5 ms·

Incidentally, GPS receivers are typically sensitive down to around -160dBm, or 0.1aW.

by zb 12y ago

mikeash 12y ago
A really fun GPS fact is that the signal is so weak by the time it reaches the surface of the Earth that it's about 1000x, or 30dB, quieter than thermal noise. Receivers still extract usable data through long-term correlation, and the fact that GPS is designed so you don't need to reliably recover every bit, or anywhere close. This, in turn, means that you can realistically use a one-bit ADC in your receiver, because that only amounts to a few more dB of loss. You're already dealing with a system built to overcome massive loss and it can tolerate a little more.
- sbierwagen 12y ago
  Additional fun fact: "[GPS receivers] being specially designed to employ anti-jam features (e.g. null steering antenna or electronically steerable antenna) to function in an environment of active or passive countermeasures" are MTCR category II dual-use missile related export-restricted components. http://www.mtcr.info/english/MTCR-April2011-Technical-Annex.pdf http://www.mtcr.info/english/MTCR-April2011-Technical-Annex.... MTCR is also why GPS receivers will refuse to provide navigation information if you're flying too high or moving too fast.
  - gonzo 12y ago
    is also why commercial GPS receivers will refuse. One can, of course, out-design all of this with software and a decent front-end.
  - gpvos 12y ago
    MTCR = Missile Technology Control Regime, an informal and voluntary partnership between 34 countries to prevent the proliferation of missile and unmanned aerial vehicle technology capable of carrying a 500 kg payload at least 300 km. https://en.wikipedia.org/wiki/Missile_Technology_Control_Regime https://en.wikipedia.org/wiki/Missile_Technology_Control_Reg...
- shubhamjain 12y ago
  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.
- zb 12y ago
  That's absolutely true of the sidebands (which contain the modulated data), but it's probably worth noting that the carrier still needs to be above the noise floor by at least around 30dBHz in order to decode anything.

Incidentally, GPS receivers are typically sensitive down to around -160dBm, or 0.1aW.

by zb 12y ago

Incidentally, GPS receivers are typically sensitive down to around -160dBm, or 0.1aW.

A really fun GPS fact is that the signal is so weak by the time it reaches the surface of the Earth that it's about 1000x, or 30dB, quieter than thermal noise. Receivers still extract usable data through long-term correlation, and the fact that GPS is designed so you don't need to reliably recover every bit, or anywhere close. This, in turn, means that you can realistically use a one-bit ADC in your receiver, because that only amounts to a few more dB of loss. You're already dealing with a system built to overcome massive loss and it can tolerate a little more.

sbierwagen 12y ago
Additional fun fact: "[GPS receivers] being specially designed to employ anti-jam features (e.g. null steering antenna or electronically steerable antenna) to function in an environment of active or passive countermeasures" are MTCR category II dual-use missile related export-restricted components. http://www.mtcr.info/english/MTCR-April2011-Technical-Annex.pdf http://www.mtcr.info/english/MTCR-April2011-Technical-Annex.... MTCR is also why GPS receivers will refuse to provide navigation information if you're flying too high or moving too fast.
- gonzo 12y ago
  is also why commercial GPS receivers will refuse. One can, of course, out-design all of this with software and a decent front-end.
- gpvos 12y ago
  MTCR = Missile Technology Control Regime, an informal and voluntary partnership between 34 countries to prevent the proliferation of missile and unmanned aerial vehicle technology capable of carrying a 500 kg payload at least 300 km. https://en.wikipedia.org/wiki/Missile_Technology_Control_Regime https://en.wikipedia.org/wiki/Missile_Technology_Control_Reg...
shubhamjain 12y ago
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.
zb 12y ago
That's absolutely true of the sidebands (which contain the modulated data), but it's probably worth noting that the carrier still needs to be above the noise floor by at least around 30dBHz in order to decode anything.