The A21 antenna is designed to help maintain tracking of GPS, GLONASS, BeiDou, Galileo, and differential correction signals in challenging environments. At times, it may be impossible to keep the antenna level away from electrical noise. A21 offers superior noise reduction with a metal base, lower profile, improved multipath mitigation, and the ability to filter out an additional 30 decibels of radio band frequencies. A21 offers superior noise rejection. The A21 is designed for use with Hemisphere GNSS Crescent® and Crescent Vector™ II receivers.