This paper presents the estimation of a 1090 MHz signal environment on an airport surface. Secondary surveillance radar and its applications share the 1030 MHz and 1090 MHz frequency bands. Many aircraft emit signals lying in the same frequency band; the consequent increase in the signal amount causes saturation and results in performance degradation. To meet aviation requirements, it is important to manage the signal amount. In this paper, we focus on the 1090 MHz signals and propose an estimation method for the signal amount on an airport surface by using a multilateration system.