The Advanced Synthetic Aperture Radar (ASAR) instrument was an active radar sensor on-board the Envisat satellite, which was operational from March 2002 until the unexpected loss of contact in April 2012. ASAR built upon the Active Microwave Instrument (AMI) carried aboard the ERS satellites.

ASAR was built by Matra Marconi Space (MMS) UK, now Airbus Defence and Space.

Applications for this sensor are many and include the study of the ocean, such as waves, sea ice extent and motion, and land surface studies, such as deforestation and ground movement.