Let me just short this discussion by posting the link to the new design doc.
There’s no need to speculate when we know there is more information available already.
There is also no need to complicate things. You don’t need the exact down time, just the down time you’re certain about. As soon as an occasional uptime check detects the node is down, it will simply assume that that just happened that instance. The increased frequency checking will start until the node is back up or disqualified. This should work just fine to get a good estimation that is in favor of the storagenode if they were offline for a while already when the first uptime check hit it.
When a satellite is down (or disconnected from the network) uptime checks should not take place. This seems fairly simple to implement, but as mentioned likely no longer needed in a production implementation.
It seems to me that this is a case where good enough is just fine as long as you air on the side of giving the storagenode the benefit of the doubt.
PS. I’m pretty sure the Nyquist frequency only applies to sinusoidal functions. Like sound waves. It doesn’t really apply here.