In Return of the Jedi, upon jumping into the Endor system, this exchange happens on the Millenium Falcon:
Lando: We've got to be able to get some kind of a reading on that shield, up or down.
Nien Nunb: [incomprehensible]
Lando: Well, how could they be jamming us... if they don't know if we're coming...
Lando (over com): Break off the attack! The shield is still up.
Wedge: I get no reading. Are you sure?
Lando: Pull up! All craft pull up!
(You can view the exchange here.)
It's clear that the Empire is using some kind of signal jamming to prevent them from taking any reading on whether the shield (projected from Endor) is active or not. But a little while later, immediately following the ground force team's success, all that's said in the space battle is:
Ackbar: The shield is down! Commence attack on the Death Star's main reactor!
Lando: We're on our way. Red group, Gold group, all fighters follow me. [laughs] I told you they'd do it!
But if the Imperial forces were jamming their sensor readings, how did they know the shield had come down? Was the Empire no longer broadcasting a jamming signal at this point? Why did they stop, if so?
I'm interested in both canon and Legends, even if canon overrides Legends.