated on a sine wave carrier, there is a clear signal there and you don't need to correlate anything to tell you that, and you don't need to keep trying different offsets - you can just demodulate using a carrier of the correct frequency and the result is correct, just with a slight phase shift relative to the local carrier and which probably isn't even relevant in the frequency domain of the signal.

The key point to me is the trying repeated offsets to try to pick out a signal well below the noise floor, and choosing the offset that provides the best correlation, compared to demodulating a very strong signal that's obviously there by just adding a carrier. The latter could be done using "correlation" if you're implementing an SDR, but it doesn't have to be, and most radio hams would prefer to think of it as a simple analogue operation instead.

[trumpdong]

You also need an accidental antenna. A tuned one that preferentially receives certain sines. Or else you are receiving sunlight with a solar panel.