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Discussion: GPS rollover?

in: kadley; kadley > 2019-04-09

Apr 9, 2019 9:53 PM # 
Evidently Saturday was GPS Week Number Rollover Day... something like Y2K that apparently happens every 19 years in GPS math. Who knew? (Answer: probably Wyatt.)

I was just googling this morning looking for GPS issues because the TomTom in my car has been acting up the past few days and learned that, here. According to TomTom's website my GPS model is not affected by the rollover day issue, so it must(?) be something else. When I put it in satellite display mode this afternoon it appeared to be only seeing 2, which doesn't seem like quite enough...?

There appears to be satellite outage information here. Just found this and need to look more closely but it appears to be saying everything is fine now.

I haven't updated my TomTom since I bought it several years ago, so trying to do that now to see if it helps with that problem or not.
Apr 9, 2019 11:23 PM # 
I'll see how it runs tomorrow. In the morning it was okay, in the afternoon it kept dropping satellites, all on the same route and similar conditions.
Apr 10, 2019 3:55 AM # 
That's odd. Though apparently they are not geosynchronous. Each one makes 2 orbits per day. See:
So given that, you would expect to see different ones between morning and afternoon.

I updated the software and satellite location file in the TomTom and now it sees 11 satellites and is working well again.
Apr 10, 2019 4:28 AM # 
Fun gps satellite fact, their clocks are super accurate because the gps calc requires it, but they're in space so time is actually different:

"Time dilation is of practical importance. For instance, the clocks in GPS satellites experience this effect due to the reduced gravity they experience (making their clocks appear to run more quickly than those on Earth) and must therefore incorporate relativistically corrected calculations when reporting locations to users. If general relativity were not accounted for, a navigational fix based on the GPS satellites would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day.[3]"
Apr 10, 2019 10:37 AM # 
Maybe I was suffering time dilation due to the excessive speed I was running. Clocks couldn't keep up.
Apr 10, 2019 1:22 PM # 
I had a chuckle at the previous sentence on that page:
Note that it is time itself rather than the function of the clock which is affected. Both effects have been experimentally observed.[citation needed]

Or, "somebody actually saw this, but I don't remember who" ;-)

But yeah it's fun to see something I studied in physics class 40 years ago and have rarely thought about since being put to practical use.

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