Many current high-end avionics installations utilize redundant Inertial Navigation Systems (INS) with very high quality Inertial Measurement Units (IMUs) in order to maintain high accuracy heading with integrity. Attitude and Heading Reference Systems (AHRSs) that provide less accurate heading are frequently used on smaller business and regional aircraft. With the advent of modernized GNSS, featuring multiple, compatible, interoperable constellations, GNSS-based dual-antenna heading can be computed with high accuracy and availability using real-time Integer Ambiguity Resolution (IAR) algorithms. For aircraft applications where the antenna spacing is fixed and known, the antenna baseline length constraint can be used to increase the robustness of the Real-Time Kinematic (RTK) baseline solution from which heading may be computed via projection to the local level navigation frame. A GNSS-based heading measurement, in a properly architected system using AHRS, can be used to either augment or replace INSs yielding a substantial reduction in overall avionics cost. Other operational benefits of GNSS-based heading include being able to replace magnetic flux detectors with a more accurate system that also works well in high latitude operations. In this paper the potential benefits and technical challenges of using GNSS-based dual-antenna heading to enable low-cost AHRS solutions for various aviation applications will be studied. Detailed simulations of the GNSS-based dual-antenna performance are conducted and presented. An Integer Least Squares IAR with integrated baseline constraint algorithm was implemented for simulation and data post-processing analyses. Tradeoffs in terms of antenna spacing, GNSS measurement noise and GNSS constellation configurations are explored. Rockwell Collins has developed and is certifying a next-generation GNSS Engine receiver that has the dual-antenna functionality to enable heading estimation. Initial flight test results for GPS L1-only performance are provided. System architecture considerations in terms of insuring overall heading integrity when utilizing GNSS-based heading to augment independent INS units will be presented in detail. Applicability to low-cost Heads-Up Display (HUD) systems and air transport applications will be described.