Downlink Aircraft Parameters (DAPs) is one of Secondary Surveillance Radar (SSR) Mode S datalink functions. It enables a ground station to obtain real-time aircraft information such as selected altitude and ground speed. DAPs improves efficiency, capacity, and safety of Air Traffic Control (ATC) service. To employ DAPs for ATC service, the reliability of DAPs is important. However, some aircraft are transmitting wrong or erroneous DAPs data. Those data hinders the use of ATC service. In order to ensure the reliability of DAPs data, Electronic Navigation Research Institute (ENRI) developed a DAPs validation system. In the first phase of the development, two DAPs validation techniques are implemented. One technique is based on the methodologies developed by Massachusetts Institute of Technology (MIT) Lincoln Laboratory. The other technique uses tests to detect errors due to transmission problems. By using these techniques, we evaluated DAPs data collected at our experimental SSR mode S ground station. The results show that the proposed techniques can detect erroneous data, and enhance the reliability of DAPs. © 2013 IEEE.