Differential GPS offers an ideal possibility for real-time avionics applications. However, two serious difficulties are faced in practical operation: interruptions in the data linking between reference station and airplane and malfunctions of the GPS-receiver due to interfering signals. The paper deals with the second problem, which becomes especially impor¬tant in an airport environment, where many transmitters operate. A particular influence on GPS-receivers have the airport radars. In extreme cases the receiver is not able to operate at all. But even if the receiver seems to operate properly, the measurements can be seriously dis¬torted. The paper describes a test with modem geodetic receivers in the vicinity of an airport radar. The theoretical background concerning the receiver operation (C/A-code-ambiguities, meas-urement noise, gross errors) and the OTF algorithm is given. A detailed data analysis shows up to what extend the radar affects the receiver performance and the precision of the code-DGPS and how it causes the OTF algorithm to fail.