A heading determination system comprises an inertial measurement unit (IMU) coupled with at least two GNSS receivers, each receiver paired with and receiving signals from a corresponding GNSS antenna, wherein the GNSS antennas are separated by an ultra-short baseline. The heading determination system receives signals broadcast by a plurality of GNSS satellites and calculates the phase difference in the signal seen among the separate GNSS antennas. Using this phase difference information, derived from comparing the signals received from a plurality of GNSS satellites, along with attitude data generated by the IMU, the heading determination system calculates a highly-accurate heading solution. A method is provided for determining a heading of a system including an IMU coupled with at least two GNSS receivers, with each receiver being paired with and receiving signals from a corresponding GNSS antenna and the antennas being separated by an ultra-short baseline.