The argument of perigee is the angle, measured in the orbital plane from the ascending node, that locates an orbit's perigee. It orients the ellipse within its plane — 0° puts perigee at the equator going north, 90° puts it at the northernmost point of the orbit.
Understanding Argument of Perigee
Placing the dwell where you want it
For any eccentric orbit, the satellite spends most of each revolution near apogee. The argument of perigee is the dial that chooses which hemisphere enjoys that dwell: Russian communications planners set ω = 270° to hang apogee over northern latitudes for hours per orbit; a science mission sampling the radiation belts might instead walk ω deliberately to sweep its apogee through different latitude bands over the mission.
Watching ω in the catalogue
In a TLE, the argument of perigee is the second angle on line 2. For most LEO satellites it drifts a few degrees per day from oblateness — harmless for circular orbits. For eccentric objects, tracking its drift matters: re-entry prediction, for instance, must know where perigee sits relative to the rotating atmosphere's densest bulge, and conjunction screening cares because the geometry of an eccentric orbit's crossings changes as ω walks.