The National Aerospace Laboratory (NAL) has been conducting basic research on the future landing navigation system of airplanes and a proposed future unmanned reentry space vehicle, using NAL research airplane Dornier Do-228 in which a Collins NAVCORE- I GPS C/A receiver was installed. A ground-to-air data link system was developed in order to realize the real time DGPS operation which improves accuracy. In this paper, experimental evaluation test results of not only GPS stand-alone operation but also DGPS operation are discussed. Since the navigation algorithm in the NAVCORE- I receiver could not be modified, this paper mainly discusses the difference of the two operations.In fixed point ground tests, ground and onboard GPS receivers were connected to the same antenna in order to avoid common errors, and both receivers output positions were compared with a reference map position. In the flight tests, approach/landing flights were conducted. Do-228 was tracked during flight by a laser tracker, which provides subsequent data on a reference aircraft's position. Onboard GPS receiver output position data was compared to the laser tracker data.From these test results, it was experimentally shown that SA error, which is added to degrade the positioning accuracy of civilian GPS users and is the largest error source, was eliminated by the DGPS operation technique. In addition, the quantitative position accuracy data of GPS/DGPS was obtained.