09-30-2020, 12:21 PM
It's hugely better if you use AGPS, which pretty much every smartphone does most of the time. If you're using Mobian you can try the proof of concept python script to fetch the AGPS data and load it into the modem:
https://gist.github.com/alastair-dm/2632...e555fa6628
If you're using a different distro you'll need to tweak it somewhat. ModemManager (as used by Phosh) gets most of its Quectel modem support from its generic Qualcom interface, but the AGPS part of that seems not to work with this modem - at least it hasn't when I've tried it. Using the Quectel AT commands as in the script does work, and gets a much faster and more stable fix than without the assistance data.
The unassisted performance isn't good, somewhat worse than an unassisted Garmin Geko or the old OpenMoko GTA02 doing a cold start. I suspect this is largely down to the antenna required to fit in the modern smartphone hardware, but there may be an unidentified interference issue similar to the early days of the GTA02. Whatever the cause, a marginal signal to noise ratio is a problem in an unassisted cold start as any interruption in the slow collection of the data from the sat signal means restarting collection. A small difference in signal makes the difference between a lock in ~2min and not getting one at all (or at least until the sats move into an advantageous position). The AGPS sidesteps this by downloading the necessary data from the internet.
I'm surprised you only get 3 sats though - I usually see more than that, but it does vary with the sat positions especially with the buildings around here. I've sometimes had no fix in 30 minutes, and sometimes got one in <2. I've not tied an unassisted cold start with another modern phone - they don't usually have an option to disable the AGPS. You'd need to remove network access and have the GPS disabled for long enough for any stored data to expire - 4hrs for ephemeris, up to 2 weeks for almanac - to make an apples-to-apples comparison.
https://gist.github.com/alastair-dm/2632...e555fa6628
If you're using a different distro you'll need to tweak it somewhat. ModemManager (as used by Phosh) gets most of its Quectel modem support from its generic Qualcom interface, but the AGPS part of that seems not to work with this modem - at least it hasn't when I've tried it. Using the Quectel AT commands as in the script does work, and gets a much faster and more stable fix than without the assistance data.
The unassisted performance isn't good, somewhat worse than an unassisted Garmin Geko or the old OpenMoko GTA02 doing a cold start. I suspect this is largely down to the antenna required to fit in the modern smartphone hardware, but there may be an unidentified interference issue similar to the early days of the GTA02. Whatever the cause, a marginal signal to noise ratio is a problem in an unassisted cold start as any interruption in the slow collection of the data from the sat signal means restarting collection. A small difference in signal makes the difference between a lock in ~2min and not getting one at all (or at least until the sats move into an advantageous position). The AGPS sidesteps this by downloading the necessary data from the internet.
I'm surprised you only get 3 sats though - I usually see more than that, but it does vary with the sat positions especially with the buildings around here. I've sometimes had no fix in 30 minutes, and sometimes got one in <2. I've not tied an unassisted cold start with another modern phone - they don't usually have an option to disable the AGPS. You'd need to remove network access and have the GPS disabled for long enough for any stored data to expire - 4hrs for ephemeris, up to 2 weeks for almanac - to make an apples-to-apples comparison.