Coherent Adaptive Subcarrier Modulation (CASM) for GPS Modernization
Philip A. Dafesh, Thanh Minh Nguyen, Steve Lazar · 1999
GPS Modernization entails the addition of a new military signal (M-code) or signals to the established C/A- and P(Y)-codes, both at L1 and L2. The signal(s) is expected to be comprised of one of a class of modulation formats described as split spectrum codes or dual offset carriers. One of the key challenges of this new signal scheme for the GPS transmitter is to transmit the C/A, P(Y), and M-code signals in a flexible efficient, constant envelope implementation, compatible with goals of the GPS modernization studies. In this work, we present the development of Coherent Adaptive Subcarrier Modulation (CASM), a high-efficiency constant-envelope spread-spectrum subcarrier modulation method for GPS that is an evolution of established subcarrier modulation techniques found in numerous space systems and complementary to recent M-code signal developments. The technique is applicable to the transmission of military acquisition (MA) and military tracking (MT) signals in a flexible and efficient manner. The high efficiency approach is presented for both combined-aperture (C/A, P(Y), MA and MT through the same upconverter amplifier chain and antenna) and separate-aperture (MA and MT transmitted out of a separate upconverter, amplifier chain and antenna from C/A and P(Y)) implementations, as termed in the current GPS modernization development. Since the more difficult challenge of the these two implementations is the combined-aperture modulation approach, the focus of the paper will be to describes that approach as a unique constant-envelope implementation of the CASM waveform. In particular, this form of the CASM waveform will be shown to be a highly efficient means of quadrature multiplexing new GPS military acquisition and tracking signals with flexibility in adjusting the relative power of combined signals without greatly altering the power efficiency of the GPS waveform.