2021 GPS Accuracy Issue Impacting Some Garmin, Suunto, other GPS Devices

Your error is in assuming every week is uniquely identified as a week-year, but that's only true in ISO week numbering.

In ISO, every week-year is exclusive and whichever week contains January 4th is the first of the year, so 2020-12-28/2021-01-03 is 2020-W53 and 2021-01-04/2021-01-10 is 2021-W01

But in the US whichever week contains Jan 1st is the first… and whichever week contains December 31st is the last of the previous year. Meaning the same week can be both X-W53 and X+1-W01 (aka the US system has overlapping / partial weeks at both start and end of year).

This is exactly the case this year (and most years, really), the week from December 27th (Sunday) to January 2nd (Saturday) contains both January 1st and December 31st, meaning it's both the last week of 2020 and the first week of 2021.

The parent comment is primarily about the non-internet case becoming uncommon enough that it isn't tested (or tested as effectively) as the internet case.

The "first" point here gives some very interesting details on the various timing of data necessary for a fix. But then it confusingly makes the claim that "the idea that GPS chip manufacturers have already forgot how to do offline cold starts is a bit silly". This claim doesn't appear to follow from the details about the offline cold starts. Perhaps some information about when these various methods of offline cold starts might help support the claim being made, but even then we'd have to keep in mind that those introducing new offline cold start methods aren't all gps manufs. We should also be careful not to discount offline warm starts or offline semi-warm starts. (iow: handling various pieces of information necessary for bootstrapping and effectively using all components possible without, say, requiring a full almanac download if a partial had been obtained previously).

The parent comment doesn't make a specific statement about what type of timing for fix when offline (either cold or warm) is reasonable. This makes it a bit funny to talk about the minimums/etc for offline cold start at all. The general thrust is that offline has become the uncommon case.

I would similarly be more cautious in the "second" point here for that reason: because of the proliferation of GPS into cell phones, I would be unsurprised if the majority of GPS devices were in devices that are "online". The GPS in those devices is primarily used for fine position information. While it's true that it can be used when the gross position estimation fails (due to lack of wifi signals or cell towers), because of the additional power consumption of the GPS radio it is less likely to be used for gross position. I would hesitate to describe this as a "prime scenario".

On the "third" point, I think we're ignoring that it's fairly easy to break offline cold starts (or make them very bad) without necessarily breaking ongoing ephemerides data. Certainly, it's very possible to break both, (given that the data is separate subframes), but one could imagine something breaking almanac assembly (for example), without breaking ephemerides subframe reception/decode/handling/etc.

I agree with the general point you're making, but this is wrong. The main benefit to getting ephemerides data is to get reasonably up-to-date orbital paths, you don't need it every 30 seconds.

The GPS device needs to project future paths anyway up to 30 seconds into the future even in the best-case scenario, so it's already making predictions based on past data.

The devices in question were already caching this data for days or weeks. It only became a problem when the server-side data was wrongly generated for whatever reason.

If there is absolutely no data cached in the device, it requires the almanac. Basically a map of all satellites in the constellation. Takes roughly 15 minutes because satellites transmit data really slow. This data is valid for a couple of months so a device can cache it a long time.

If the almanac is present, it provices the coarse information. The device still requires precise status of the satellites it is receiving signal from. Each satellite transmits that within something like 30 seconds. So it would take 30 seconds for a precise fix.

If the device knows everything about the constellation, can process the satellite signal immediately and pinpoint the location. If you are using a mobile phone, it should always be in this state because the required data is provided fast via internet.

In case of other devices like offline watches, the almanac might be synchronised from time to time when you connect it to your phone or computer. And the fix would take up to 30 seconds.