4 ms·

> surprisingly GPS signals come from satellites. AFAIK there are about 20-40 GPS satellites. Let's say each covers at least 1/50 of the Earth's surface area,

by csense 13y ago

> surprisingly GPS signals come from satellites. AFAIK there are about 20-40 GPS satellites. Let's say each covers at least 1/50 of the Earth's surface area, about 500 million km^2. That's 10 million km^2 per satellite. What's the max power generation you can fit on reasonable-sized satellite that's going to be in orbit for decades? I'm guessing not more than 10 kW. So 10 kW / 10 million km^2 means you have .001 watt per km^2. GPS signals being easy to jam isn't surprising. What's surprising is that you can detect them with equipment that's small and cheap enough to fit in a cellphone!

mikeash 13y ago
Where it really gets amazing is when you discover that the GPS signal is around 20-30dB below the noise floor. That is to say, ambient noise drowns out the signal by a factor of 100-1000. Long-term correlation manages to pull out usable information from this extremely noisy signal. An interesting consequence of this is that you don't need a very fancy receiver, hardware-wise, because your software already has to be extremely sophisticated. For example, you can use a 1-bit ADC rather than trying to be more sophisticated about exactly how strong the signal is at any given moment.
- jordanthoms 13y ago
  That's crazy... I think I'll contain my frustration more now when my GPS takes a few minutes for a cold lock. It's even more crazy because with the latest GPS systems in phones, I'm able to get a fix with only a very small portion or even no sky visible.
  - mikeash 13y ago
    Interestingly, the cold lock acquisition time is usually gated by the time required to download the almanac from the satellites, rather than acquiring the actual GPS signal. The almanac (which is the stuff that basically tells you where the satellites are, so you can figure out where you are based on that) gets sent on a side channel at a very slow rate, something like 50bps, such that it takes about 30 seconds before you have all the data, and any interruption can mean it takes multiples of that. The main advantage cell phones have is the fact that they can download that data from the cell network rather than waiting for it to dribble in from the satellites directly. The ability for GPS to (sometimes) work indoors these days is indeed pretty crazy. The signals are already incredibly weak, and you'd think having a roof and walls and pipes and wiring in the way would make it impossible to read them.
    - jordanthoms 13y ago
      I guess when the signal is already that weak, some roofs etc doesn't make that much difference. I'm excited for the new GPS satellites, GPS works well already but with modern technology in the satellites it should be much better.

by csense 13y ago

mikeash 13y ago

Where it really gets amazing is when you discover that the GPS signal is around 20-30dB below the noise floor. That is to say, ambient noise drowns out the signal by a factor of 100-1000. Long-term correlation manages to pull out usable information from this extremely noisy signal. An interesting consequence of this is that you don't need a very fancy receiver, hardware-wise, because your software already has to be extremely sophisticated. For example, you can use a 1-bit ADC rather than trying to be more sophisticated about exactly how strong the signal is at any given moment.

jordanthoms 13y ago
That's crazy... I think I'll contain my frustration more now when my GPS takes a few minutes for a cold lock. It's even more crazy because with the latest GPS systems in phones, I'm able to get a fix with only a very small portion or even no sky visible.
- mikeash 13y ago
  Interestingly, the cold lock acquisition time is usually gated by the time required to download the almanac from the satellites, rather than acquiring the actual GPS signal. The almanac (which is the stuff that basically tells you where the satellites are, so you can figure out where you are based on that) gets sent on a side channel at a very slow rate, something like 50bps, such that it takes about 30 seconds before you have all the data, and any interruption can mean it takes multiples of that. The main advantage cell phones have is the fact that they can download that data from the cell network rather than waiting for it to dribble in from the satellites directly. The ability for GPS to (sometimes) work indoors these days is indeed pretty crazy. The signals are already incredibly weak, and you'd think having a roof and walls and pipes and wiring in the way would make it impossible to read them.
  - jordanthoms 13y ago
    I guess when the signal is already that weak, some roofs etc doesn't make that much difference. I'm excited for the new GPS satellites, GPS works well already but with modern technology in the satellites it should be much better.