4 ms·

I don't have lot of knowledge of Signal processing but how is that possible? Isn't it like detecting sound of a pin drop against massive noise of loud speakers?

mnw21cam 12y ago
http://www.aholme.co.uk/GPS/Main.htm http://www.aholme.co.uk/GPS/Main.htm Basically, if you are sampling the thermal noise with a 1-bit ADC, you are measuring the times of the zero crossings. The GPS signal will affect the times of those zero crossings. The thermal noise will be random, and therefore over the long term will even out. The GPS signal is a very low bit rate (50 bits per second), so you can gather enough statistics to identify each bit.
- mikeash 12y ago
  The 50bps signal is a side channel that tells the receiver where the satellites are. The main signal is actually slightly over 1Mbps. Decoding that one is really interesting. I won't claim to fully understand it, but my weak understanding is that it's not really 1Mbps of unique data, but rather a long-term pattern that's known to all parties. All the receiver needs to know is the time offset of the pattern, which it can do by listening for a long time and seeing where the small biases line up with the known pattern. In terms of listening to a pin drop against loud speakers, imagine that the pin is being played by a drummer and you know what beat he's playing, and your job is just to figure out what part of the song he's playing at the moment. If you listen long enough, the very small bias he applies to the extremely loud noise can be teased out, because you know what you're listening for, you just don't know exactly how it lines up in time. Do this with several drummers playing the same beat, and you'll get different time offsets for different drummers. Given at least four drummers (satellites), and you can take those differences and the known locations of the drummers (satellites) and figure out where you are.

mnw21cam 12y ago

http://www.aholme.co.uk/GPS/Main.htm http://www.aholme.co.uk/GPS/Main.htm Basically, if you are sampling the thermal noise with a 1-bit ADC, you are measuring the times of the zero crossings. The GPS signal will affect the times of those zero crossings. The thermal noise will be random, and therefore over the long term will even out. The GPS signal is a very low bit rate (50 bits per second), so you can gather enough statistics to identify each bit.

mikeash 12y ago
The 50bps signal is a side channel that tells the receiver where the satellites are. The main signal is actually slightly over 1Mbps. Decoding that one is really interesting. I won't claim to fully understand it, but my weak understanding is that it's not really 1Mbps of unique data, but rather a long-term pattern that's known to all parties. All the receiver needs to know is the time offset of the pattern, which it can do by listening for a long time and seeing where the small biases line up with the known pattern. In terms of listening to a pin drop against loud speakers, imagine that the pin is being played by a drummer and you know what beat he's playing, and your job is just to figure out what part of the song he's playing at the moment. If you listen long enough, the very small bias he applies to the extremely loud noise can be teased out, because you know what you're listening for, you just don't know exactly how it lines up in time. Do this with several drummers playing the same beat, and you'll get different time offsets for different drummers. Given at least four drummers (satellites), and you can take those differences and the known locations of the drummers (satellites) and figure out where you are.