The very low power levels associated with GNSS signals make them vulnerable to jamming, whether intentional or not. This is a particular concern for military users on missions where the availability of GNSS information is critical. Since conventional antennas are non-directional, they are not aimed at any particular point in space. Interference is received in the same way as satellite signals.
The principle behind controlled radiation pattern antennas (CRPAs) involves modifying the non-directional antenna pattern to form a dynamically controlled pattern that optimises the GNSS signal-to-noise ratio. The technique of interference rejection is when the antenna is controlled to minimise the power level of interference sources (“nulls” are introduced into the antenna pattern in the direction of the jammers).