I just found an excellent article that I wish I had seen before I wrote my previous blog, 50/50. It was created by a UPS pilot, who is also a cyber wiz, Ken Hoke. This gives a very good explanation of the North Atlantic Track System (NATS). It also talks about the data uplink systems that came into use after the time I was flying the 747-100s at UPS in the early 90s.
North Atlantic Track System