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Recent twts in reply to #iefub6q

One thing I’ve learned from locking down my Android phone (see #pknsrda):

The data for assisted GPS does not come from Google or, better yet, A PUBLIC SERVICE, but from a server hosted by the hardware manufacturer. Without regularly fetching fresh A-GPS data, the GPS performance is much worse (https://en.wikipedia.org/wiki/Assisted_GNSS).

This means that the hardware manufacturer has (more or less) direct control over whether I’m able to use GPS or not. This isn’t an Android setting, it’s buried deep within the device, no way to change the URL. If that manufacturer decides one day to cut me off, for whatever reason, or goes bankrupt or whatever, then I’ll have to buy a new phone.

And of course, this data transfer is encrypted as well, so I don’t know what my phone sends to those servers.

All this smartphone business is such a clusterfuck. I should have never bought one of those things.

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@movq@www.uninformativ.de Surely it isn’t as bad as this though right? 🤔 I mean reading the Wikipedia page on A-GPS, it sounds to me like it sends enough data to the nearest tower to give you enough data to warm-up the GPS receivership, presumably with GPS coordinates of the tower itself? I can imagine this data payload would include the Date/Time, IEMI and possibly your SIM details and Phone number (why not). It could be worse 🤣

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I can’t imagine it’s a static URL either, it surely has to be a unicast address of some sort. Relying on a static URL sounds like an utter disaster to me 🤔 I’d have to read the open spec on this…

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@prologic@twtxt.net Hmm, have you used a GPS device 15, 20 years ago? I had one in my car. It would take a long time until it got a first “fix” of your location. That’s because it can take up to 12 minutes until you have gathered all the data directly from the satellites. These days, GPS trackers on smartphones get a fix within seconds, maybe 30 seconds tops, because they get pre-seeded with (approximated) satellite positions via A-GPS.

We also not only have the USA’s GPS these days but also other satellite systems like the EU’s Galileo or Russia’s Glonass. A-GPS helps you get “in contact” quickly with more satellites, which enhances the precision quite a lot.

So, yeah, you can use it without A-GPS. But it would be very annoying and imprecise. I bought a new phone last year and A-GPS was broken on that one (I saw no internet traffic at all), which made it basically useless, to the point where I wouldn’t want to use it at all. I sent it back and bought another model.

To my knowledge, the only way to use GPS without something like A-GPS is to have it turned on all the time, so you get regular updates directly from the satellites.

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I’ll make an experiment: I’ll keep blocking all the phone’s internet traffic and then we’ll see how bad the GPS performance will get in a couple of hours/days. 😅 (If I got it all wrong and it still works fine, that’d be great!)

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The GPS satellites transmit an almanac, a (coarse) list of all satellite positions:

https://www.e-education.psu.edu/geog862/node/1739

That’s apparently crucial for a low “time to first fix” and, as I understand it, that’s where A-GPS comes into play: Downloading this information from the satellites takes about 12.5 minutes, but downloading it via the internet (A-GPS) is much faster.

So the question is: How long is this data valid for? It’s a bit hard to find information on this … It looks like it’s valid for several weeks:

https://flysight.ca/wiki/index.php/Almanac_and_ephemeris

If true, it would mean the situation is much less dramatic than I thought. 😅 I go on a walk every couple of days and that gives the device more than enough time to download an updated almanac. So, I guess I should be fine without A-GPS if I regularly use (standard) GPS for an hour or so. 🤔

We’ll see. This might take a couple of months to find out. 😂

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@movq@www.uninformativ.de Well I used to have a handheld GPS device, probably before I lost most of my sight. I didn’t really feel that it took ~12m to get a fix, it was usually much faster. You may just find that all this A-GPS thing is all just bullshit anyway and just an excuse to collect and store your GPS location on some random web server that someone else owns 🤣

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@prologic@twtxt.net I sure hope you’re right. 😅 I’d love nothing more than not having to rely on the internet for this. 🤞

(I clearly remember sitting in my car and waiting an eternity to get a fix, though. I’d regularly start the GPS device and then continue to load up my bags/stuff into the car because it took so long. 😅 Maybe it was just a shitty device, who knows …)

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Okay, GPS performance has degraded a lot over the last few days.

  • Time to first fix is a couple of minutes now, instead of 5-30 seconds.
  • Accuracy is reduced greatly, probably because the phone can one lock on to about 6-12 satellites, this used to be around 30 satellites.

In theory and under good conditions, you need 4 satellites to get a fix. But in reality, there are rarely “good conditions”, there are always buildings, hills, or trees nearby, so you need as many satellites as you can possibly get.

It’s not completely useless (yet), but it’s not great. I think I’m gonna lift some firewall restrictions. 🫤

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