Each reindeer in flight from the North Pole is tracked. We see them on radar no matter where in the world they go.
We track them for their health and safety.
If we see something wrong near where they are flying we try to steer them in another direction. This could be anything from bad weather to rogue airplanes. There is a lot of bad stuff in the sky that we want to keep the reindeer away from.
New technology allows us to do this. For example, old technology like sleigh bells make it so you can hear reindeer whether they are high or low. But within that sleighbell now are some high tech components such as heart monitors, weather trackers, and video cameras. Nearly every part of reindeer flight is observed while reindeer are in flight.
Santa uses so many reindeer on Christmas that tracking them all requires a lot of elves.
We can track reindeer in teams or individually. We have it set up so that alarms trigger observation when it is needed. We know almost immediately what is going on with a reindeer when we need to know it.
If you thought tracking Santa was a big job, you should try tracking each individual reindeer.
It takes a lot of technology.