Because of the limited availability of the spectrum allocated for navigation systems, the numerous navigation signals broadcast by Modernized GPS and Galileo system will have to be combined and employ bandwidth-efficient modulations. Indeed, the GPS modernization scheme entails the addition of the new military signal (M-code) to the established C/A and P(Y) codes at the same carrier frequency. One of the most important questions is how to combine this new signal with the legacy ones at the payload level, while maintaining good performance at reception. This problem also exists for Galileo since: - in the L1 band, the Open Service (OS) signal (two channels) and the Public Regulated Service (PRS) signal must be transmitted on the same carrier. - in the E6 band, the Commercial Service (CS) signal must be transmitted with the PRS signal. The Interplex modulation, a particular phase-shiftedkeyed/ phase modulation (PSK/PM), was chosen to transmit all these signals because it is a constant-envelope modulation, thereby allowing the use of saturated power amplifiers with limited signal distortion ([STF, 2002; Rajan and Irvine, 2005]). The main objective of this paper is to study the Interplex modulation, as it is used for the GPS and Galileo signals. In a first part we will present the Interplex modulation, its general formulation and its application for the multiplexing of three signals. Then, we will be interested in the application of this modulation to the GPS L1 signals and the Galileo L1 signals. We will give the general expression of the GPS L1 Interplex signal and show the modification which must be made on the general expression of the Interplex modulation to apply it to the case of the combination of the C/A and P(Y) codes with the new M-code. With regards to the Galileo signals we will study two different cases. In the first case we will consider that the OS signal is a classical BOC(1,1). In the second case we will make the study, assuming that the OS signal is the new signal called Composite Binary Coded Symbol (CBCS), recently published ([Hein et al., 2005]). To conclude, the theoretical formula of the power spectrum densities of the GPS L1 Interplex signals and of the Galileo L1 Interplex signal are given.