In Return of the Jedi, upon jumping into the Endor system, this exchange happens on the Millennium 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.