Ground based GPS antennas for differential applications may be subject to intentional or other interference signals incident at low elevation angles. Described GPS antenna systems are usable to provide an antenna pattern having a vertically-steerable null. An array of vertically spaced radiator units having omnidirectional azimuth characteristics provides a primary reception pattern. Vertically intermixed radiator units employed on a separate or shared basis provide an auxiliary reception pattern. By subtractively combining the auxiliary pattern with the primary pattern and adjusting the relative signal level of the auxiliary pattern a vertically-steerable pattern null is provided. The antenna system may include an adaptive control system responsive to an antenna output signal to derive a steering signal to adjust the relative signal level of the auxiliary pattern to steer the vertically-steerable null to provide interference suppression. Antenna systems and methods are described.