4 ms·

From reading the thread, I have a question that maybe HN knows the answer to. I feel like I haven't had to do a cold start on new GPS chips in a really long ti

From reading the thread, I have a question that maybe HN knows the answer to. I feel like I haven't had to do a cold start on new GPS chips in a really long time. Even in terrible sky conditions, I've never had to wait 12 minutes to download the almanac. Do chips just ship from the factory with this data, or is it not needed anymore. (I know about AGPS, but that's not involved. This happens on receivers with no possible way of communicating to the outside world other than the serial port I'm reading navigation messages from.) Even if I do a reset + cold start in u-center or similar, I still don't have to wait 12 minutes. I wonder why.

fh973 3y ago
It may have flash. Parts of the AGPS data are valid for weeks and may help the cold start.
deleted 3y ago
[deleted]
robinduckett 3y ago
Drone GPS chips on FPV quad copters still need significant time to acquire satellites (several minutes)
throw0101b 3y ago
> Do chips just ship from the factory with this data, or is it not needed anymore. While the general orbits of the satellites (each is uniquely identifiable) can be pre-shipped, orbits do fluctuate, so the data would still need to be updated / fine tuned. > The receiver is missing or has inaccurate estimates of its position, velocity, the time, or the visibility of any of the GPS satellites. As such, the receiver must systematically search for all possible satellites. After acquiring a satellite signal, the receiver can begin to obtain approximate information on all the other satellites, called the almanac. This almanac is transmitted repeatedly over 12.5 minutes. Almanac data can be received from any of the GPS satellites and is considered valid for up to 180 days. * https://en.wikipedia.org/wiki/Time_to_first_fix https://en.wikipedia.org/wiki/Time_to_first_fix But a general fix can do done without any initial data, and is sped up by being near the previous last fix and caching some data.
pithon 3y ago
Discarding assisted GPS where GPS data messages are obtained through about channel such as cellular or Wifi... Almanac gives coarse satellite position information (and some other stuff), good enough to know which ones are probably visible and therefore prioritize signal acquisition attempts which used to be very very costly in terms of signal processing. That's the message that takes up 12.5 minutes to piece together. Nowadays you can just brute force all possible satellite signals and there's no need to wait around for the almanac information. Each satellite signal broadcasts precise satellite location information (ephemeris), which takes maybe 30 seconds to get a few frames I believe. So that's basically the bottleneck for a modern chipset which starts with zero information and relies solely on the GPS signals to navigate.
- fsh 3y ago
  I believe this might be the correct answer. The signal from a GPS satellite is incredibly weak (way below the thermal noise of a typical amplifier) and can only be detected by correlating it with the satellite's unique gold code. In addition, the satellites move pretty fast which leads to sizeable Doppler shifts of their carrier frequencies (tens of kHz). This has to be taken into account in the signal demodulation. Classical GPS receivers use the almanac (and a reasonably accurate local clock) to determine which satellites are probably in view, and with which Doppler shifts. I would not be surprised if modern GPS chips had enough compute power to simply correlate the received signal with all gold codes and at all reasonable Doppler shifts. The almanac is then no longer necessary.
  - AstralStorm 3y ago
    Once the initial code is had, you can just store the almanac and reuse it most of the time with reasonable accuracy to make the computation faster. Usually that approach cuts the sync time by factor of ten, even if the real ephemeris diverges from the general almanac.
  - ianburrell 3y ago
    The almanac is still needed, it has the orbital positions and those change over days. I think current GPS receivers can receive from lots if not all satellites at once. Different satellites are transmitting different parts of almanac. The receiver downloads the almanac in parallel.
29083011397778 3y ago
The third and fourth requests even de-Googled (Qualcomm) phones make is to download the GPS almanac. You don't have to wait because it downloads over wifi or cellular, which have gotten pretty quick. There was a recent kerfuffle about this [0]. [0] https://blog.brixit.nl/nitrokey-dissapoints-me/ https://blog.brixit.nl/nitrokey-dissapoints-me/

fh973 3y ago

It may have flash. Parts of the AGPS data are valid for weeks and may help the cold start.

deleted 3y ago

[deleted]

robinduckett 3y ago

Drone GPS chips on FPV quad copters still need significant time to acquire satellites (several minutes)

throw0101b 3y ago

> Do chips just ship from the factory with this data, or is it not needed anymore. While the general orbits of the satellites (each is uniquely identifiable) can be pre-shipped, orbits do fluctuate, so the data would still need to be updated / fine tuned. > The receiver is missing or has inaccurate estimates of its position, velocity, the time, or the visibility of any of the GPS satellites. As such, the receiver must systematically search for all possible satellites. After acquiring a satellite signal, the receiver can begin to obtain approximate information on all the other satellites, called the almanac. This almanac is transmitted repeatedly over 12.5 minutes. Almanac data can be received from any of the GPS satellites and is considered valid for up to 180 days. * https://en.wikipedia.org/wiki/Time_to_first_fix https://en.wikipedia.org/wiki/Time_to_first_fix But a general fix can do done without any initial data, and is sped up by being near the previous last fix and caching some data.

pithon 3y ago

Discarding assisted GPS where GPS data messages are obtained through about channel such as cellular or Wifi... Almanac gives coarse satellite position information (and some other stuff), good enough to know which ones are probably visible and therefore prioritize signal acquisition attempts which used to be very very costly in terms of signal processing. That's the message that takes up 12.5 minutes to piece together. Nowadays you can just brute force all possible satellite signals and there's no need to wait around for the almanac information. Each satellite signal broadcasts precise satellite location information (ephemeris), which takes maybe 30 seconds to get a few frames I believe. So that's basically the bottleneck for a modern chipset which starts with zero information and relies solely on the GPS signals to navigate.

fsh 3y ago
I believe this might be the correct answer. The signal from a GPS satellite is incredibly weak (way below the thermal noise of a typical amplifier) and can only be detected by correlating it with the satellite's unique gold code. In addition, the satellites move pretty fast which leads to sizeable Doppler shifts of their carrier frequencies (tens of kHz). This has to be taken into account in the signal demodulation. Classical GPS receivers use the almanac (and a reasonably accurate local clock) to determine which satellites are probably in view, and with which Doppler shifts. I would not be surprised if modern GPS chips had enough compute power to simply correlate the received signal with all gold codes and at all reasonable Doppler shifts. The almanac is then no longer necessary.
- AstralStorm 3y ago
  Once the initial code is had, you can just store the almanac and reuse it most of the time with reasonable accuracy to make the computation faster. Usually that approach cuts the sync time by factor of ten, even if the real ephemeris diverges from the general almanac.
- ianburrell 3y ago
  The almanac is still needed, it has the orbital positions and those change over days. I think current GPS receivers can receive from lots if not all satellites at once. Different satellites are transmitting different parts of almanac. The receiver downloads the almanac in parallel.

29083011397778 3y ago

The third and fourth requests even de-Googled (Qualcomm) phones make is to download the GPS almanac. You don't have to wait because it downloads over wifi or cellular, which have gotten pretty quick. There was a recent kerfuffle about this [0]. [0] https://blog.brixit.nl/nitrokey-dissapoints-me/ https://blog.brixit.nl/nitrokey-dissapoints-me/