GPS unable to lock position, A-GPS not helping

[BulkAdhesive]

I make use of my Bangle.js 2's GPS feature several times a week. This is always done in conjunction with manually updating A-GPS data through the A-GPS Data Downloader App connected to the Bangle.js Gadgetbridge app. In my experience this is the only way to get a position lock before starting an activity; just leaving the watch to acquire satellites on its own never seems to work.

However on the last two occasions, despite doing the usual manual A-GPS update, it has failed to acquire a GPS lock at all. I haven't updated any firmware/apps on the watch recently and can't think of anything that might have broken it. Have there been any back-end changes that might be causing an issue?

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I've noticed the updater app showing the usual Updating A-GPS... takes ~10 seconds and A-GPS updated. touch to close. But instead of taking the claimed 10 seconds to complete (like it usually does), it seems to finish much faster. Looking at the code, data appears to be pulled from https://www.espruino.com/agps/casic.base64. That endpoint returns HTTP 200 but the response appears to be empty. I.e.

$ curl -I https://www.espruino.com/agps/casic.base64
HTTP/1.1 200 OK
Date: Tue, 16 Sep 2025 13:42:54 GMT
Server: Apache/2.4.29 (Ubuntu)
Last-Modified: Tue, 16 Sep 2025 13:10:02 GMT
Accept-Ranges: bytes
Cache-Control: max-age=0, no-cache, no-store, must-revalidate
Pragma: no-cache
Expires: Wed, 11 Jan 1984 05:00:00 GMT
Access-Control-Allow-Origin: *

Is this the source of the A-GPS data? If so, I'm assuming it should be returning something. If I'm on the right track, I've got a few more questions to ask later.

[gfwilliams]

Thanks for the report and looking into it, that was extremely helpful!

We actually download the AGPS data from https://api.smawatch.cn/epo/ble_epo_offline.bin and host it on espruino.com to avoid spamming SMA's server with every user. However it looks like their certificate expired, and so the code to get the agps failed. I've just changed it to ignore certificates and it should hopefully be back to normal!

[BulkAdhesive]

Thanks for the quick reply. I can confirm espruino.com now returns a base64 response but I haven't had a chance to test it with the watch. Presumably it's fixed.

This is my first time looking into A-GPS. I assume ble_epo_offline.bin provided by the vendor is effectively a black box. I.e. the specifics of how its produced are not publicly available. If so, what guarantees (if any) do you have that this data will be available for the foreseeable future? It would be rather unfortunate if it just went away and GPS on Bangle devices was effectively rendered useless.

[gfwilliams]

To be fair the Bangle will get a GPS lock without AGPS - it just takes a while (ideally you just leave it stationary outside for a bit).

ble_epo_offline.bin is kind of a black box - it is possible to figure out what the commands in it are - they're setting the ephemeris data. In theory it might be possible to just run a Bangle with GPS permanently on and then get the data from it and put it into that file (or even convert the files from another manufacturer such as Ublox), but the chip isn't well documented.

The GPS is the AT6558 and the ephemeris data comes from the manufacturer (I haven't been able to find the official source, hence getting it from SMA). But it should ideally be possible to find a better, more reliable source of data - potentially even one that contains data that is valid for a longer period (Ublox have some that's valid for a week).

edit: more GPS info at https://www.espruino.com/Bangle.js2+Technical#gps

[BulkAdhesive]

When I first started using my watch about a year ago, I recall attempting to acquire GPS lock with just the watch itself on a couple of occasions. Both times I was stationary, outdoors, and under a cloudless sky for at least 20 minutes. Even if it had got a lock in that time, the cost to battery life isn't worth it. Those two failures are what prompted me to always prime the watch with A-GPS before starting an activity. I should give it another go and see what happens.

Good to know about ble_epo_offline.bin. For my use case, Bangle.js GPS functionality is practically contingent on it's continued availability. If the SMA source ever goes away, and no other vendor-provided solution can be found, your suggestion of sourcing data from an actual device may have some merit.

In the meantime, it might be worth having some additional error handling in the various A-GPS acquisition apps. E.g. alerting users to an empty response from the server rather than just HTTP success. It would make diagnosing this specific problem a little easier.

[gfwilliams]

I guess ideally on my server I should have some code to alert me if there is a problem, but to be honest we've been grabbing the GPS data for almost 4 years and this is the first time there's been an issue, so it's probably not worth spending too much time on.

[BulkAdhesive]

Following up this issue after doing some testing. For reference, my process/experience was always

1. make sure watch has bluetooth connection to phone
2. start A-GPS Data Downloader App on the watch
3. start the download process (touch to start); downloading would take ~10 seconds; A-GPS updated. touch to close message displayed
4. start Run+; GPS lock acquired in ~20 seconds

Yesterday (~20hrs ago) I attempted to do the same

• watch showed the usual A-GPS updated. touch to close message (which, as we've found, doesn't necessarily confirm success; manually loaded https://www.espruino.com/agps/casic.base64 in the phone browser to confirm something was there)
• started Run+; no lock acquired in the usual ~20 second window; continued to wait (outdoors, stationary, clear sky)
• lock eventually acquired after ~18 minutes

I repeated the process, but this time used Assisted GPS Updater (AGPS) through the Gadgetbridge App Manager. I saw options to select how long I wanted the A-GPS data to be valid for, in number of days up to a week. Which is different to the usual options for choosing GNSS system (GPS, BDS, GLONASS, etc.) I chose 7 days and it seemed to update successfully. However it took ~6 minutes to obtain a lock. Again outdoors, stationary, clear sky, leading me to believe lock had been acquired without the data. Have any changes been made to assistedgps recently? I can't see any recent commits related to it.

I've tried rebooting the watch for good measure (not that I expect it to do anything), but I can't seem to get a lock in ~20 seconds like I used to be able to do. Perhaps data supplied by the vendor has somehow changed. If there's anything else you'd like me to try, let me know.

[gfwilliams]

Thanks for the update - I've just looked a bit further and the original AGPS file on the server (https://api.smawatch.cn/epo/ble_epo_offline.bin) hasn't been updated since Sept 12th. I'm chasing up with SMA to find out what's going on and whether there's a way for me to get AGPS data direct from the manufacturer.

[gfwilliams]

Please can you try now? SMA have updated their server and the file should be being updated again.

[BulkAdhesive]

After pulling A-GPS data like I usually do, I was able to get a GPS fix in ~10 seconds. So I'd say it's working again. Thanks for looking into this.

Was there anything further from SMA as to where they source the data from? There's always a chance of something like this happening again.

[gfwilliams]

Great!

as to where they source the data from?

Sadly no, no information. I contacted the maker of the GPS chip too and didn't get a response