Several kinds of cooperative surveillance systems are proposed for future onboard surveillance system. They exchange the surveillance information via data link for each system. The automatic dependent surveillance-broadcast, ADS-B, is one of them for aeronautical purpose. It is under standardization phase in the international civil aviation organization, ICAO. One of the candidates for ADS-B data link channel is 1090 MHz with sharing its capacity with conventional radar beacon systems. The system performance estimation for future environment is the issue for introducing this system. Especially, the signal interference is an important issue.

This paper presents the result of signal environment measurement in 1030 MHz channel, which is used for interrogations to elicit the reply in 1090 MHz channel. First, the concept of future signal environment is introduced with mode S extended squitter, and the conventional method for signal environment estimation is discussed. Secondary, the issues are discussed on the signal environment estimation using the measured results. Also, some newly observed phenomena are shown. They increase the interrogation rate around ground interrogators and the reply mode failure at onboard transponder. Finally, some important items are addressed for establishing a signal environment estimation method.