If all satellites are off by 13 milliseconds then you end up with all good signals but a bad position. Not the case here, but just saying.

An aircraft in inclement weather using GPS to find an airport might also have a problem with a 4km error.

I don't think so either. I also used to be a SETI@Home supporter and supplied them with over 2 years of CPU time. Then I found THE Extraterrestrial Intelligent life form and did not even need a computer or telescope. I have since dedicated my CPU time to things that are more important to me.

My SETI@Home Stats - http://setiathome.berkeley.edu/show_user.php?useri d=133483
The source of my own personal SETI discovery - http://www.hoei.com/blog/tlb/