Exercise 2: sample answer

2 microseconds at about 300 metres per microsecond is roughly 600 metres of error.

The satellites carry atomic clocks because the whole system depends on every satellite broadcasting exactly the right time; a small error there spoils every receiver's result. A receiver does not need an atomic clock because it uses the signals from several satellites to solve for its own clock error as well as its position (this is why a minimum of four satellites is used for a position in three dimensions; check). That lets a cheap quartz clock in a phone do the job.