How did NORAD notice Kosmos 954 was making erratic manoeuvres?

Question

In Wiki for Kosmos 954 there is:

In mid-December North American Aerospace Defense Command, which had assigned the satellite the Satellite Catalog Number 10361, noticed Kosmos 954 making erratic manoeuvres, changing the altitude of its orbit by up to 50 miles, as its Soviet operators struggled to control their failing spacecraft.

This is great mental shortcut which I don't understand. How did NORAD noticed Kosmos 954 making erratic manoeuvres? By what technical means?

This is similar quesiton to "How do you recognise an island?"

Answer 1

NORAD uses radar to track the position of satellites. When you observe a satellite a few times, you can calculate its orbit and predict where the satellite will be in the future.

When a satellite manoeuvres, it changes its orbit so it won't be at the predicted position. If your tracking system is good enough (=you need frequent observations), you can see when a satellite changes course.

Changing course is a rare occurrence for a satellite. Many satellites don't maneuver at all (or just once, to reach their desired orbit after launch). Spy satellites maneuver more often (to look at specific targets, or to make it hard to predict where/when a satellite will be overhead), but these are generally simple maneuvers. A satellite making lots of manoeuvres in a short time, or making orbit changes that don't make sense would stand out.