The development of Ground Based Augmentation Systems (GBAS) to support aircraft Category II/III approach and landing operations is underway. Given improvements in GBAS ground stations, airborne multipath is apparently the largest error source affecting the accuracy and integrity of GBAS. This paper describes a program which has been designed and launched to investigate and quantify the effects of multipath from the airframe as well as from short delay ground bounce multipath during landing operations. The program includes development of an electromagnetic modeling capability which predicts amplitude, time delay and phase characteristics of airframe multipath. The program also includes the collection of a very large amount of GPS code and carrier phase measurement data recorded during flight testing of the 777-300ER and 737-NG airplanes. The flight test observations will ultimately be used to validate the modeling capability. Then, the modeling capability will be exercised to develop an Airborne Multipath Model (AMM) suitable for standardization. The paper describes the overall program, describes the electromagnetic modeling and gives details of the test setup and data collection program. This program is expected to be an important step in the development of GBAS to support CAT II/III approach and landing operations.