This invention relates to a system for radio navigation and communication and which may be employed for either or both.
It has also been found that digital data modulation can be added to the navigation code modulation without increasing the energy requirements. Moreover, the navigation modulation may be used for developing synchronization signals for the digital modulation.
In radio communication systems for the transmission of digital data, synchronization bits are transmitted with the data for the achievement of frame and bit synchronization at the receiver. High power levels at each bit (e.g. to achieve a 12-14 db signal to noise ratio) are often needed to ensure recognition of the received synchronization bits. At lower power levels (e.g. 6 db), a failure to recognize the synchronization bits leads to loss of synchronization with the received data and substantial bit errors result. Thus, it has been found desirable to provide a synchronization system which is suitable at low power levels.